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Photoelectron spectroscopy of gases Vroom, David Archie S. 1966

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The University of British Columbia FACULTY OF GRADUATE STUDIES PROGRAMME OF THE FINAL OPAL EXAMINATION , FOR THE DEGREE OF DOCTOR OF PHILOSOPHY of DAVID A„ VROOM B.Sc (HonSo) University of Br i t i s h Columbia, 1963 MONDAY, NOVEMBER 28TH, I966 at 3»30 p»m, IN ROOM 261, CHEMISTRY BUILDING COMMITTEE IN CHARGE Chairman: F, A„ Kaempffer C. A., McDowell C E „ Brioxi F. W. Dalby External Examiner: E. Piers A. V. Bree Dc Co Walker Sir Harrie Massey, F<,R„S. Department of Physics University College Gower Street, London, W.C.I England Research Supervisor: D. C„ Frost PHOTOELECTRON SPECTROSCOPY OF GASES ABSTRACT Photoelectron spectroscopy i s the study of the kinetic energies of the photoelectrons ejected from gaseous species by monochromatic radiation., Sub-tration of these kinetic energies from the incident photon energy yields the binding energies of the orbitals from which the electrons were removed. The work to be described f a l l s into two main partso The f i r s t i s concerned with the development of a new type of spectrometer to measure such kinetic energies. To date s a l l photoelectron spectrometers designed specifically for gaseous samples have employed a retarding f i e l d of cylindrical geometry to energy-analyze the emitted photoelectrons. Con-sideration of the angular distribution with which electrons are ejected during the photoionization process indicates that a spectrometer u t i l i z i n g a retarding f i e l d of spherical geometry should give a stopping curve., the f i r s t d i f f e r e n t i a l of which is close to the true photoelectron kinetic energy spectrum. Instruments of both cylindrical and spherical geometry were constructed and their properties tested. Conclusive evidence for the -superiority of the spherical system is presented together with details of i t s construction and operation. The second part of this thesis contains the results of photoelectron spectroscopic studies on twenty-one atoms and molecules (Ar, Kr, Xe s H2, HD, I>2, N2, CO, 02, NO, HF, HC1, HBr, HI, F 2, C l 2 , Br 2, I 2 , N-0,.N02 and l l ^ j . The energies of the ionic states could be obtained to a pre-cision of OoOl eVo, and they agree well with available spectroscopic data. In many instances new ionic states are found, and where possible they are correlated with states predicted by molecular orbital theory. Relative transition probabilities to the various ionic states are also obtained by this method. They are, in nearly every case, the only experimental values availabl The spacihgs and relative probabilities for formation of ionic vibrational levels have been measured for certain states i n H2, HD, D2, N2, CO, 0 2, NO, F 2, K 20 and N0 2 and the values obtained compared with spectroscopic and calcu-lated data where this i s available. GRADUATE STUDIES Fi e l d of Study: Chemistry Topics i n Physical Chemistry Seminar in Chemistry Quantum Chemistry Theoretical Chemistry Topics i n Inorganic Chemistry J. A . R. Coope A* V. Bree Ho Bartlett J. Ao R„ Coope W„ C. L i n C.. A , McDowell C. E. Brion W. Ro Cullen N. Bartlett Ro C„ Thompson Spectroscopy and Molecular Structure A . V* Bree Cu Reid B. A . Dunell Chemical Kinetics Topics i n Organic Chemistry Related Studies: Modern Physics Computer Programming PUBLICATIONS • E„ Ao Ogryzlo N, Basco Do C, Walker D„ E. McGreer J. Po Kutney F. McCapra* Mo Bloom A . Go Fowler Do C. Frost, Co A. McDowell, D. A. Vroom - "Photoelect-ron Spectroscopy with"a Spherical Analyzer. The Vib-rational Energy Levels of H 2 + 0" Phys. Rev. Letters ; p^, 612, (1965). D„ Co Frost, Jo S„ Sandhu, D. A„ Vroom - "A single Grid Photoelectron Spectrometer for Gases"» Nature 212 3 60k, (1966). Do C„ Frost, C„ A, McDowell, D» A a Vroom - "Photoelect-ron Kinetic Energy Analysis i n Gases by means of a Spherical Analyzer". Prop „. Roy. _ Soc. A (19.6? ) 0 PHOTOELECTRON SPECTROSCOPY OF GASES by DAVID A. S. VROOM B.Sc. University of B r i t i s h Columbia, 1963 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in the Department of CHEMISTRY We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA October, 1966. In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for ex-tensive copying of this thesis for scholarly purposes may be gran by the Head of my Department or by his representatives. It is understood that copying or publication of this thesis for finan-cial gain shall not be allowed without my written permission. Department of Chemistry  The University of British Columbia Vancouver 8, Canada Date November 29, 1966-ABSTRACT Photoelectron spectroscopy is the study of the kinetic energies of the photoelectrons ejected from gaseous species by monochromatic radiation. Subtraction of these kinetic energies from the incident photon energy yields the binding energies of the orbitals from which the electrons were removed. The work to be described falls into two main parts. The first is concerned with the development of a new type of spectrometer to measure such kinetic energies. To date, all photoelectron spectrometers designed specifically for gaseous samples have employed a retarding field of cylindrical geometry to energy-analyze the emitted photoelectrons. Consideration of the angular distribution with Which electrons are ejected during the photoioriization process indicates that a spectrometer utilizing a retarding field of spherical geometry should give a stopping curve, the first differential of which is close to the true photoelectron kinetic energy spectrum. Instruments of both cylindrical and spherical geometry were constructed and their properties tested. Conclusive evidence for the superiority of the spherical system is presented together with details of its construction and operation. The second part of this thesis/contains the results of photoelectron spectroscopic studies on twenty-one atoms and molecules (Ar, Kr, Xe, H2, HD, D2, N 2 > CO, 0 2 > NO, HF, HC1, , HBr, HI, F 2, C l 2 , Br 2, I2» N 20, NO2 and NH^). The energies of the io n i c states could be obtained to a precision of 0.01 ev., and they agree, well with available spectro-scopic data. In many instances new ionic- states.are found, and where possible they are correlated with states predicted by molecular o r b i t a l theory. Relative t r a n s i t i o n p r o b a b i l i t i e s to the various ionic states are also obtained by th i s method. They are, i n nearly every case, the only experimental values available. The spacings and r e l a t i v e p r o b a b i l i t i e s for formation of ionic v i b r a t i o n a l levels have been measured for certain states i n H^, HD, D2, N 2, CO, 0 2, NO, ? 2, N 2Q and N0 2 and the 1 values obtained compared with spectroscopic and calculated data where th i s i s available. ACKNOWLEDGEMENT I wish to express my gratitude to Dr. D. C. Frost for suggesting the problem and for his invaluable help and guidance throughout the course of this investigation; I also wish to thank Professor C. A. McDowell for his continued interest in this work. I would also like to thank Dr. C. E. Brion, Dr. G. J. Krige and Mr. G. E. Thomas for their helpful discussions and assistance during the preparation of this thesis. I take this opportunity to acknowledge the skillful assistance of the technical staff of the Department: of Chemistry during the development of the instruments and to thank Miss Donna Symons for typing the manuscript. I wish to express my appreciation to the National Research Council of Canada for generous financial support in the form of National Research Council Bursaries and Scholarships. TABLE OF CONTENTS Page ABSTRACT i i ACKNOWLEDGEMENT i v CHAPTER ONE - INTRODUCTION 1 1.1 Determination o f I o n i z a t i o n P o t e n t i a l s 2 1.1.1 Absorption Spectroscopy 3 1.1.2 E l e c t r o n Impact 5 1.1.3 P h o t o i o n i z a t i o n 7 1.1.4 Photoelectron Spectroscopy 8 CHAPTER TWO - THEORETICAL 11 2.1 The I o n i z a t i o n Process 11 2.1.1 Production of Photoelectrons 11 2.1.1.1 D i r e c t I o n i z a t i o n 11 2.1.1.2 A u t o i o n i z a t i o n 15 2.1.1.3 Angular Dependence o f Photoelectron Emission 18 2.1.2 I o n i z a t i o n o f Atoms 20 2.1.3 I o n i z a t i o n and D i s s o c i a t i v e I o n i z a t i o n o f Molecules 20 2.2 Design o f Retarding F i e l d Analyzers 23 vi Page 2.2.1 The Parallel Plate Analyzer 23 2.2.2 The Concentric Sphere Analyzer 25 2.2.3 Effects of Electron Reflection and Magnetic Fields 26 2.3 Evaluation of Experimental Results 28 2.3.1 Calculation of Atomic and Molecular Energy Levels 28 2.3.2 The Francks-.Gondon. Principle 30 2.3.2.1 General. Considerations 30 2.3.2.2 Application, to Photoelectron Spectroscopy 34 2.3.2.3 The Energy of Fragment Ions 37 2.3.3 Assignment of Atomic and Molecular Energy Levels 38 2.3.4 Spin-Orbit Coupling in Atoms and Molecules 41 CHAPTER THREE - EXPERIMENTAL 43 3.1 Construction of the Photoelectron Spectrometer 43 3.1.1 General Features of Photoelectron Spectrometers; 43 3.1.2 The Light Source 45 3.1.3 The Grid and Collector Systems 46 3.1.3.1 Cylindrical System 46 3.1.3.2 Spherical, System 49 3.1.4 Associated Electronics 52 3.1.5 Vacuum System 56 3.2 Operation of the Photoelectron Spectrometer 59 v i i Page 3.2.1 General Operation 59 3.2.2 Positive Ion Curves 60 3.2.3 First Differential Curves 61 3.3 Treatment of Data 64 3.3.1 Cylindrical System 64 3.3.2 Spherical System 64 CHAPTER FOUR - RESULTS AND: DISCUSSION 65 4.1 Characteristics, of-the Spectrometers; The Rare Gases - Argon, Krypton, Xenon 65 4.1.1 Evaluation of the Instrument 65 4.1.2 Results and Discussion 71 4.2 Hydrogen, Deuterium Hydride and Deuterium 78 4.2.1 General Considerations 78 4.2.2 Experimental 78 4.2.3 Results and Discussion 79 4.3 Nitrogen 92 4.3.1 General Considerations 92 4.3.2 Results and Discussion 93 4.4 Carbon Monoxide 102 4;4.1 General Considerations 102 4.4.2 Results and Disucssion 102 4.5 Oxygen 109 4.5.1 General Considerations 109 4.5.2 Results and Discussion 111 v i i i 4.5.3 The 0 + Fragment Ion 4.5.4 Transition Probabilities Ionic Levels of Q2 4.6 Nitric Oxide 4.6.1 ; General Considerations 4.6.2 Results and Discussion 4.7 The Hydrogen Halides 4.7.1 General Considerations 4.7.2 Experimental 4.7.3 Results and Discussion 4.8 The Halogens 4.8.1 General Considerations 4.8.2 Experimental 4.8.3 Results and Discussion 4.9 Nitrous Oxide 4.9.1 General Considerations 4.9.2 Results and Discussion 4.10 Nitrogen Dioxide 4.10.1 General Considerations 4.10.2 Results and Discussion 4.11 Ammonia 4.11.1 General Considerations 4.11.2 Results and Discussion CHAPTER FIVE - CONCLUSION REFERENCES Page 118 to the 121 123 123 124 132 132 133 134 152 152 153 155 171 171 172 177 177 179 186 186 186 190 192 LIST OF TABLES Page 2 2 I. The P3^2 - P^ 2 Ground State Separation in Ar +, Kr+, and Xe+ 76 2 2 II. , P^ 2 / P ^ 2 Transition Ratio for Ionization of Ar, Kr, and Xe at 584 A 76 III. Ionization Potentials of H 2 , H D , and D 2 84 85 IV. Vibrational Spacings for the H * (X 2I +) State • * • 6 V. Relative Transition Probabilities for H 2 (xV, v" = 0) - H * (xV, v'= 0-8) for 584 A Radiation 86 VI. Ionization Data for HD (X1!*, v" = 0) g •*• H D + (X2Z+, v' = 0-9) for 584 A Radiation 90 g VII. Ionization Data for D 2 (X1!*, v" = 0) -»• D t (X 2Z +, v' = 0-10) for 584 A Radiation 91 VIII. Ionization Potentials of Nitrogen 96 IX. Relative Transition Probabilities to Some Ionic o States of Nitrogen for 584 A Radiation 97 X. Ionization Data for N2 (X1!*, v" = 0).•"-»•'. N* (X 2Z +, v' = 0,1) for 584 A Radiation 97 ^ g XI. Ionization Data for N-CX1^, v" •» 0 ) -*• 2V g' J N2 ^ "u» V' = °"6) f o r 5 8 4 ^  R a d i a t i o n 9 9 XII; Equilibrium Interatomic Distances of Some and N* Electronic States XIII. Ionization Potentials for Carbon Monoxide XIV. Relative Transition Probabilities to the Ionic States of Carbon Monoxide for 584 A Radiation 1 1 XV. Ionization Data for CO (X £*, v" = 0) CQ+ (X2Z+, v' = Oyl) for 584 A Radiation XVI. Ionization Data for CO (X1!*, v" = 0) -+ C0+ (A2H, v' = 0-6) for 584 A Radiation XVII. Equilibrium Internuclear Distances of Some CO and C0+ Electronic States XVIII. Ionization Potentials for Oxygen XIX. Equilibrium Internuclear Distances of Some ©2 and 0* Electronic States XX. Vibrational Spacings for © 2 (X32.~, v" = 0) H- 0* (xV, v' = 0-4) XXI. Relative Transition Probabilities for 0_ (X3Z", v" =0) 0* (X2H , v' = 0-4) 2 g' -1 2 g' XXII. Relative Transition Probabilities to the O^  Ionic Levels for 584 A Radiation XXIII. , Ionization Potentials of Nitric Oxide XXIV. Ionization Data for NO (X 2^, v" = 0) -> N0+ ;CX1Z+, v' = 0-4) for 584 A Radiation XXV. Equilibrium Internuclear Distances for Some Molecular and Ionic States of Nitric Oxide X I Page XXVI. Relative Transition Probabilities to Some, e Ionic States of Nitric Oxide for 584 A Radiation 131 XXVII. Ionization Potentials of the Hydrogen HaflHer 139 XXVIII. Equilibrium Internuclear Distance for the Grotritd and Ionic States of the Hydrogen Halides 141 XXIX. 2 2 n 3/ 2 " n y2 Doublet Separations of the Hydrogen Halides 142 XXX. Ionization Data for HF (X1!*,- v" =0) - HF+ ( V , v'= 0-2) for 584 A Radiation 143 XXXI. Data Required for Calculation of DI for the Ionic States of the Hydrogen Halides 147 2 2 + XXXII; Dissociation Energies for the IL and £ Ionic States of the Hydrogen Halides 148 XXXIII. Relative Transition Probabilities to the Ionic States of the Hydrogen Ha/lides for 584 A Radiation 150 XXXIV. Ionization Potentials of the Halogens 160 XXXV. Ionization Data for F_(X 1Z +,v" = 0) 2V g' •> F* ( 2n g, v' = 0-3) for 584 A Radiation 162 2 9 XXXVI. n 3^ 2 - ^1/2 D o u b l e t Separations of the Halogens 164 o XXXVII. Data Required for Calculation of D» for the Ionic States of the Halogens 165 XXXVIII. Dissociation Energies of the Ionic States of the Halogens 166 x i i Page XXXIV. Relative Transition Probabilities to the Ionic. States of the Halogens for 584 A Radiation 168 XL. Ionization Potentials for Nitrous Oxide 174 XLI. Ionization Data for the Second Ionic Level of o Nitrous Oxide for 584 A Radiation 175 XLII. Relative Transition Probabilities to the Ionic Levels of Nitrous Oxide for 584 A Radiation 176 XLIII. Ionization Potentials of Nitrogen Dioxide 181 XLIV. Relative Transition Probabilities to the Ionic Levels of Nitrogen Dioxide for 584 A Radiation 183 XLV; Vibrational Data for the Highest Ionic State o of Nitrogen Dioxide for 584 A Radiation 185 XLVI< Ionization Potentials of Ammonia 187 XLVI.I. The Relative Transition Probabilities to the o Ionic Levels of Ammonia for 584 A Radiation 189 LIST OF FIGURES Page 1. . The Ionization of Oxygen 4 2. The Autoionization Process 17 3. The Franck-Condon Principle 33 4. Application of Franck-Condon Principle to Photoelectron Spectroscopy 36 5. The Microwave Cavity 47 6. The Cylindrical Photoelectron Spectrometer 48 7. The Spherical Photoelectron Spectrometer 50 8. A Spherical Grid 53 9. Schematic Diagram of Electron Retarding System 55 10. The Vacuum System 57 11. The Differentiating Circuits 62 12. Evaluation of Instrumental Characteristics 68 13. Comparison of Measured and Calculated Photoelectron Distributions for Xenon 70 14. Photoelectron Spectrum of Argon 72 15. Photoelectron Spectrum of Krypton 73 16. Photoelectron Spectrum of Xenon 74 17. First Differential Photoelectron Spectrum of Argon 75 18,. Comparison of Cylindrical and Spherical Analyzer Results for Hydrogen 80 xiv Page 19. Photoelectron Spectrum of Hydrogen 81 20. Photoelectron Spectrum of Deuterium Hydride 82 21. Photoelectron Spectrum of Deuterium 83 22. Hypothetical Photoionization Curve for Hydrogen 88 23. Photoelectron Spectrum of Nitrogen 94 24. Photoelectron Spectrum of Nitrogen - Krypton Mixture 95 25. Photoelectron Spectrum of Carbon Monoxide 103 26. Photoelectron Spectrum of Oxygen 112 27. First Differential Photoelectron Spectrum of Oxygen 113 28. Positive Ion Retarding Curve for Oxygen 119 29. Photoelectron Spectrum of Nitric Oxide 125 30. Photoelectron Spectrum of Hydrogen Fluoride 135 31. Photoelectron Spectrum of Hydrogen Chloride 136 32. Photoelectron Spectrum of Hydrogen Bromide 137 33. Photoelectron Spectrum of Hydrogen Iodide 138 34. The Bond Dissociation Energies of the Hydrogen Halides 146 35. Sample Inlet System for Corrosive Gases 154 36. Photoelectron Spectrum of Fluorine 156 37. Photoelectron Spectrum of Chlorine 157 38. Photoelectron Spectrum of Bromine 158 39. Photoelectron Spectrum of Iodine 159 40. Photoelectron Spectrum of Nitrous Oxide 173 41. Photoelectron Spectrum of Nitrogen Dioxide 180 42. Photoelectron Spectrum of Ammonia 188 CHAPTER ONE INTRODUCTION Spectroscopic techniques have been used extensively to study the electronic and nuclear structure of atomic and molecular species. In fact much of chemistry, which has as its foundation the properties of bound electrons, can be understood by application of information obtained from studies of this nature. Thus, for atoms, an understanding of the arrangement and angular momenta of the electrons surrounding the nucleus, discovery of the electron spin, and theoretical interpretation of the periodic system are some of the important concepts which have evolved from spectroscopic studies. In addition, these fundamental principles of atomic spectra have enabled the processes of molecular formation to be better understood and have supplied a basis for development of the theories of molecular spectra. Basically, spectroscopy involves a study of the transitions between the various energy levels, or states, of atoms or molecules. Whereas in atoms,, one has only to consider transitions between electronic states, the additional degrees of freedom in molecules may lead to transitions involving electronic vibrational and rotational states. Transitions between the various states correspond to absorption or emission of energy by the system, and, providing dissociation does not occur either by ejection of an electron, or, in the case of a molecule, by rupture of a bond, the energy changes 2 must be quantized. This quantum r e s t r i c t i o n no longer a p p l i e s i n cases where d i r e c t i o n i z a t i o n occurs. Consequently i o n i z a t i o n can occur at any energy above a c e r t a i n l i m i t corresponding to the removal of the most l o o s e l y bound e l e c t r o n (the i q n i z a t i o n p o t e n t i a l ) . Removal of any e l e c t r o n other than the most l o o s e l y bound one r e s u l t s i n the formation of an e x c i t e d s t a t e of the i o n . The minimum energy r e q u i r e d to form an e x c i t e d i o n i s r e f e r r e d to as an inner i o n i z a t i o n p o t e n t i a l . A knowledge of the f i r s t and inner i o n i z a t i o n p o t e n t i a l s of a species i s of b a s i c s p e c t r o s c o p i c i n t e r e s t . Because the energy r e q u i r e d f o r removal of an e l e c t r o n i s very n e a r l y equal to the b i n d i n g energy of the o r b i t a l from which i t i s removed, i o n i z a t i o n p o t e n t i a l s can be used to e s t a b l i s h the absolute o r b i t a l b i n d i n g energies i n the n e u t r a l species (1). I o n i z a t i o n p o t e n t i a l s are a l s o u s e f u l i n other branches of chemistry, f o r example i n thermochemical c a l c u l a t i o n s of bond energies and i n the determination of the s t a b i l i t y o f v a rious chemical compounds (2). The r e l a t i v e p r o b a b i l i t i e s f o r i o n i z a t i o n from the various o r b i t a l s i s a l s o of i n t e r e s t , and there are at present very few r e l i a b l e sources of t h i s i n f o r m a t i o n , important i n nuclear f u s i o n research (3) and i n s t u d i e s of the upper atmosphere (4). 1.1 Determination of I o n i z a t i o n P o t e n t i a l s Several methods have been developed f o r the measurement of i o n i z a t i o n p o t e n t i a l s . The accurate determination of these values i s o f great importance s i n c e t h e o r e t i c a l approaches to the problem 3 have' been r e l a t i v e l y u n successful. The c a l c u l a t e d . i o n i z a t i o n p o t e n t i a l s u s u a l l y - l i e o utside the e r r o r l i m i t s .of -the; measured values. The more important experimental techniques which are a v a i l a b l e are discussed below. 1.1.1. Absorption Spectroscopy O p t i c a l spectroscopy provides the most accurate means of determining i o n i z a t i o n p o t e n t i a l s . In t h i s method data obtained from an absorption spectrum i s f i t t e d i n t o a Rydberg s e r i e s , the convergence l i m i t o f which gives the energy r e q u i r e d f o r i o n i z a t i o n . This method, although very accurate, can only be used i n cases where a n a l y s i s of^the spectrum allows i d e n t i f i c a t i o n of a s u f f i c i e n t number of Rydberg l e v e l s . Consequently i t i s u s u a l l y p o s s i b l e to determine i o n i z a t i o n p o t e n t i a l s i n t h i s manner only f o r atoms (5,6) and a few simple molecules (7 ,8 ) . Often only one or two of the many i o n i z a t i o n p o t e n t i a l s a s s o c i a t e d with-a simple molecule can be derived i n t h i s way. A t y p i c a l absorption spectrum i s shown i n Figure 1(a) which de p i c t s the r e s u l t s obtained by Huffman, Larrabee, and Tanaka (9) f o r the oxygen molecule. F i v e i o n i z a t i o n p o t e n t i a l s are i n d i c a t e d , and a l l except the f i r s t are obtained as l i m i t s of Rydberg s e r i e s . The many peaks observed above the- i o n i z a t i o n t h r e s h o l d may be a s s o c i a t e d w i t h Rydberg s e r i e s l e a d i n g to inner i o n i z a t i o n p o t e n t i a l s . Figure 1(a) gives the t o t a l absorption spectrum f o r oxygen. The r e l a t i v e number of photons absorbed at any wavelength depends on 4 Figure 1. The Ionization of Oxygen. 5 the number of processes occurring at that energy. For atoms, the absorption cross-section will closely approximate the ionization cross section whereas for molecules other processes, for example dissociation, may occur. This means that absorption spectroscopy is not always a valid means of obtaining molecular photoipnization cross sections. 1.1.2 Electron-Impact' A widely applicable method for the determination of ionization potentials involves electron bombardment of the sample gas. The energy of the electron beam is gradually increased from a value below the species' ionization potential until the ionization threshold is observed. This type of experiment can be done in a simple total ionization apparatus (10) or may employ a mass spectrometer for ion detection (11-13). Normally, the electron beam used to ionize the sample gas is obtained by thermionic emission from a heated filament, and consequently, the energy distribution of the electrons is of the order of 1 electron-r volt (ev.). This leads to difficulties in determining the onset energy of the first ionization process (1) and usually makes the observation of inner ionization potentials impossible. Attempts have been made to decrease the electron beam energy spread; the two most widely used methods being the electron velocity selector (14-16) and the retarding potential difference (RPD) technique (17,18). The first of these provides electrostatic velocity selection of the thermionic electron distribution from a filament to obtain a narrow energy band of electrons. In the latter technique a 6 narrow electron energy distribution is simulated by electronically retarding the emitted electron energy distribution at two energies separated by approximately 0.1 ev. The ion current produced by each 'sliced' distribution is measured and the difference between them ;; is the current produced by the difference between the two electron distributions, ideally a 0.1 ev. band. The ion current differences are plotted as a function of electron energy, and an ionization efficiency curve is obtained. Figure 1(c), which clearly shows the first and inner ionization potentials of oxygen, represents an RPD curve obtained by Frost and McDowell (19). Two mathematical methods of obtaining inner ionization potentials from ionization efficiency curves have been described. Morrison (20) has reported a technique which involves the elimination of the electron energy spread by deconvolution of the ionization efficiency curve produced using a conventional ion source. More recently a much simpler method for revealing fine structure in such a curve has been developed by Winters, Collins, and Courchene (21). Despite the large number of workers in the field, the experimental difficulties inherent in monoenergetic electron impact studies have limited the number of reproducible results obtained. The ionization potentials measured by electron impact are often slightly greater than those obtained by spectroscopic techniques. This might arise from one or more of three factors: calibration error in the electron energy scale, the difference in threshold law for 7 the two processes, or from a difference in the mechanism of the ionization process for charged particles as opposed to neutral ionizing radiation. 1.1.3 Photoionization The photoionization technique may be regarded as a combination of the methods of optical spectroscopy and electron impact. The experimental methods are the same as those used in electron impact except that a beam of photons is substituted for the beam of electrons. This technique is advantageous in that the energy of the radiation is easily calibrated and a narrow photon energy distribution may be obtained without difficulty. There are,; however, some disadvantages inherent in the method. A light source producing sufficiently intense radiation in the energy range under consideration is required. A vacuum ultraviolet; monochromator, preferably without optical windows must be employed. Such a system does not produce radiation of high intensity and requires very efficient vacuum pumping. Apart from experimental difficulties, photoionization curves usually exhibit peaks which tend to obscure the inner ionization potentials. These peaks result from auto-ionization, a process involving radiationless transitions from bound molecular; excited states of the species to the ionization continuum. Figure 1(b) depicts the photoionization curve obtained by Mak (22) for oxygen - a good example of a strongly autoionizing species. The photoionization technique has been used by several workers to obtain ionization potentials both with (23,24) and 8 without (25,26) mass analysis. Total ionization cross sections may also be obtained from photoionization data, but the relative cross sections to the various ionic states present cannot be found as they are usually obscured by autoionization peaks. 1.1.4 Photoelectron Spectroscopy Photoelectron Spectroscopy was first developed as a method of obtaining accurate ionization potentials by Vilessov, Kurbatov and Terenin (27,28) in 1961. The principleof the method, the photoelectric. effect, was first give"n by Einstein, who postulated that when a photon has more than sufficient energy to eject an electron from a substance the excess energy manifests itself as kinetic energy of the particles produced. Consequently i f a photon of energy hv (h, Planck's Constant; v , frequency of the radiation) ionizes a gaseous sample, the resulting photoelectron and ion will part with a total kinetic energy given by hv less the appropriate ionization potential of the species, I. This may be expressed as K.E. = hv - I (1-1) As shown in a,later section, conservation of momentum ensures that the photoelectron carries away virtually all of this energy. , It ,. follows that i f a gas is irradiated by a monochromatic photon beam, there will result as many groups of photoelectrons as there are ionic energy levels attainable through absorption of an incident photon. Kinetic energy measurements will enable the ionization potential I, in each case, to be deduced from equation (1-1), and the group intensities 9 will be proportional to the relative transition probabilities to the respective energy levels. The electron kinetic energies may conveniently be determined by using retarding potential techniques, in which case the photoelectron intensity is plotted as a function of retarding voltage. Such a curve exhibits a series of steps, the onset of each representing the collection of the electrons from a particular ionization process. Photoelectron spectroscopy circumvents many of the difficulties encountered in the other methods available for the determination of ionization potentials. Like photoionization and electron impact methods, i t is applicable to a wide range of substances, but unlike these methods the instrumentation is relatively simple and consequently the results are much more easily obtained. The problems associated with the detection of low ion currents near threshold which arise in electron impact seldom occur since the cross-section for photon impact at the ionization threshold is usually far greater than i t is for particle impact. Ionization potentials can be compared directly with the accurate spectroscopic values reported in the literature, and, since the substance is always irradiated at one wavelength, auto-ionization, which is a resonant process, cannot obscure the ionization processes. Two disadvantages associated with the method are the lack of mass analysis to assist in the identification of the ions formed, and the necessity of calibrating the energy scale to obtain accurate ionization potentials. This latter factor can introduce a slight 10 error (usually about +_ 0.01 ev.) into the absolute values of the measured ionization potentials. Figure 1(d) shows the first derivative of the photoelectron retarding curve for oxygen obtained in this work. The peaks on the curve represent transitions to the zeroth and excited vibrational levels corresponding to the appropriate electronic level of the ion. Auto-ionization does not mask the energy levels and accurate values can be obtained for the various ionization potentials. On the direct stopping curve (Figure 26), the heights of the steps are proportional to the transition probabilities from the ground state to the various ionic states for radiation of the wavelength used here. This study presents the results; obtained by photoelectron spectroscopy for a variety of atoms and simple molecules. The first and inner ionization potentials have been measured and the corresponding electronic states identified. The relative transition probabilities to different levels have been derived from the curves. The experimental methods used are superior to those which have been reported in the literature. Vilessov, Kurbatov and Terenin used a Lozier type apparatus (32) coupled to a vacuum ultraviolet monochromator (27,28) to measure the kinetic energy of the photo-electrons, but lithium fluoride optical windows restricted their incident energy to below 11.7 ev. Schoen (29) used a similar instrument without optical windows. Recently Turner and Al-Joboury (30-31) presented results obtained with an instrument in which they had eliminated both optical windows and monochromator. They used a 11 o light source producing monochromatic radiation of wavelength 584 A (21.21 ev. photon energy). The spectrometer used in this study is similar to that described by Al-Joboury and Turner (31) although several modifications have been made which significantly improve the resolution and general performance. CHAPTER TWO T H E O R E T I C A L 2.1 T h e I o n i z a t i o n P r o c e s s A t o m s o r m o l e c u l e s may b e i o n i z e d b y p a r t i c l e s o r l i g h t q u a n t a o f s u f f i c i e n t e n e r g y . F o r m o s t s p e c i e s t h e e n e r g y r e q u i r e d i s i n e x c e s s o f 4 e v . (33). T h e i o n i z a t i o n p r o c e s s may b e a d i r e c t t r a n s i t i o n o f a b o u n d e l e c t r o n i n t o t h e i o n i z a t i o n c o n t i n u u m , o r may t a k e p l a c e b y a n i n d i r e c t p r o c e s s , f o r e x a m p l e a u t o i o n i z a t i o n . 2.1.1 P r o d u c t i o n o f P h o t o e l e c t r o n s 2.1.1.1 D i r e c t I o n i z a t i o n T h e minimum e n e r g y r e q u i r e d f o r d i r e c t r e m o v a l o f t h e m o s t l o o s e l y b o u n d e l e c t r o n o f a n a t o m o r m o l e c u l e c o r r e s p o n d s t o t h e i o n i z a t i o n p o t e n t i a l o f t h a t s p e c i e s . I f m ore t h a n t h i s minimum amount o f e n e r g y i s s u p p l i e d t o t h e s y s t e m , d i r e c t i o n i z a t i o n c a n s t i l l o c c u r , w i t h t h e e x c e s s e n e r g y a p p e a r i n g a s k i n e t i c e n e r g y o f t h e p a r t i c l e s l e a v i n g t h e i o n i z a t i o n s i t e . T h e r e m o v a l o f more t i g h t l y b o u n d e l e c t r o n s f r o m a n a t o m o r m o l e c u l e c o r r e s p o n d s t o t r a n s i t i o n s i n t o i o n i z a t i o n c o n t i n u a , t h e o r i g i n s o f w h i c h l i e h i g h e r i n e n e r g y t h a n t h a t o f t h e f i r s t i o n i z a t i o n p o t e n t i a l . H e r e a g a i n a n y e n e r g y i n e x c e s s o f t h a t r e q u i r e d f o r i o n i z a t i o n i s t r a n s f o r m e d i n t o k i n e t i c e n e r g y o f t h e r e s u l t a n t 12 p a r t i c l e s . I n some c a s e s t h e r e m o v a l o f a s i n g l e e l e c t r o n c a n g i v e r i s e t o m o r e t h a n o n e i o n i c s t a t e , t h a t i s t h i s p r o c e s s c a n h a v e more t h a n o n e R y d b e r g s e r i e s l i m i t a s s o c i a t e d w i t h i t . T h i s s i t u a t i o n a r i s e s when t h e i o n f o r m e d b y r e m o v a l o f t h e e l e c t r o n i s i n some e x c i t e d s t a t e . T h e s e s t a t e s , w h i c h a r i s e t h r o u g h s u c h p r o c e s s e s a s s p i n - o r b i t i n t e r -a c t i o n o r s i m u l t a n e o u s e x c i t a t i o n o f a n e l e c t r o n o t h e r t h a n t h e o n e b e i n g r e m o v e d , a r e o t h e r s o u r c e s o f i n n e r i o n i z a t i o n p r o c e s s e s ( 3 4 ) . I n a t y p i c a l p h o t o i o n i z a t i o n e x p e r i m e n t a g a s i s s u b j e c t e d t o a beam o f m o n o c h r o m a t i c r a d i a t i o n o f v a r i a b l e e n e r g y a n d t h e i n t e n s i t y o f i o n i z a t i o n i s m e a s u r e d a s a f u n c t i o n o f t h e i o n i z i n g r a d i a t i o n e n e r g y . I n t h i s way t h e t h r e s h o l d e n e r g y a n d e n v e l o p e o f t h e i o n i z a t i o n e f f i c i e n c y c u r v e a r e d e t e r m i n e d . I t h a s b e e n shown b o t h t h e o r e t i c a l l y ( 3 5 , 3 6 ) a n d e x p e r i m e n t a l l y (37 ) t h a t t h e p r o b a b i l i t y f o r i o n i z a t i o n r i s e s s h a r p l y a t t h e t h r e s h o l d o f e a c h p r o c e s s a n d t h e n t e n d s t o a s s u m e a c o n s t a n t v a l u e . A b o v e t h r e s h o l d , i o n i z a t i o n w i l l o c c u r w i t h a c e r t a i n p r o b a b i l i t y w h i c h d e p e n d s o n t h e w a v e l e n g t h o f t h e i m p i n g i n g r a d i a t i o n a n d t h e n a t u r e o f t h e s a m p l e g a s . I n g e n e r a l , t h e p r o b a b i l i t y f o r i o n i z a t i o n t e n d s t o d e c r e a s e t o h i g h e r e n e r g y . T h e r e a r e , h o w e v e r , e x c e p t i o n s t o t h i s , f o r e x a m p l e a r g o n , w h e r e t h e c r o s s s e c t i o n f o r i o n i z a t i o n b y p h o t o n s i n c r e a s e s f o r a b o u t 5.5 e v . a b o v e t h r e s h o l d ( 3 8 ) . When p r o c e s s e s s u c h a s e l e c t r o n c a p t u r e a n d i o n - p a i r f o r m a t i o n a r e e x c l u d e d , s i m p l e i o n i z a t i o n b y p a r t i c l e s c a n b e shown t o h a v e a low p r o b a b i l i t y a t t h r e s h o l d , r i s i n g t o a maximum a t a n 13 e n e r g y w e l l a b o v e t h e o n s e t ( 3 9 , 4 0 ) . T h i s d i f f e r e n c e f r o m t h e p h o t o i o n i z a t i o n c a s e c a n b e a t t r i b u t e d t o t h e number o f p a r t i c l e s p r e s e n t a f t e r i o n i z a t i o n h a s o c c u r r e d ( 4 1 ) . T h e g e n e r a l t e n d e n c y f o r t h e p h o t o i o n i z a t i o n p r o b a b i l i t y t o d e c r e a s e a t h i g h e r i o n i z i n g e n e r g i e s i m p l i e s t h a t t h e maximum p r o b a b i l i t y f o r p r o d u c t i o n o f i o n s i n a p a r t i c u l a r s t a t e w i l l o c c u r a t o r n e a r t h e t h r e s h o l d o f t h a t p r o c e s s . T h u s i f r a d i a t i o n o f e n e r g y j u s t s u f f i c i e n t t o r e m o v e a n i n n e r s h e l l e l e c t r o n i m p i n g e s o n a s p e c i e s , a c o n s i d e r a b l e n u mber o f t h e i o n s f o r m e d w i l l a r i s e f r o m t h i s i n n e r s t a t e . F o r p a r t i c l e i o n i z a t i o n h o w e v e r , t h e p r o b a b i l i t y o f r e m o v a l o f a n o u t e r s h e l l e l e c t r o n i s a l w a y s much g r e a t e r t h a n t h a t f o r a n i n n e r s h e l l e l e c t r o n . F u r t h e r m o r e , u n l i k e p a r t i c l e i o n i z a t i o n , i n w h i c h t h e p r o b a b i l i t y f o r m u l t i p l e i o n i z a t i o n i s a p p r e c i a b l e , t h e c h a n c e o f r e m o v i n g m o re t h a n o n e e l e c t r o n f r o m a s p e c i e s b y p h o t o n i m p a c t i s v e r y s m a l l . T h e s e d i f f e r e n c e s c a n b e a s c r i b e d t o t h e f a c t t h a t a n i n c o m i n g p h o t o n d o e s n o t p e r t u r b t h e a t o m i c o r m o l e c u l a r s y s t e m i n t h e same m a n n e r as a s l o w e l e c t r o n ( 3 3 ) . To a f i r s t a p p r o x i m a t i o n , t h e c r o s s - s e c t i o n f o r p h o t o -i o n i z a t i o n c a n b e c a l c u l a t e d b y c o n s i d e r i n g t h e i n t e r a c t i o n o f a . p h o t o n w i t h a n a t o m i c s y s t e m t o b e . a n a l o g o u s t o t h e i n t e r a c t i o n o f a n e l e c t r o m a g n e t i c wave w i t h a n e l e c t r i c d i p o l e . E v e n t h o u g h n o p e r m a n e n t d i p o l e e x i s t s i n a n a t o m i c s y s t e m i n t h e a b s e n c e o f a n e l e c t r o m a g n e t i c f i e l d , t h e a c t i o n o f t h e i n c o m i n g r a d i a t i o n i n t r o d u c e s n o n - z e r o m a t r i x e l e m e n t s i n t o t h e wave e q u a t i o n d e s c r i b i n g t h e e l e c t r o n i c s y s t e m . T h e s e , n o n - z e r o m a t r i x e l e m e n t s c o r r e s p o n d t o a n 14 overlap of two of the states of the system and make possible transitions between them. In the case of ionization the two states involved are a bound state of the neutral species and some ionization continuum associated with the removal of one of the bound electrons. The matrix element describing the overlap is therefore composed of a bound state wave function and a continuum wave function which describes both the ion and the ejected electron. If the wavelength of the incident radiation is long compared with the distance over which the wave functions are appreciable, the dipole approximation described above is satisfactory, and the operator describing the interaction of the two functions may be taken to be the dipole operator (33). Using the electric dipole approximation and assuming only a single electron to be involved in the transition, the expression for the photoionization cross-section is (42). o. = 327T 4m e 1 FT vvC / S». (Ir.) Y.dt (2-1) _,3 U K i i . * J J •> 3h c l J where a is the photoionization cross-section at some v • radiation frequency v . m,e are the mass and charge respectively of,an electron. h,c are Planck's constant and the velocity of light respectively. OK is the degeneracy of the initial state. £ , J denote sums over the init i a l and final states i j respectively. 15 V i s the v e l o c i t y of the e j e c t e d e l e c t r o n . C i s a f a c t o r to allow f o r the d i s t o r t i o n of the wave P f u n c t i o n of the passive e l e c t r o n s . i s the normalized wave f u n c t i o n f o r the bound i n i t i a l 1 s t a t e . c h a r a c t e r i z e s the d i p o l e moment operator. i s the normalized wave f u n c t i o n f o r the i o n and eje c t e d e l e c t r o n . d t i n d i c a t e s that the summation i s over a l l space. Equation (2-1) i n d i c a t e s that the c r o s s - s e c t i o n depends not only on the degree of overlap of the wave fu n c t i o n s d e s c r i b i n g the i n i t i a l and f i n a l s t a t e s , but a l s o on the frequency of the i n c i d e n t r a d i a t i o n and the v e l o c i t y of the e j e c t e d e l e c t r o n . Since the c r o s s -s e c t i o n f o r p h o t o i o n i z a t i o n g e n e r a l l y decreases with i n c r e a s i n g i n c i d e n t photon energy and e j e c t e d e l e c t r o n v e l o c i t y , i t f o l l o w s that the amount of overlap of the i n i t i a l and f i n a l s t a t e s must decrease r a p i d l y as the i o n i z i n g energy i s increased above the i o n i z a t i o n t h r e s h o l d . e x a c t l y f o r hydrogen-like systems (42). For more complicated systems, approximations must be used to o b t a i n c r o s s - s e c t i o n s and they o f t e n introduce u n c e r t a i n t i e s i n t o the values obtained. The d i p o l e approximation, equation (2-1), can be solved 2.1.1.2 A u t o i o n i z a t i o n Removal of an e l e c t r o n from an atom or molecule by a method 16 which does not involve a transition directly into the continuum is known as autoionization (or preionization). This process of indirect ionization is depicted in Figure 2. Series 1 represents the Rydberg levels leading to some ionization potential and series 2 the Rydberg levels leading to a higher one. If an electron is excited to a bound level in series 2 which is higher in energy than the ionization potential associated with series 1, then there exists the possibility of a radiationless transition from the bound state into the continuum. The electron emitted in this manner will have a kinetic energy equal to the difference between the energy of the bound state and the lower ionization potential. Unlike direct ionization, which can occur at any energy above the ionization threshold, autoionization is a resonant process occurring only at discrete energies which correspond to the bound levels of series 2. The process is manifested as a number of peaks on the ionization efficiency curve as shown in Figure 2. The theoretical aspects of autoionization have been investigated extensively. The phenomenon was first postulated by Shenstone (43) to explain the broadening and asymmetry of some absorption peaks above the ionization potential of certain species. This broadening was subsequently explained by invoking perturbation theory (34). The selection rules governing these radiationless transitions, which are discussed by Herzberg (44), have been tested and found to hold for the hydrogen molecule by Beutler and J linger (7). e l e c t r o n e x i t e n e r g i e s series 2 series 1 V 1' 1 2nd I.R st IP . p h o t o - i o n c u r r e n t pre-ionization of atoms after exitation to levels above the first I. P. 18 Fano (45) postulated configuration interaction between the discrete autoionizing levels and the continuum, and has studied theoretically the asynmetric peaks obtained for these levels. He has been able to predict the position and intensity shifts produced in a Rydberg series due to autoionization. 2.1.1.3 Angular Dependence of Photoelectron Emission In the design of photoelectron energy analyzers, the angular distribution of the electrons must be considered. The currently unknown nature of the distribution requires that the analyzer be constructed to measure the emission over the largest possible solid angle. If this is not done, the possibility arises that the results obtained using only a small solid angle might be a function not only of the ionization process occurring, but also of the angle of measurement. This latter possibility arises due to the prediction that electrons with different angular momenta in the bound state may be emitted with different angular distributions (46, 159). The Einstein equation (Equation (1-1)) indicates that, in the photoionization of a particle in the gaseous state, all the energy which is not absorbed by the ionization process will manifest itself as kinetic energy of the resultant ion-electron pair. Conservation of momentum demands that these two particles possess the same momentum. Because of the large disparity in mass between an ion and an electron, the electron must be ejected at a high velocity and therefore carry away most of the excess kinetic energy. In the least favourable case studied here, that of the hydrogen molecule, the mass difference is 19 approximately 4000 to 1 and the electron is therefore produced with about 4000 times the kinetic energy of the ion. The method of obtaining ionization potentials by measurement of photoelectron energies is therefore not subject to errors arising from ionic kinetic energies within the precision of the measurements of this work. The angular dependence of emitted photoelectrons has been studied theoretically by Heitler (47) using the Born approximation. He has shown, for a non-relativistic case, that the angular distribution of photoelectrons per unit solid angle is: dN a Sin 2e (2-2) dQ (1-6 cose) where dN is the number of electrons produced per unit solid angle, dft is the unit of solid angle. 6 is the angle between the incident photon beam and the emitted photoelectrons. 6 is the velocity of the emitted photoelectrons expressed as a fraction of the speed of light. For photons that have an energy comparable with the ionization potentials of the species being studied, as is the case in the experiments described here, the photoelectron velocities are much less than the speed of light and consequently the term 8cos6 in equation (2-2) is far smaller than unity. The angular distribution of 2 photoelectrons should therefore be proportional to sin 6 . For unpolarized light this corresponds to a dumb-bell shaped distribution 20 rotated about the axis of the incoming radiation. Equation (2-2) was obtained using the Born approximation and is therefore usually considered as being valid only in the range in which the energy of the ejected photoelectrons is large compared to the ionization potential of the species. The present study indicates, nevertheless, that the spatial arrangement of the photoelectrons 2 closely approximates that of sin 9 . This observation has been supported by the experimental results of Schoen (29). 2.1.2 Ionization of Atoms The only possible product that can result from removal of a single electron from an atom is an atomic ion. This process may. be represented by equation (2-3). A + hv •-»• A+ + e (2-3) The ion may be formed in its ground state or any available excited state. Since these states have definite energies and there is no other means of giving energy to the ionized species ( i . e . no vibrational or rotational excitation) the photoelectron retarding spectrum would be expected to give a sharp step corresponding to each electronic level. 2.1.3 Ionization and Dissociative Ionization of Molecules Molecules, unlike atoms, may have more than one product as the end result of an ionization process. That is to say, that apart from forming the parent molecular ion, which may.be electronically, vibrationally, and/or rotationally excited, there exists the 21 possibility of bond rupture. The products resulting from this process may be a charged and neutral fragment, or, in the case of ion pair formation, two charged particles. The amount of vibrational excitation that will be present in any molecular ion formed depends on the difference in equilibrium internuclear distance between the ground molecular state and the final ionic state. Ionic dissociation is either equivalent to a transition to a vibrational level above the dissociation limit of some ionic state, or to a transition to an unstable ionic state r In the former case, whether or not dissociation occurs is governed by the same factors that determine the amount of vibrational excitation. These factors will be discussed later in this chapter. The major processes resulting from the photoionization of molecules may be summarized as follows: Simple Ionization XY + hv •*• XY+ + e (2-4) Fragmentation of Parent ton XY + hv -+ XY+ + e (2-5) Dissociative Ionization XY + hv + X+ + Y + e (2-6) Ion Pair Formation XY + hv -»• XY* L + Y" (2 Any of the products of the above reactions may be electronically excited. 23 2.2 Design of Retarding Field Analyzers The most widely employed means for measurement of the kinetic energy of charged particles is the electrostatic retarding field. In the work reported here* two types of retarding analyzers were employed: one possessing cylindrical geometry, and the other spherical. For the purposes of this discussion the first system may be regarded as a parallel plate analyzer and the second as a concentric sphere analyzer. Although the analyzers can be used for any charged particles, only electrons will be considered here. A brief summary of the conclusions reached by Simpson (48) regarding the theoretical, resolution of these two analyzers is given below. 2.2.1 The Parallel Plate Analyzer Consider the following experimental arrangement. A beam of monoenergetic electrons is directed through an aperture in a flat plate at a fixed electrostatic potential. It is then retarded by a variable electric field applied between this plate and a collector plate parallel to i t . If the beam has no angular divergence a plot of the electron intensity against the retarding voltage should show a sharp cut off when the retarding field reaches a value such that the potential barrier which the electrons must overcome is equal to the electron kinetic energy. This ideal case is not realized in practice since the beam will always be somewhat divergent due to a) interelectron repulsion and b) inconsistencies of the electric field in the region of the aperture due to field penetration (the 24 electrostatic lens effect). The momentum of any electron must then be resolved into its components perpendicular to, and parallel to the collector plate. This division is represented as momentum component parallel to collector = tancj> momentum component perpendicular to collector (2-8) where <|> is the angle between an 'unretarded electron' trajectory and the perpendicular component. Since only the perpendicular component is affected by the retarding potential, and since the kinetic energy is proportional to the square of the velocity, the energy resolution for an initially monoenergetic beam of electrons of finite angular divergence is ^ = 1 - (cos2 40 = sin 2 4 (2-9) where AE is the observed energy spread E is the energy of the particles. The AE in equation (2-9) is therefore an apparent energy spread which is introduced due to the angular divergence of the electron beam. This implies that in any experiment where the kinetic energy of the electron beam is measured by a parallel plate analyzer, the angular divergence of the beam must be kept to a minimum. In photoelectron spectroscopy, as indicated in section 2.1.1.3 , the electrons are emitted with a wide angular distribution. 25 A parallel plate retarding analyzer would therefore not be expected to give the optimum energy resolution. Vilessov, Kurbatov and Terinin (27,28) and Schoen (29) have attempted to overcome this problem in their photoelectron experiments by employing a Lozier Tube (32) to eliminate this effect. 2.2.2 The Concentric Sphere Analyzer For systems in which an electron beam with a wide angular distribution is (or gives the appearance of having been) produced at a point source, an ideal retarding curve can be obtained with an analyzer of spherical geometry. The analyzer generally consists of a spherical grid concentric with a larger spherical collector. The electrons are produced at the center of the system and.the retarding field is applied between the two spherical electrodes. The paths followed by the electrons are not as simple in this case as in the parallel plate analyzer, but they may be found analytically. If an analyzer consisting of an inner spherical grid of radius a, and an outer collector of radius b is considered, then i t can be shown (48) that the energy resolution is AE ,a.2 . 2 . . -p- = (T-) sin Y (2-10) where y is the angle between an 'unretarded electron' trajectory and a radius to the surface of the collector. The angular divergence y in equation (2-10) arises in the same manner 26 as for the parallel plate system, i.e. due to interelectron repulsions and electrostatic lens effects. If the photoelectrons are generated at a point source, as may be arranged in a photoelectron spectroscopy experiment, then from the above discussion, i t would appear that a spherical retarding analyzer wquld be suitable for the measurement of their kinetic energies. In a later section of this thesis where the results of photoelectron measurements on both types of system are compared, the superiority of the spherical analyzer will be demonstrated. 2.2.3 Effects of Electron Reflection and Magnetic Fields To measure accurately the number of electrons possessing a particular kinetic energy, total collection of the beam in the retarding analyzer must be achieved. Reflection of electrons from the collector surface is one factor which may reduce the collection efficiency. For a polished collector surface, the amount of reflection occurring will depend on the type of metal used in construction and on the incident energy of ,the electrons striking the surface. (The greater the energy, the greater the probability of reflection). This dependence of the amount of reflection on the electron energy results in retarding curves which increase in intensity as the retarding potential is increased from zero, a maximum in electron current being reached just before the beam is stopped. This problem can be minimized by applying a coating of low electron reflectance material to the surface of the collector. In this work a coating of colloidal carbon (Aquadag) was used. 27 As indicated in equations (2-9) and (2-10), i f the best resolution is to be obtained with a retarding field analyzer, the angular divergence of the beam from a line perpendicular to the collector must be minimized. The presence of any magnetic fields in the analyzer region will cause the electrons to deviate from this perpendicular line with a resultant decrease in energy resolution. This effect can be decreased either by introducing an opposing magnet field to cancel the original or by shielding the retarding region with a high permeability material such as 'Mu-Metal':. The latter method was used in the work reported here and served to reduce the magnetic field in the retarding region from 0.5 gauss to less than 0.1 gauss. This resulted in an increase in resolution from 0.25 ev. to 0.15 ev. (Resolution defined as the ability of the instrument to completely separate adjacent ionic energy levels). 28 2.3 Evaluation of Experimental Results A suitable means of evaluating the experimental results is necessary i f the maximum information is to be extracted from the photoelectron retarding spectra. The most satisfactory means of attaining this objective would be comparison of the results obtained with theoretical predictions. In most cases however, such calculations are not reliable and other methods must be employed to supply the necessary information. In this section some of the methods used to calculate the molecular energy levels will be briefly discussed and following that a more qualitative means of interpretation in terms of the Franck-Condon Principle will be discussed. 2.3.1 Calculation of Atomic and Molecular Energy Levels Solutions to the Schroedinger equation will give the exact energy levels of a particular atomic or molecular system. With the exception of one electron systems, such solutions are not possible and various approximations must be used to obtain the desired wave functions. The particular method of approximation employed is determined by the size and nature of the species being considered, and the accuracy desired in the results. Many theoretical calculations, especially those entirely theoretical in nature do not yield values that are within an electron volt of the experimentally determined values. Calculations such as these are judged not only on their ability to give the correct energy but also on whether they give the correct order for the energy levels. 29 Most c a l c u l a t i o n s of the energies of e x c i t e d s t a t e s of molecules have stemmed from M u l l i k e n ' s molecular o r b i t a l c o n s i d e r a t i o n s and attempts to put these ideas on a more q u a n t i t a t i v e b a s i s . The s e l f - c o n s i s t e n t - f i e l d methods f o r closed s h e l l e l e c t r o n i c s t a t e s introduced by Roothaan (49) and l a t e r extended to open s h e l l s by Pople and Nesbet (50) have been used e x t e n s i v e l y to o b t a i n p u r e l y t h e o r e t i c a l estimates of energy l e v e l s . Another method in v o l v e s the assumption that f o r higher Rydberg s t a t e s (perhaps even with n = 3) the e l e c t r o n moves i n an atomic type o r b i t a l i n the p o t e n t i a l f i e l d o f the core n u c l e i and, other e l e c t r o n s . This then allows the energy l e v e l s of the molecule to be estimated (51,52). This method though not as accurate as the s e l f - c o n s i s t e n t - f i e l d procedure, i s much e a s i e r to compute. Apart from the above methods, there are s e m i - c l a s s i c a l c a l c u l a t i o n s such as the Rydberg-Klein-Rees (RKR) method (53) f o r diatomic molecules. In t h i s method the p o t e n t i a l energy curves f o r the various s t a t e s are obtained from the observed r o t a t i o n a l and v i b r a t i o n a l energy l e v e l s of the system. Semi-empirical c a l c u l a t i o n s have a l s o been used. One such method, the method of equivalent o r b i t a l s , f i r s t conceived by Lennard-Jones (54, 55) and H a l l and Lennard-Jones (56) has been used by H a l l (57, 58) and Fueki (59J to o b t a i n the e l e c t r o n i c s t a t e s of many saturate d hydrocarbons. In general, c a l c u l a t i o n s of e l e c t r o n i c l e v e l s of atoms and molecules are i n a c c u r a t e . Experimentally obtained values are, 30 therefore, of value in testing the theoretical models. The best test of experimental values is comparison with spectroscopic values. 2.3.2 The Franck-Condon Principle 2.3.2.1 General Considerations This principle, first proposed by Franck (60) and later formulated mathematically by Condon (61) states that i f an electronic transition occurs in a time much shorter than that required for a single molecular vibration, then i t may be assumed that the nuclei will have very nearly the same velocity and position after the transition as they had before i t . This implies that when a transition between two levels occurs, the relative positions of the nuclei will be the same immediately before and after the transition. Herzberg (44) has given a quantum mechanical formulation of the Franck-Condon principle. The transition probability between two states, characterized by the total wave functions ¥ and V , can be represented as the square of the matrix element of the electric dipole moment. R = / r * M f" d T (2-11) where R is the transition moment. M is the electric dipole moment operator summed over a l l electrons. Neglecting rotation, and using the Born-Oppenheimer approximation, the electronic ¥ and vibrational f parts of the total wave function e v r 31 may be separated into electron and nuclear parts. Now, using the fact that the electronic part of the dipole operator does not depend on the nuclear coordinates, and that the nuclear part of the dipole operator does not depend on the electronic coordinates, the transition moment R can be written in two parts, one depending only on the nuclear coordinates and the other only on the electron coordinates. If the Franck-Condon principle that there is no change in nuclear coordinates during the time required for an electronic transition is now applied, the term which depends on the nuclear coordinates can be set equal to a constant R<e. The square of the resulting expression is proportional to the transition probability I. I «-|Re I 2 [ / V *v d r ] 2 ( 2 _ 1 2 ) where dr indicates integration over the nuclear coordinates r, t *' " 2 The term [ J ¥ ¥ dr] the Franck-Condon factor, is proportional, to the transition probability. Recently, many workers (62,63,64) have calculated Franck-Condon factors for excitation and direct ionization. The majority of these workers have used Morse potentials to describe the potential energy curves of the upper and lower states. Values calculated in this way should be accurate for the first few vibrational levels where the Morse potential 'fits' the actual curve well and should decrease in accuracy at higher vibrational,levels. Dunn (65) has attempted to overcome this problem by using the known potential curves for hydrogen and deuterium in his calculations. 32 The amount of overlap of the two wave functions V" and • i f. determines the magnitude of the Franck-Condon factor, v Theoretically, a finite probability exists for transitions to all possible vibrational levels since the vibrational wave functions approach zero asymptotically. An effective Franck-Condon regiqn for transitions from the initial state can be defined in terms of the maximum and minimum internuclear distances from which observable transitions can occur. Transitions will be observed only to those upper states which have vibrational levels of high probability within this internuclear separation range. The width of the Franck-Condon o region is effectively quite small (0.1 to 0.2 A for molecules with medium to large bond strengths (66)), however it must be noted that the width is really determined by the sensitivity of the measuring device. Figure 3 illustrates the application of the Franck-Condon principle to ionization phenomena. The transitions are all taken as originating from the zeroth vibrational level of the initial electronic state. The maximum probability for transition occurs in the center of the Franck-Condon region where the in i t i a l state wave function has its maximum. Figure 3(a) represents a transition between two states for which the equilibrium internuclear distances (r^) are similar. Here, the Franck-Condon principle requires that the most probable transition be to the lowest vibrational level of the1upper ionic state. The situation depicted by Figure 3(b) corresponds to a 34 moderate change in r on formation of the ion. From the diagram i t can be seen that the maximum overlap of the wave functions would occur i for the v = 3 level of the upper state, and i f the Franck-Condon principle holds, the maximum transition probability would be expected to this level. There will also be transitions to the other vibrational levels, the probability of which will be determined by the amount of overlap of the appropriate wave functions. Two terms are often used to describe the transitions 1 occurring. A transition to the v =0 level of the upper electronic state is called an 'adiabatic' transition. This process requires the least amount of energy to reach the appropriate electronic level. The 'vertical' transition is the second type, and corresponds to the most probable transition. It is represented by the solid lines in Figure 3. In Figure 3(a) the vertical and adiabatic processes are equivalent. Figure 3(c) shows the situation with a large change in r . Here the vertical transition intersects the ionic potential energy curve at an energy above the dissociation limit. This results in dissociation of the ion into charged and neutral fragments which share the excess kinetic energy E. There is a finite probability of transition over the entire width of the Franck-Condon region. The fragments, therefore, have a spread in kinetic energy about the most probable energy E. 2.3.2.2 Application to Photoelectron Spectroscopy The number of vibrational levels that are populated on the 35 f o r m a t i o n o f a p a r t i c u l a r i o n i c s t a t e i s a n i n d i c a t i o n o f t h e c h a n g e i n r o c c u r r i n g o n i o n i z a t i o n . T h i s i n f o r m a t i o n o f t e n a s s i s t s i n a s s i g n m e n t o f t h e i o n i c s t a t e f o r m e d . P h o t o e l e c t r o n s p e c t r o s c o p y , b e c a u s e o f i t s a b i l i t y t o m e a s u r e t h e r e l a t i v e number o f i o n s f o r m e d i n a n y s t a t e , c a n b e u s e f u l f o r s u c h a s s i g n m e n t s . F o r e x a m p l e , i f t h e r e i s no c h a n g e i n i n t e r n u c l e a r d i s t a n c e u p o n i o n i z a t i o n o f a d i a t o m i c m o l e c u l e , t h e m o s t p r o b a b l e t r a n s i t i o n w i l l b e t o t h e l o w e s t v i b r a t i o n a l l e v e l o f t h e i o n i c s t a t e , w i t h v e r y l o w p r o b a b i l i t y t o t h e h i g h e r l e v e l s . T h i s l e a d s t o a p h o t o e l e c t r o n r e t a r d i n g s p e c t r u m s h o w i n g on e s i n g l e s h a r p s t e p . F i g u r e 4 i l l u s t r a t e s two c a s e s w h e r e a c h a n g e i n r g o c c u r s o n f o r m a t i o n o f t h e i o n . T h e t r a n s i t i o n f r o m t h e l o w e r s t a t e t o c u r v e B r e p r e s e n t s a c a s e w h e r e r g d e c r e a s e s . H e r e t r a n s i t i o n s t o o n l y a few v i b r a t i o n a l l e v e l s a r e p o s s i b l e , i . e . a s u f f i c i e n t o v e r l a p o f t h e v i b r a t i o n a l w a v e f u n c t i o n s o c c u r s o v e r o n l y a s m a l l e n e r g y r a n g e . T h e r e s u l t i n g p h o t o e l e c t r o n s p e c t r u m , , s k e t c h e d t o t h e l e f t o f c u r v e B, shows o n l y a few v i b r a t i o n a l s t e p s b e f o r e c o m p l e t e l y l e v e l i n g o f f . F o r c u r v e C, h o w e v e r , r g i s l a r g e r a n d t h e p o s i t i o n o f t h e F r a n c k - C o n d o n r e g i o n i s s u c h t h a t a l a r g e number o f v i b r a t i o n a l l e v e l s a n d p a r t o f t h e d i s s o c i a t i o n c o n t i n u u m a r e a v a i l a b l e f o r t r a n s i t i o n s . T h e p h o t o e l e c t r o n s p e c t r u m e x p e c t e d h e r e i s d e p i c t e d t o t h e l e f t o f c u r v e C. T h e r e i s a g r e a t d e a l o f v i b r a t i o n a l s t r u c t u r e o n t h e t h r e s h o l d a n d t h e c u r v e c o n t i n u e s t o r i s e a b o v e t h e d i s s o c i a t i o n l i m i t . T h i s c o n t i n u e d r i s e i s due t o t h e p h o t o e l e c t r o n s f o r m e d d u r i n g d i s s o c i a t i v e i o n i z a t i o n . I f a t r a n s i t i o n o c c u r s t o a r e p u l s i v e i o n i c c u r v e , a s f o r e x a m p l e i n F i g u r e 3 ( d ) , t h e 3 6 Figure 4. Application of Franck-Condon Principle to Photoelectron Production. 37 photoelectron retarding curve obtained will reflect the nature of the transition probability to the curve, i*e. the transition probability will be low at both the onset and completion of the process with the maximum probability occurring at some intermediate point corresponding to the vertical transition. From the preceding discussion.it is evident that a knowledge of the shape of the photoelectron stopping curve for a certain process can give valuable information about the type of process that is occurring and can be of great assistance in the assignment of ionic states. 2.3.2.3 The Energy of Fragment Ions . The charged and neutral fragments which result from dissociative ionization of a molecule may be produced with kinetic energy. Figures 3(c and d) illustrate two processes which lead to formation of energetic fragments. In both cases the mean kinetic energy available for the fragments is denoted by E and represents the energy supplied to the system above that required to produce the fragments at rest in their appropriate electronic states. The partition of this kinetic energy between the fragments will be in the inverse ratio of their masses. Therefore i f two fragments are formed and i f the energy and identity of one is known, the total kinetic energy possessed by the fragments may be obtained. This information may, in favourable cases, allow the dissociation energy of,the appropriate ionic state to be established. The kinetic energy of positively charged fragments formed 38 in photoionization may be measured in an experiment analogous to photo-electron spectroscopy. In this case the retarding voltages are reversed to retard positively charged particles. Consideration of the ion retarding curves enables one to ascertain which of the two possible processes (see Figure 3(c and d)) gave rise to the energetic fragments. For the case illustrated in Figure 3(c), for transitions above the dissociation limit of a bound state, the fragment ions will be formed at all energies from zero to some maximum determined by the width of the Franck-Condon region. The retarding curve will therefore rise continually from this maximum value to zero retarding potential. For the case of a transition to a repulsive curve (Figure 3(d)) only a certain range of ion kinetic energies occurs. The retarding curve obtained here will resemble that obtained for photoelectrons emitted from transitions to these repulsive curves (see section 2.3.2.2). 2.3.3 Assignment of Atomic and Molecular Energy Levels As the complexity of an electron-nuclear system increases, the assignment of the various possible electron configurations becomes more difficult. For atoms, diatomic molecules, and their related ions, the electronic configurations can be described in terms of the orbital and spin angular momenta associated with each. For polyatomic molecules, assignments are made in terms.of the symmetry of the configuration. For atoms, the order of f i l l i n g of the electron orbitals is well known. In terms of Russell-Saunders (L-S) coupling the electronic term symbols may be determined directly by application of 39 simple rules such as the Pauli Principle and Hund's Rules (34). Hund (67), Mulliken (68,69,70) and others (54,71,72) have developed the concepts of molecular orbital theory in an attempt to describe the behaviour of an electron in the complex field of a molecular system. Atomic orbitals are used as a basis for these molecular orbitals. The electronic term symbols for the states of diatomic, species may be derived in three different ways (44). The first of these, the separate atom method, is a procedure in which the electronic configuration of a molecule is built up by bringing together the atoms of which i t is comprised; i.e. the question of what molecular states result from the combination of given states of the separate atoms is investigated. In this method, which finds application in cases where the resultant molecule has a large equilibrium internuclear distance, the components of the orbital angular momentum of each of the atoms along the axis joining them is added vectorially to give the orbital angular momenta of all the possible molecular states arising from the two atomic states used. The multiplicity of these states will be determined in a similar manner by vector addition of the spin angular momenta of the separate atoms. The electrons in the resulting molecular orbitals are designated according to the atomic levels from which they are believed to have originated and according to their molecular, symmetry; ie. they are designated as 2so, 2pir, etc. where the 2s or 2p refers to the atomic orbital from which the electron came, and the a or * designates the symmetry of the molecular orbital. 40 T h e u n i t e d a t o m m e t h o d i s t h e s e c o n d means o f o b t a i n i n g t h e e l e c t r o n i c t e r m s y m b o l s o f a d i a t o m i c m o l e c u l e . H e r e t h e p o s s i b l e e l e c t r o n i c s t a t e s a r e o b t a i n e d b y h y p o t h e t i c a l l y s p l i t t i n g a n a t o m o f c o r r e c t n u c l e a r c h a r g e i n t o t h e d e s i r e d c o m p o n e n t s . I n t h i s m e t h o d , d e v e l o p e d f o r s p e c i e s o f s m a l l r t h e s p l i t t i n g o f t h e u n i t e d a t o m h a s t h e $££6ct o f q u a n t i z i n g t h e a n g u l a r momentum o f t h e a t o m a l o n g t h e i n t e r n u c l e a r a x i s . T h e s p i n o f t h e m o l e c u l a r s t a t e i s t h e same a s t h a t o f t h e a t o m i c s t a t e f r o m w h i c h i t o r i g i n a t e s . T h e i n d i v i d u a l e l e c t r o n s a r e d e s i g n a t e d , a s i n t h e s e p a r a t e a t o m c a s e , b y r e f e r e n c e t o t h e i r m o l e c u l a r s y m m e t r y a n d a t o m i c o r i g i n . T h e f i n a l means o f o b t a i n i n g t h e m o l e c u l a r e l e c t r o n i c c o n f i g u r a t i o n i s a n a l o g o u s t o t h a t e m p l o y e d f o r a t o m s . U s i n g t h e P a u l i P r i n c i p l e t h e e l e c t r o n s a r e a d d e d t o t h e a v a i l a b l e m o l e c u l a r o r b i t a l s i n d e c r e a s i n g o r d e r o f t h e i r b i n d i n g e n e r g y , t h e n u c l e i b e i n g c o n s i d e r e d a s f i x e d . H e r e a n o t h e r means o f l a b e l i n g t h e m o l e c u l a r o r b i t a l s m u s t b e u s e d a s t h e a t o m i c o r i g i n o f t h e e l e c t r o n s n o l o n g e r , h a s a n y m e a n i n g . T h i s m e t h o d u s e s l e t t e r s a n d m o l e c u l a r s y m m e t r y t o d e s i g n a t e t h e o r b i t a l s , f o r e x a m p l e z a . T h e r e l a t i v e e n e r g i e s o f t h e v a r i o u s e l e c t r o n i c s t a t e s o f a p a r t i c u l a r d i a t o m i c a r e n o t a l w a y s u n a m b i g u o u s . C o r r e l a t i o n d i a g r a m s w h i c h e q u a t e t h e e q u i v a l e n t s t a t e s o b t a i n e d f o r t h e u n i t e d a t o m a n d s e p a r a t e a t o m m o d e l s c a n b e d r a w n . T h e r e a r e , h o w e v e r , many c a s e s o f c r o s s i n g o f m o l e c u l a r l e v e l s a t some i n t e r n u c l e a r d i s t a n c e i n t e r -m e d i a t e b e t w e e n t h o s e u s e d t o c o n s t r u c t t h e two m o d e l s , a n d t h e p a r t i c u l a r o r d e r i n w h i c h t h e o r b i t a l s o f a g i v e n d i a t o m i c s p e c i e s 41 f i l l therefore depends on the nature of that species. Polyatomic molecules may no longer be described in terms of orbital and spin angular momenta. Mulliken (73-76) has shown how more complex polyatomic molecules may be represented in terms of molecular orbits which are characterized with respect to their symmetry. That is.each eigenfunction of the molecule is a basis for one of the irreducible representations of the molecular point group. 2.3.4 Spin-Orbit Coupling in Atoms and Molecules Splitting of electronic terms in atoms is observed in all cases where the electronic shell is not half or completely filled. This splitting, which is the result of coupling of the spin and orbital angular momenta of the atomic orbitals, can be shown for one electron atoms to vary as the fourth power of nuclear charge (77). This rapid increase in multiplet spitting is found to hold qualitatively for multi-electron cases. Calculation of accurate spin-orbit splittings is possible for atomic species (78). Molecules generally have filled-she11 configurations and consequently have no spin-orbit splitting in the ground state. Molecular ions on the other hand will have incomplete orbitals and spin-orbit coupling may occur. The calculation of molecular spin-orbit splittings is not straightforward and l i t t l e work has been done. In cases where an electron is removed from a molecular orbital which is atomic in character, the spin-orbit splitting will be very similar to that found for the atomic species. An empirical formula for obtaining the splitting in these molecules is (79) 4 2 A V = [ 3 (3 P 2 - + 2 (3 P 2 - % ) ] ( 2 _ 1 3 ) where Av is the spin orbit splitting in cm 1 ( and 3 3 3 - 1 ?2> PQ a r e 'the levels of the atomic ion in cm CHAPTER THREE EXPERIMENTAL The results presented in this study were obtained on two photoelectron spectrometers, both of which were constructed in this department. Initially, an instrument of cylindrical^geometry, similar to that described by Al-Joboury and Turner (31) was used. The shape of the photoelectron retarding curves obtained on this instrument were such as to indicate a photoelectron spatial distribution close 2 to sin 6 as predicted (section 2.1.1.3). In order to provide an energy analyzing field always parallel to the electron trajectories, a new spectrometer of spherical symmetry was constructed. This instrument yielded photoelectron retarding curves of essentially 'stepped' shape. In this chapter the detailed construction and operation of the two spectrometers is described. 3.1 Construction of the Photoelectron Spectrometer 3.1.1 General. Features of Photoelectron Spectrometers Photoelectron spectrometers may be considered as comprising three main sections: the light source, the ionizing region, and the electron analyzer. Each must possess specific characteristics i f meaningful results are to be obtained. The light source must provide monochromatic radiation of an 44 energy greater than that required to ionize the sample under investigation. The majority of sources available produce either line spectra or continua in the energy region 8-25 ev. Consequently i f these sources are to be used to supply radiation for a photoelectron spectrometer, some type of vacuum ultraviolet monochromator must be incorporated. However, since the intensity of radiation from such a monochromator is generally low, i t is difficult to use such a source in practice because of the low resultant photoelectron currents. These problems have been circumvented in the present study by employing a low pressure discharge in helium as a source of radiation. The radiation produced in this fashion is.essentially monochromatic in the energy range under consideration, and hence does not require the use of a monochromator. It is also sufficiently intense to ensure a photoelectron current intense enough for direct measurement by a vibrating reed electrometer. The ionization region must be so constructed that the photoelectrons are emitted in a field-free region. This is essential i f accurate values of the photoelectron kinetic energies are required. It will be shown later that the quality of the results obtained is critically dependent on the geometry of this region. The electron analyzer must be designed so that i t discriminates as l i t t l e as possible against electrons emitted at different angles from the ionization region. If the solid angle subtended by the analyzing field at the ionizing region is large, one can be reasonably sure that all ejected photoelectrons are being 45 collected, and the problems discussed in the previous chapter regarding the angular distribution of these electrons can be avoided. A conventional electron velocity selector, of the 127° type (14), was therefore rejected in this study in favour of two retarding field electrostatic energy analyzers, which permitted the collection of photoelectrons emitted over a large solid angle. 3.1.2 The Light Source A low pressure microwave discharge in helium, first used as a far ultraviolet source by Frost and McDowell (80), provided the photon source in both spectrometers. The majority of the emission o has a wavelength of 584 A, corresponding to an energy of 21.21 ev. This spectral line arises from the 2 *P -»• 1 *S resonance transition in helium (34), and is of sufficient energy to ionize all gases with the exception of neon and helium. Figures 6 and 7 include a schematic diagram of the light source. Unpurified commercial helium (Canadian Liquid Air Co.) was introduced into the discharge region at a pressure of approximately 1 mm. of Hg. Fine pressure control was maintained with an Edwards High Vacuum OSIC stainless steel needle valve. The pressure in the discharge region was further stabilized by a constriction at the lower end of the quartz tube through which the helium flowed. The constriction (see Figures 6 and 7) also served to facilitate differential pumping of the helium between the light source and the vacuum chamber. The discharge was produced in a microwave cavity by,power 46 f r o m a R a y t h e o n M i c r o w a v e T y p e CMD-4 m i c r o w a v e g e n e r a t o r . T h i s u n i t p r o d u c e d m i c r o w a v e r a d i a t i o n a t a f r e q u e n c y o f 2450 mc/s. w i t h a maximum p o w e r o u t p u t o f 125 w a t t s . A s c a l e d r a w i n g q f t h e m i c r o w a v e r e s o n a n t c a v i t y , w h i c h i s o f t h e t y p e d e s c r i b e d b y Z e l i k o f f , W y c h o f f , A u . s c h e n b r a n d a n d L o o m i s (-81), i s shown i n F i g u r e 5. T h e c a v i t y was c o n s t r u c t e d o f b r a s s w i t h t h e i n n e r s u r f a c e s s i l v e r p l a t e d t o g a i n t h e h i g h e s t p o s s i b l e p o w e r t r a n s f e r t o t h e d i s c h a r g e t u b e . F o r c e d a i r c o o l i n g was u s e d t o p r e v e n t m e l t i n g o f t h e q u a r t z t u b e . T h e c a v i t y was s o p l a c e d t h a t t h e m o s t i n t e n s e r e g i o n o f t h e d i s c h a r g e o c c u r r e d j u s t a b o v e t h e c o n s t r i c t i o n i n t h e q u a r t z t u b e . T h e l i g h t s o u r c e was f i x e d t o t h e v a c u u m c h a m b e r , w h i c h c o n t a i n e d t h e g r i d a n d c o l l e c t o r a s s e m b l y , b y means o f a k o v a r s e a l . A c o l l i m a t i n g c a p i l l a r y , 5 cm. i n l e n g t h a n d 0.5 mm. i n t e r n a l d i a m e t e r , s e r v e d a d u a l p u r p o s e i n d i r e c t i n g t h e p h o t o n f l u x i n t o t h e i o n i z a t i o n r e g i o n a n d h e l p i n g t o e l i m i n a t e h e l i u m f r o m t h e i o n i z a t i o n r e g i o n . C a r e h a d t o b e t a k e n when i n s t a l l i n g t h e l i g h t s o u r c e , w h i c h was c o n s t r u c t e d i n o n e p i e c e , t o e n s u r e t h a t t h e c o l l i m a t i n g c a p i l l a r y d i r e c t e d t h e p h o t o n beam i n t o t h e c e n t e r o f t h e i o n i z a t i o n r e g i o n . 3.1.3 T h e G r i d a n d C o l l e c t o r S y s t e m s T h e two i n s t r u m e n t s u n d e r d i s c u s s i o n d i f f e r o n l y i n t h e g e o m e t r y o f t h e i o n i z a t i o n r e g i o n a n d p h o t o e l e c t r o n c o l l e c t o r . 3.1.3.1 C y l i n d r i c a l S y s t e m : F i g u r e 6 i s a s c h e m a t i c d i a g r a m o f t h e c y l i n d r i c a l p h o t o -e l e c t r o n s p e c t r o m e t e r . T h i s i n s t r u m e n t was s i m i l a r t o t h a t d e s c r i b e d M I C R O W A V E C A V I T Y MICROWAVE IMPUT FORCED AIR COOLING QUARTER WAVE CHOKE Figure 5. The Microwave Cavity. P H O T O E L E C T R O N S P E C T R O M E T E R 48 to microtherm unit air helium microwave cavity sample inlet -collimating capillary /Teflon disks to fast rotary pump (light source exhaust) to J vibrating reed to trap and oil diffusion pumps •glass seals Figure 6 . The C y l i n d r i c a l Photoelectron Spectrometer. 49 by Al-Joboury and Turner (31). It consisted of two grids and a collector, co-axial with one another. Ionization took place along the length of the axis of the grid-collector system which was held in place by two teflon disks. These disks not only insulated the three electrodes from one another, but being cut with concentric grooves into Which the electrodes fitted, ensured the rigidity of the assembly while maintaining the correct relative positions between the electrodes. Additional stability was provided by three metal rods of equal length which spanned the gap between the disks, at a position outside the collector, and were firmly attached at.each end to the teflon. The length of the grid-collector system was 7 cm; the diameters of the electrodes were 1 cm, 2.2 cm and 4.5 cm. The grids were constructed of brass mesh (30 x 30 mesh 0.005" diameter wire) formed on a brass frame for rigidity. The collector was constructed from a brass tube. All electrodes were gold plated to facilitate cleaning and to prevent contamination. The electrode system was housed in a brass vacuum chamber containing outlets for the pumping and $ample introduction lines and electrical leads. This chamber had a flange which allowed the spectrometer to be easily removed for cleaning. The electrode system was assembled on this flange and all electrical.connections were made prior to replacing i t in the vacuum housing. 3.1.3.2 Spherical System: The spherical photoelectron spectrometer is shown in Figure 7. The basic design of this instrument was the same as that / so PHOTOELECTRON SPECTROMETER reedv^ Figure 7. The Spherical Photoelectron Spectrometer. 51 of the cylindrical system with the exception that the ionization region was restricted to a small volume at the center of a spherical electrode system. The position and size of this region were defined by two photon conducting tubes, of gold plated brass. Both of these tubes made physical contact with the inner grid, thus ensuring that all the metal surfaces in the center of the electrode system were at the same potential, and therefore that the ionization region was field free. The upper tube was merely an extension of the collimating capillary and was fitted with a lip which rested on the inner grid to prevent the tube from slipping iflto the ionization region. Tt was insulated from the other electrodes by means of a glass spacer, which was, as nearly as possible, out of sight of the photon beam. The lower tube fitted snugly through a hole in the inner grid and was insulated from the other electrodes by a glass spacer which supported the inner grid. Teflon spacers, coaxial with this glass spacer, supported the outer grid and electron collector. An earthed metal ring was placed between these two teflon spacers to prevent electrical leakage from grid to collector. All the electrodes were constructed as separate hemispheres. The grids, which were 1.5" and 2" in diameter were made from brass mesh (30 x 30 mesh, 0.005" diameter wire). They were constructed by first compressing the mesh between a ball bearing and a hemispherical depression in a lead block. The ends of the mesh were then trimmed and soldered to brass rings, suitably shaped to pass over the photon conducting tubes. Before being gold plated the grids 52 w e r e e l e c t r o p o l i s h e d t o a n a p p r o x i m a t e t r a n s p a r e n c y o f 7 5 % . ( T h i s i n c r e a s e i n t r a n s p a r e n c y r e d u c e d t h e number o f r e f l e c t e d e l e c t r o n s a n d i n c r e a s e d t h e s i g n a l i n t e n s i t y ) . T h e two h a l v e s o f e a c h g r i d w e r e f i n a l l y f a s t e n e d t o g e t h e r b y w i r e s p a s s i n g t h r o u g h s m a l l h o l e s d r i l l e d i n t h e b r a s s r i n g s . A d i a g r a m o f t h e two h a l v e s o f a g r i d a r e shown i n F i g u r e 8. T h e c o l l e c t o r , 3" i n d i a m e t e r , was t u r n e d f r o m s o l i d b r a s s a n d g o l d p l a t e d . F o u r s c r e w s t h r o u g h f l a n g e s l o c a t e d o n t h e r i m s o f t h e h e m i s p h e r e s h e l d t h e two h a l v e s t o g e t h e r . A n a q u e o u s c o a t i n g o f 'Aquadag' was a p p l i e d t o t h e i n n e r s u r f a c e o f t h e c o l l e c t o r t o m i n i m i z e e l e c t r o n r e f l e c t i o n . T h e e n t i r e s y s t e m was t h o r o u g h l y c l e a n s e d o f a n y g r e a s e o r o i l b e f o r e b e i n g a s s e m b l e d o n t h e l o w e r f l a n g e o f t h e b r a s s v a c u u m c h a m b e r . C o r r e c t a l i g n m e n t was e n s u r e d b y t h e a c c u r a t e c o n s t r u c t i o n a n d p o s i t i o n i n g o f t h e g l a s s a n d t e f l o n s p a c e r s . T o o b t a i n t h e b e s t r e s o l u t i o n i n t h e i n s t r u m e n t , t h e c a n was c o v e r e d w i t h ' M p - m e t a l ' t h u s m i n i m i z i n g t h e e f f e c t o f s t r a y m a g n e t i c f i e l d s o n t h e e l e c t r o n t r a j e c t o r i e s . 3.1.4 A s s o c i a t e d E l e c t r o n i c s U s i n g t h e s y s t e m o f two g r i d s a n d a c o l l e c t o r , t h e r e w e r e two p o s s i b l e means o f o b t a i n i n g a p h o t o e l e c t r o n r e g a r d i n g c u r v e . T h e f i r s t o f t h e s e was t h e m e t h o d e m p l o y e d b y A l - J o b o u r y a n d T u r n e r ( 3 1 ) i n w h i c h t h e p h o t o e l e c t r o n s w e r e r e t a r d e d b y a p p l y i n g a s t e a d i l y i n c r e a s i n g p o s i t i v e p o t e n t i a l t o t h e i n n e r g r i d w i t h r e s p e c t t o t h e o u t e r g r i d . I n t h i s mode o f o p e r a t i o n t h e o u t e r g r i d was m a i n t a i n e d a t a n e g a t i v e Figure 8. A Spherical Grid. 54 p o t e n t i a l w i t h r e s p e c t t o t h e g r o u n d e d c o l l e c t o r i n o r d e r t o t u r n b a c k a n y e n e r g e t i c i o n s w h i c h may e s c a p e t h e i o n i z a t i o n r e g i o n . A l t h o u g h t h i s m e t h o d was u s e d i n t h e i n i t i a l e x p e r i m e n t s , i t d i d n o t g i v e t h e b e s t r e s o l u t i o n . T h e p o o r r e s o l u t i o n was a s c r i b e d t o two f a c t o r s . I n t h e f i r s t p l a c e , t h e e x t e n t o f f i e l d p e n e t r a t i o n f r o m t h e o u t e r g r i d i n t o t h e i o n i z a t i o n r e g i o n was n o t c o n s t a n t b u t i n c r e a s e d w i t h r e t a r d i n g p o t e n t i a l . S e c o n d l y , e l e c t r o n s w h i c h h a d j u s t s u f f i c i e n t k i n e t i c e n e r g y t o o v e r c o m e t h e r e t a r d i n g f i e l d w e r e e s s e n t i a l l y s t o p p e d a t t h e o u t e r g r i d , a n d t h e p o s s i b i l i t y e x i s t e d t h a t t h e r e s u l t a n t l o w e n e r g y e l e c t r o n d i s t r i b u t i o n w o u l d r e l a x b e f o r e b e i n g c o l l e c t e d . A n a l t e r n a t e m e t h o d o f o b t a i n i n g a p h o t o e l e c t r o n s p e c t r u m , w h i c h was s i m i l a r t o t h a t u s e d b y S c h o e n ( 2 9 ) , i n v o l v e d r e t a r d i n g t h e e l e c t r o n s b e t w e e n t h e i n n e r g r i d a n d t h e c o l l e c t o r . A s c h e m a t i c o f t h i s e l e c t r i c a l s y s t e m i s shown i n F i g u r e 9. T h e p h o t o e l e c t r o n m e a s u r e m e n t s w e r e made b y a p p l y i n g a g r a d u a l l y i n c r e a s i n g p o s i t i v e r e t a r d i n g p o t e n t i a l t o t h e i n n e r g r i d w i t h r e s p e c t t o t h e g r o u n d e d c o l l e c t o r . A c o n s t a n t p o t e n t i a l ( u s u a l l y 1-3 v o l t s ) b e t w e e n t h e i n n e r a n d o u t e r g r i d s s e r v e d t o t u r n b a c k a n y e n e r g e t i c p o s i t i v e i o n s . T h i s mode o f o p e r a t i o n y i e l d e d t h e b e t t e r r e s o l u t i o n a n d was u s e d t o o b t a i n a l l t h e r e s u l t s r e p o r t e d i n t h i s s t u d y . T h e s c a n n i n g c i r c u i t c o n s i s t e d o f a m o t o r d r i v e n p o t e n t i o m e t e r s u p p l y i n g a p o t e n t i a l t o t h e i n n e r g r i d o f t h e s p e c t r o m e t e r . One e n d o f t h i s p o t e n t i o m e t e r , ( a 1 0 t u r n , t w i n s h a f t e d Beckman M o d e l . A - R S , 2 0 K f i h e l i p o t ) was a t g r o u n d p o t e n t i a l , w h i l e t h e o t h e r e n d was a p o t e n t i a l d e t e r m i n e d b y t h e v o l t a g e t a p o n a B u r g e s s 22 1/2 V s e r i e s ' C , m u l t i t a p light source S P H E R I C A L P H O T O E L E C T R O N S P E C T R O M E T E R 56 dry battery. During a scan, the helipot was driven through a friction clutch at a constant speed by a Heller 2T60 variable speed motor. The scan speed was reproducible. A Digitec Model 202 digital D.C. voltmeter was used to continuously monitor the applied retarding voltage. The fixed potential between the inner ;and outer grids was supplied by a Burgess 3V series 'A' dry battery. The photoelectron current was measured with a Cary Model 31 Vibrating^Reed Electrometer with a TO11 ohm input resistor. This type of electrometer was ideal for these experiments because of its low noise level and absence of zero drift. The output from the electro-meter was fed into a Leeds and Northrup "Speedomax" Type G. lOmV strip chart recorder. 3.1.5 Vacuum System The vacuum systems employed for both the cylindrical and spherical instruments were similar. Figure 10 depicts the vacuum system for the spherical spectrometer. It consists ^ essentially of three sections: the sample handling system, the high vacuum system and the light source. The first of these comprised the sample inlet ports and a five liter storage bulb connected to the spectrometer by means of a sintered glass leak. The effusion rate of this leak was such that for a pressure of about 1 mm. of Hg in the sample bulb, the pressure in the spectrometer was approximately 10 mm. of Hg. An oil diffusion pump (C.E.C. Type MCF 60) coupled with a dry ice cooled pyrex cold trap formed the high vacuum system for the spectrometer. The pressure in the vacuum chamber was measured 58 with an N.R.C. Equipment Corporation ionization gauge. As can be seen in the Figure both the sample and"high vacuum systems used the same rotary pump (Welch Duo Seal, No. 1405 H). The light source consisted of a needle valve, to control the helium flow, the discharge tube-y arid a pumping line leading to a rotary pump (Welch Duo Seal, No. 1405 H). The cylindrical spectrometer used a vacuum system identical to that just described but with two exceptions. A glass mercury diffusion pump was used in place of the. oil diffusion pump and only one rotary pump was used to evacuate the entire system. This latter condition imposed a restriction on.the pressure Of helium that could be used in the light source of this spectrometer, i.e. the helium pressure had to be kept low so that the backing pressure for the diffusion pump did not exceed that required for the pump to operate. 59 3.2 Operation of the Photoelectron Spectrometer 3.2.1 General Operation In a typical run the spectrometer was operated with an ultimate vacuum of less than 1 x 10"^ mm. of Hg and a sample pressure _3 Q'f about 1 x 10 mm. of Hg. Under these conditions the sample pressure was always at least one hundred times that of the background gases and hence any contribution from background gases to the photo-electron current could be neglected. In a series of tests on the effect of sample pressure on the resolving power of the spherical _3 spectrometer i t was found that for sample pressures above 7 x 10 mm. of Hg the resolution began to decrease. This loss of resolution was attributed to scattering of the electrons by the gas molecules. For molecules of average photoionization cross-section a measurable photo-electron current could be obtained down to a pressure of less than -4 10 mm. of Hg. Under normal operating conditions photoelectron currents of about 10 to 10 ^  amp. were obtained. In the operation of the light source care had to be taken to ensure that pure helium flowed in the discharge tube. By flushing the helium system for some time before operation of the instrument all contaminating gases could be eliminated. To start the light source the power output of the microwave unit was set to 90%; the discharge was then initiated with a Tesla coil, after which the helium pressure was adjusted to give a photon beam of sufficient intensity. The shape of the photoelectron retarding curve was affected 60 by a background current arising from photoelectrons which were produced on the walls of the collimating capillary and photon conducting tubes of the spectrometer. To compensate for this background, a run was first made on the sample gas under consideration, scanning in the direction of decreasing retarding voltage at a speed of approximately one volt per minute. A run with no sample gas present was then recorded directly under the previous trace; and the resultant curve was subtracted directly from the total curve.This procedure was facilitated by the fact that no structure was observed in the background curve. 3.2.2 Positive Ion Curves With suitable modifications of the grid potentials positive ion curves could be obtained in the same manner as photoelectron retarding curves. Because the photoelectrons produced could have high kinetic energies i t was necessary to use a relatively large potential between the inner and outer grids of the spectrometer to prevent any electrons from reaching the collector (see Figure 9). The positive ions, on the other hand, have a low kinetic energy, and in this case i t was therefore desirable to scan the retarding voltage over a small region on either side of zero (ground potential). To perform these experiments the 22 1/2 V and 3V batteries and their polarities were simply interchanged from the arrangement used for photoelectron retarding curves. An additional bucking voltage of 1.5 V was applied between the potential divider and ground to permit scanning past zero. 61 3.2.3 First Differential Curves The first differential ofathe direct photoelectron retarding curve was periodically used in this study to clarify indistinct structure in a curve. Two methods of obtaining these first differential curves were employed. The first involved placing a simple R-C circuit between the photoelectron collector and the input to the vibrating reed electro-12 meter, operated in this case with a 10 ohm input resistor. Figure 11(a) depicts this differentiating circuit. This method of obtaining first differential curves suffered from two disadvantages. Firstly, the long time constant of the electrometer when used with a 12 10 ohm input resistor demanded that the retarding curve be scanned very slowly i f the best resolution was to be obtained. Because a differential curve is a measure of the rate of change of slope of a curve this restriction on the speed of scan severely affected the maximum strength of the differential signal that could be obtained. Secondly, the time constant of the R-C circuit was such that there was an inherent loss of resolving power regardless of how slowly the spectrum was scanned. An alternate, and more suitable method for obtaining first differential curves required the modification of a Philbrick Research Corp. P65 AU solid state operational amplifier. This circuit, illustrated in Figure 11(b) was placed between the vibrating reed electrometer and the recorder. With this arrangement the electrometer could be used with a 10** ohm resistor which allowed for rapid scanning and 6 2 D I F F E R E N T I A T I N G C I R C U I T S ELECTRON COLLECTOR ELECTROMETER a) SIMPLE DIFFERENTIATING CIRCUIT R, ' 1011 OHM C,' VARIABLE AIR CAPACITOR RECORDER R] • . 4.7K OHM R2 *3 <=} Bl Oi 1 M OHM 33 K OHM 1.5K OHM 1 /ufargd 1.4 VOLT OPERATIONAL AMPLIFIER ELECTROMETER b) OPERATIONAL AMPLIFIER DIFFERENTIATING CIRCUIT Figure 11. The Di f f e r e n t i a t i n g C i r c u i t s . 63 and i t s corresponding increase i n the magnitude of the d i f f e r e n t i a l s i g n a l . The f a s t time response of t h i s d i f f e r e n t i a t i n g c i r c u i t ensured that there was no l o s s of r e s o l u t i o n due t o excessive time constants i n the e l e c t r o n i c c i r c u i t s . I t d i d , however, a l s o r e s u l t i n the d i f f e r e n t i a t i o n of any short term noise on the photoelectron s i g n a l which l e d to r a t h e r n o i s y f i r s t d i f f e r e n t i a l curves. 64 3.5 Treatment of Data 3.3.1 Cylindrical System The initial photoelectron retarding curves obtained in this study were run on the cylindricaiKsystem at a stage when the development of the experimental technique and the investigation of the shape of the spectra were of primary importance. The results presented using this technique werertfrerefore not corrected for background, the spectra being plotted on the assumption that the background was a straight line. Furthermore, the ionization potentials were calculated from the recorded retarding potential without attempting to fix a point on the scale by introducing the sample and a suitable standard simultaneously, (see below). 3.-3.2 Spherical System > . . The majority of the results presented in this study were obtained using .the spherical spectrometer. Data obtained on a large number of gases are presented as plots of the difference between the total curve and the background against either the applied retarding voltage or the absolute energy for ionization. In a l l cases, at least ten runs were obtained under optimum conditions for each sample, and results are quoted with errors which apply to the standard deviations of the mean. The ionization potentials were measured directly from the recorder traces. Precise values were determined by performing a scan with a mixture of the sample gas and a suitable rare gas having a 65 spectroscopically known;ionization potential. This procedure fixed the first ionization potential of the molecule and all higher processes were measured with respect to this value. Relative transition probabilities from the molecular o ground state to the various ionic levels for 584 A radiation were measured directly from the recorder charts. Since in the spherical system i t could be safely assumed that most of the electrons produced in ionization to each state were collected, the relative intensity of the photoelectron current arising from a particular process was proportional to the relative transition probability to that state. To obtain accurate probabilities i t was essential that the photon beam intensity and the sample pressure remained constant during a run. Tests showed that these requirements were met in this instrument. CHAPTER FOUR RESULTS AND DISCUSSION The first section of this chapter discusses the ionization of the rare gases; the results obtained at various stages of development are used to indicate how the final instrument evolved. The photoelectron retarding curves shown demonstrate the advantages of spherical over cylindrical geometry, the importance of magnetic shielding and the effect of coating the collector with 'Aquadag' . The following sections will describe the results obtained for the various molecules studied. In these sections the use of transition probabilities and threshold shape for interpretation of the curves will be demonstrated. 4.1 Characteristics of the Spectrometers; The Rare Gases -Argon, Krypton, Xenon 2 The P term describing the singly-charged rare gas ions is 2 split by spin-orbit interaction giving rise to the g r o u n c' 2 and the P^ /2 ^^TSt e x c i t e d states. As these are the only two ionic o states accessible using 584 A radiation, two groups of photoelectrons-would be expected. 4.1.1 Evaluation of the Instrument Many workers have studied the ionization of the rare gases using several different techniques (absorption spectroscopy (82,83), 67 electron impact (16,18), photoionization (84,85) and photoelectron spectroscopy (31)). The results obtained using these techniques, except perhaps those obtained by electron impact, are consistent and well understood. Study of these gases therefore gave a means of evaluating the accuracy and reliability of the spectrometer. Argon was obtained from the Canadian Liquid Air Company. mass spectrometric analysis, showed no detectable impurities present. The krypton and xenon were obtained 'spectroscopically pure' from the Airco Company. All runs were performed as previously " 3 - 3 described, with a sample pressure of between 0.5 x 10 and 1 x 10 mm. of Hg in the ionization chamber. Theoretically argon should give a photoelectron retarding curve consisting of two sharp steps separated in energy by the 2 spectroscopic difference between the ground P 3 / 2 a n c* ^ r s t excited 2 1^/2 s ^ 3 ^ 3 . Knowing this, argon was used to evaluate the performance of the spectrometers. Figure 12(a) shows the results obtained using the cylindrical analyzer with no 'Aquadag' or'Mu-metal' incorporated. The curve does not resemble the predicted spectrum. The two sloping curves obtained are indicative of the retarding curves expected for analysis of an electron beam with wide angular divergence by a parallel plate analyzer (see section 2.2.1). An attempt to determine the angular distribution associated with low energy photoelectron production was attempted by comparing the retarding curve obtained>for Xe on the cylindrical analyzer 2 with the curves calculated assuming both sin 6 (see section 2.1.1.3) 68 Figure 12. Evaluation of Instrumental Characteristics. 69 and isotropic distributions. The large energy separation between the two ionic states of Xe made i t the most suitable species for this experiment. The results of this comparison, depicted in Figure 13, indicate that the electrons must be emitted with some intermediate distribution. Cylindrical instruments, which have been used by a l l other workers in the f i e l d (27,29,30) are useful for obtaining ionization potentials, but the failure of the curves to level off after each process makes the estimate of the transition probabilities very d i f f i c u l t . Vilessov et al, (27) and Schoen (29) have attempted to overcome this problem by using a Lozier tube analyzer (32) to eliminate a l l the electrons not travelling directly toward the collector. The problem was overcome in this work by constructing the instrument of spherical geometry where a l l the electrons travel perpendicular to the collector surface. Figure 12(b) represents the results obtained for Ar using the spherical design without 'Aquadag' or'Mu-metal'. In the curve the two ionic states can just be distinguished. Although the curve does resemble a step function i t decreases slowly in intensity to lower retarding voltages due to electron reflection (see section 2.2.3). This d i f f i c u l t y was overcome by coating the surface of the collector with 'Aquadag'. Figure 12 (c) shows the Ar curve when the effect of electron reflection has been decreased. In order to improve the resolution of the spectrometer an attempt was made to eliminate.all magnetic fields from the analyzer 70 Figure 13. Comparison of Measured and Calculated Photoelectron Distributions for Xenon. 71 region by surrounding the instrument with'Mu-metal'. Figure 12(d) illustrates the increase in resolution obtained; the curve is now very close to the theoretically predicted step. A l l results reported in the remainder of this chapter, using the spherical spectrometer, were obtained with these modifications. 4.1.2 Results and Discussion Typical photoelectron stopping curves for Ar, Kr, and Xe are shown in Figures 14, 15, and 16 respectively. On each curve the vertical arrows indicate the spectroscopically known values for the doublet separation. In a l l cases the doublet was completely resolved and to further illustrate the instrumental resolution (Figure 17) a tracing of a f i r s t differential curve for Ar is included. This curve was obtained using the operational amplifier circuit described in section 3.2.3. Table I compares the values obtained for the doublet separations with those derived by other methods. A l l experimental values obtained in this study are quoted in the appropriate tables with their standard deviations. According to the s t a t i s t i c a l weight of each ionic level 2 the relative transition probabilities for ionization to the P 3 / 2 2 and P j y 2 states °f the rare gases should be 2:1 in favour of the 2 ^3/2 s t a t e - The transition probabilities measured here and those obtained by Comes and Salzer (87), using a retarding technique in which they measure the ratio of the electrons arising from the two states are given in Table II. lR,ARBITRARY UNITS ZD CD ro ro I 7Z L_ 74 Figure 16. Photoelectron Spectrum of Xenon. 75 Figure 17. First Differential Photoelectron Spectrum of Argon. 76 Table I 2 2 The " 1^/2 Ground State Separation in Ar +, Kr +, and Xe+ (ev.) Ion Spectroscopic (86) Electron (79) This Impact Work Ar + 0.178 0.2 0.183 + 0.005 Kr + 0.666 0.68 0.676+0.006 Xe+ 1.306 1.29 1.312 + 0.006 Table II 2 / ^1/2 Transition Ratio for Ionization of Ar, Kr, and Xe at 584A Comes and Salzer (87) This Work 2.14 : 1 1.96 +_ 0.02 : 1 1.69 : 1 1.73 +_ 0.02 : 1 1.66 : 1 1.68 +_ 0.02 : 1 The ratio obtained for Ar is close to the expected value but those for Kr and Xe are much less than 2:1. Comes and Salzer (87) have attempted to explain this difference in terms of autoionizing 2 transitions stealing intensity from the ionization continuum. This explanation seems, however, unlikely in view of the fact that no autoionizing transitions are known in the rare gases in the region o of 584 A (84). A more probable explanation may lie in the variation of photoionization cross-section with the wavelength of the Molecule Ar Kr Xe 77 impinging radiation (equation 2-1). For Ar the thresholds of both the 2 2 P^2 a n c* ionization continua lie 'at very nearly the same energy and hence the variation in cross-section of each process with wave-length of the impinging radiation should be similar, thereby preserving the 2:1 ratio for all ionizing energies including 21.21 ev. For Kr and Xe however, the doublet splitting is larger and consequently 2 the lower P3/2 ionization continuum may have undergone a significant 2 change in cross*-section before the onset of the state. This means 2 2 that the ratio of the transition probabilities to the P3/2 a m * 1^/2 states may vary considerably from the 2:1 ratio for a particular 2 wavelength above the Pj/2 o n s e t even though the ratio of the two processes measured at the onset of each may be the predicted value. This concept is discussed more fully in the following section on H^, HD and D2, for which independent experimental data is readily available. 78 4.2 Hydrogen, Deuterium Hydride and Deuterium 4.2.1 General Considerations From simple l.c.a.o. m.o. calculations the ground state configuration of the hydrogen molecule and its isotopic analogues may be written as H2 d s ' o - ) 2 V (4-1) The only molecular orbital (the (liso )) must be bonding and therefore the removal of an electron would be expected to give an increase in r_ in the ionic state. From the considerations of section 2.3.2.2 this should result in a photoelectron retarding curve which shows considerable vibrational structure. Other studies of the ionization of hydrogen have only been able to detect the first five vibrational levels. Due to the interference of autoionization peaks, photoionization (88) and absorption studies (83) do not reveal any vibrational levels at a l l . In the electron impact studies of Kerwin, Marmet and Clarke (89) evidence for the first five vibrational levels was seen, but the results of other workers seem to be complicated by interference from rotational levels and autoionization processes (90). Al-Joboury and Turner (31) and Schoen (91) have considered the photoelectron spectrum of hydrogen. 4.2.2 Experimental All the data for was obtained using tank hydrogen supplied by the Canadian Liquid Air Co. Mass spectrometric analysis showed no evidence of impurities in the gas. The HD was obtained with a purity 79 of 98 atom % (min.) from the I s o t o p i c Products D i v i s i o n , Merck, Sharp and Dohme, Canada Ltd. Deuterium was s u p p l i e d by Matheson of Canada Ltd. w i t h a p u r i t y of 99.5 atom % (min.). The r e s u l t s were obtained i n the manner p r e v i o u s l y described; the only v a r i a t i o n being the need f o r somewhat higher sample pressures (of the order of 4-5 microns) to o b t a i n a s u f f i c i e n t l y intense photoelectron c u r r e n t . The large number of processes to be measured, and the low p h o t o i o n i z a t i o n c r o s s - s e c t i o n f o r these molecules combined to make the pressure increase necessary. Krypton was used as a c a l i b r a t i n g gas i n a l l cases. 4.2.3 Results and D i s c u s s i o n In a p r e l i m i n a r y report on the c o n s t r u c t i o n of the s p h e r i c a l spectrometer (92) some e a r l y r e s u l t s on were given comparing the v i b r a t i o n a l s t r u c t u r e obtained on t h i s apparatus w i t h the from the c y l i n d r i c a l system. This comparison i s shown i n Figure 18. In l a t e r experiments up to nine v i b r a t i o n a l l e v e l s could be detected i n H 2 and HD and up to ten i n D^- Figures 19, 20 and 21 depict a c t u a l recorder t r a c i n g s of the r e t a r d i n g curves f o r these molecules. Table I I I compares the i o n i z a t i o n p o t e n t i a l s obtained f o r these gases with the s p e c t r o s c o p i c a l l y known values (44) and those obtained from p h o t o i o n i z a t i o n experiments (88). 00. C to 00 33 O <6 O Ui 3 C tJ 83 r» •! (A »->• (A H) O 0 3 H O = H» o. n 1 x o QQ to 3 3 CL • 1 »-•• o 63 a CL sr A w-o > 3 S3 1 U J O o r r h -o U l o i r T r T 1 r CYLINDRICAL SPHERICAL 0-0 J L. J L J I I L RETARDING VOLTAGE 00 o RETARDING POTENTIAL (VOLTS) oo 5.5 5.0 4.5 4.0 3.5 3.0 RETARDING POTENTIAL (VOLTS) DISSOCIATION RETARDING POTENTIAL (VOLTS) 84 Table III Ionization Potentials of H^, HD and (ev) Molecule Spectroscopic (44) Photoionization (88) This Work H2 15.427 15.42 15.43 + 0.01 HD 15.46 15.45 + 0.01 D„ 15.46 15.47 + 0.01 In Table IV the vibrational energy spacing for the first nine 2 + + levels of the X Z state of H. are compared with the calculated g 2 vibrational spacings of Wacks (63) and those obtained using electron impact (89). The three sets of data are in good agreement indicating that the Morse potential function used to describe the interatomic forces required for the calculation is very close to the actual function describing the eneTgy well. The relative transition probabilities were obtained by measuring the intensities of the photoelectron groups directly from the retarding curves. The probabilities were taken to correspond to the relative transition probabilities for transitions from the 1 + " Hj, X £ , y =0 ground state to the various vibrational levels of the + 2 + ° ti0, X Z state. As there is no evidence for autoionization at 584 A . g (83) the transition probabilities might be expected to be close to those for direct ionization. Table V compares the calculated Franck-Condon factors of Wacks (63) and Halmann and Laulicht (64) with the 85 Table IV V i b r a t i o n a l Spacings f o r the H* ( X 2 Z + ) State (ev.) Av' C a l c u l a t e d (63) E l e c t r o n Impact (89) This Work 0-1 0.269 0.268 0.27 0.01 1-2 0.254 0.266 0.25 + 0.01 2-3 0.239 0.235 0.24 + 0.01 3-4 0.223 0.245 0.23 + 0.01 4-5 0.208 0.20 0.21 + 0.01 5-6 0.193 0.20 + 0.01 6-7 0.177 0.19 + 0.01 7-8 0.161 0.17 + 0.01 8-9 0.146 0.16 + 0.01 86 Table V Relative Transition P r o b a b i l i t i e s for H^CXV, V M o O ) + H* ( x V , v'=0-8) o for 584 A Radiation Calculated Electron This v' Wacks(63) Halmann(64) Impact(89) Work 0 0.4706 0.47038 0.53 0.48 +_ 1 0.8938 0.89375 0.91 i 0.90 +_ 2 1.0000 1.00000 i 1.00 1.00 3 0.8678 0.86776 0.35 0.91 +_ 4 0.6508 0.65051 0.35 0.74 + 5 0.4461 0.44573 0.58 + 6 0.2889 0.28877 0.44 +" 7 0.1812 0.19013 0.28 +_ 8 0.1118 0.11697 0.15 + 87 electron impact values of Kerwin, Marmet and Clarke (89) and the results obtained in this work. For convenience all values have been normalized at v'=2, the most probable transition. The 584 A relative transition probabilities clearly differ from the calculated values; the higher vibrational levels being more heavily pppulated at the higher energies. The discrepancies may arise through invalid assumptions in the Franck-Condon calculations, but a more probable reason is that the difference comes about in the same manner as the discrepancies between observed and predicted transition probabilities to the two ionic states of the rare gases. This hypothesis may best be explained with the aid of the curve shown in Figure 22. The portion of the curve between threshold and the dissociation limit at 18.07 ev.(44) was constructed from the calculated vibrational spacings and Franck-Condon factors given by Wacks (63). Above 18.07 ev. the curve was continued to 21.21 ev. using the absorption spectrum of Cook and Ching (83). Their value for the absorption coefficient at 18.07 ev. was 270 cm"1 but'it decreased to 170 cm 1 at 21.21 ev. It would be expected, as shown in the Figure, that the ionization cross-section to each vibrational level would decrease within this energy region. From Figure 22 i t can be seen that the probability of ionization to the higher vibrational levels o will have decreased less than that to the lower levels;for 584 A radiation. In other words, relative transition probabilities to the various vibrational levels are believed to be energy-dependent with the higher levels being favoured at higher energy. t ; 3 0 0 U J o LL_ L U § 2 0 0 o N 0 0 o o h-o 0L 0 K,= 270 cm r l 1 I I H2+ Dissociation y /Limit — Ki =170 cm -H 15 16 7 18 19 2 0 21 22 23 24 PHOTON ENERGY (eV) 89 The results obtained for HD and D 2 show the same trends as those for H 2. In Tables VI and VII the results for the vib r a t i o n a l spacings and t r a n s i t i o n p r o b a b i l i t i e s for HD and D 2 are compared with the calculated values of Wacks (63). The t r a n s i t i o n p r o b a b i l i t i e s are normalized at v'=2 for HD and v'=3 for D„. 90 Table VI Ionization Data for HD(xV,v" =0) HD+ r . x V , v' =0-9) © o o for 584 A Radiation v' Erv'l" Efv* 11 (ev*) Transition Probabilities Calculated(63) T h i s W o r k Calculated(63) This Work 0 - - 0.3619 0.37 +_ 0.02 1 0.235 0.23 +_ 0.01 0.7919 0.84 +_ 0.04 2 0.224 0.23+0.01 1.0000 1.00 3 0.212 0.22 +_ 0.01 0.9625 0.96 +_ 0.02 4 0.201 0.20 +_ 0.01 0.7875 0.84 +_ 0.04 5 0.189 0.20 +_ 0.01 0.5780 0.71 +^ 0.04 6 0.178 0.19+0.01 0.3982 0.60+0.03 7 0.166 0.17 +_ 0.01 0.2611 0.45 +_ 0.04 8 0.154 0.16 _+ 0.01 0.1667 0.32 +_ 0.04 9 0.143 0.15+0.01 91 Table VII Ionization Data for D2 ( x V.v" =0) •*• D* (X2Z*,V =0-10) o for 584 A Radiation V 1 E(v') - E(v'-1) (ev.) Transition Probabilities Calculated(63) This Work Calculated(63) This Work 0 - - 0.2197 0.19 +_ 0.02 1 0.194 0.20 + 0.01 0.5897 0.56 +_ 0.03 2 0.186 0.19 +0.01 0.8901 0.86 + 0.03 3 0.178 0.18+0.01 1.0000 1.00 4 0.171 0.17 + 0.01 0.9354 0.94 +_ 0.03 5 0.163 0.17 +_ 0.01 0.7728 0.87 +_ 0.04 6 0.155 0.16 +_ 0.01 0.5798 0.77 +_ 0.04 7 0.148 0.15 + 0.01 0.4165 0.66 +_ 0.04 8 0.140 0.15 +_ 0.01 0.2831 0.55 +_ 0.03 9 0.132 0.14 +_ 0.01 0.1862 0.39 + 0.04 10 0.125 . 0.13 + 0.01 92 4.3 Nitrogen 4.3.1 General Considerations. Mulliken (70) has shown that the electronic structure of the ground state of the nitrogen molecule can be written in terms of molecular orbitals as N2 KK(2sog)2 (2So u) 2 (2pt u) 4 (2po g) 2 , V (4-2) where the term KK represents the closed inner shells. If the orbitals are in order of decreasing binding energy from left to right, the first ionization potential must correspond to removal of an electron from + 2 + the (2pa ) molecular orbital forming N2 in its ground X Z state (18). The second ionization potential of nitrogen would then involve the removal of an electron from the ^pn^) orbital leaving N 2 + in its first 2 excited A I I U level (18). This electronic configuration may split 2 2 due to spin-orbit interaction to give rise to the u a n d n3/2 u states but the splitting would be less than 0.01 ev. and could not be detected here (18). In the molecular orbital scheme above, the third most loosely bound group of electrons would lie in the ^scij) molecular orbital and removal of one of them would be expected to give rise to the second excited state.of the ion, the B 2I* ionic state (18). The absorption spectrum and ionization of N2 has been extensively investigated by many workers. Mulliken (94) in 1934 was able to identify Rydberg series leading to the first and third ionization potentials of the molecule. The second ionization potential proved more difficult 93 to identify spectroscopically, and i t was not untiT 1953 that Worley (95) identified a Rydberg series leading to a limit between the known ioniz-ation potentials. The location of this ionic'state had been predicted earlier by Meinel (96) using data obtained from analysis of aurora spectra. Frost and McDowell (18) have been able to correlate the Rydberg series limits with electron impact results obtained using the RPD technique. Other electron impact values also agree well although there is some,doubt as to the effect oa autoionizing transitions on the shape of the electron impact curve in the region of the second ionization potential (14,21). Recently several workers have re-investigated the vacuum ultraviolet absorption spectrum of nitrogen (83,97) and several photoionization studieshave been made (22,98,93). The results of the latter experiments show evidence of a great deal of autoionization contributing to formation of the ion. 4.3.2 Results and Discussion The photoelectron retarding curve-obtained for this molecule shows three ionization processes below 21.21 ev. Figure 23 represents the nitrogen photoelectron retarding spectrum. Table VIII compares the spectroscopically known ionization potentials for the first three processes in nitrogen with the values obtained here, in the photoelectron experiments of Al-Joboury and Turner, and by- electron impact. The agreement between the results is good. Krypton was used to calibrate the energy scale, and Figure 24 shows the photoelectron retarding curve for a mixture of nitrogen and krypton". This curve is typical of the results obtained when krypton-sample mixtures were run. I R , A R B I T R A R Y U N I T S I R , A R B I T R A R Y U N I T S — i 1 1 1 — + ro ro ro 3D ro ro — <=3 + X ro M + •a 7\ ro + x m ro + 00 CD ro + 0D ro M c + S6 96 Table VIII Ionization Potentials of Nitrogen (ev.) Ionic Spectroscopic Electron Photoelectron Spectroscopy State (99) Impact(18) Al-Joboury(31) This Work X V 15.576 15.63 15.57 15.58 + 0.01 • 6 A 2n 16.693 16.84 16.72 16.70+0.01 u — B 2 I + 18.757 18.76 18.72 18.80+0.01 From the retarding curves, the r e l a t i v e t r a n s i t i o n p r o b a b i l i t i e s to each i o n i c state for 584 A radiation could be measured. These values are l i s t e d i n Table IX along with the values estimated from Schoen's Figure 6 (29). Both sets of values are normalized 2 at the A II state, u The vi b r a t i o n a l structure observed on the f i r s t two ionization processes can be seen i n Figure 23. Table X compares 2 + the values obtained here for the X £ state vibra t i o n a l spacings with the values obtained from Herzberg (44) and the t r a n s i t i o n p r o b a b i l i t i e s with the Franck-Condon factors of Halmann and Laulicht (64). The t r a n s i t i o n p r o b a b i l i t i e s are normalized at v'=0. 97 Table IX Relative Transition Probabilities to Some Ionic States of Nitrogen for 0 584 A Radiation Ionic Relative Transition Probability State Schoen (29) This Work X 2Z + 0.59 0.55 + 0.03 g A 2n 1.00 1.00 u B 2E + 0.14 0.12 + 0.02 u — Table X Ionization Data for N o(X 1Z +v"=0) •> N * ( X 2 I + = 0 , 1 ) 2 g J 2 g' o for 584 A Radiation v' E(v') - E(v'-l) (ev.) Transition Probabilities Calculated This Work Calculated This Work 0 - 1.0000 1.00 1 0.29 0.33 + 0.01 0.1004 0.054 + 0.01 98 The high probability for the occurrence of the 0-0 transition (Table X) indicates, that no significant change in r g occurs when an electron is removed from the (2pa ) molecular orbital. This suggests that l i t t l e bonding character can be assigned to this molecular orbital. The measured vibrational spacing is significantly larger than the value from Herzberg (44). This difference manifests itself in the observed transition probability. Since the calculated Franck-Condon factor depends on Herzberg's value for the vibrational spacing, (64), i t will appear larger than would be obtained using the, measured value (Table X) because the calculated value lies closer to the minimum of the curve and hehce more in the Franck-Condon region. Much more vibrational structure appears in the second process. Comparison of the spectroscopic vibrational spacings of Worley (95) and the calculated Franck-Condon factors of Halmann and Laulicht (64) with the experimental values is made in Table XI. The relative transition probabilities are normalized at the v'=l vibrational level. The amount of vibrational structure observed and the shape of the retarding threshold are indicative, of the removal of a bonding electron with its concomitant increase in r^ (see section 2.3,2.2). The agreement between the spectroscopic and photoelectron vibrational spacings is good. The transition probabilities however show deviations from the calculated data for higher vibrational levels similar to those observed in H* (see Table V) and they are 99 Table XI Ionization Data for ^ ( x V . v ' s O y M*2^2\,v "0-6) e for 584 A Radiation. , E(v') - E(v'-l) (ev.) Transition P r o b a b i l i t i e s v Spectroscopic(95) This Work Calculated(64) This Work 0 -- - 0.7875 0.85 +_ 0.02 1 0.24 0.23+0.01 1.0000 1.00 2 0.23 0.23+0.01 0.7251 0.71+0.03 3 0.23 0.22 +_ 0.01 0.3978 0.46 + 0.03 4 0.21+0.01 0.1846 0.21+0.03 5 0.20 0.01 0.0769 0.14 + 0.03 6 0.19 + 0.01 0.0298 0.08 + 0.05 100 ascribed to the same phenomenon. The ionization process arising from the removal of the third most tightly bound electron is seen in Figure 23 to be similar in appearance to that corresponding to the first ionization potential although much less probable. This would indicate l i t t l e change in r with this process. In the foregoing section, an attempt has been made to predict changes in. r from the shape of the photoelectron retarding curve and, hence determine the bonding nature of the electron being removed. For the first and third ionization processes l i t t l e change in r g is predicted while for the second process a considerable increase would be expected. Table XII gives the known r values for the ground molecular state and the first three ionic states. These figures support the assumptions made above. Table XII Equilibrium Interatomic Distances of Some N2 and N 2 + Electronic States (44) N*(xV, A 2n , B 2Z +) 2V g' u' u' 1.116 1.18 1.075 State * (A) e. 1.094 101 Wainfan, Walker and Weissler have measured the absolute o cross-section for the photoionization of nitrogen at 584 A to be 23 x -18 2 10 cm /molecule (136). Using this figure, the cross-section for the formation of each of the ionic states available may be calculated from the relative transition probabilities given in Table IX. The -18 2 -18 2 values obtained are 7.2 x 10 cm /molecule, 14.0 x 10 cm /molecule -18 2 2 + 2 and 1.6 x 10 cm /molecule for formation of the X E , A IIu and .2 + . . . B E y ionic states respectively. 102 4.4 Carbon Monoxide 4.4.1 General Considerations Carbon monoxide and nitrogen are isoelectronic, and may be expected to exhibit similar spectroscopic behaviour. Thus for carbon monoxide, the orbitals available for ionization, their relative energies, and the ionic states obtained by removal of an electron from them would be expected to be similar to those for nitrogen (section 4.3.1). The absorption spectrum of carbon monoxide in the wavelength o region below 1000 A has been studied by a number of workers (100-103). ... 2 Tanaka (100) in 1942 reported Rydberg series converging to the A n first excited state and the B 2Z + second excited state of the ion, while in 1943 Takamine, Tanaka and Iwata (101) observed a series leading to the X 2Z + ground ionic state. Photoionization (104,22) and Photoelectron Spectroscopy (29,105) have also been used to obtain the ionization potentials of this molecule. Fox (106) Using the RPD electron impact technique has been able to detect a l l three of the ionic levels reported here. He obtained values in agreement with the Rydberg series convergence limits. 4.4.2 Results and Discussion Figure 25 represents the photoelectron retarding curve for carbon monoxide. Comparison'of this curve with the retarding curve for nitrogen, Figure 23, shows the striking similarity in the ionization processes- for these molecules. The energy scale for C0+ was not calibrated, and the value of the first ionization potential l R A R B I T R A R Y U N I T S i i 1 1 X ro M + 1 1 1 • - * : _ o o + — 55 — — i s > i « — < 00 -i ! •* i l l 1 j i i i • ! : Y - . 1 1 ' K I i 1 • • * 1 i . i i i • i *. — — <5 DO ro — IN) O — I'* 1 1 1 • • • • M + — ro 1 t 1 1 1 : £01 1G4 was therefore assumed to be the spectroscopic one. The higher ionization potentials were determined as differences from the first. Table XIII compares the known spectroscopic, electron impact and previous photoelectron spectroscopic.values with the results obtained here. The agreement between the: spectroscopic values and those found in this work is very good. Table XIII Ionization Potentials for Carbon Monoxide (ev.) Ionic Spectroscopic Electron Photoelectrpn Spectroscopy State (102) Impact(106) Al-Joboury(105) This Work X E + 14.013 13.89 13.89 14.01 +_ 0.01 A 2n 16.536 16.4 16.58 16.55 +_ 0.01 B 2Z* 19.674 19.0 19.67 19.67 + 0.01 Comparison of the values in Table XIII with those for N2. (Table VIII) shows, that the energy required to remove a bonding electron 2 to form the A n ionic state is similar for both molecules, but for CO the ground X 2E + ionic state is formed at a lower energy. This i 105 indicates that the CO(2pa) orbital is higher in energy than the N 2 (2pa ). The opposite is the case for: the B2E* ionic state (removal of a (2sa) electron) where more energy is required for C0 than for N2. The relative transition probabilities to the various ionic states, listed in Table XIV, are similar to those found for the same transitions in N 2 (Table IX). The values obtained are compared to those of Schoen's Figure 9 (29). The probabilities are normalized to the A 2n state. Table XIV i Relative Transition Probabilities to the Ionic o States of Carbon Monoxide for 584 A Radiation-T . Transition Probability Ionic • ' State Schoen (29) This Work X 2Z+ 0.61 0.58 + 0.03 A 2n 1.00 1.00 B 2Z + 0.14 0.17 + 0.03 Figure 25 shows vibrational structure associated with the first two states of C0+. Table XV compares the values obtained here for the vibrational spacing between the first two vibrational levels 106 of the X Z* state of CO* with the calculated values of Wacks (107), as well as the measured relative transition probabilities with the Franck-Condon factors of Wacks (107) and Halmann and Laulicht (64). The agreement for the relative transition probabilities (normalized i at the v =0 level) i s much better than for nitrogen. The high i transition probability to the v =0 level indicates that an electron of essentially non-bonding character is being removed. Table XV Ionization Data for CO(X1Z+,v"=0) -+ C0 +(X 2Z +, v'=0,l) o for 584 A Radiation i v E,. ' - E. ' (ev.) (v ) (v -1) Calculated(107) This Work 0 1 0.271 0.32 + 0.01 Transition Probabilities Calculated This Work Waoks(107) Halmann (64) 1.000 1.0000 1.00 0.0377 0.03747 0.039 +0.03 Formation of the A n ionic state by removal of an electron from the (2pTr) molecular orbital gives rise to vibrational structure over a wide energy range on the photoelectron retarding curve, indicating the bonding character of this orbital. Table XVI compares the values obtained here with the vibrational spacings calculated by Wacks (107) and the transition probabilities with the Franck-Condon 107 Table XVI Ionization Data for COCxV, v"=0) + C0+(A2H, v'=0-6) o for 584 A Radiation v E, \-E. ' . (ev.) Transition Probabilities (v ) (v -1) Calculated(107) This Work Calculated This Work Wacks(107) Halmann(64) 0 - - 0.367 0.3686 0.38 + 0. .02 1 0. 190 0.20 + 0.01 0.819 0.8186 0.83 + 0. .02 2 0. 187 0.19. + 0.01 1.000 1.0000 1.00 3 0. 184 0.19 + 0.01 0.893 0.8930 0.89 + 0. .03 4 0. 180 0.19 + • 0.01 0.653 0.6541 0.65 •+ 0, .03 5 0. 177 0.18 + 0.01 0.417 0.4175 0.49 + 0, .04 6 0. 173 0.17 0.01 0.241 0.2414 0.28 + 0, .04 108 factors of Wacks (107) and Halmann and Laulicht (64). The third ionization process, as seen from the photoelectron retarding curve, resembles the first process, indicating that the (2sor) molecular orbital is largely non-bonding in character. A similar result was found for • The assignment of the bonding character of the molecular orbitals involved in formation of the ionic levels obtained here can be tested by consideration of the r values of these states and that e. . of the ground molecular state (see Table XVII). Table XVII Equilibrium Internuclear Distances of Some CO and C0+ Electronic States (44) COfxV) C0+(X2Z+ , A2n , B2Z+) 1.128 1.115 1.244 1.169 State r e (A) The values for the X E + and B E + ionic levels are very close to the ground state value indicating l i t t l e change in bond order due to removal of an electron from the appropriate orbital. The 2 A II ionic state, however, has an internuclear distance considerably larger than the ground state indicating removal of a bonding electron as previously supposed. 109 4.5 Oxygen 4.5.1 General Consideration Molecular oxygen, because of its importance in upper atmosphere reactions and other chemical processes, has been the subject of numerous spectroscopic studies. In particular, vacuum ultraviolet absorption studies have led to identification of four ionization processes above the first (for which no Rydberg series has been observed). The first ionization potential has been determined by other techniques. The ground state of the oxygen molecule has the electronic configuration (70) 0 2 KK (2s0 g) 2(2sa u) 2(2pa g) 2(2 p ii u) 4(2pu g) 2 , V (4-3) The first ionization potential will refer to removal of an electron 2 from the outer antibonding (2piT ) orbital to give rise to the X II g & ground state of the ion. The energy required for this process has been determined by electron impact (19,108,110),'photoionization (22,104,109) and photoelectron (105) experiments. The values obtained by a l l methods have been in good agreement. Mulliken has shown (70) that removal of an electron from the second most tightly bound orbital (2puu) may result in the two 4 2 electronic ionic states I I u and n^. Frost and McDowell (19), using the RPD electron impact technique have, in the light of spectroscopic data, been able to show that these states correspond to the second 4 2 ( a l l ) and third ( A H ) ionic states. 110 Price and Collins ( 6 } observed a Rydberg series leading 2 to a limit- higher in energy than the A JI third ionic state. The 4 -ionic state corresponding to this limit has been assigned b E and has been shown by Frost and McDowell (19) to be the fourth level of the ion. This state arises from removal of an electron from the (2pOg) molecular orbital. A fifth ionization potential of oxygen is known to be below the 21.21 ev. energy limit associated with the experiments described here. The ionic state resulting from this series limit was observed by Frost and McDowell (19) who, from energy considerations, assigned the process to the removal of an electron from the (2sov) molecular orbital. Rydberg series have been observed to this level (111) which 2 -has been assigned E by Gilmore (112). For oxygen, two dissociation limits lie lower in energy than o the 584 A radiation used in these experiments. The first of these, at 18.73 ev. above the ground state of the molecule, results from 2 4 2 dissociation of the X II , a H , and the A n bound ionic states g' u' u and in addition is the limit of several repulsive curves. The products are an 0(3P) atom and an 0+(^S°) ion (112). The other limit 4 -which lies at 20.70 eV arises from the dissociation of the b E g ionic state and again is also the limit for several repulsive curves. Here the products are 0(1D) and 0 +( 4S°) (112). As a number of the repulsive curves leading to the dissociation limit at 18.73 ev. pass through the Franck-Condon region below 21.21 ev. (112) the production of fragment ions with kinetic energy might well be expected. I l l 4.5.2 Results and Discussion Figures 26 and 27 represent the photoelectron retarding curve for and its first derivative respectively. The latter curve was obtained by manual differentiation of a recorder tracing of a retarding curve for C^. Figure 27 shows four distinct processes contributing to the photoelectron current. These processes can be associated with the first, second, fourth and fifth known ionic levels. The series limit for the third level lies close to that for the second, causing i t to become lost in the vibrational levels associated with the second level. Al-Joboury, May and Turner (105) were also unable to detect this level in their photoelectron spectrum of oxygen. Table XVIII compares the ionization potentials obtained by various techniques for the formation of the molecular ionic states. Krypton was used as the calibrating gas for the values reported here. Table XVIII Ionization Potentials for Oxygen (ev.) Ionic Spectroscopic Photoionization Electron Photoelectron Spectroscopy State (9) (104) Impact (19) Al-Joboury(105) This Work x 2n g a 4 n u 12.075 12.21 12.10 12.10 +_ 0.01 16.107 16.30 16.26 16.13 + 0.01 A 2 n u 16.824 17.18 b V g 18.173 18.42 18.18 18.19 +_ 0.01 V g 20.308 21.34 20.31 20.30 +_ 0.01 •USSXXQ j o uina^ oods Uoj+Doxoo^oqd ••92 ojnStj I R f A R B I T R A R Y U N I T S ro 1 x ro =1 OS o ro + 'Ul ro 3 > ro ro JM S ** 1 N V M V . 1«° i ZU 114 The values presented in Table XVIII are in good agreement with one another. One additional point arising from the data merits consideration. As mentioned previously, the first ionization potential of oxygen is not known spectroscopically and the photoionization value of Watanabe (104) is generally taken to be the most accurate value. Higher ionization potentials are not detected in the photoionization studies due to the interference of autoionizing peaks (22) which completely obscure the shape of the ionization continuum. Photoelectron spectroscopy (105) which in a l l other molecules considered has detected all known levels, misses the third known value here. The only experimental method which shows a l l the known ionic levels is electron impact (19,21,108). This divergence of results obtained by photon and electron impact provides a measure of support for the proposal by Briglia and Rapp (113) that the Franck-Condon Principle may not hold in electron impact and that the ionization process depends on the nature of the ionizing radiation. Thus in,electron impact the probability for exciting the vibrational levels of an electronic state might differ considerably from that for photon impact-A comparison of the r g values for the five ionic states of oxygen makes i t possible to predict the shape of the photoelectron retarding curve. Table XIX gives the known internuclear separations for the ground molecular state and several ionic states. Table XIX indicates a significant decrease in the r g value 2 for the X II groun.d state as would be expected for removal of an 8 electron from the (2pir ) antibonding molecular orbital. As discussed 115 in section 2.3.2.2 , this should give rise to a photoelectron retarding threshold which exhibits vibrational structure over only a small retarding range before leveling off as the potential energy curve passes out of the Franck-Condon region (see Figure 4). Figure 26 shows that 2 the X Jig retarding threshold does exhibit this behaviour. Table XIX Equilibrium Internuclear Distances of Some ©2 and 0^* Electronic States (44) State G v ( x V ) 0-+(X2n a4n A2n 2V g 2 v g u u r (A) 1.207 1.1227 1.3813 1.4089 b V V 1.2795 — Table XX shows the spacing for the first four vibrational levels observed on the removal of a(2pir^) antibonding electron. The results are compared with the values calculated by Wacks (107) and the electron impact values of Brion (108). The agreement between the calculated values and those obtained here is good, indicating that the model used for the calculation of the Franck-Condon factors is 2 a good approximation to the actual X 11^  potential energy curve. The relative transition probabilities to the vibrational states obtained in this work are compared with the calculated Franck-Condon factors of Wacks (107) and Halmann and Laulicht (64) and the 116 Table XX Vibrational Spacings for 0 2 ( X 3 l " , v"=0) -+ 0*(X 2n , v'=0-4) (ev.) © .. • * g A v Calculated(107) Electron Impact(108) This Work 0- 1 0.228 0.26 0.23 + 0.01 1- 2 0.225 0.25 0.23 + 0.01 2- 3 0.220 0.24 0.22 + 0.02 3- 4 0.216 0.21 + 0.02 Table XXI Relative Transition Probabilities for 0 2 (xV ,v"=o) + 02(x2ng,v'=o-4) i Calculated Electron V Wacks(10) Halmann(64) Impact(108) This Work 0.60339 0.60321 0.3 0.43 + 0.02 1 1.00000 1.00000 1.0 1.00 2 0.67349 0.67364 0.8 0.93 + 0-03 3 0.23569 0.23052 0.43 + 0.04 4 0.04538 0.04544 0:.14 + 0.02 117 electron impact values of Brion (108) in Table XXI. The calculated Franck-Condon factors dp not agree with the values measured in this work. Although this difference may arise from inaccuracies in the calculations or in the manner proposed in the discussion of the ioniza-tion of H2 (section 4.2.3), there is another possibility in this case. For H2 the disparity between calculated and measured values increased to higher vibrational levels while for Q2 there is a large difference even for the lowest values, i t is therefore conceivable that another process is contributing to the formation of the ground state 0 2 +, and since the absorption spectrum of 0^ shows many peaks in the region of 584 A (114) this discrepancy may be due to the occurrence of auto-ionization. Such an effect has been observed for H 2 + by Doolittle and Schoen (91). The contribution of autoionization to each ionization continuum will depend on certain selection rules (44) and the relative number of ions formed in each would then differ from that predicted by the Franck-Condon factors. 1 Removal of an electron from the bonding ^pir^) molecular orbital results in the a IT and A IIu states, both of which have r values considerably larger than the ground molecular state (see Table XIX). Considerable vibrational structure would therefore be expected for these states. Figure 27 illustrates that this is indeed : .- 4 the case, the vibrational structure associated with the lower a n state obscuring the onset of the upper state. This vibrational structure could not be completely resolved but the vibrational spacing is estimated to be 0.11 ev. for the first two levels with the most 118 probable transition being to the fifth vibrational level of the a IT state. The difference in internuclear distance between the ground 4 - -• • molecular state and the b E g ionic state is small compared to that for 4 2 the a II and A n levels and therefore less vibrational structure u u would be expected. Figure 27 supports this prediction, showing only 4 _ three levels of the b Z state excited, with the maximum probability g being to the lowest. The vibrational spacing of these levels is approximately 0.13 ev. Figure 27 illustrates the similarity between the fourth and fifth ionization thresholds. For the fifth process the inter-nuclear distance would appear to be slightly larger than the value for the fourth level as the second vibrational level is the most highly populated. The lower vibrational spacings for this level are estimated to be 0.16 ev. ' • 4.5.3 The 0* Fragment Ion Figure 28(a) shows the positive ion retarding curves for oxygen and argon and a curve representing the difference between them. As the Ar + ions have virtually zero kinetic energy, subtraction of the argon curve from that of oxygen results in a plot corresponding to the retarding curve for the 0 + fragment ions produced with finite kinetic energy. This curve is shown with its first derivative in the lower part of Figure 28(a). It is noted that the curve shape resembles the retarding energy distribution that would be expected i f the fragment ions arose through transitions to a repulsive oxygen ionic.curve 120 (see section 2.3.2.3). Similar curves have been obtained by Schoen (115). Figure 28(a) indicates that the maximum kinetic energy possessed by the fragment ions is 1.23 +_ 0.01 ev. If momentum is conserved both fragments will have an equal amount, and hence for the process 0 2 0 + 0 + + e (4-4) both fragments will have the same kinetic energy. The maximum total kinetic energy of the fragments is therefore 2.46 ev. If, as shown in Figure 28(b), there is a finite transition probability to a repulsive curve at 21.21 ev., the dissociation limit of the repulsive state may be calculated. The value obtained here is 18.75 ev.which agrees well with the known dissociation limit at 18.73 ev. (112). The first derivative of,the 0 + kinetic energy retarding curve shows that the most probable transition leads to production of fragments with 0.91 +_ 0.01 ev kinetic energy. This information locates the point on the repulsive 0* curve which lies directly above the center of the it v =0 level of the ground molecular state. This point lies at 20.55 ev. In Figure 28(a), i f i t is assumed that all the oxygen atomic, and molecular ions produced are being collected when the retarding curve levels off, then i t can be seen that approximately 7.5 % of the total o ionization occurring at,584 A results in the production of fragment ions with kinetic energy. This data permits an estimate to be made of the cross-section, at 584 A, for the formation of 0 ions, (one of the major components of 121 the upper atmosphere (116)). The total photoionization cross-section for C>2 has been measured by Wainfan, Walker and Weissler (116). The value obtained from their results for the photoionization cross-section o -18 2 at 584 A is 27^  x 10 cm /molecule. Assuming that 7.5% of the ions + -18 formed are fragments, the cross-section for 0 formation is 2.0 x 10 2 cm /molecule. 4.5.4 Transition Probabilities to the Ionic Levels of 0,, Table XXII compares the relative transition probabilities to each observed level of the ion with the values computed from Schoen's 4 Figure 11 (29). The probabilities are normalized to the a-II ionic level. The agreement between the two sets of results is not good. The difference might be due to the geometry of Schoen's cylindrical apparatus. Table XXII Relative Transition Probabilities to the 0^  o Ionic Levels for 584 A Radiation Transition Probability Ionic Schoen(29) ! This Work Level X2n 0.90 0.50 + 0.03 g -a4n , A2n 1.00 1.00 u' u b 4Z + 0.64 +_ 0.04 g } 1.36 2E" 0.57 + 0.03 122 The information in Table XXII coupled with the absolute photoionization cross-section of oxygen measured by Wainfan, Walker and Weissler (136) allows the cross-section for formation of each of the ionic states at 584 A to be calculated. The values obtained, after allowance for the 0 + dissociative ionization cross-section (see section 4.5.3), are 4.7 x 10 - 1 8 cm2/molecule, 9.3 x 10~18 cm2/molecule, - 1 8 2 - 1 8 2 2 6.0 x 10~ cm /molecule and 5.3 x 10 cm /molecule for the X IIg, 4 2 4 + 2 -a II - A I , b I and I ionic states respectively, u u' g g ^ 3 123 4.6 Nit r i c Oxide 4.6.1 General Considerations Much confusion exists in the literature concerning the absolute, energy of the f i r s t and inner.ionization potentials of n i t r i c oxide. Although the molecule has been studied by a number of different techniques (absorption spectroscopy (118-120), photoionization (121-125), electron impact (89) and photoelectron spectroscopy (105)) no single, technique has been able to reproduce a l l the ionization potentials reported by any one of the others. The ground state configuration of n i t r i c oxide is (70) NO KK(2sa g) 2(2sa u) 2(2po; g) 2(2pTr u) 4(2pTr g) 1 , X 2n. (4-5) This configuration is very similar to that for oxygen (section 4.5.1) and a similar photoelectron spectrum might therefore be expected. Removal of an electron from the (2piT g) molecular orbital results in formation of the ground state of N0+. The ionization, potential can be obtained by an indirect spectroscopic method using either the data of Tanaka (118) or Huber (120). As in oxygen, formation of the ground ionic state results from removal of an electron •if i from an antibonding orbital and should therefore give rise to*a decrease in r . e Tanaka (118) observed the convergence of three Rydberg series in NO to states above the f i r s t ionization potential. Huber (120) used the same data to assign two of theni. The lowest of the three he assigned 124 to the formation of the ion in a E + state, and the second, to 3 formation of the ion in a A state. The two ionization potentials correspond to removal of an electron from the second most tightly bound molecular orbital, (2piT u) „ The third Rydberg level observed by Tanaka (118) has been assigned (see Gilmore (112)) as the N0+ A*n state, arising from removal of an electron from the (2pag) molecular orbital. Data from this A*IT Rydberg limit, when combined with that obtained from the A->X f i r s t negative band system allows the f i r s t ionization potential of NO to be obtained. 3 A n state of the ion should exist at an energy lower than 1 that of the II state. Gilmore (112) used an uncharacterized band reported by Zapesochny et al,(126) to estimate the location of this 3 1 level between the A and n states mentioned above. 4.6.2 Results and Discussion Figure 29 shows the photoelectron retarding curve for NO, with a direct recorder trace of the f i r s t ionization process; inset. The scale was fixed using krypton- The values obtained here, as well as those from other experiments and calculations (127) are recorded in Table XXII. The theoretical values reported are a combination of the known spectroscopic f i r s t ionization potential (120) and the calculated energy separations of the ionic states of the molecule obtained by Lefebvre-Brion and Moser (127) using approximate Hartree-Fock orbitals. The results obtained here are in rough agreement with the results from other experimental methods. •apixo OTJ^TN jo unru+aads uoJioaiaoioi jd *6£ aanStj l R , A R B I T R A R Y U N I T S 126 Table XXIII Ionization Potentials of Nit r i c Oxide (ev.) Ionic State Spectroscopic Theoretical (127) X V a I 9.26 (120) 14.2 (118) 9.26 14.96 Photoelectron Spectroscopy Al-Joboury (105) This Work 9.23 9.32 + 0.01 14.84 + 0.01 16.55 (118) 15.99 15.4 16.53 15.72 +0.02 16.62 +0.01 16,9 (112) 16.95 17.18 + 0.01 18.4 (118) 19.44 18.24 ,18.24 + 0.01 The value obtained for the f i r s t ionization potential of NO, 9.32 ev., is slightly higher than values reported elsewhere. Removal of this most loosely bound electron should, because of i t s antibonding character, lead to a retarding threshold similar in shape to that for removal of the most loosely bound electron in 0^ (see section 4.5.2). Figure 29 shows that this is in fact the case and the pertinent data on the five vibrational levels observed is recorded in Table XXIV which includes a comparison of these results with those obtained by other methods. Equilibrium internuclear distances are again useful in assigning the f i r s t and inner ionic levels of the molecule. Table XXV summarizes the r values for some NO and NO* states. e . 127 Table XXIV Ionization Data for NO(X2IL, v"=0) •+ NO+(X1Z+, v'=G-4) o for 584 A Radiation a) Vibrational Spacing (ev.) Av' Calculated Photoionization Spectroscopic Electron This Work (107) (124) (128) Impact(89) 0- 1 0.290 0.290 0.291 0.29 0.30+_0.01 1- 2 0.287 0.288 0.286 0.26 0.29 +_ 0.01 2- 3 0.283 0.277 0.283 0.28 +_ 0.01 3- 4 0.278 0.295 0.278 0.28 +_ 0.01 b) Relative Transition Probabilities (normalized for v'=l) Calculated Photoionization ; This Work v' Wacks(107) Halmann(64) (122) 0 0.478 0.4781 0.79 0.59 +_ 0.03 1 1.000 1.0000 1.00 1.00 2 0.917 0.9170 1.00 0.81 +_ 0.02 3 0.484 0.4843 0.67 0.40+0.02 4 0.163 0.1629 0.16 + 0.03 128 Table XXV Equilibrium Internuclear Distances for Some Molecular and Iqnic States of Nitric Oxide. State N0(2IL) N0+(X1Z+, aV, 3A, 3n, AJn) o r e(A) 1.151 1.062 1.17 1.23 1.15 1.193 Tanaka's Rydberg series limit (118) at 14.2 ev., assigned as a 3 £ + by Huber (120) must correspond to the ionization potential A' i of 14.84 ev. found here. The calculated value of Lefebvre-Brion and Moser agrees with that reported in this work, and furthermore Gilmore (112) indicates that the value of 14.2 ev. is in some doubt. Figure 29 shows that this process has a low cross-section, which could account for the failure of Al-Joboury, May and Turner (105) to locate, i t . Examination of their results, however, does suggest the onset of some process in the region where the threshold was found in this work. From the shape of the retarding curve at the onset of this process (see Figure 29), i t would appear that a non-bonding electron is being removed. This observation is consistent with the slight change in internuclear distance occurring on formation of the 3 E + state (see Table (XXV)). The next ionization potential to be observed had an onset of 15.72 ev. Al-Joboury, May and Turner (105) in their photoelectron experiments, are the only other workers who have observed this process. 129 Figure 29 indicates that the electron being removed has considerable bonding character as the curve increases in intensity over a considerable range of retarding voltages. Assignment of this state is not obvious from the available evidence. There are three states which could give rise to the observed 3 - 1 -break at 15.72 ev. Two of these the I and I , dissociate at an energy of 21 ev., the second dissociation limit of the ion (112). If either of these states lay at 15.72 ev., their dissociation energy would be similar to that for the ground molecular state. That is, the ionization process occurring would hot change the bond strength of the species, and the threshold observed would correspond to the removal of a non-bonding electron. As this does not agree with the observed threshold, i t is concluded that neither of these is the correct one. The third state available is the: which dissociates at the lowest ionic dissociation limit, 20.1 ev." (112). This state is unknown but would have the desired characteristics, for the process involves the simultaneous removal and excitation of two electrons from the bonding (2piru) molecular orbital. The excited electron goes to the (2pir ) antibonding molecular orbital. Although this would result in the desired increase in r i t is difficult to ascertain i f i t would be e possible energetically. Nevertheless the unknown state has been tentatively labelled ^ Z+. Figure 29 indicates that the.fourth ionization potential results from removal of a non-bonding electron as confirmed by the values 3 in Table XXV. This level, which has been assigned at A, is well 130 known, and the values obtained for i t are in good agreement with each other. 3 Transitions to the II ionic level were observed here but were not reported by Al-Joboury, May and Turner (105). Gilmore estimated the energy of this level from a reported band in the spectrum of NO+ (112) to be 16.9 ev., in reasonable agreement with the value of 17.18 ev. reported here. The consistency of these values with the calculated value of 16.94 ev. establishes the energy of this l e v e l . The highest ionization potential observed, corresponding to formation of the A*n state, is well known spectroscopically. Since the electron removed is from the bonding (2pa^) orbital, an increase in r g for the ion would be expected, thereby giving rise to a photoelectron retarding curve which increases continuously in intensity over a considerable range of retarding voltages. This is confirmed by examination of Figure 29. From the experimental data i t was possible to obtain the relative transition probabilities to several of the ionic states for o 584 A radiation. The importance of NO in upper atmosphere studies is well known and a knowledge of the cross-sections for the various ionization processes that can occur is important. Table XXVI summarizes the relative transition probabilities obtained. The values are . 3 normalized to the A level. 131 Table XXVI Relative Transition Probabilities to some Ionic o States of Nitric Oxide for 584 A Radiation. Ionic Transition State Probabilities 1 I + 0.36 + 0.03 a 3E + 0.06 +_ 0.02 3A 1.00 AXn 0.51 + 0.03 132 4.7 The Hydrogen Halides 4.7.1 General Considerations A lack of accurate data on the location of the ionic levels of the hydrogen halides prompted their study during the photoelectron spectroscopic work reported in this thesis. The ground state of the hydrogen halides has the electronic configuration (70) HX , ZZ (nsa)2 (npo)2 (npir)4 , V . (4-6) where the ZZ term refers to the closed atomic shells of the particular halogen atom involved (the K,L,M and N for iodine down to the K shell . for fluorine). The discussion below shows that the electrons associated with the two outer orbitals, the (npir) and (npo) orbitals, have non-bonding and bonding character respectively. The non-bonding behaviour of the former orbital arises from its atomic character. The first ionization potential of molecules with the configuration given in equation (4-6) corresponds to removal of an electron from the (npir) non-bonding orbital. The resultant electronic 2 configuration, IK will be split by spin-orbit interaction to give the 2 2 2 two components 3^/2 a n c* ^1/2' t n e 3^/2 s t a t e lyi n2 lower in energy. The magnitude of the splitting which results from the removal of an electron having essentially atomic character would be expected to be roughly equal to that observed for the appropriate free atom. Removal of an electron from the outermost molecular orbital of the hydrogen halides will therefore give rise to two ionization processes separated 133 in energy by approximately the spin-orbit splitting found for the appropriate halogen atom. Removal of an electron from the (npo) molecular orbital corresponds to the third ionization potential of the hydrogen halidea. 2 + The resultant ion will be formed in a E electronic state. The electron removed in this case is bonding in nature and an increase in r g would therefore be expected, giving rise to vibrational structure in the photo-electron stopping curve. 4.7.2 Experimental The experimental results were obtained in the manner s previously described. The energy scale was calibrated in all cases with krypton. Due to the instability and corrosive properties of some of the gases used, special precautions had to be taken to ensure the gases were as pure as possible. The HF, obtained from the Matheson Co., could not be introduced into the ionization region in the normal manner due to its ability to attack glass. To overcome this difficulty the sample was introduced directly into the vacuum chamber from a lecture bottle by means of a teflon tube connected to a ground glass joint on the bottom flange of the spectrometer. (see Figure 7). The flow of gas was controlled by a monel needle valve attached directly to the head of the cylinder. HC1, HBr and HI were introduced in the conventional manner. The first two gases were obtained, free from impurities, from the Matheson Co. The experimental results verified the purity of the gases. Because of its instability, the HI was prepared immediately before use. It was produced by the action of water on a mixture of red 134 phosphorus and iodine (129). The gas was prepared at atmospheric pressure and purified by trap to trap vacuum distillation. The HI obtained in this manner showed no signs of impurities. 4.7.3 Results and Discussion Figures 30, 31, 32 and 33 depict the photoelectron retarding 2 2 curves for HF, HC1, HBr and HI respectively. The ^y2' 1/2 d o u b l e t was observed in each case with the exception of HF where the resolution of the apparatus was not sufficient to separate the states. For HC1 the doublet could be observed more easily from the first derivative curve (see inset Figure 31). The energies for the onset of the various processes observed in this study are compared in Table XXVII with the values obtained by other methods. The agreement between the values obtained by other methods and those reported here, particularly in the case of the 2 E + state, is not good. The shape of the photoelectron retarding curves (Figures 30, 2 31, 32 and 33) for all the molecules in the region of the I L ground ionic levels is typical of that which may be expected when a non-bonding electron is removed. Thus only one process is observed, with no vibrational structure, levelling off immediately above the threshold. A threshold of this shape indicates that l i t t l e change in r g occurs on 2 formation of the ionic states. The values for the known equilibrium internuclear separations of the ground and ionic states of HC1 and HBr, given in Table XXVIII, confirm this. 135 Figure 30, Photoelectron Spectrum of Hydrogen.Fluoride. Ti H-OQ c a T3 3* G rt O (D 1—• <D O rt •i O 3 W X) <D O rt i-t C 3 o SC X o. o o 3* O H* a-a t O Q: H 1 ' 1 ' 1 ' 1 ' HCl + -1 1 1 1 • 1 1 1 1 1 1 1 1 1— • Mi 12.7 13.0 • • • - > • • - IL -• • • • • 1 . 1 i . i _ l 1 1 1 • 13.0 14.0 15.0 16.0 (2I.2I-R) eV 170 18.0 ON 137 Figure 32. Photoelectron Spectrum of Hydrogen Bromide. Table XXVII Ionization Potentials of the Hydrogen Halides (ev.) Molecule Ionic Spectroscopic Electron Photoionization This Work State Impact (79) (104) HF 2 n . 15.77 16.06 + 0.01 I — 2 l * 16.88(a) 16.91 16.48 +_ 0.01 HC1 2 n 3 / 2 12.72 12.74 12.75+0.01 2 n i / 2 12.85 +_ 0.01 2I+ 16.29 (b) 15.92 16.28 +_ 0.01 HBr 2 n 3 / 2 11.82 11.62 11.71 +_ 0.01 2 n i / 2 12.60 12.03 +_ 0.01 2 E + 14.91 (c) 14.32 15.31 +_ 0.01 HI 2 n 3 / 2 10.44 10,38 10.42 +_ 0.01 2 n i / 2 11.094 (130) 11.14 11.08 +_ 0.01 2 E * 13.27 14.03 + 0.01 (Continued) 140 Table XXVII (Continued) Notes: a) Assuming the Rydberg series limit; observed by Johns 2 + and Barrow (131) refers to the E state. b) Using Watanabe's (104) figure of 12.74 ev. for the first 2 2 + ionization potential and the spectroscopic ' ^ separation of 3.56 ev. (132). c) Using Watanabe's (104) figure of 11.62 ev. for the first 2 2 + ionization potential and the spectroscopic 3^/2 ~ ^ separation of 3.29 ev. (133). 141 Table XXVIII Equilibrium Internuclear Distance for the Ground and Ionic States of the Hydrogen Halides (44). o Molecule State r g (A) HC1 1 t * 1.274 HC1+ 2jli 1 , 3 1 5 2 + ZE 1.514 HBr V 1.413 HBr+ 2n i 1.459 2 E + 1.666 142 2 2 The n3/2» ^1/2 douD-"-e't separations for a l l the species, with the exception of HF+ are listed in Table XXIX. The table includes the spectroscopically known values, the values calculated using equation (2-13) and the electron impact values obtained by Frost (79). The correlation between the values is good. Table XXIX 2 2 " n i / 2 D°ublet Separations of the Hydrogen Halides (ev) Ion Calculated Electron Spectroscopic This Work Impact (79) HC1+ 0.08 — 0.08 (133) 0.10 +_ 0.01 HBr+ 0.35 0.78 0.33 (133) 0.32+0.01 HI + 0.71 0.70 0.66 (130) 0.66 + 0.01 The inner ionization process corresponding to formation of the 2 E + ionic state should result in a photoelectron retarding curve exhibiting evidence of vibrational structure i f a bonding electron is removed. The amount of structure w i l l depend on the difference in r g between the ground molecular state and the 2 E + ionic state. These r values are listed in Table XXVIII, and i t is clear, that for HC1 and 143 HBr a large increase does indeed take place on formation of the upper ionic state. For HF, as illustrated in Figure 30, vibrational structure associated with the 2 Z + ionic state was detected, table XXIX l i s t s the pertinent data concerning this structure. The majority of the vibrational excitation is to the v'=0 level, indicating that the difference, in nuclear separation is slight. There is however an appreciable probability for transitions occurring to the next two levels, after which the retarding curve levels off in the same manner observed for the 0^ *^ ng s t a t e ( s e e Figure 26). Since this state of 0* is known to correspond to the removal of an antibonding electron, one may infer that the same is true for the state of HF+. Additional evidence in support of this proposal is presented later. Table XXX Ionization Data for HF(X^E+,v"-0) + HF +( 2£ +,v'=0-2) for 584 A Radiation v' E(v') - E(v'-l) (ev.) Relative Transition Probabilities normalized to v'=0 0 1.00 1 0.34 + 0.01 0.37 +_ 0.03 2 0.32 •+ 0.02 0.06 + 0.04 144 The shape of the retarding curve for HC1 (see Figure 31) suggests that an inner ionization process corresponding to the removal of a bonding electron occurs, although the vibrational structure i s not resolved. Furthermore, the curve indicates that for the 2 £ + state . the most probable transition (vertical transition) lies at about 0.2 eV above the onset. This shift in the vertical transition corresponds o to an increase of 0.23 A in r for the upper state. 2 + The I threshold of HBr (Figure 32). closely resembles that of HC1. In this case however, the most probable transition occurs at about 0.35 eV above onset, and the related increase in r is e 0.252 A. 2 + For HI, the shape of the E threshold (Figure 33) appears to dif f e r somewhat from that of both HC1 and HBr. The energy range over which the threshold extends is much narrower for HI than that observed for the other two molecules and,.in addition, the photoelectron intensity levels off quickly after the most probable transition has occurred. This threshold resembles the curve obtained for the positive ion retarding study of oxygen (section 4.5.3) which was shown to result from transitions to a repulsive 0* state. For this reason, and because of considerations discussed below, i t appears that this transition is to a dissociative ionic curve of HI +. Using the ionization potentials obtained in this work in conjunction with certain spectroscopic data, a lower limit for the bond dissociation energies of the ionic states detected here may be calculated. It is assumed that a l l of these states dissociate at 145 the lowest dissociation limit of the ion. This assumption can be jus t i f i e d by the following arguments. F i r s t l y , the bond dissociation energies obtained (see Table XXXII) show the trends expected, ie. the 2 + 2 I bond strengths are weaker than the IL for every molecule, and the bond strengths decrease as the atomic weight of the halogen atom increases. Secondly, as w i l l be shown below, the shape of the photo-electron curves supports a low dissociation limit for the 2 E + ionic states. Figure 34 defines the ground molecular state, the lowest ionic states, and the energies required for calculation of the dissocia-o tion energies of a typical HX molecule. The energies are; Do, the energy required to dissociate the ground molecular state; 1^, the ionization potential of the halogen atom; and 1^, the 0-0 ionization potential of the HX molecule. The sum of the f i r s t two energies gives the dissociation limit of the ionic state and the difference between this value and 1^ yields Do for the particular ionic state being considered. If 1^ does not correspond to the adiabatic 0-0 transition then the bond energy obtained is a lower limit of the actual value. Table XXXI l i s t s the data required for these calculations. The dissociation energies obtained in the above manner are compared with spectroscopic values in Table XXXII. For HC1+, HBr +, and HI + the agreement between the spectroscopic and calculated values for the dissociation energy of the lowest ionic level is good. o Comparison of the Do values for HC1, HBr, and HI from Table XXXI with o + + + the Do values for the lowest ionic state of HC1 , HBr and HI shows 146 2 1 . 2 1 eV INTERNUCLEAR DISTANCE Figure 34. The Bond Dissociation Energies of the Hydrogen Halides. 147 Table XXXI Data Required for Calculation of D„ for the Ionic States of the Hydrogen Halides a) Dissociation Energies of the X *E + Ground Molecular State (ev.) Molecule HF HC1 HBr HI Do 5.86 (134) 4.43 (44) 3.75 (44) 3.06 (44) b) Ionization Potential of the Halogen Atoms (ev.) Atom F Cl Br I I, 17.42 13.01 11.84 10.45 / 148 Table XXXII 2 2 + Dissociation Energies for the FL and E Ionic States of the Hydrogen Halides (ev.) o. Do Ion State Spectroscopic This Work HF+ V 3.64 (135) 7.22+0.01 V 6.80 + 0.01 HC1+  2J[Z/2 4.48 (44) 4.45 +_ 0.01 2 n 1 / 2 4.49 +_ 0.01 2 E + 1.06 + 0.01 HBr+ 2 n 3 / 2 3.50 (44) 3.88 ^ 0.01 2 n i / 2 3.56 + 0.01 2 E + 0.28 + 0.01 HI+ 2 n 3 / 2 3.11 (44) 3.09 +_ 0.01 2 n i / 2 2.43 +_ 0.01 2 E + dissociates 149 that the ground molecular and ionic states have v;ery nearly the same bond strength, supporting the fact that the electron being removed to form this state is non-bonding. For the 2 E + state of these ions, however, the bond is much weaker than that for the ground molecular 2 state. For HF, a different situation exists. For both the II. and E + ionic states the bond is much stronger than in the ground molecular state, indicating that the removal of any electron from the molecule results in a more stable species. Table XXXII indicates that for HI+ the 2 E + state is not formed, i.e. the ionization potential measured lies above the calculated dissociation limit of this ionic state. The state must therefore be either completely dissociative, or, i f formed, must have a very weak bond and large r g . Further evidence that this is the case comes from the shape of the HI+ 2 E + photoelectron retarding threshold (Figure 33) which, as pointed out above, resembles the curve found for formation of 0 + from dissociative ionization (see section .4.5.3). This result supports the assumption that the 2 E + state of the hydrogen halides dissociates at the lowest ionic dissociation limit. The relative transition probabilities to the various ionic states were derived from the experimental results. The first part of Table XXXIII lists the values for the transition probabilities 2 to the doublet components of the IL level and the second part, the 2 + 2 relative E , r L transition probabilities. Table XXXIIIa shows that the relative transition probabilities for exciting the doublet components is almost the same for all the 150 Table XXXIII Relative Transition Probabilities to the Ionic o States of the Hydrogen Halides for 584 A Radiation a) Doublet Transition Probabilities ^ (normalized to ^1/2^ State HC1+ HBr+ HI + 2n 0.94 +_ 0.04 0.94 + 0.01 0.95 +_ 0.02 2 n 1 / 2 1.00 1.00 1.00 2 2 + b) I I . . , E Transition Probabilities 2 1 (normalized to I K ) State HF+ HC1+ HBr+ HI + 2 n . 1.00 1.00 1.00 1.00 1 2 Z + 0.57 + 0.01 0.52 + 0.01 1.08 + 0.03 0.45 + 0.03 151 hydrogen halides. This is in contrast to the rare gases where there was a marked difference between the various atoms. The relative probability for transitions occurring to 2 + 2 the ? state with respect to the lower IL level is nearly the same for HF, HC1 and HI while for HBr the probability is much higher. An autoionizing level of HBr at 584 A, i f one exists, could be responsible for this difference., 152 4.8 The Halogens 4.8.1 General Considerations Little experimental data is available on the location of the ionic levels of the halogen molecules and that which has been obtained is often not in agreement. The ground state electronic structure of these molecules may-be represented as (70) X2, ZZ(nsa p) 2(nsa u) 2(npa g) 2(npn u) 4(npn g) 4, hg* • (4-7) where ZZ represents the closed atomic shells (K, L, M and N for iodine down to K alone for fluorine). The two outer molecular orbitals (npn ) and (npf ) are both thought to be mainly atomic in character and hence would be expected to give photoelectron retarding thresholds which are similar and exhibit the characteristics associated with the removal of a non-bonding electron. The ionic levels formed by 2 2 removal of electrons from these two orbitals are II and II respectively, u g the latter lying lower in energy. The actual energy difference between the two states will depend on the overlap between the wave functions 4 3 for the (np7i) and (npir) atomic groups. Mulliken (117) predicted that the overlap should be quite small resulting in the levels being closely spaced. Evidence to the contrary was observed here. Removal of an electron from either of the outer two molecular orbitals (equation (4T7)) of these molecules to produce an ion in one 2 of the II electronic levels introduces the possibility of observing splitting of the configurations due to spin-orbit interaction. If the 153 II levels lie far enough apart such that no overlap can occur, then> in order of increasing energy, ionization could yield the following states: 2 2 2 2 n n n IT ll3/2 g' J 1l/2 g' J13/2 u' l ll/2 u' The energy separation of the doublets would be expected to be similar to that measured for the hydrogen halides, and slightly smaller than that for the relevant halogen atoms (see Table XXXVI). The next inner molecular orbital available for ionization is the bonding (npo ). Removal of an electron from this orbital would result in formation of the ionic state with a consequent weakening of the interatomic bond. The photoelectron retarding curve for the threshold of this process should therefore resemble that observed for the similar process in the hydrogen halides, i.e. a sloping threshold indicative of unresolved vibrational structure. 4.8.2 Experimental The photoelectron retarding curves were obtained in the manner previously described, krypton being used as the calibrating gas. However, due to the corrosive powers of fluorine and the low vapour pressure of iodine, special methods of sample introduction had to be used. Fluorine was introduced into the spectrometer from a stainless steel vessel as illustrated in Figure 35. This vessel was filled with fluorine, supplied by the Matheson Co., to a pressure of about one atmosphere, using a stainless; steel vacuum system. The container was 154 AUXILIARY SAMPLE SYSTEM I — PHOTOELECTRON SPECTROMETER STAINLESS STEEL CONTAINER Figure 35. Sample Inlet System for Corrosive Gases. 155 then connected to the sample inlet system at a point such that the sintered glass leak was bypassed. The flow of fluorine was controlled by an Edwards High Vacuum stainless steel needle valve. To ensure that only pure fluorine was introduced into the spectrometer, the storage vessel was cooled with liquid nitrogen before opening the. needle valve and was maintained at that temperature throughout the duration of the run. Due to its low vapour pressure, iodine could not be introduced into the spectrometer through the molecular leak. Iodine crystals were therefore placed in a pyrex tube sealed at one end, and the vapour introduced directly into the ionization region through the needle valve used for fluorine (see Figure 35). In this manner a sufficient iodine pressure could be maintained. 4.8.3 Results and Discussion The photoelectron retarding curves for F 2, C l 2 , Br 2, and I 2 are given in Figures 36, 37, 38 and 39 respectively. Processes which can be associated with removal of electrons from the (npu .), (npir ) and (npo ) molecular orbitals are detected for all species. Table XXXIV compares the onset energies for the various processes with values obtained spectroscopically, by photoionization and by electron impact. The agreement between the first ionization potentials obtained by the various methods is good, but for the higher processes the electron impact and photoelectron results tend to differ. The value of 14.09 ev. obtained by Frost and McDowell (137) for the Q 156 Figure 36. Photoelectron Spectrum of Fluorine. 14.0 15.0 (21.21-R)eV 16.0 170 in Figure 38. Photoelectron Spectrum of Bromine. 3 _<M M 3 159 o LO" o o ro O CM CVJ (M CM CM ' ' ' ' I I I L o d - I ° Figure 39. Photoelectron Spectrum of Iodine; Table XXXIV Ionization Potentials of the Halogens (ev.) 160 Molecule Br, Ionic Spectroscopic State u g u V g 3/2 g 1/2 g u V g "3/2 g n i / 2 8 "II 3/2 u 'l/2 u V g Electron Photoionization Impact Ref.(104) unless Ref (141) otherwise noted unless other-wise noted. 15.7 (139) 15.83 (140) 16.04 (84) This Work 11.63 14.09 10.69 11.05 11.97 13.72 9.35 9.97 10.91 11.72 13.64 11.48 10.55 9.28 15.63 + 0.01 17.35 +_ 0.01 18.46 + 0.01 11.50 + 0.01 14.11 + 0.01 15.94 +_ 0.01 10.51 +_ 0.01 10.90 +_ 0.01 12.52 + 0.01 14.44 +_ 0.01 :9.33 + 0.01 9.96 + 0.01 10.87 +_ 0.01 11.68 + 0.01 12.79 + 0.01 161 second ionization potential of C l 2 was assigned on the basis of spectroscopic evidence (138) to the 2 E + state. The results presented s 2 here would indicate that the 14.09 ev. state should be II. Frost u and McDowell (137) did not detect a level that corresponds to the state of Cl*, and, as shown later, this can be explained in terms of the bond dissociation energy of this state. 2 The energy differences between the onsets of the IT and 2 n u states for the halogens are 1.72 +_0.01, 2.61 +_ 0.01, 2.01 +_ 0.01 and 1.54 +_ 0.01 ev. for F2+>. Cl 2 +» Br 2 + and I 2 + respectively. These energy differences are large compared to the doublet splittings of the appropriate atomic,species (see Table XXXVI) and would seem to contradict Mulliken's prediction (117) that the levels might lie close 4 together. This indicates appreciable overlap of the (npir) and (npir)"* atomic groups. The non-bonding character of the electrons removed in the 2 2 l 'v- ' ! formation of the II and n ionic: levels would be expected to be g u f reflected in sharp steps for the photoelectron thresholds (as observed for the hydrogen halides). From the appearance of the retarding curves i t is evident Ithat this condition is not met in a l l cases. For fluorine (Figure 36) the first process shows an increase, in photoelectron intensity over a range of about one volt, while for the second process the intensity increases gradually for more than a volt. This behaviour would appear to correspond to the removal of an antibonding electron in the first case and the removal of a bonding 2 2 + electron in the second. The ^3/2 " ni/2 d o u D ^ e t separation in F 2 162 could not be separated by the instrument but vibrational structure 2 was observed on the II threshold (see inset Figure 36). The pertinent s data on this structure is presented in Table XXXV (the relative transition probabilities being normalized to v =1.) Table XXXV Ionization Data for F„(X1Z+, v"=0) -+ F*(2II , v'=0-3) 2V g' 21" g' o for 584 A Radiation. i i i v E(v )-E(v -1) (ev.) Relative Transition Probability 0 - — 0.78 1.0.05 1 0.14 +_ 0.01 1.00 • 2 0.13 i 0.01 0.80 +.0.06 3 0.12 + 0.02 — No vibrational structure could be resolved on the threshold 2 of the process leading to the IK state. In Figure 37 the two photoelectron retarding thresholds for 2 2 chlorine arising from ionization to the n and II ionic states are 6 g u dissimilar: indicating that the bonding contributions of the (3pir ) and s (3piru) molecular orbitals are not the same. The first process, the sharper, of ;theitwo, would appear to correspond to the removal of a non-bonding:, or slightly antibondingjelectron. The latter possibility arises since the width of the step (approximately 0.5 V) is more than would be expected for a totally non-bonding electron. The probability for the 163 2 second process, corresponding to the formation of the rtu level, increases over an energy range of nearly 1.5V, and can therefore be associated with the removal of an electron which has bonding character. 2 2 The " ^1/2 doublet c o u l d not be separated for either of the lower ionic levels due to unresolved vibrational structure on the threshold which obscures the onset of the upper process I g 2 2 + The n , IIu photoelectron retarding thresholds for Br^ (Figure 38) resemble those for Cl*, indicating that the bonding character of the molecular orbitals must be similar for the two species. + 2 2 For Br™ the II doublet has been separated: the n doublet states are 2 g ^ u obscured by unresolved vibrational structure. The doublet separation 2 obtained for the IF state is compared in Table XXXVI with the values obtained for HBr, the values obtained by electron impact, and with the 2 2 atomic doublet separation. The n IIu photoelectron retarding thresholds for I* resemble those of Cl* and Br*, except that both, doublets are now separated. This separation proves that the lowest ionic 2 levels of these molecules must in fact be the II configurations. The shape of the retarding thresholds also confirms the antibonding character of the (5pTr ) molecular orbital and the bonding character of the ( S p i T y ) • The doublet separations obtained for iodine are given in Table XXXVI. The agreement between the electron impact and photoelectron doublet separations in Table XXXVI is excellent. The values for the 2 hydrogen halides are similar to those found for the n halogen doublets indicating the orbitals involved have nearly the same atomic character. 164 Table XXXVI 2 2 n3/2.'"" 1^/2 doublet Separations of the Halogens (ev.) Ion States Hydrogen Electron Atomic This Work Halides Impact(141) Separation(86) Br*, 2n-._ - 2n. 0.32 0.36 0.46 0.39 + 0.01 2 3/2g l/2g -i ! 2 n , / 0 - 2 n i / 0 0.66 0.62 0.94 0.63+0.01 2 3/2g l/2g -2 2 n3/2u" ni/2u 0.81 0.82 + 0.01 For fluorine the shape of the 2E* photoelectron retarding 8 threshold (Figure 36) corresponds to the removal of a bonding electron. The maximum transition probability appears to occur at about 0.3 V above the ionization onset, indicating that the change in r g on ionization is appreciable. In Figures 37, 38 and 39, the photoelectron retarding curves for Cl*, Br* and I* a l l show similar structure in the region of the 2E* threshold, indicating that a similar ionization process is occurring 8 in a l l three molecules. The threshold, which rises over a region of about 0.5 V in*all cases, is comparable to that obtained for the retarding curve associated with the'2Z* state of HI*, (see section 4.7.3). For this reason i t would appear that the photoelectrons arising in this process originate in a dissociative process. To test the suppositions regarding the bonding character of 165 the various molecular orbitals from which ionization is occurring the dissociation energies of the various ionic states were calculated. The method used for calculation is the same as that employed for the hydrogen halides (see section 4.7.3). The spectroscopic data required is listed in Table XXXVII. The dissociation energies obtained using this data and the measured ionization potentials (see Table XXXIV) are recorded in Table XXXVIII where they are compared with other available data. Table XXXVII Data Required for Calculation of Do for the Ionic States of the Halogens. a) Dissociation Energy of the X*Z+ ground molecular state Molecule ? 2 C l 2 Br 2 I 2 Do (ev.) 1.63 (139) 2.48 (44) 1.97 (44) 1.54 (44) b) Ionization Potentials of Halogen Atoms (86) Atom F Cl Br I I A (ev.) 17.42 13.01 11.84 10.45 2 For F 2 the bond dissociation energies indicate that the II state results from removal of an antibonding electron (that is, the ionic bond is stronger) as previously indicated, while, contrary to the result obtained from consideration of curve shape, the formation 166 Table XXXVIII Dissociation Energies of the Ionic States of the Halogens (ev.) Ion State Spectroscopic Do Electron Impact This Work V 3.3 (139) 3.18 (140) 3.42 +_ 0.01 1.70 +0.01 0.59 + 0.01 Cl. 4.01 (140) 3.99 +_ 0.01 1.38 +_ 0.01 dissociates Br, 3/2 g 1/2 g u 3.26 (140) V g 3/2 g 1/2 g 3/2 u "n V 1/2 u + g 2.64 (137) 3.30 +_ 0.01 2.91 £ 0.01 1.29 +_ 0.01 dissociates 2.66 +_ 0.01 2.03 +_ 0.01 1.12 + 0.01 0.31 + 0.01 dissociates 167 2 of the Hu state involves l i t t l e change in bond energy, indicating that a non-bonding electron is involved. The 2Z + ionic state has a * g weaker bond than the molecule indicating removal of a bonding electron, (as was previously indicated by the shape of the retarding curve). o Comparison of the D0 values for the ground molecular state (Table XXXVII) with the dissociation energies calculated here for the ionic states of Cl^, B^, and 12 supports the conclusions reached above on the basis of the photoelectron curve shapes. The (npir ) molecular orbital therefore has antibonding character and the (npi^) bonding character. Table XXXVIII shows that all of these molecules undergo dissociative ionization at energies below the 2Z g onset, as was previously postulated. This indicates that either the 2Z + ionic states s are not stable, or that the bonds are very weak and the r g values large. The possibility of a higherdissociation limit for this state can be ruled out from energetic considerations based on the energy of the first excited states of the atomic ions. Dissociation of this upper state probably accounts for the failure of Frost and McDowell (141) to observe the 2 £ + ionic state of Cl„. g 2 Information on the relative transition probabilities to the various ionic states could also be obtained from the experimental results. Table XXXIXa lists the relative transition probabilities to the components 2 2 of the "• ^1/2 doublets where separation could be detected. The 2 values are normalized to the Hjy2 s t a t e * n a ^ cases. The probabilities for the 2 n g components of Br* and I*, are similar to each other and to the appropriate hydrogen halide doublets (see Table XXXIII). 168 Table XXXIX Relative Transition Probabilities to the Ionic States of the Halogens for 584 A Radiation. a) Doublet Transition Probabilities Ion State Relative Transition Probability Br* 2n_.- 0.98 + 0.01 2 3/2 g — 2 n . l . o o 1/2 g I* 2n_._ 0.96 + 0.01 2 3/2 g — 2 ni/2 g 1.00 \/2 u 0.77 + 0.01 \/2u 2 2 2 + b) II , n , Z Transition Probabilities 1 g' u' Ion State Relative Transition Probability F * 'n 1.00 2 g n 0.28 + 0.02 u — 2Z+ 0.79+0.04 g -cit 2n l.oo 2 "g 2 n 0.74+0.03 u — 2 I + 0.29 + 0.02 (Continued) Br, u V g u g 1.00 0.85 +_ 0-04 0.28 + 0.02 169 1.00 0.83+0.06 0.27 + 0.03 170 This supports the earlier evidence found in studying the doublet separations that the (npir) orbitals in the HX molecules are similar to the (npir ) orbitals in the X^  molecules. 8 2 2 The relative transition probabilities to the II , II and 2Z* ionic levels are listed in Table XXXIXb. With the exception of fluorine, the relative probabilities for transitions to any particular 2 ionic state are similar. The probabilities are normalized to the ionic level. There is no apparent reason for the discrepancy observed in the case of F^. It is possible that the differences might be due to the presence of an autoionizing level at 584 A which affects the population of the various levels in accordance with certain selection rules (44). 171 4.9 Nitrous Oxide 4.9.1 General Considerations Triatomic molecules which possess sixteen electrons in their outer shell are known to have a linear structure (142). Nitrous oxide falls into this class of compounds. Due to its unsymmetrical structure its molecular orbital description is more complicated than for symmetrical isoelectronic triatomic molecules such as carbon dioxide (142). The first ionization potential of N^ O has been observed spectroscopically by Price and Simpson (143) and by Duncan (144), while Tanaka, Jursa and LeBlanc (145) have identified two Rydberg series in addition to that leading to the first ionization potential. Curran and Fox (146) have studied N20 using the RPD technique and have located a number of breaks in the ionization efficiency curve. Evidence for four ionization processes was seen in the photoelectron spectrum obtained by Al-Joboury, May, and Turner (147), of which three could be associated with the Rydberg series limits of Tanaka, Jursa and LeBlanc (145). Mulliken (142) gave the molecular orbitals for nitrous oxide, in order of decreasing binding energy, as 2 2 2 2 N20, (s+s+s,o) , (s+a-s,o) ,(s-s+s,a) ,(o-+a-a,a) , 4 4 1 + (Tr+7r+tr,ir) (TT+TT-TT , IT) , Z (4-8) The first electron removed would be expected to arise from the (TT+7r-ir,ir) molecular orbital. Mulliken (142) indicated that this orbital might have some antibonding character, the electron involved 172 being associated with the N-0 bond. This ionization process was observed by Price and Simpson (143), by Duncan (144) and by Tanaka, Jursa and LeBlanc (145) . The latter workers were able to detect splitting of this level due to spin-Orbit coupling, the doublet separation being 0.04 ev. Removal of an electron from the (TT+TT+TT,^) molecular orbital leads to formation of the first excited state of the ion. In Mulliken's notation (142) this corresponds to removal of an electron with bonding character. Tanaka, Jursa and LeBlanc (145) observed Rydberg series limits leading to the zeroth and first vibrational levels of the corresponding ionic state. The next orbital available for electron removal is the bonding (a+a-a,a) orbital. This level (which has not been observed spectroscopically), was first reported by Al-Joboury, May and Turner (147) although they assigned the process as arising from removal of an electron from the (TT+IT+TT,IT) molecular orbital. Evidence will be presented later to support Mulliken's prediction. Tanaka, Jursa and LeBlanc (145) observed a Rydberg series converging to a higher limit (20.10 ev). They designated the process occurring as arising from removal of an electron from the (srs+s,a) molecular orbital. According to Mulliken's description (142) this molecular orbital should be antibonding. 4.9.2 Results and Discussion Table XL compares the ionization potentials obtained from the photoelectron retarding spectrum, presented in Figure 40, with the values obtained by other methods. Krypton was used to fix the energy scale. 'sptxo sno-iiTN j o mnjcioads uoai03xao^oi{r] 'Ofr sJ-nStj I R , A R B I T R A R Y UNITS £11 174 Table XL Ionization Potentials for Nitrous Oxide (ev.) Molecular Spectroscopic Photoionization Photoelectron Spectroscopy Orbital (145) (104) Al-Joboury(1^7) This Work Ionized ( T T + T T - T T , T T ) 12.89 12.90 12.82 12.90 +_ 0.01 (TT+ T T + T T , IT) 16.39 16.37 : 16.41 + 0.01 (a+o-a,a) 17.67 17.74 + 0.02 (s-s+s,o) 20.10 20.10 20.10 +_ 0.01 The shape of the first photoelectron retarding threshold (see Figure 40) is indicative of the removal of a non-bonding electron. This contradicts Mulliken's proposal that the electron removed from the most loosely bound molecular orbital is antibonding in character. The second ionization potential, as seen from Figure 40, has vibrational structure associated with i t . This implies either a change in a bond length or, as this is a triatomic molecule, a change in the geometry of the species on formation of the ion. The maximum transition probability is to the lowest of the vibrational levels. However the excitation of the higher vibrational levels indicates tr^ at the electron being removed has slight bonding character. Table XLI presents the pertinent vibrational data associated with this level. This level can be correlated with the (iT+iT+Tr,Tr)molecular orbital, which would not be expected to be as strongly bonding in character as the a orbitals discussed in the next paragraph. 175 Table XLI Ionization Data for the Second o Ionic Level of Nitrous Oxide, for 584 A Radiation < i i v E(v )-E(v -1) (ev) Relative Transition Probabilities Spectroscopic(145) This Work (normalized to v =0) 0 — 1.00 1 . 0.18 0.15 + 0.01 0.23 +_ 0.03 2 0.17 0.15 +_ 0.02 0.08 +_ 0.04 Removal of an electron from the (a+a»o,o) molecular orbital leads to the third ionization process observed. The shape of the photoelectron retarding curve for this process indicates the removal of a strongly bonding electron as would be expected from the character of this orbital (142). Mulliken predicted (142) that the fourth most strongly bound electrons should have antibonding character. Examination of Figure 40 supports this hypothesis. From the experimental data, the relative transition probability to each level could be obtained. The values found are recorded in Table XLII, and are normalized to the lowest molecular orbital. 176 Table XLII Relative Transition Probabilities to the Ionic o Levels of Nitrous Oxide for 584 A Radiation Molecular Relative Transition Probability Orbital Ionized CTT+TT-IT , IT) 1.00 O+Tr+ir.w) 0.64 +_ 0.04 (o+o-a.g) 1.61 +_ 0.06 (s-s+s,a) 0.66 +_ 0.06 177 4.10 Nitrogen Dioxide 4.10.1 General Considerations Several authors have reported ionization potentials for nitrogen dioxide. The values have been determined by various methods: direct electron impact (148, 149), indirect electron impact (150) (i.e.) deduced from the ionization potential of CH,jN02, the appearance potential of the NO* fragment and the H^ C-NO^  bond energy), absorption spectroscopy (151, 152), photoionization (24,104) and photoelectron spectroscopy (133). The reported values are not in good agreement and indicate the need for further study. Triatomic molecules which possess seventeen electrons, such as nitrogen dioxide are known to have bent structures, as opposed to those with only sixteen which are linear (142) . It is therefore apparent that the extra electron must reside in the vicinity of the central atom. Mulliken (154) has given the molecular orbital structure for N02, in order of decreasing binding energy, as N02 ( l a p 2 , ( l b 2 ) 2 , ( 2 a i ) 2 , (2b 2) 2, ( l b p 2 , ( 3 a i ) 2 , ( l a 2 ) 2 , (3b 2) 2, (4a 1) 1 (4-9) This indicates that the first ionization potential involves removal of the odd electron associated with the nitrogen atom. Since this process involves removal of an 'atomic' electron the resultant ion, which is isoelectronic with carbon dioxide, would be expected to be linear. Price and Simpson (151) have indicated that from considerations of the Franck-Condon Principle, the photoionization cross-section for this 178 process would be expected to be very low due to the large change in geometry. In nitrogen dioxide, the next two most loosely bound electrons reside in the (3b2) and (la 2) molecular orbitals. These are predicted by Mulliken (154) to have similar binding energies of about 13 ev. They may be thought of as two components of a doublet since in a linear sixteen electron molecule (for example CO^) the two orbitals have the same energy. The splitting arises due to a change in geometry. These two orbitals are associated with the non-bonding electrons of the oxygen atoms, and consequently l i t t l e change in geometry would be expected when ionization from them takes place, Mulliken (154) indicates that the molecular orbitals (3a^), (lbp and (2b,,) will lie at approximately 17 ev. The first two arise in the same manner as the two orbitals just mentioned, that is they form a closely-spaced doublet with the orbitals being largely non-bonding in character. The (2b2) molecular orbital is of a bonding character. The next electron removed arises, according to equation (4-10), from the (2a^) molecular orbital. This electron, which Mulliken (154) predicts will have about 20 ev. binding energy, is also bonding in character. The two remaining orbitals, the (lb 2) and (la^) will, according to Mulliken's description (151), have a binding energy of over 21 ev.; transitions to them would therefore not be observed in photoelectron spectroscopy at 21.21 ev. 179 4.10.2 Results and Discussion N02 was studied in the manner previously described, argon being used for the calibrating gas. Because of the shape of the threshold arising from the first ionization process, the second ioniza-tion potential at 12.92 ev. was used to establish the energy scale for the other ionization potentials. Figure 41 depicts the photoelectron retarding spectrum obtained for N02- A total of seven ionization processes were detected below 21.21 ev. As shown in Table XLIII, Al-Joboury and Turner (153) also observed seven processes which agree with the values obtained here. Results obtained by other methods are included in the Table but for the most part the agreement between the values is poor. The reasons for the molecular orbital assignments given here as well as for some of the major discrepancies in the Table will be discussed below. The wide range of values observed for the first ionization potential can be explained in terms of a change in geometry of the species on removal of the most loosely bound electron. As previously mentioned, Price and Simpson (151) predicted a low cross-section for this process. The photoionization experiments, which give the lowest values, may be assumed to give the first ionization potential of this molecule. Frost, Mak, and McDowell (24) show in their results that this process has indeed a very low cross-section. The ionization potential obtained by indirect electron impact methods agrees with the low photoionization value. I R , A R B I T R A R Y UNITS 08T 181 Table XLIII Ionization Potentials of Nitrogen Dioxide (ev.) Molecular Photo- Electron Impact Spectro- Photoelectron Orbital ionization (Direct) (Indirect) scopic Spectroscopy Ionized Al Joboury(147) This Work 4ax 9.76 (158) 10.5(22) 9/91(150) 10.97 10.91 +_ 0.02 9.80 (24) 11.(149) 3b2 11.7 (158) 11.62(158 12.82 12.92+0.01 12.3 (151) l a 2 13.48 13.64 _+ 0.01 3ax 13.98(148) 14.01 14.14 +_.0.01 lb x .14.37 14.59 + 0.01 2b2 16.79 17.31 +_ 0.03 2ax 18.87(152) 18.86 18.90 +_ 0.01 182 The higher first ionization potentials obtained by direct electron impact must correspond to formation of the ground ionic state in some highly excited vibrational state available to a vertical or near vertical transition. The photoelectron spectroscopy values, (the highest obtained), must arise in a manner similar to that for electron impact. As previously discussed (section 2.1.1.1) the photoionization cross-section generally decreases with increasing energy of the ionizing o radiation and it would appear in this case that for 584 A radiation the cross-section for the onset of the process forming the ground ionic state is so small that i t is not detected until well above threshold. The next four ionization potentials observed are quite closely spaced, as seen in Figure 41, and the thresholds are of similar shape. The lower two of these four excited ionic levels have nearly the same probability of formation from the ground molecular state, as shown in Table XLIV. The same is true for the upper two levels. These four levels can therefore be regarded as two sets of adjacent, non-overlapping doublets. The similarity of the photoelectron retarding threshold shapes supports this observation. The lower doublet must correspond to removal of electrons from the (3b2) and la^) molecular orbitals. As previously mentioned, Mulliken (154) has shown that these two molecular orbitals should form a closely-spaced doublet with a binding energy of about 13 ev. The results obtained are therefore in agreement with Mulliken's predictions (154). Furthermore, the shape of the retarding curve for these 183 thresholds indicates removal of a non-bonding electron as predicted. Table XLIV Relative Transition Probabilities to the Ionic Levels 0 of Nitrogen Dioxide for 584 A Radiation (Normalized to the 2a. Molecular Orbital). Molecular Orbital Relative Transition Probability Ionized 4 a l 0. + 0.02 3b2 0. ,36 + 0.02 l a2 0. ,37 + 0.03 3 a l 0. ,12 + 0.01 l b l 0. 12 + 0.02 2 b2 0. .75 + 0.04 2 a l 1, ,00 The second doublet must Correspond to removal of electrons from the (3a^) and (lb^) molecular orbitals. As in the case above, this doublet arises from the splitting of a degenerate orbital in the related sixteen electron linear molecule. The binding energy of these orbitals is somewhat less than the approximate value of 17 ev. predicted by Mulliken (154) but the lower half of the doublet corresponds to the value of 13.98 ev. obtained by Collin (148). Examination of Figure 41 indicates that these levels also arise from orbitals which are largely non-bonding in character. 184 The sixth ionization potential at 17.31 ev. has been designated as arising from removal of an electron from.the (2b^) molecular orbital. This level occurs close to the value predicted by Mulliken (154) and the shape of the retarding curve for this threshold indicates that the electron is indeed bonding in character. The highest ionization potential, obtained corresponds to the only ionic state which is known accurately from spectroscopic data. The state arises from the removal of an electron -from the bonding (2ap molecular orbital. Vibrational structure could be resolved on this threshold confirming the bonding nature of the orbital. The data concerning this vibrational structure is recorded in Table XLV. The vibrational transition probabilities are normalized to the v'=0 level. With the exception of the ground ionic state (which is believed to have linear geometry) i t would appear that the other observed ionization processes do not involve large changes in molecular geometry. 185 Table XLV Vibrational Data for the Highest Ionic State of Nitrogen Dioxide for 584 A Radiation E(v') - E(v'- 1) (ev.) Relative Transition Probabilities 0 1.00 1 0.17 +_ 0.01 0.74 +_ 0.04 2 0.15 + 0.02 0.30 + 0.03 186 4.11 Ammonia 4.11.1 General Considerations The ionization potentials of ammonia have been determined experimentally by photoionization (104, 155), RPD electron impact (156) and photoelectron spectroscopy. A number of theoretical calculations have been performed for this molecule, the latest being an LCAO-SCF calculation in which the ionization potentials for the first two molecular orbitals were estimated (157) . The ammonia molecule, which has symmetry in the ground state, can be represented in terms of the molecular orbital formula (76) N H 3 , (la 1) 2(2a 1) 2(le) 4(3a 1) 2, 1A 1 (4-10) The first ionization potential corresponds to removal of an electron from the (3a^) molecular orbital. This orbital is non-bonding, with the electron pair being located on the nitrogen atom. Ionization from the (He) doubly degenerate orbital is the only other process occurring below 21.21 ev. This orbital is known to be the major bonding orbital in the molecule, as i t spans the three N - H bonds. Removal of an electron might therefore be expected to result in a substantial change in molecular parameters. 4.11.2 Results and Discussion The photoelectron retarding spectra for N H ^ were obtained in the usual manner, krypton being used as a calibrating gas. The 187 ionization potentials obtained are compared in Table XLVI with the results obtained by other methods. Table XLVI Ionization Potentials of Ammonia (ev.) Photoionization Electron Theoretical Photoelectron (155) Impact(156) (157) Spectroscopy Al-Joboury (153)This Work ^ a p 10.07 10.40 10.26 10.16 10.35 + 0.01 (le) *15 15.31 15.50 15.02 14.95 +_ 0.02 Note: The theoretical values were obtained using a minimal basis set for the LCAO-SCF computations. The NH* formed by removal of an electron from the (3a^) molecular orbital is isoelectronic with the methyl radical. Since CHj is planar i t might be expected that the ground state of NH*. would also be planar. Figure 42 represents the photoelectron retarding curve for ammonia. The shape of the threshold for the first ionization process resembles that observed for NO^ (see section 4.10.2). As for N02> a large discrepancy was found between the first ionization potentials measured by different;methods. It is proposed that these two features are characteristic of molecular species which undergo a change in geometry on being ionized. Thus, the N02 molecule is bent in the ground state whereas the NO*, ground state is planar, and Molecular Orbital Ionized •T3TU0UMIV jo uiruaosds uo.i}39i90}Ot{d 'Zb s-cnSij I R , ARBITRARY UNITS T 1 1 r 881 189 the NHj molecule exhibits C j v symmetry i n the ground state whereas the NH* ground state has symmetry. Removal of an electron from the bonding (le) molecular o r b i t a l results i n a photoelectron threshold which i s t y p i c a l for a process of t h i s nature (see Figure 42); i.e. the removal of a bonding electron results i n a photoelectron spectrum which increases i n in t e n s i t y over a large energy range, i n t h i s case 1.5 V. The r e l a t i v e t r a n s i t i o n p r o b a b i l i t i e s for formation of the two i o n i c states for transitions from the ground molecular state are given i n Table XLVII. As was the case f o r NO^, the p r o b a b i l i t y of formation of the i o n i c state which involves a change i n geometry i s f a r less than i n the cases where only bond lengths change. Table XLVII The Relative Transition P r o b a b i l i t i e s to the Ionic Levels of Ammonia for 584 A Radiation. Molecular Orbi t a l Relative Transition Ionized Probability (3a r) 0.14 + 0.02 (le) 1.00 CHAPTER FIVE CONCLUSION The results presented in this study have shown conclusively that a photoelectron spectrometer of spherical geometry can be used to study the ground and excited states of atomic and molecular ions. In many ways this method is superior to those used previously to observe ionization phenomena, as i t can offer information not only on the location of an ionic state but also on the nature of the electron being removed and on the relative probabilities of formation of the ionic states. This last property is particularly useful, as information on the cross-sections for the formation of individual ionic states at energies above threshold has not previously been available. Furthermore,the absence of interference from such processes as autoionization makes this technique more versatile than other methods. As only 21.21 ev. radiation has been used here, the relative transition probabilities measured are reliable only for that energy. A logical extension of this work would therefore be the coupling of a spherical photoelectron spectrometer to a vacuum ultraviolet monochromator. This would allow the probability of formation of the various ionic states to be measured as a function of ionizing energy. Such information used in conjunction with absolute measurements of the 191 total ionization cross-section would enable the actual cross-section for formation of each ionic state to be obtained. Data of this nature is of basic interest in upper atmospheric studies. Use of monochromatic radiation of energy greater than that used here (21.21 ev) would allow observation of higher ionic levels associated with removal of inner shell electrons. Preliminary experiments of this type are being attempted in this laboratory. Under the correct conditions an A.C. discharge in helium will emit strong o o radiation at 303 A (40.9 ev.). This emission line, like the 584 A line used here, is the only source of emission in the energy region of interest and i t could be used to extend the present studies to higher energy without the requirement of a monochromator. Another obvious extension of this work is a quantitative measurement of the angular dependence of photoelectron emission. As was mentioned in section 2.1.1.3, bound electrons may be emitted with an angular distribution which is a function of their angular momentum. 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