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Photoelectron spectroscopic studies of some polyatomic molecules Sandhu, Jagjit Singh 1967

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PHOTOELECTRON SPECTROSCOPIC STUDIES OF SOME POLYATOMIC MOLECULES by JAGJIT, SINGH, SANDHU M.Sc. Panjab University, Chandigarh, 1964 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Department of CHEMISTRY We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1967 In presenting this thesis in pa r t i a l fulfilment of the requirements for an advanced degree at the University of B r i t i s h Columbia,, I agree that the Library shall make i t freely available for reference and study. I further agree that permission-for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of (ZfvfrrvU\ The University of B r i t i s h Columbia Vancouver 8, Canada i i ABSTRACT o The 584 A photoelectron spectra from eight polyatomic molecules (CtijI, CHjCl, CH^Br, CH 3 CH0, CH3COCH3, SF 6, CH3CN and C2Hj.CN) are described and shown to give a l l the i o n i z a t i o n p o t e n t i a l s less than 21.21 eV i n each case. The r e s u l t s are interpreted i n terms of the e l e c t r o n i c structures of these molecules as given by molecular o r b i t a l theory. They are compared with r e s u l t s from other sources, and agreements and differences explained. A b r i e f account of other e x i s t i n g methods used for the determination of i o n i z a t i o n p o t e n t i a l s with t h e i r advantages and disadvantages i s given. The major components of the instruments are b r i e f l y discussed, and use of a Single-Grid Photoelectron Spectro-meter i n the detection of f i n e structure i n the photoelectron spectra i s pointed out. ACKNOWLEDGEMENT The work described i n t h i s thesis was done under the supervision of Dr. D. C. Frost to whom I would l i k e to express my sincere appreciation.. I would also l i k e to thank Professor C. A. McDowell f o r his i n t e r e s t i n the work. I wish to express my gratitude to Dr. C. E. Brion, Dr. D. A. Vroom and Mr. G. E. Thomas f o r t h e i r h e l p f u l discussions, to the technical s t a f f of the Chemistry Department f o r t h e i r s k i l f u l assistance and to Miss Donna Symons for typing the manuscript. 1 i v CONTENTS Page ABSTRACT i i ACKNOWLEDGEMENT i i i CHAPTER ONE - INTRODUCTION 1 A. Ionization Potentials 3 1. Introduction 3 2. Adiabatic and V e r t i c a l Ionization Potentials 3 3. Determination of i o n i z a t i o n p o t e n t i a l s 5 a) Optical Spectroscopy 5 b) Electron Impact 6 c) Photoionization 8 d) Photoelectron Spectroscopy 9 • B. The Franck-Condon P r i n c i p l e 13 CHAPTER TWO - EXPERIMENTAL 16 A. Introduction . 16 B. The Samples used and t h e i r handling system 18 C. Photoelectron.Spectrometer 22 1. The Photon Source 22 2. The Photo-electron Analyzer 24 3. Operation of the Spectrometer 24 4. Vacuum System 26 CHAPTER THREE - RESULTS AND DISCUSSION 28 1. Methyl Halides 28 2. Acetaldehyde 42 3. Acetone 47 4. Sulfur Hexafluoride 5. Methyl Cyanide 6. Ethyl Cyanide CHAPTER FOUR - CONCLUSION REFERENCES . LIST OF TABLES The Samples used and t h e i r o r i g i n 20 O r i g i n and p u r i t y of Methyl Halides 28 Ionization Potentials of Methyl Halides 34 2 2 E ^ 2 - doublet separations of the Methyl Halides 40 Ionization Potentials of Acetaldehyde 45 Ionization Potentials of Acetone 49 Ionization Potentials of Methyl Cyanide and Ethyl Cyanide. 54 ( v i i LIST OF FIGURES 1. Potential energy curves 4 2. Comparison of C y l i n d r i c a l and Spherical Analyzer Results f o r Hydrogen 12 3. The Franck-Condon P r i n c i p l e 14 4. The Spherical Photoelectron Spectrometer 17 5. Photoelectron Spectrum of Argon obtained (a) with double, (b) with single g r i d i n the energy analyzer 19 6. The Sample Handling and Vacuum System 21 7. A Spherical Grid 23 8. Schematic Diagram of Electron Retarding System 25 9. Photoelectron Spectrum of Methyl Iodide 29 10. Photoelectron Spectrum of Methyl Bromide 30 11. Photoelectron Spectrum of Methyl Chloride 31 12. Electron energy l e v e l diagram f or the Methyl Halides 32 13. Photoelectron Spectrum of Acetaldehyde 43 14. Photoelectorn Spectrum of Acetone 48 15. Photoelectron Spectrum of Sulfur Hexafluoride 51 16. Photoelectron Spectrum of Methyl Cyanide 55 17. Photoelectron Spectrum of Ethyl Cyanide 56 18. Electron energy l e v e l diagram f o r HCN, CH CN and C„H CN 57 1 CHAPTER I  INTRODUCTION Although, a. 1 arge;; amount vof' information i s available on the inner ionization- potentials- (1.P's) of diatomic molecules, and polyatomic molecules with- r e l a t i v e l y high..jdegxees"of symmetry, s i m i l a r data are not-readily available' for' unsymmetrical,.polyatomic molecules. The i o n i z a t i o n of. polyatomic molecules with r e l a t i v e l y high degrees of symmetry such, as methyl halides and benzene can be treated t h e o r e t i c a l l y i n somewhat the same fashion as diatomic molecules (1). I f , now, one. considers larger, unsymmetrical poly-atomic molecules such as acetaldehyde, the t h e o r e t i c a l considerations applied i n the above cases can no longer be employed since the number of e l e c t r o n i c and v i b r a t i o n a l states of the system i s too large. This thesis i s mainly concerned with discussions of i o n i z a t i o n p o t e n t i a l s obtained by studying the k i n e t i c energy d i s t r i b u t i o n of photoelectrons ejected from polyatomic molecules (both 0 symmetric and unsymmetric) when subjected to 584 A (21.21 eV) ra d i a t i o n . In t h i s way the i o n i z a t i o n p o t e n t i a l s of each of the molecules which l i e below 20 eV have been measured. Besides y i e l d i n g values f o r the energy le v e l s of the excited states of the various molecular ions, the present studies are also of int e r e s t f o r the following reason. They provide one way i n which molecular o r b i t a l theories of the e l e c t r o n i c structures of these molecules may be tested. This i s because i t i s known from s e l f -consistent f i e l d molecular o r b i t a l theories (2,3,4,5), that T. Koopmans' theorem indicates (6) to a good approximation that the 2 energy of an electron in- a given molecular o r b i t a l i s equal to minus the I.P. r e f e r r i n g to the removal of an electron from that o r b i t a l ; and i t follows that once the several I.P.'s of a molecule are known, the energies of the various o r b i t a l s are likewise known. 3 A. Ionization Potentials 1. • Introduction Ionization p o t e n t i a l s are one of the most important properties of a molecule, and when available they can shed much l i g h t upon i t s behaviour. The i o n i z a t i o n p o t e n t i a l of a polyatomic molecule may be used (a), to help determine which o r b i t a l i n the molecule i s the most loosely bound (7,8); .(b)-> t 0 deduce information about bond strengths, bond lengths, bond order, and the d i s t r i b u t i o n of e l e c t r i c charge i n the molecule (9); and (c), to provide data f o r the c a l c u l a t i o n of e l e c t r o n i c wave functions. (2,10). It i s generally recognized, moreover, that there i s a.relationship between i o n i z a t i o n p o t e n t i a l and chemical r e a c t i v i t y . Ionization p o t e n t i a l s are thus of both t h e o r e t i c a l and p r a c t i c a l importance, and i t i s desirable to have methods f or t h e i r accurate measurement. Theoretical c a l c u l a t i o n s of i o n i z a t i o n p o t e n t i a l s are.extremely complex even f o r atoms above helium and i n most cases give a very inaccurate value f o r the I.P. Several experimental methods have been developed f o r I.P. measurement. The more important of these are o p t i c a l spectroscopy, electron and photon impact i o n i z a t i o n , and photoelectron spectroscopy. 2. Adiabatic and V e r t i c a l Ionization Potentials Two d i f f e r e n t values of the i o n i z a t i o n p o t e n t i a l often have to be considered: (1) The adiabatic I.P., which corresponds to a t r a n s i t i o n from the zeroth v i b r a t i o n a l l e v e l of the molecular ground state to that of the i o n i c ground state. Spectroscopic i o n i z -ation p o t e n t i a l s usually correspond to these 'adiabatic' values, since POTENTIAL ENERGY 5 the 0-0 transition can often be obtained either directly or by extrapolation. The ionization:potentials obtained by photoionization are also generally adiabatic... The vertical I,p. corresponds to the most probable ionization transition from the ground, state of the molecule. Ionization potentials measured by electron impact may or may not refer to the ion in its ground vibrational state... The difference, between the adiabatic and vertical potentials can best be illustrated with, a diagram, Figure I, where RX is the potential energy curve, fox,the normal electronic state of the molecule, and RX+ is the potential energy curve for the molecule ion. It is apparent; from Figure I, that the vertical I.P. will usually exceed the adiabatic value because usually there is a difference of r g between molecule and,.ion. Experimentally, the difference is usually between 0.02 to 0.5 eV (1). 3. Determination of Ionization Potentials Various methods have been used for the determination of ionization potentials, and some of them will now be described. a) The Optical Spectroscopic Method This method involves the study of that portion of the far-ultraviolet absorption spectrum produced by Rydberg transitions. The most useful form in which to express the Rydberg series is v = A - R (n+a)2 6 where: A and a are constants f o r a p a r t i c u l a r molecule and n takes on i n t e g r a l values representing the d i f f e r e n t Rydberg bands. v = frequency i n cm * of the t r a n s i t i o n R = Rydberg constant This method has been used to obtain accurate values f o r about 40 molecular i o n i z a t i o n p o t e n t i a l s . However, i t cannot be applied to those molecules which give continuous or d i f f u s e spectra i n which i t i s d i f f i c u l t to deduce the Rydberg convergence l i m i t s . 'b) The Electron Impact Method This method uses a heated filament (usually tungsten) as a source of electrons which are passed through the gas under i n v e s t i g a t i o n . The i n t e n s i t y of the ions produced i s measured as a function of electron energy, and mass analysis i s usually employed. Experimentally, a mixture of the gas under study (for which an i o n i z a t i o n p o t e n t i a l i s desired) and a gas whose i o n i z a t i o n p o t e n t i a l i s known (and w i l l serve as the reference f o r e s t a b l i s h i n g the energy scale) i s subjected to electron bombardment. The i n e r t gases Ar, Kr, etc. are frequently used as standards, since t h e i r i o n i z a t i o n p o t e n t i a l s are known with high accuracy (11). A p l o t of ion current vs. electron energy provides an " i o n i z a t i o n e f f i c i e n c y curve". Now, when one has experimentally found the i o n i z a t i o n e f f i c i e n c y curves, the problem becomes one of evaluating these curves to y i e l d the desired I.P. Various techniques have been developed to do t h i s . Nicholson (12) gives an account of eight methods a v a i l a b l e . It should be noted, however, that a l l of these 7 methods are e s s e n t i a l l y empirical, and even the better ones have d e f i n i t e shortcomings. The usual mass spectrometer ion source i s unsatisfactory for precise i o n i z a t i o n studies at low electron energies because of the large spread i n electron energies i n the beam (13). The other major sources of d i f f i c u l t y are (a) the acceleration of the electrons by an unknown contact p o t e n t i a l difference between the cathode and the i o n i z a t i o n chamber and (b) an unknown acceleration of the electrons i n t h e i r passage through the i o n i z a t i o n chamber because of the presence of the e l e c t r i c f i e l d necessary to extract the ions. The e f f e c t of the electron energy d i s t r i b u t i o n i s to obscure f i n e structure i n the i o n i z a t i o n e f f i c i e n c y curve. One s o l u t i o n to the^ problem of resolving the f i n e structure which may be present i n these curves i s to reduce the energy spread of the i o n i z i n g electron beam. The two most widely used methods for reducing the energy spread of electron beams are electron v e l o c i t y s e l e c t i o n (14,15) and the Retarding-Potential-Difference (RPD) method (16). Preliminary 2 studies, using the electron energy selector indicate that the P3 /2 and components of the A r + ground state doublet can be resolved. More recent studies on hydrogen show that f i n e structure i s c l e a r l y present on the i o n i z a t i o n e f f i c i e n c y curve f o r the H* ion. This i s a t t r i b u t e d to the formation of the ion i n i t s various excited v i b r a -t i o n a l states. Recently mass spectrometric studies of the rare gases using an electron v e l o c i t y s e l e c t o r (17) have revealed i n t e r e s t i n g structure i n the curves for Kr + and A r + . Using the 8 R.P.D. technique Hickam and coworkers (18) and Frost and McDowell (19) were able to detect the higher excited states of N* and other ions (20, 21). Electron impact methods s u f f e r from the defect that i n the i o n i z a t i o n e f f i c i e n c y curves produced by t h i s method structure i s sometimes observed at energies which do not correspond to the known excited states of the ion. Fox and Hickam (22), (in 1954), suggested that such structure i n t h e i r electron-impact curves was due to auto-i o n i z a t i o n (the same as pre-ionization) of a highly excited neutral state of the atom or molecule concerned, and that t h i s phenomenon i s of widespread occurence. Strong evidence i n support of t h e i r suggestion comes from the photoionization curves f o r many diatomic molecules, where pre-ionization i s c l e a r l y indicated. c) Photoionization Most of the work i n t h i s f i e l d has been done by using a monochromator to provide a beam of photons of known narrow energy spread. A gas to be studied i s i r r a d i a t e d by these photons, and the r e s u l t i n g photoion current measured as a function of photon energy (23, 24). The f i r s t i o n i z a t i o n p o tentials of more than a hundred molecules have recently been reported by Watanabe (25) using t h i s technique, and his r e s u l t s are probably the most accurate to date (since he finds excellent agreement with spectroscopic values where these are a v a i l a b l e ) . This method has, however some drawbacks (26), the most serious of which i s that because of the lack of mass analys one has to be sure that the sample being studied i s quite free of impurities with lower i o n i z a t i o n p o t e n t i a l s . Fragment ion formation may also i n t e r f e r e with the "parent" photoion curve. 9 This d i f f i c u l t y can be overcome by combining the photo-i o n i z a t i o n source with a mass, spectrometer. This technique has been used by several workers (27, 28, 29) to obtain the f i r s t and the inner i o n i z a t i o n p o t e n t i a l s . o f several molecules. The main advantage of the method i s that i t i s very much easier to obtain a beam of u l t r a v i o l e t radiation, with a narrow energy spread, and of accurately known energy, than .an electron beam with s i m i l a r properties. Secondly, one does not need a c a l i b r a t i n g gas to be introduced along with the sample i n order to c a l i b r a t e the energy scale. A disadvantage of the photoionization method i s that often i t becomes d i f f i c u l t to detect inner i o n i z a t i o n p o t e n t i a l s owing to the structure i n d i c a t i n g them being obscured by p r e - i o n i z a t i o n peaks. d) Photoelectron Spectroscopy Although widely used i n the determination of work functions of s o l i d s , photoelectron spectroscopy has only recently been applied to the study of the I.P.'s of molecules in the gas phase. It enables the d i r e c t measurement of a l l the i o n i z a t i o n p o tentials of a molecule smaller than the value of the i o n i z i n g r a d i a t i o n (in our case 21.21 eV). The method was f i r s t developed by Vilesov, Kurbatov and Terenin (30), who used a Lozier-type apparatus combined with a vacuum u l t r a v i o l e t monochromator to measure the k i n e t i c energies of photo-electrons produced on i o n i z a t i o n . In 1962, Turner and Al-Joboury (31,32) reported a new method, f o r the measurement of k i n e t i c energy of photo-ejected electrons ,which employs t;he helium resonance l i n e c (584 A) as the e x c i t a t i o n source. Schoen (33), i n 1962 started 10 s i m i l a r studies using a Seya-Namioka monochromator; but without the disadvantage of o p t i c a l windows that the Vilesov et a l instrument had. The p r i n c i p l e of the method i s that a photon of energy hv causes the emission of a photo-.electron of k i n e t i c energy given very c l o s e l y by (hv - I.P.). Conservation of momentum r e s u l t s i n energy p a r t i t i o n between the electron.and the ion i n the inverse r a t i o of t h e i r masses; i n the leasts favourable case (H^) the mass 4 r a t i o i s 10 to 1 and thus the error i n equating the electron 4 energy with (hv - I.P.) i s 1 part i n 10 ; for larger ions the error i s correspondingly l e s s . V i r t u a l l y a l l the excess energy i s therefore c a r r i e d away by the photoelectron, so one can write K.E = hv - I.P. (1-1) where h = Planck's constant v = frequency of r a d i a t i o n I.P.= Ionization p o t e n t i a l of the atom or molecule. It follows that i f a gas i s i r r a d i a t e d by a monochromatic photon beam there w i l l r e s u l t as many groups of photoelectrons as the ion has energy levels attainable through absorption of an incident 21.21 eV photon. Kinetic energy measurements w i l l enable the I.P. i n each case to be deduced from equation (1-1), and the r e l a t i v e group i n t e n s i t i e s w i l l be proportional to 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 the appropriate i o n i c states. Electron k i n e t i c energies may conveniently be determined by using retarding p o t e n t i a l techniques. The pl o t of the photo-electron i n t e n s i t y as a function of retarding voltage gives the 'integrated' photoelectron spectrum, 11 which i s usually found to consist of series of steps when a spherical analyzer i s employed. In t h i s technique we are thus examining a process or processes occurring i n the i o n i z a t i o n continua. The important difference from the Rydberg series convergence method, the photoion current method of Watanabe, and from mass spectrometric appearance p o t e n t i a l measurements using electron impact i s that photoelectron spectroscopy i s not a threshold technique. It w i l l usually be free from the l i m i t a t i o n s imposed by autoionization processes which frequently render higher i o n i z a t i o n l i m i t s d i f f u s e or unobservable by other methods. However, one does have to use a c a l i b r a t i n g gas (Ar, Kr) i n order to c a l i b r a t e the energy scale. The Instrument used i n t h i s work. In t h i s work a sph e r i c a l g r i d analyzer i s used to measure the k i n e t i c energy d i s t r i b u t i o n of photoelectrons produced by the o i n t e r a c t i o n of 584 A r a d i a t i o n with gas molecules, an arrangement d i f f e r i n g s i g n i f i c a n t l y from other e x i s t i n g instruments. With spherical grids the electron retarding f i e l d i s always p a r a l l e l to the electron t r a j e c t o r y , and t h i s r e s u l t s i n a d e f i n i t e increase i n r e s o l u t i o n , (as shown i n Figure 2), over instruments with conventional c y l i n d r i c a l grids (30, 31, 33). 1 f i 1 1 1 r t r T 1 1 1 1 1 1 p CYLINDRICAL SPHERICAL .0-0 j i I L « i • • I • I I I 1 1 I 1 L RETARDING VOLTAGE Figure 2 Comparison of C y l i n d r i c a l and Spherical Analyzer Results f o r Hydrogen. 13 B. The Franck-Condon P r i n c i p l e (34,. 35) As pointed out e a r l i e r , t r a n s i t i o n s .induced by photon and electron impact obey the Franck-Condon P r i n c i p l e , The p r i n c i p l e states that an e l e c t r o n i c t r a n s i t i o n within a molecule takes place so rap i d l y i n comparison to the v i b r a t i o n a l motion that, immediately afterwards the nuclei s t i l l have very nearly the same r e l a t i v e p o s i t i o n and v e l o c i t y as before the !'jump". In order to apply t h i s p r i n c i p l e to i o n i z a t i o n phenomena l e t us consider Figure 3, i n which are drawn the p o t e n t i a l energy curves f o r a diatomic molecule AB i n i t s ground e l e c t r o n i c and various i o n i c excited states. Figure 3 (a) represents a case where the interatomic distance i s the same for the molecular ion and the neutral molecule; i n t h i s case the appearance p o t e n t i a l w i l l equal the adiabatic I.P. In t h i s case the most probable t r a n s i t i o n w i l l be to the lowest v i b r a t i o n a l l e v e l of the i o n i c state, with very low p r o b a b i l i t y to the higher l e v e l s . This gives r i s e to a single sharp step i n the photoelectron energy spectrum, i n d i c a t i n g the removal of a non-bonding electron. When the interatomic distances for the two states d i f f e r appreciably, as depicted i n Figure 3 (b), i t i s to be expected that some of the t r a n s i t i o n s w i l l lead to v i b r a t i o n a l l y excited AB + ions, and others w i l l lead to d i s s o c i a t i o n of the AB + ion to give(A + B. or A + B +, etc. depending on the l i m i t of the p o t e n t i a l energy curve of the upper e l e c t r o n i c state at i n f i n i t e internuclear separation). This w i l l lead to a photoelectron retarding curve e x h i b i t i n g a great deal of v i b r a t i o n a l structure, and then a continuous r i s e to perhaps above the d i s s o c i a t i o n l i m i t . INTERNUCLEAR DISTANCE INTERNUCLEAR DISTANCE INTERNUCLEAR DISTANCE (o) tb.) (c) s Figure 3 The Franck-Condon Principle 15 I f the upper curve i s repulsive, as i n Figure 3 (c), then the t r a n s i t i o n s are no longer to discre t e v i b r a t i o n a l l e v e l s but to a continuum. They w i l l r e s u l t i n d i s s o c i a t i o n of the molecule ion AB +. In such a case, the r e l a t i v e k i n e t i c energies of the fragments + (A and B i n t h i s example) w i l l have some d i s t r i b u t i o n l y i n g between E and E . y x The e l e c t r o n i c t r a n s i t i o n p r o b a b i l i t y i s proportional to the square of the v i b r a t i o n a l overlap i n t e g r a l (the i n t e g r a l over the product of the v i b r a t i o n a l wavefunctions of the two states involved), when the v a r i a t i o n of e l e c t r o n i c perturbation i n t e g r a l s with i n t e r -nuclear separation i s small (36, 37). Recent work by Ni c h o l l s (38) and others has been concerned with the c a l c u l a t i o n of these Franck-Condon factors f or polyatomic as well as diatomic molecules. 16 CHAPTER II  EXPERIMENTAL A. Introduction The photoelectron spectrometer used i n t h i s work has been f u l l y described previously (39), and i s shown schematically i n Figure 4. Only a b r i e f d e s c r i p t i o n i s given here. The major advantage the present spectrometer has over others of i t s type i s that i t has a spherical g r i d energy analyzer- A l l the previous work reported i n t h i s f i e l d has been done using c y l i n d r i c a l -g r i d energy analyzers. In these systems electron c o l l e c t i o n i s most e f f i c i e n t for those ejected at 90 e to the photon beam and so the c o l l e c t o r current i s very s e n s i t i v e to the angular d i s t r i b u t i o n of the photo-2 electrons. This d i s t r i b u t i o n i s roughly a function of sin 0 (40), where 9 i s the angle between photon beam and ejected photoelectron t r a j e c t o r y . These d i f f i c u l t i e s are overcome i n the spherical analyzer used i n t h i s work, wherein photoelectrons are produced i n a small volume at the centre of the spherical g r i d system, so that e j e c t i o n i s always normal to the retarding f i e l d . This gives r i s e to greatly improved res o l u t i o n owing to the 'stepped' photoelectron stopping curve. One important modification was made during these studies. Previously, the spherical grids were made of brass mesh ( 30 x 30 mesh 0.005 inch gauge brass) which was gold plated a f t e r etching. In th i s work, new grids constructed from s t a i n l e s s s t e e l mesh (50 by 0.003 17 helium to microtherm unit sample inlet grid assembly collector to fast rotary H B S B S^ pump (light source exhaust) collimating capillary to trap and oil diffusion pumps glass seals to vibrating Piguflfc 4 The Spherical Photoelectron Spectrometer 18 inch) were used. The new grids were more spherical and transparent, 56% against 45% with the etched brass. Both these factors helped i n improving the r e s o l u t i o n . Further improvement i n r e s o l u t i o n came with the use of only one spherical g r i d i n the analyzer (41). The retarding f i e l d i s then applied between the single erstwhile 'inner' g r i d and the c o l l e c t o r . By t h i s method a l l the p o s i t i v e ions (which were previously r e p e l l e d by a p o s i t i v e p o t e n t i a l between the two grids) now reach the c o l l e c t o r . This presents no problem, however, since the p o s i t i v e current background remains constant. This arrange-ment decreases the electron s c a t t e r i n g between the central i o n i z a t i o n region and c o l l e c t o r and improves the r e s o l u t i o n by a f a c t o r of two. The photoelectron stopping curve f o r argon i s used to i l l u s t r a t e the energy r e s o l u t i o n attainable with one and two grids i s shown i n Figure 5. The former arrangement r e s u l t s i n a peak h a l f width of only 0.045 eV, compared with double t h i s for the l a t t e r , and w i l l obviously be advantageous i n the detection of f i n e structure i n photo-electron spectra. In the work reported here two spherical grids were used, since the single g r i d modification only occurred very recently. B.. The Samples used and t h e i r handling system. The compounds used i n t h i s work and t h e i r o r i g i n s are shown i n Table I. Mass spectral analysis showed them to be free from any impurities which might cause ambiguities i n the photoelectron stopping curves. COLLECTOR CURRENT (l) Figure 5 . 'Photoelectron Spectrum of Argon obtained (a) with double, .(b) with single grid i n the energy analyzer. 20 Table I Sample Grade Origin Methyl Iodide Spectroscopic Baker and Adamson Products Methyl Bromide Reagent B r i t i s h Drug House Methyl Chloride Matheson of Canada Ltd. Methyl Cyanide Spectroscopic Eastman Kodak Ethyl Cyanide Reagent Eastman organic chemicals Acetaldehyde Reagent Eastman organic chemicals Acetone Spectroscopic Eastman organic chemicals Sulfur Hexafluoride Matheson of Canada Ltd. Sample Handling System Figure 6 shows a diagram of the sample handling and vacuum system. Gases are introduced d i r e c t l y into the r e s e r v o i r R (to a pressure of ^ 1 mm Hg) from the gas sample container. Taps T^ and T^ are i n i t i a l l y open while taps 7^, 7^ and T^ are closed. A f t e r the sample i s introduced into the re s e r v o i r , taps T^ and T^ are closed and T^ opened, so that the vapour (after i t s i n i t i a l expansion into R) enters the spectrometer through a 'metrosil' molecular leak RESERVOIR N E E D L E V A L V E S A M P L E T U B E L E A K C @ = 3 '< HELIUM TL" v DIFFUSION T R A P ROTARY OIL P U M P p u M P Figure 6. The Sample Handling and Vacuum System. S P E C T R O M E T E R ] ROTARY OIL P U M P 22 (sintered glass) at the required rate. The leak dimensions are such that about a millimeter r e s e r v o i r pressure r e s u l t s i n an i o n i z a t i o n -4 gauge pressure of 5 x 10 mm Hg. The i o n i z a t i o n chamber pressure i s probably an order of magnitude greater than t h i s . For l i q u i d samples the procedure followed was s l i g h t l y d i f f e r e n t . They are f i r s t placed i n the tube and degassed by repeated freezing i n l i q u i d nitrogen, pumping o f f and melting. F i n a l l y the vapour i s introduced into the r e s e r v o i r . The system i s evacuated by a Welch duo-seal rotary o i l pump and i s c o n t r o l l e d by tap T^. A l l the taps are greased with Apiezon M. C. The Photoelectron Spectrometer The e s s e n t i a l components of the spectrometer, shown i n Figure 4 , are: 1) The Photon Source and 2) The Photo-electron Energy Analyzer. 1. The Photon Source e The photon source i s the 584 A (21.21 eV) resonance emission from a microwave discharge i n helium. Tank helium (Canadian Liquid A i r Co) at a pressure of ^20 microns flows through the a x i a l quartz discharge tube. Flow control i s achieved with an Edwards high vacuum OSIC s t a i n l e s s s t e e l needle valve. The discharge takes place i n a resonant cavity, and the power i s supplied by a Raytheon Microtherm 100 watt 2450 mc/s generator. The inside of the cavity i s s i l v e r plated for high e f f i c i e n c y . D i f f e r e n t i a l pumping and a 5 cm long 0.5 mm Figure 7. A Spherical G r i d . 24 diameter co l l i m a t i n g c a p i l l a r y e f f e c t i v e l y prevent the helium from entering the analyzer. The source i s forced a i r cooled. 2. The Photoelectron Analyzer The analyzer consists of two concentric spherical grids and a c o l l e c t o r . The grids (1.5 and 2 inches i n t e r n a l diameter) are pressed from 50 by 0.003 inch diameter s t a i n l e s s s t e e l mesh, giving a t h e o r e t i c a l transparency of 75% each. A diagram of the grids appears i n Figure 7. As shown i n Figure 4, the inner g r i d i s kept i n p o s i t i o n by two photon conducting tubes of gold plated brass. These two tubes are equipotential to the inner g r i d , and so the space enclosed by the inner g r i d and the two tubes i s e l e c t r i c f i e l d r f r e e . The c o l l e c t o r , i n t e r n a l diameter 3 inches, i s turned from s o l i d brass and i s gold plated. The inner surface i s coated with a c o l l o i d a l suspension of graphite ("Aquadag"). The purpose of t h i s i s to minimize the r e f l e c t i o n of photoelectrons. The whole analyzer i s enclosed i n a cy l i n d e r of "mumetal" to reduce i n t e r n a l magnetic f i e l d s and so maximize the re s o l u t i o n . A l l the spacers are made of pyrex, and e l e c t r i c a l leakage between the outer g r i d and the c o l l e c t o r i s minimized by placing an earthed n i c k e l disc between the two t e f l o n spacers. The sample gas flows i n through 1/4 inch diameter holes i n the c o l l e c t o r near the photon beam axis. 3. Operation of the Spectrometer In the central f i e l d - f r e e region the photon beam interacts with sample molecules to form ions and photon-electrons. A constant light source SPHERICAL PHOTOELECTRON SPECTROMETER Figure 8. Schematic Diagram of Electron Retarding System. 26 p o t e n t i a l difference of 3 v o l t s i s maintained between the inner and outer grids - s u f f i c i e n t to prevent p o s i t i v e ions from passing through the outer g r i d into the electron retarding f i e l d . The photoelectron retarding voltage i s applied between the inner g r i d and the c o l l e c t o r from a ten turn 20,000 ohm double-ended h e l i p o t . The c o l l e c t o r i s grounded through the v i b r a t i n g reed electrometer. An electron ejected from a molecule i n the f i e l d free region moves to the inner g r i d . It i s then accelerated to the second g r i d and decelerated as i t moves towards the c o l l e c t o r . The photoelectron energy spectrum i s scanned by decreasing the retarding p o t e n t i a l difference between the inner g r i d and the c o l l e c t o r at 1 v o l t per minute by a H e l l e r 2 T 60 v a r i a b l e speed motor, connected through a f r i c t i o n clutch to the helipot spindle. This method of scanning the electron energy spectrum i s s i m i l a r to that used by Schoen (33), but d i f f e r s from that used by Al-Joboury.and Turner (31,32). A Cary model 31 Vibrating.Reed Electrometer amplifies the c o l l e c t o r current, f i n a l l y displayed on a Leeds and Northrup s t r i p chart recorder. The photoelectron retarding voltage i s read from a d i g i t a l voltmeter, and a push button event control marker i s used to produce spiked reference pulses every 0.1 v o l t on the chart. A f t e r the completion of every scan, the run i s repeated with no sample gas present and the background so obtained subtracted from the o r i g i n a l signal to obtain the true photoelectron spectrum. 4. Vacuum System Since s c a t t e r i n g of the electrons by neutrals could d i s t o r t the retarding p o t e n t i a l curves, i t i s desirable to work at as low a 27 pressure as possi b l e . The main spectrometer i s pumped by a C.E.C. 'MCF 60' o i l d i f f u s i o n pump f i t t e d with a 'dry ice'-cooled cold-trap, and backed by a Welch duo-seal rotary o i l pump. The same rotary o i l pump i s used to evacuate the sample r e s e r v o i r . An i o n i z a t i o n gauge mounted over the cold trap i s used to measure sample pressure. The background pressure i s usually about 1 x 10"^ mm Hg. The l i g h t source d i f f e r e n t i a l pumping i s provided by a s i m i l a r Welch duo-seal rotary o i l pump to that used f o r the main vacuum system. 28 CHAPTER III The Methyl Halides Experimental d e t a i l s The sources and p u r i t i e s of the sample gases are given below. Krypton was used as a c a l i b r a t i n g gas and the i o n i z a t i o n Table II Sample Grade Ori g i n Methyl Iodide Methyl Bromide Methyl Chloride Spectroscopic Reagent Baker and Adamson products B r i t i s h Drug House Matheson of Canada Ltd. po t e n t i a l s were determined with i t and the unknown compound both i n the photoelectron spectrometer at the same time. P r i o r to each set of experiments the r e s o l u t i o n of the instrument was tested by i t s a b i l i t y to resolve the argon doublet (of 0.18 eV-separation) . The _4 gas pressures used i n t h i s work were of the order of 5.x 10 mm. of mercury for CH 3I, CH^Br and CH3C1. Experimental Results The photoelectron current versus i o n i z i n g energy curves 29 o CVl 00 CD T3 o z <4-( 9 o a> a. to c DCs o 3 <M CH»Br •r •••••••• 10 12 14 16 Figure ? 107 Photoelectron Swe 18 20 Spectrum of Methyl Bromide. i 1 1 1 1 1 — 1 1 r T T C H . C I _LZ L II Mr I L J ! I I I L 13 15 17 (21.21-R)^y 19 Figure 11. Photoelectron Spectrum of Methyl Chlori<Ie\ 21 i I w CH.F CHjCI CH8Br CH3I Figure 12. Electron energy le v e l diagram f o r the Methyl Halides. 33 corrected for background are p l o t t e d i n Figures 9, 10 and 11 r e s p e c t i v e l y . Figure 12 shows the o r b i t a l energy levels for the three methyl halides found here and i n the electron impact studies of Frost and McDowell (42) using monoenergetic electrons. The photoelectron stopping curves for methyl iodide and methyl bromide shown i n Figures 9 and 10 c l e a r l y indicate the formation of these ion species 2 2 i n four different, states. These are deduced to be the E j / 2 ' E3/2* 2 2 and E state. This i s i n agreement with the electron impact studies done by Frost and McDowell (42) Tsuda, Melton and Hamill (43) and the spectroscopic studies of Price (44) . The corresponding components of the CH^Cl* ground states are not resolved however. Frost and McDowell (42) were also unable to resolve t h i s doublet, but Tsuda, Melton and Hamill (43) report i t s separation i n t h e i r electron impact studies. For t h i s ion only three d i f f e r e n t processes have been detected, and they lead to CH^Cl* i n the ground 2E and 2 2 excited A and E e l e c t r o n i c states. Reasons for the assignment of the i o n i z a t i o n processes are given i n the discussion, and the various electron impact, spectroscopic and photoionization i o n i z a t i o n potentials of the methyl halides found by d i f f e r e n t workers are summarized i n and belong to the pyramidal symmetry . Their e l e c t r o n i c structure may be represented (47> by the formula: Table I I I . Discussion The methyl halides a l l have a s i m i l a r nuclear arrangement ( l S c ) 2 [ S a j ] 2 ( n S ^ ) 2 (Tie) 4 [ o a j 2 (npi^e) (3-1) Table III Ionization p o t e n t i a l s of the Methyl Halides. (eV) Compound Spectroscopic (44) Photoionization Watanabe Nicholson (45) (46) .Electron ..Impact Frost § McDowell (42) Tsuda § Hamill (43) This Work Methyl Iodide (1st I.P.) 9.49 t \ / 2 ) 9.54 9.55 9,51 < El/2> 9.50 9.56 + 0.01 C2e1/2> (2nd I.P.) 10.11 ^ 3 / 2 ^ 10.12 10.09 C 2 E 3 / 2 ) 10.00 10.19 +_ 0.02 ( 2 E 3 / 2 ) (3rd I.P.) 11.22 11.20 12.29 +_ 0.02 ( 2A X) (4th I.P.) 13,14 2 ( E3/2, 1/2^ 13.10 14.30 + 0.01 Table III (Continued) Compound Spectroscopic (44) Photoionization Watanabe Nicholson (45) (46) Electron Frost $ McDowell (42) Impact Tsuda § Hand11 (43) This Work Methyl Bromide (1st I.P.) 10.49 10.53 10.52 10.53 10.50 10.55 0.01 ( 2 E 1 / 2 ) ^ 1 / 2 ^ (2nd I.P.) 10.80 10.86 10.85 10.80 10.86 _+ 0.02 ( 2 E 3 / 2 ) ( 2 e3/2^ (3rd I.P.) 11.62 11.50 13.22 +_ 0.03 2 2 . ( A x) ( A x) (4th I.P.) 12.94 12.90 14.79 *_ 0.05 E3/2, 1/2) Table III (Continued) Compound. Spectroscopic (44) Photoionization Watanabe Nicholson (45) (46) Electron Impact Frost § McDowell (42) Tsuda c, Hamill (43) This Work Methyl Chloride (1st I.P.) 11.17 ( 2 E I / 2 ) 11.28 11.26 11.42 < E l / 2 ) 11.30 11.29 +0.02 < 2 E I/2> (2nd I.P.) 11.25 ( 2 E 3 / 2 ) 11.34 11.40 (3rd I.P.) 12.07 ( 2A X) 11.90 14.14 +_ 0.04 ( 2A X) (4th I.P.) 13.02 ( 2E 13.20 15.14 + 0.05 3/2, 1/2) 37 where n = 3, 4, or 5 f o r CH^Cl, CH^Br and CH^I r e s p e c t i v e l y . The inner electrons are omitted and the o r b i t a l s are written i n order of decreasing binding energy. Mulliken (47) has discussed the molecular o r b i t a l formulae of these compounds and has shown that t h e i r structures can be understood better i f they are compared with that of methane. The structure of CH 4 (symmetry Td) i s (48). ( l S c ) 2 ( S a p 2 ( p t 2 ) 6 , 1 A 1 (3-2) I f we suppose one H atom s l i g h t l y displaced the symmetry can be reduced to As a consequence of t h i s operation there are c e r t a i n a l t e r a t i o n s i n the molecular o r b i t a l s . From group theory i t i s known that the completely symmetrical representations of a l l symmetry groups c o r r e l a t e . The 2 6 methane [Sa^] o r b i t a l i s completely symmetrical but the [pt^j i s not, and so i t i s e s s e n t i a l to determine how the l a t t e r o r b i t a l correlates with the representations of the methyl halide symmetry group C^- It i s also known from group theory (47, 49) that the t r i p l y degenerate T^ correlates with the A^ and E representations of the group so when the CH^ symmetry i s reduced from T^ to C^v the t r i p l y degenerate [pt^] o r b i t a l s w i l l s p l i t up into a symmetric [aa^] and a doubly degenerate p a i r [iTe] and lead to an expression s i m i l a r to that given i n (3-1) for the methyl halides: [ l S c ] 2 [ S a x ] 2 b e ] 4 [ o a ^ 2 , ^ - (3-3) The twofoldly degenerate [ire] here should have nearly the 38 same energy as the [ire] i n the methyl halides. Each i s confined e s s e n t i a l l y to the CH^ r a d i c a l , and the only differences are due to secondary e f f e c t s i n shape and f i e l d of force within the CH^ which r e s u l t when a C-I i s substituted f or a C-H bond. In equation (3-2), the ( ) brackets r e f e r to mainly atomic non-bonding o r b i t a l s and the square brackets r e f e r to molecular o r b i t a l s , although [ire] i s completely, and [Sa^] la r g e l y l o c a l i z e d on the CH 3 r a d i c a l . The o r b i t a l [a] i s the main C-H bonding o r b i t a l . The f i r s t i o n i z a t i o n p o t e n t i a l corresponds to the removal of an electron from the (npu^e) o r b i t a l , non-bonding and la r g e l y l o c a l i z e d on the halogen atom. Some evidence for t h i s comes from the shape of the photoelectron stopping curves for the methyl halides i n Figures 9, 10 and 11 re s p e c t i v e l y , where a sharp step for the f i r s t process i s seen i n d i c a t i v e of the removal of a non-bonding electron. 2 This would leave the molecular ion i n a E e l e c t r o n i c state, having 2 2 two components, ^1/2 a n c * E3/2' c* u e t 0 s P i n _ o r b i t a l i n t e r a c t i o n . When the molecular ion CH^X i s formed i t i s to be expected that the si n g l y occupied o r b i t a l w i l l be to a large degree l o c a l i z e d around the halide atom. I f t h i s i s the case then the separation of the i o n i c ground state s p i n - o r b i t a l components for each methyl halide would be expected to approximately equal the separation pertaining i n the corresponding halogen atom and hydrogen halides. Furthermore, since f or atoms the doublet energy separation r e s u l t i n g from spin-o r b i t a l i n t e r a c t i o n i s proportional to the fourth power of the 2 2 atomic number (50), i t follows that the ~ E l / 2 § r o u n c * s t a t e separations f o r the CH,X+ ions should increase with the atomic weight 39 of the halogen atom from the chloride to the iodide. The experimental r e s u l t s i n Table IV bear out t h i s expectation (51). It + + i s c l e a r from Figures 9 and 10 that f o r CH-jI and CH^Br t n e doublets due to s p i n - o r b i t a l i n t e r a c t i o n have been resolved. That t h i s i n t e r p r e t a t i o n i s correct i s shown by f a r u l t r a v i o l e t studies of these molecules by Price (44), photoionization studies by Nicholson (46), and electron impact studies by Frost and McDowell (42) and Tsuda, Melton and Hamill (43). In 1936, Price found two Rydberg series i n t h i s spectral region for methyl iodide and t h e i r l i m i t s were 0.62 eV apart. Mulliken (47) interpreted these l i m i t s as corresponding to the 2 two s p i n - o r b i t a l components of the E i o n i c ground state. The value of 0.63 eV found i n t h i s work agrees well with the spectroscopic value. The values 0.58 eV and 0.50 eV found by Frost and McDowell (42) and Tsuda, Multon and Hamill (43) by electron impact studies are s l i g h t l y lower. In the case of methyl bromide, Price (44) found two Rydberg series having a separation of 0.31 eV between t h e i r l i m i t s . Frost and McDowell (42) found t h i s separation to be 0.32 eV, and Tsuda, Melton 2 and Hamill (43) found a value of 0.30 eV. In t h i s work the ^-^12 " 2 E ^ 2 separation i s found to be 0.31 eV, again i n good agreement with the previous studies. For CH^Cl*, the 2E ground state doublet separation would be expected to be less than 0.1 eV. From Figure 11, i t i s clear that we have been unable to resolve these components. However Price (44) found two Rydberg series having a separation of 0.08 eV between t h e i r l i m i t s . Table IV Sp i n - o r b i t a l Interaction energies of Methyl Halide Ions (eV) Ion Worker Method Doublet Separation + This work Photoelectron spectroscopy 0. 61 Price (44) Spectroscopic 0. .62 Frost § McDowell (42) Electron impact 0. .59 Tsuda, Melton $ Hamill (43) Electron impact 0. 50 Nicholson (46) Photoionization 0. 57 Morrison (52) Photoionization 0. ,60 This Work Photoelectron spectroscopy 0 .31 Price (44) Spectroscopic 0. .31 Frost and McDowell (42) Electron impact 0 .32 Tsuda, Melton § Hamill (43). Electron impact 0 .30 Nicholson (46) Photoionization 0 .33 The next inner i o n i z a t i o n p o t e n t i a l above those associated with the methyl halide doublet ground state should r e f e r , according to equation (3-5to the removal of an electron from the C-X o r b i t a l [aa^]. This would lead to the formation of CH^X* ions i n t h e i r excited e l e c t r o n i c states. Through a comparison of data of t h i s work f or 41 three methyl halides i t i s possible to say that the process just described i s correct. The value found i n th i s work f o r the removal of an electron from the CH I [oa.] o r b i t a l i s 12.29 eV, f o r CH_Br 3 •!• and CH3C1 being 13.22 and 14.14 eV respectively. The next i o n i z a t i o n p o t e n t i a l above that involving the [aa^] o r b i t a l should, from equation (3-1), r e f e r to the removal of an electron from the doubly degenerate [ire] 4 bonding o r b i t a l l o c a l i z e d i n the 4 methyl group. The [ire] o r b i t a l , being derived from the [pt^] o r b i t a l of methane when the symmetry i s al t e r e d from Td to C ^ , should have an I.P. approximately equal to the f i r s t I.P. of methane, 13.16 eV. The values observed f o r t h i s i o n i z a t i o n process i n t h i s work are 14.30 eV for methyl iodide, 14.79 eV f o r methyl bromide, and 15.14 eV f o r methyl chloride. The increase i n value could be e a s i l y accounted f o r by CH^ being more p o s i t i v e l y charged due to the presence of the e l e c t r o -negative atoms chlorine, bromine and iodine i n the methyl halides. 42 Acetaldehyde Acetaldehyde, acetone and formaldehyde are the simplest set of compounds containing the group >C=0. It i s well known that the double bond i n these compounds i s associated with high p o l a r i t y and hence i t i s to be.expected that the molecules' i o n i z a t i o n p o t e n t i a l s w i l l be strongly influenced by charge t r a n s f e r . Previous r e s u l t s are a v a i l a b l e f o r the f i r s t i o n i z a t i o n p o t e n t i a l s of these molecules and w i l l be ref e r r e d to below. The work to be described here shows how the binding energies of some of the more strongly bound o r b i t a l s d i f f e r from the values obtained by Sugden and Price (53), who studied these molecules using a monoenergetic (photo) electron impact technique. They, found a number of breaks i n the i o n i z a t i o n e f f i c i e n c y curves which they assumed to be inner I.P.'s Experimental d e t a i l s The acetaldehyde was an Eastman Kodak sample and was of reagent grade p u r i t y , and a pressure of 5 x 10~ 4 mm of mercury was maintained i n the system. The acetone used was spect r o s c o p i c a l l y pure and the c a l i b r a t i o n gas was krypton i n both cases. Experimental Results The photo-electron stopping curve f o r acetalhyde i s shown in Figure 13. The curve shows c l e a r l y that there are four d i s t i n c t processes leading to i o n i z a t i o n and these occur at 10.22 + 0.01, 12.89 + 0.1, 13.98 +.04 and 15.10 + 0.02 eV. C H , C H O _L_^  L 10 12 14 (21.2>R)eV Figure 13. Photoelectron S{ 16 18 Spectrum of Acetaldehyde. 2 0 _1_ »^  44 Discussion Mulliken (54) has described the e l e c t r o n i c structure of acetaldehyde as follows (IS.electrons are omitted) (2S) 2 [ S ] 2 [ S ] 2 [x+x] 2 [ y ] 2 [Z+Z] 2 [ n ] 4 ( 2 P y ) 2 , XA 0 CH HCC-0 CO HCC HCC-0 CH 0 (3-4) The e l e c t r o n i c structure i s , e s s e n t i a l l y that of 2 4 formaldehyde with the CH^ o r b i t a l s [S] [ n ] added, and i n [y], [Z+Z.], and the second [S], the C of CO i s bonded to one H and one C instead of to two H's as i n H^CO. No representation symbols have been used i n equation (3-4), the symmetry of ^CO ( C 2 y ) having been l a r g e l y destroyed. S t r i c t l y speaking there i s i n t e r a c t i o n and mixing amongst a l l the various o r b i t a l s given i n equation (3x4) . From equation (3-4) the f i r s t i o n i z a t i o n p o t e n t i a l of acetaldehyde would be expected to r e f e r to the removal of a (2 py) electron from the 0 atom. Walsh (55) found three Rydberg series a l l leading to the same l i m i t at 10.18 eV, and he deduced that a non-bonding, electron was being removed since the series members were remarkably free from v i b r a t i o n a l structure. Mulliken (54) has also shown that the lowest i o n i z a t i o n p o t e n t i a l i n these simple aldehydes corresponds to the removal of a non-bonding 2 p^ electron from the oxygen atom. The f i r s t i o n i z a t i o n p o t e n t i a l observed i n t h i s work at 10.22 eV therefore should correspond to i o n i z a t i o n of the 2 p v non-bonding o r b i t a l . The shape of the stopping curve confirms the non-bonding character of the o r b i t a l . The f i r s t i o n i z a t i o n p o t e n t i a l found i n t h i s work agrees well with photoionization values of 10.21 eV by Watanabe (45), and of 10.25 eV by HurzeTer, Inghram and Morrison (56) 45 The value of 10.4 eV found by Sugden and Price (53) using the electron impact technique seems to be rather high. The f i r s t four i o n i z a t i o n potentials found i n t h i s work are given i n Table V. Also included are the r e s u l t s of Sugden and Price (53). • Table V Ionization Potentials of Acetaldehyde (eV) This Work Electron impact • Sugden and Price (53) 10. .22 0. .01 10, ,40 12. .89 + 0. ,01 11. ,30 13. .98 + 0. .04 12. .30 15. .10 + 0. .02 13, .50 It i s evident from Table V that the agreement between the f i r s t i o n i z a t i o n p o t e n t i a l obtained by the two methods i s good, but for higher energy processes the values d i f f e r widely. The electron impact r e s u l t s , however could be complicated by interference from pre-i o n i z a t i o n . We assign the value at 12.89 to i o n i z a t i o n from the C-C bond, bearing i n mind the possible h y b r i d i s a t i o n between the y and S bonding electrons i n t h i s respect. The i o n i z a t i o n p o t e n t i a l of C-H. i s 11.8 eV, and t h i s corresponds to the removal of an electron from 46 the O C bonding (a o r b i t a l . The higher value i n our case could be due to the presence of the strongly electro-negative oxygen atom. In equation (.3*4) the [Z+Z] and [x + x ] electrons provide the a and II bonding of the carbonyl group. The predicted i o n i z a t i o n p o t e n t i a l f o r [x + x £] from valence state data on the basis of the 1 0 o simple equation (54) I > y [I C2pQ) + I C2p ) ] , with I = 17.17 eV f o r 2p oxygen atom (57) I = 11.21 eV for 2p of carbon atom (57) i s s l i g h t l y greater than 14.2 eV. The charge trans f e r i s la r g e l y due to these electrons, and consequently a reduction i n the i o n i z a t i o n p o t e n t i a l i s predicted. These q u a l i t a t i v e arguments show that the value at 13.89 eV i s by no means unreasonable for the i o n i z a t i o n p o t e n t i a l of the [x + x £] o r b i t a l . As pointed out by Mulliken (54) the i o n i z a t i o n p o t e n t i a l f or [II] of CH^ i s predicted to be approximately the same as f o r [ n ] of CH, i n CH, or CH_I, for which the estimated value i s 14.4 eV. The 3 4 3 fourth i o n i z a t i o n p o t e n t i a l found i n t h i s work at 15.1 eV, therefore, may well correspond to removal of an electron from [ n ] of the CH^ bonding o r b i t a l . This value i s somewhat larger than that estimated by Mulliken probably because the CH^ group here i s more p o s i t i v e l y charged owing to the presence of the strongly electro-negative oxygen atom. Interaction with (2 p ) 0 should also tend to increase the i o n i z a t i o n p o t e n t i a l of [ n ], the i o n i z a t i o n p o t e n t i a l of (2p y) 0 being decreased at the same time. 47 Acetone The experimental r e s u l t s obtained f o r acetone are shown i n Figure 14. In the case of acetone there are seen to be four d i f f e r e n t i o n i z a t i o n processes leading to i o n i z a t i o n . They correspond to i o n i z a -t i o n p o t e n t i a l s of 9.72 _+ 0.01, 12.11 +_ 0.02, 13.8 +_ 0.04 and 15.40 +_ 0.01 eV, and as expected, they f a l l generally into l i n e with the r e s u l t s for acetaldehyde. For acetone a configuraton s i m i l a r to equation (3-4) can be written, but with an [S] and [n] group f o r each CH^. The molecular o r b i t a l formula (54) i s : [ 2 S ) 2 [ S ] 2 [ S ] 2 [ S ] 2 [x + x ] 2 [ y ] 2 [ Z + Z ] 2 [ n ] 4 [ n ] 4 ( 2 p y ) 2 ; l A , 0 CH 3 CH 3 ^ - 0 C-0 pc ^C-0 CH 3 CH 3 0 (3-5) The lowest I.P. (9.72 eV) obviously corresponds to the non-bonding electron 2p y, and has been observed as the culmination of a Rydberg series by Duncan (58) at 10.20 eV, appreciably higher than the value found i n the present work. Watanabe's (45) photoionization value for t h i s process i s 9.69 +_ .01 eV i n excellent agreement with the present value. The electron impact method gives the f i r s t i o n i z a t i o n p o t e n t i a l of acetone as 9.92 eV (Morrison and Nicholson (59)), 10.2 eV (Sugden and Price (53)), and 10.1 eV (Noyes (60)). Recently A l -Joboury and Turner (61) found the f i r s t i o n i z a t i o n p o t e n t i a l of acetone to be 9.67 eV by photoelectron spectroscopy. It i s noted that the electron impact figures are' considerably higher than the value found i n t h i s work, and t h i s i s probably due to the r e l a t i v e l y low i o n i z a t i o n cross-section at threshold The i o n i z a t i o n p o tentials obtained i n t h i s work together with the r e s u l t s of Al-Joboury and Turner, and Sugden and Price are given i n Table 6. 14 ( 2l.2l-R)eV Figure 14. Photoelectron Spectrum of Acetone. 49 Table 6 Ionization p o t e n t i a l s of Acetone (eV) This Work Al-Joboury and Sugden and Price Turner (53) (613 .. 9. .72 + 0, .01 9. .67 10, .2 12. .11 + 0, .02 12, .16 11. .3 13. ,80 + 0, .04 14, .15 « 12, ,2 15. .40 + 0, .01 15, .55 13, .6 No spectroscopic data i s available on the inner i o n i z a t i o n p o t e n t i a l s of acetone however i t i s clear from the Table 6 that evidence f o r the four i o n i z a t i o n processes seen i n the photo-electron spectrum obtained by Al-Joboury and Turner (.61) i s i n good agreement with the present work. Such i s not the case with the values obtained, by Sugden and Price (53) for three inner i o n i z a t i o n p o t e n t i a l s . Sulfur Hexafluoride ; The s u l f u r hexafluoride molecule i s known to have the form of a regular octahedron from the r e s u l t s of electron d i f f r a c t i o n measurements and from studies of the Raman and i n f r a r e d spectra (62). No photoelectron spectrum of t h i s molecule has been reported previously. The f a r - u l t r a v i o l e t absorption spectrum has been studied by Moe and Duncan (64) and Codling (65) . Using the electron-impact method, 50 Dibeler and Mohler (66), Marriot and Craggs (67) and Fox and Curran (68) have indicated that the most probable i o n i z a t i o n product i s the SF* ion and that i t s i o n i z a t i o n threshold i s about 15.8 eV. Very recently, Dibeler and Walker (69) measured the photoionization e f f i c i e n c y between 1050 and 600 A by means of a combined vacuum-UV monochromator and mass spectrometer while Simpson, Kuyatt and Millczarek (70) measured the absorption spectrum of SF^ i n the f a r u l t r a v i o l e t by electron impact. Experimental The sample of SF^ was obtained from Matheson of Canada Ltd. and was of high p u r i t y . Argon was used as a c a l i b r a t i n g gas. Experimental Results The photoelectron spectrum from SF^, determined at an incident o photon energy of 584 A, i s shown i n i t s integrated form i n Figure 15. Electron current i s p l o t t e d against (21.21 eV minus R, the retarding voltage) so that the binding energy can be read d i r e c t l y o f f the abscissa. There are four i o n i z a t i o n thresholds c l e a r l y i n evidence, and these occur at 15.35, 16.71, 18.11 and 19.5 eV. Discussion The f i r s t two thresholds observed at 15.35 eV and 16.71 eV during t h i s work correspond with SF* photoionization thresholds obtained by Dibeler and Walker (69), and the t h i r d With what appears to be a d e f i n i t e increase i n t h e i r photoionization e f f i c i e n c y at about 18.2 eV. The process occuring at 19.50 eV could e i t h e r be due to SF* formation or to another SF* threshold, there appears to be a s l i g h t increase 51 16 17 18 19 20 (21.21 -R)eV — a) t o 3 •—i d - i . rt , X 4) X r. U 3 <4H i — I 3 c/o «4-l O U +-> o a, w c o o r-> a> o *-» o a. in u • 3 •H 52 i n Dibeler's curve at t h i s energy. Fox and Curran (68) have reported electron impact (RPD) SF* thresholds at 15.85, 17.0 and 18.0 eV. A l -though the f i r s t i s 0.56 eV higher than the SF* photoionization thresh-hold, the other two correlate reasonably well with our 16.7 and 18.1 values, Although no mass analysis i s employed i n t h i s work the ions prbducted below 19 eV can be taken to be almost excl u s i v e l y SF* since '•'••+• -4 + the r e l a t i v e abundance of SF,. i s only 3x10 that of SF r and other o b p o s i t i v e ions have appearance p o t e n t i a l s above 19 eV. Direct compari-son of our photoelectron spectrum may therefore be made with the SF* fragment photoionization curve from SF^. The r e l a t i v e (maximum) i n t e n s i t i e s of the 15.29 and 16.53 eV processes as measured by Dibeter and Walker are about 1:2, whereas ours are about 1:3. The difference i s e i t h e r due to the e f f e c t s of p r e i o n i -zation unresolved i n the d i r e c t i o n i z a t i o n experiment or to a greater f a l l - o f f . i n the cross-section f o r the f i r s t process at a photon energy of 21.21 eV. It i s i n t e r e s t i n g to note that the U-V absorption cross-section of SF^ derived from i n e l a s t i c electron scattering measurements 6 (70) shows two maxima of almost equal i n t e n s i t y at energies correspon-ding to our f i r s t two thresholds. That our r e l a t i v e i n t e n s i t i e s are so much more i n agreement with Dibeler and Walker's supports t h e i r sugges-t i o n that electron impact non-ionizing processes may be responsible for the discrepancy. Methyl and Ethyl Cyanides No work concerned with the inner i o n i z a t i o n p o t e n t i a l s of 53 these molecules has been reported i n the l i t e r a t u r e , even though methyl halides have been studied by many workers. It was the purpose of th i s work to determine the inner i o n i z a t i o n p o t e n t i a l s of these molecules and to attempt to correlate them with the e l e c t r o n i c structures as given by McDowell (71). Experimental . The methyl cyanide was a spect r o s c o p i c a l l y pure sample. The ethyl cyanide was supplied by Eastman organic chemicals and was found to be free from detectable impurities (mass spectral a n a l y s i s ) . Krypton was again used as a c a l i b r a t i n g gas. Experimental Results The photoelectron stopping curves f o r CH^CN* are shown i n Figures 16 and 17 re s p e c t i v e l y . The energies at which the steps are observed i n the curves are given i n Table 7 below. Figure 18 gives the o r b i t a l energy l e v e l diagram for HCN, CH^CN and C2H,_CN. The values f o r HCN are taken from electron impact studies (72) using the R.P.O. tech-nique. Discussion Infra-red studies (73) have shown that the methyl cyanide molecule has symmetry C^v l i k e the methyl h a l i d e s . Its el e c t r o n i c structure can be derived by the same method employed for the methyl hal i d e s , i . e . i t i s regarded as a de r i v a t i v e of a methane which has been d i s t o r t e d so that i t has a symmetry C^ v McDowell (71) has des-cribed the e l e c t r o n i c structure of methyl cyanide as follows: [ S n + S c , a a ^ 2 [ S n - S c , o a j 2 [ S & i ] 2 [ o ^ , ^ ] 2 [ n n + n c , e ] 4 [He] 4 (3-6) 54 Table 7 Ionization p o t e n t i a l of methyl cyanide and ethyl cyanide (eV) Compound Ionization Potentials (eV) Methyl Cyanide - •.•'•/. 12.23 + 0.01 12.52 + 0.01 13.17 + 0.04 • 15.29 + 0.10 Ethyl Cyanide 11.8 + 0.02 12.83+0.02 13.47 + 0.02 16.29 + 0.03 (The inner o r b i t a l s are omitted) In t h i s formulation the [lie] o r b i t a l s are assumed to be 2 l a r g e l y l o c a l i z e d i n the methyl group. -[a +ac,a'^.] i s the main c - c 4 bonding o r b i t a l , the [n +n ,e] represents the two degenerate H bonding o r b i t a l s of the CN group which are mutually perpendicular. The spectroscopic value of 11.96 eV reported by Cutler (74) f o r the f i r s t i o n i z a t i o n p o t e n t i a l of methyl cyanide i s based on a poorly characterized Rydberg series of only three, members, Miss Cutler mentions several experimental d i f f i c u l t i e s ' encountered i n her work. The f i r s t i o n i z a t i o n p o t e n t i a l observed i n t h i s work at 12.23 eV i s in good agreement with the photoionization data of 12.22 + o.ol eV by Watanabe (25), 12.205 +0.004 eV by Nicholson (46) and 12.33 eV by Mak (75). Previous determinations of the electron impact f i r s t i j - n r — — i 1 1 1 — — ™ T * i i r CH3CN / -i I i i L t I I 1 1 L 12 14 16 18 20 (21.21-R)eV Figure 16. Photoelectron Spectrum of Methyl Cyanide. 12 C2KpN 14 16 18 20 (2!2I-R)6V F l g u r e 1 7 , p h o t o e i e c t r o n s P e c t r u m ° f E t h y l C y a n i d e ' HON CH3CN G*H-GN EiPiire IS. Electron energy le v e l 2 , 5 W " , , ^igram f o r H C N , CH3CN and C 2 H s C N . 58 i o n i z a t i o n p o t e n t i a l have been made by McDowell and Warren (12.52 eV) (59). The f i r s t i o n i z a t i o n p o t e n t i a l of methyl cyanide refers to the removal of a [a^ + a ,.a^[ bonding electron, the bonding character being determined from the shape of the photoelectron stopping curve f o r thi s process. The i d e n t i f i c a t i o n of t h i s process i s assured for the following reasons. The d i s s o c i a t i o n energy of.the C-C bond i n ethane i s 3.68 eV and D (CH^CN). i s known to be 5.8 eV (77). Relative d i s s o c i a t i o n energies can be taken as a reasonable measure of the r e l a t i v e firmness with which electrons are bound i n the bonding o r b i t a l s . It i s , therefore, to be expected that the i o n i z a t i o n p o t e n t i a l of an electron i n the ' [ a £ + o a^] o r b i t a l of CH^CN would be higher than that f or an electron i n the [a a^] o r b i t a l of ethane, i . e . greater than 11.8 eV. Further support comes from the fact that a methyl group when attached to a resonating system behaves as i f i t were conjugated to the attached group (78, 79). The contribution of these conjugated structures can be e a s i l y seen from the C-C bond distance i n the molecule. The C-C bond distance i n t h i s case i s o 1.459 A (80) which corresponds to about 17 percent double bond character and hence to a 17 percent contribution of these conjugated structures. Thus the f i r s t i o n i z a t i o n p o t e n t i a l of CH^CN could well l i e at 12.23 eV. As pointed out by McDowell (71) there w i l l be some i n t e r -action between the II o r b i t a l s of the methyl and CN groups. This w i l l lead to two new molecular o r b i t a l s of the type +• Hq^) and ( I I - , , - n „ M ) , and so two new energy levels w i l l a r i s e . It i s therefore 59 suggested that the second i o n i z a t i o n p o t e n t i a l found here at 12.5 eV ref e r s to the removal of an electron from the ( n „ , , + n _ x r ) o r b i t a l , L r i j CN and that the t h i r d i o n i z a t i o n p o t e n t i a l at 13.17 eV arises from i o n i z a t i o n of the ( n „ u - n _ . t ) o r b i t a l . The fourth i o n i z a t i o n p o t e n t i a l at 15.29 eV i s presumed to re f e r to the removal of an electron from the [S^ + S^,, a a^] bonding o r b i t a l . Spratley (72) found a value of 16.59 eV i n case of HCN for occurence of t h i s process. The increase i n value i n the l a t t e r case can be at t r i b u t e d to highly e l e c t r o p o s i t i v e nature of the hydrogen atom. Ethyl Cyanide Ionization potentials are observed f o r C2H^CN at 11.8 eV, 12.83 eV, 13.47 eV and 16.29 eV, The value found for the f i r s t i o n i z a t i o n p o t e n t i a l at 11.8 eV i s i n f a i r agreement with the electron impact figure of 11.85 eV found by Morrison and Nicholson (59) and the photoionization value of 11.84 eV found by Watanabe (25). This process w i l l r e f e r to the removal of an electron from the C-C bond. In the case of C2H^CN the f i r s t i o n i z a t i o n p o t e n t i a l i s lower, while the second and t h i r d i o n i z a t i o n p o t e n t i a l s are higher than the corresponding ones i n CH^CN. The differences be understood on the basis of hyperconjugation. In case of CH^CN we expect the C-C bond to be somewhat strengthened by becoming an acceptor bond drawing i t s new strength from the "donor" bonds on eit h e r side (79). This i s not true i n the case of C_Hj.CN, however since as pointed out by 60 Coulson (78) the degree of hyperconjligation i s a maximum i n the case of a CH 3 group and to a lesser extent for a CH 2CH 3 and other a l k y l r a d i c a l s . One would therefore expect the C-C bond to be weaker and the two II bonds to be stronger i n the case of C^HgCN. It seems from t h i s information that the value of 11.8 eV associated with the i o n i z a t i o n from the C-C bond would not be unreasonable. The values of 12.83 eV and 13.47 eV are assigned to the two II o r b i t a l s (these o r b i t a l s a r i s e from the i n t e r a c t i o n of n o r b i t a l s of methyl group with n o r b i t a l s of the CN group). The fourth I.P. at 16.29 i s presumed to r e f e r to the removal of an electron from the N-C a bonding o r b i t a l . 61 CONCLUSION In the present work, i t has been shown that the photoelectron spectroscopic technique can. be used to determine I.P.'s of molecules to an accuracy of 0.01 eV. The method i s capable of i n d i c a t i n g the existence of excited e l e c t r o n i c states of molecular ions, and i n favourable cases i t i s also; possible to determine the bonding or a n t i -bonding nature of the ionized o r b i t a l from the shape of the photoelectron stopping curve. In addition, the ordinary photon and electron impact techniques f a i l to d i s t i n g u i s h between i o n i z a t i o n l i m i t s and auto-i o n i z a t i o n i n the strong resonance t r a n s i t i o n s of inner electrons. The photoelectron method i s not d i r e c t l y affected by autoionization, and the inner I.P.'s are not obscured by these autoionization peaks. 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