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Photoionization studies of molecules by mass spectrometry Mak, Danny Shiu Hung 1962

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PHOTOIONIZATION STUDIES OF MOLECULES BY MASS SPECTROMETRY by DANNY, SHIU HUNG, MAK B . S c . M c G i l l U n i v e r s i t y , I960 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 t h e s i s as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1962 In presenting t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree t h a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I f u r t h e r agree that permission f o r extensive copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department o r by h i s r e p r e s e n t a t i v e s . I t i s understood t h a t copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be allowed without my w r i t t e n permission. Department of CHEMISTRY The U n i v e r s i t y of B r i t i s h Columbia, Vancouver 8, Canada. - i i -ABSTRACT This t h e s i s i s an account of work on the photoionization of molecules using a beam of monochromatic l i g h t of considerably narrow band width. A mass spectrometer was used to d i f f e r e n t i a t e the ions formed by photon impact, and to measure t h e i r i n t e n s i t y . At the beginning of the t h e s i s , a b r i e f account of the h i s t o r i c a l developments leading to the present work i s described. A few e x i s t i n g methods f o r the determination of i o n i z a t i o n p o t e n t i a l s , and t h e i r advantages and l i m i t a t i o n s are pointed out. The major e s s e n t i a l components of the instrument are itemised, and t h e i r s p e c i a l c h a r a c t e r i s t i c s b r i e f l y discussed. Normal procedures and maintainance of the instrument, and simple c a l c u l a t i o n s and the major sources of error are also included. Diagrams and tables are added i n order to make the text easier to understand. The i o n i z a t i o n and d i s s o c i a t i o n of s i x molecules were studied, namely, ammonia, nitrogen dioxide, nitromethane, n i t r i c oxide, benzene and a n i l i n e . Their photoionization e f f i c i e n c y curves are interpreted, and the i o n i z a t i o n p o t e n t i a l s of the parent ions were determined. They are compared with -the reported r e s u l t s from other sources, and the agreements and d i f f e r e n c e s explained. - i i i -A conclusion i s included i n the end of the t h e s i s . The l i m i t a t i o n s of t h i s instrument at the present stage are pointed out, and improvements are suggested. The choice of molecules f o r t h i s -work i s mentioned, and an outline f o r further work has also been included. We accept t h i s abstract as conforming to the required standard - i v -Acknowledgment This t h e s i s , which i s being submitted as p a r t i a l f u l f i l m e n t of the requirements f o r the degree of Master of Science i n the Un i v e r s i t y of B r i t i s h Columbia, i s an account of work done under the combined supervision of Professor C. A. McDowell and Dr. D. C. Frost i n the Department of Chemistry of the U n i v e r s i t y of B r i t i s h Columbia from September I960 to A p r i l 1962. It i s a pleasure to acknowledge the guidance throughout t h i s work of Professor C. A. McDowell and Dr. D. C. Frost f o r t h e i r i n t e r e s t i n t h i s work, and f o r t h e i r many h e l p f u l suggestions and discussions. I should l i k e to express my gratitude to my colleagues, Drs. C. E. Brion, D. V. George and Mr. G. E. Styan who read the manuscript and made many suggestions. I am indebted to the t e c h n i c a l s t a f f s at the Un i v e r s i t y of B r i t i s h Columbia, e s p e c i a l l y Mr. F. G. Bloss f o r h i s s k i l f u l assistance. I wish to thank the National Research Council of Canada f o r grants i n a i d of t h i s work. In e a r l i e r phases, we were gre a t l y a s s i s t e d by support from the Geophysical Research Directorate of the United States Air- Force, Cambridge Research Centre, and I wish to extend my thanks to that body f o r i t s generosity. -V-TABLE OF CONTENTS PAGE Chapter I. H i s t o r i c a l Introduction 1 Chapter I I . Molecular I o n i z a t i o n P o t e n t i a l s 7 11.1 Introduction 7 11.2 Comparison between adiabatic and v e r t i c a l I o n i z a t i o n P o t e n t i a l s 8 I I . 5 Determination of I o n i z a t i o n P o t e n t i a l s 10 a. Spectroscopic Measurements 10 b. E l e c t r o n Impact Studies 11 c. Photon Impact Studies 15 Chapter I I I . Experimental 14 I I I . 1 Introduction 14 111.2 Monochromator 16 a. Light Source 16 b. Grating System 17 c. Detecting and Recording 19 d. Energy Conversion Scale 19 111.3 Mass Spectrometer 21 a. Mass Spectrometer Equation 21 b. Gas Handling System 23 c. Ion Source 24 d. Analyser and Electromagnet 26 e. E l e c t r o n M u l t i p l i e r 27 f. V i b r a t i n g Reed Electrometer 28 - v i -TABLE OF CONTENTS (cont'd.) PAGE Chapter III.4 Vacuum System. 29 a. Analyser Tube 29 b. Monochromator 30 c. Gas Handling System 30 d. Light Source 30 1 1 1 . 5 Operation of the Machine 31 1 1 1 .6 Experimental Techniques 32 a. Samples 32 b. Procedure 32 c. Photoionization E f f i c i e n c y Curve 34-111 .7 Sources of E r r o r 34-Chapter IV. Io n i z a t i o n and D i s s o c i a t i o n of Ammonia 37 Chapter V. Photoionization of Nitrogen Dioxide 4-5 Chapter VI. 1 Photoionization of Nitromethane 5 5 VI. 2 Photoionization of N i t r i c Oxide 60 Chapter VII. Photoionization of Aromatic Compounds 66 VII. l Photoionization of Benzene 66 VII.2 Photoionization and D i s s o c i a t i o n of A n i l i n e 71 - v i i -TABLE OP CONTENTS (cont'd.) PAGE Chapter VIII. The Conclusion. 7 7 Bibliography 80 Abstract i i Acknowledgment i v L i s t of Tables v i i i L i s t of Figures i x - v i i i -LIST OF TABLES PAGE Table 1. 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 Ammonia 42 2. 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 Nitrogen Dioxide 51 3 . The I o n i z a t i o n P o t e n t i a l of N i t r i c Oxide 6 3 < 4. The I o n i z a t i o n P o t e n t i a l of Benzene 6 7 - i x -LIST OP FIGURES To follow page Figure 1. Adiabatic and V e r t i c a l I o n i z a t i o n P o t e n t i a l s 8 2 . The Monochromator and Mass Spectrometer 14 3. Gas Handling System 17 4 . Light Source 17 5. Ion Source 24 6 . Photoionization E f f i c i e n c y Curves f o r Ammonia 38 7 . Photoionization E f f i c i e n c y Curves f o r Nitrogen Dioxide 47 8. I o n i z a t i o n E f f i c i e n c y Curves f o r N 0 2 + and K r + by E l e c t r o n Impact 49 9. Photoionization E f f i c i e n c y Curve f o r nitromethane 5 6 1 0 . Photoionization E f f i c i e n c y . Curve f o r N i t r i c Oxide 61 1 1 . Photoionization E f f i c i e n c y Curve f o r Benzene 67 1 2 . Photoionization E f f i c i e n c y Curves f o r A n i l i n e 7 1 1 CHAPTER I. H i s t o r i c a l Introduction.) Since the discovery of the i o n i z a t i o n of gases by X-rays, and the ph o t o e l e c t r i c e f f e c t of l i g h t on s o l i d s and metals, several i n v e s t i g a t i o n s have been made to determine the mode of i o n i z a t i o n of gases when exposed to u l t r a v i o l e t r a d i a t i o n . Hertz v y j i n 1887, when performing experiments on the sparking between electrodes, observed that u l t r a v i o l e t r a d i a t i o n could be used to ioniz e * gases. ( pa") Lenard v J i n 1901, c a r r i e d out some experiments with spark terminals of aluminium, and found that a i r was made conducting under the a c t i o n of a very absorbable kind of u l t r a v i o l e t l i g h t . Later, i o n mo b i l i t y studies by Bloch> J suggested that the p o s i t i v e c a r r i e r i n Lenard's experiments was not a p o s i t i v e i o n of a i r but a dust p a r t i c l e which l o s t negative e l e c t r i c i t y i n accordance with the ordinary p h o t o e l e c t r i c e f f e c t at the surfaces of s o l i d s . Stark v ' J has investigated the e f f e c t of u l t r a v i o l e t l i g h t on the conductivity of gases and has obtained p o s i t i v e r e s u l t s with c e r t a i n complex organic vapors: anthracene, diphenylmethane, diphenylamine and a-naphthylamine. Professor Lyman demonstrated the act u a l i o n i z a t i o n of a i r by the u l t r a v i o l e t l i g h t which passed 2 through a f l u o r i t e p l a t e . The i o n i z a t i o n of a i r commenced at some wavelength between 1230S and and increased very- r a p i d l y with decreasing wavelength. During the period 1920-1930, experiments on photoionization were conducted mostly with vapors of the a l k a l i metals, e s p e c i a l l y Caesium, Rubidium and Potassium. The i o n i z a t i o n p o t e n t i a l s of these metals are low enough to permit measurements i n the quartz region f o r they have a cutoff wavelength at 1450$ or about 8 . 5 5 ev. The experimental techniques were, as a whole, grea t l y improved. However, lack of high r e s o l v i n g power, and the r e l a t i v e l y low s e n s i t i v i t y of the instruments used have l e f t the data of many of these workers i n a somewhat uncertain state. The use of a quartz window does not allow the transmission of photons with energies greater than about 8 . 5 5 ev. Consequently many i n t e r e s t i n g molecules with i o n i z a t i o n p o t e n t i a l s greater than t h i s could not be studied. A considerable amount of data concerning the photoionization of a l k a l i metal vapors C18 39} has been c o l l e c t e d and reviewed by several writers. y A f t e r 1 9 3 5 , very l i t t l e work concerning photoionization appeared i n the l i t e r a t u r e , and the study of i o n i z a t i o n of molecules seemed to be dominated mainly by the e l e c t r o n impact studies using mass spectrometers. Many in v e s t i g a t o r s have used the 3 e l e c t r o n impact method to measure the i o n i z a t i o n p o t e n t i a l s of molecules and ions; to determine accurately the energy needed to s p l i t up a polyatomic molecule i n t o s p e c i f i e d r a d i c a l s , and to determine the i o n i z a t i o n p o t e n t i a l s of r a d i c a l s . Hundreds of papers have already been published i n t h i s f i e l d , and the i o n i z a t i o n p o t e n t i a l s of more than four hundred molecules have been studied. In many cases, t h i s method has provided the only a v a i l a b l e r e s u l t s . The d i f f i c u l t y of obtaining an el e c t r o n beam with s u f f i c i e n t l y low energy spread and accurately known energy caused much of the information from the ele c t r o n impact method to be inaccurate. The form of the i o n i z a t i o n p r o b a b i l i t y curve f o r a single process i s such that when several are superimposed, the r e s o l u t i o n of separate threshold p o t e n t i a l s i s d i f f i c u l t . In the l a s t ten years, photoionization studies have resumed t h e i r steady pace, l a r g e l y due to the development of ingenius designs f o r grating monochromators, and b e t t e r instruments f o r measuring the photon r a d i a t i o n . (22} Johnson et a l . v J i n 1 9 5 1 , found that coating the photo-electron m u l t i p l i e r s with a layer of fluorescent material rendered them s a t i s f a c t o r y f o r the measurement of extreme u l t r a v i o l e t r a d i a t i o n i n t e n s i t y . Sodium s a l i c y l a t e was found to be best suited f o r s e n s i t i z i n g a photo-electron m u l t i p l i e r , since i t i s 4 stable, does not evaporate i n vacuo and gives reproducible r e s u l t s up to 850JL Furthermore i t s response i s excellent and i t s quantum e f f i c i e n c y i s nearly constant. The development of a commercial low-cost grating (45*) (55*) monochromator by Seya v J and Namioka w^ y helped to f a c i l i t a t e the studies of photoionization. It i s b a s i c a l l y a 1 or 3-meter monochromator, with a 3 0 , 0 0 0 grooves per inch grating located on a Rowland c i r c l e . The r e s o l u t i o n of such a monochromator i s better than fo r the range between 0 - 3 , 0 0 0 # . Watanabe et a l . * ^ ^ i n 1953* used a vacuum monochromator to produce photoionization, and reported photoionization measurements i n the vacuum u l t r a v i o l e t region. The t o t a l absorption cross s e c t i o n of a molecule was measured i n an absorption c e l l , and the i o n i z a t i o n p o t e n t i a l was found to correspoind to the long wavelength l i m i t of the i o n i z a t i o n continuum. Hisi.imethod does not give any information about the products which are formed by Photoionization, and no mass analyses have been introduced to d i f f e r e n t i a t e between parent and fragment ions. The t h r e s h o l d - i o n i z a t i o n p o t e n t i a l s of more than a hundred molecules were reported i n 1959 by Watanabe.(57) j j i s r e s u i t s are comparable to those by the spectroscopic method and are superior to those 5 obtained by e l e c t r o n impact studies. Since 1 9 5 5 , several groups of people have t r i e d to combine the monochromator with a mass spectrometer. Lossing and Tanaka, combined a monochromator with a Nier type mass spectrometer, using a krypton discharge lamp as the l i g h t source. Mass spectra of propane and butadiene were studied. A radio-frequency mass spectrometer i n combination with a monochromator was introduced i n 1957 by Herzog and Marmo. Several gases have been studied by t h i s method, and the products of d i f f e r e n t mass are d i f f e r e n t i a t e d with the mass spectrometer. A l l these instruments have the disadvantage that a l i t h i u m f l u o r i d e window i s needed to separate the l i g h t source from the main part of the instrument. Lithium f l u o r i d e does not allow the transmission of u l t r a v i o l e t r a d i a t i o n of wavelength shorter than 1 , 1 0 0 $ or of energy higher than 11.4 ev. Consequently, molecules with i o n i z a t i o n p o t e n t i a l s higher than t h i s energy could not be studied using these instruments. The Instrument used i n t h i s Work. Our instrument has succeeded i n overcoming t h i s d i f f i c u l t y . Two powerful f r a c t i o n a t i n g o i l d i f f u s i o n pumps, backed by duo-seal rotary o i l pumps, have allowed 6 _7 the maintenance of an ultimate vacuum of 5 x 10 ' mm Hg. within the instrument. Any gas p a r t i c l e s leaving the l i g h t source are pumped out before they reach the i o n i z a t i o n chamber, so that a l i t h i u m f l u o r i d e window i s unnecessary i n t h i s case. We have obtained photoionization of molecules with i o n i z a t i o n p o t e n t i a l s i n the higher energy range. 7 CHAPTER I I . Molecular I o n i z a t i o n P o t e n t i a l s . 1. Introduction. During the l a s t f o r t y years, a very considerable number of accurately determined molecular i o n i z a t i o n p o t e n t i a l s have been evaluated. These have been made possible by photoionization methods, spectroscopic measurements, e l e c t r o n impact studies, t h e o r e t i c a l c a l c u l a t i o n , semi-empirical c a l c u l a t i o n and charge t r a n s f e r methods. This f i e l d of knowledge, being both extensive and p r e c i s e , i s very important i n the study of molecular structure. In t h i s work, we are mainly i n t e r e s t e d i n the determination of the f i r s t and second i o n i z a t i o n p o t e n t i a l s of molecules. The removal of the f i r s t most l o o s e l y bound el e c t r o n from one of the molecular o r b i t a l s , with the formation of a s i n g l y charged p o s i t i v e ion, corresponds to the f i r s t or threshold i o n i z a t i o n p o t e n t i a l . The energy needed to remove the second electron, again with the formation of a s i n g l y charged p o s i t i v e ion, i s termed the second or inner i o n i z a t i o n p o t e n t i a l . The term i o n i z a t i o n p o t e n t i a l used i n t h i s work r e f e r s to the adiabatic i o n i z a t i o n p o t e n t i a l unless otherwise stated. 8 2. Comparison between the Adiabatic and V e r t i c a l I o n i z a t i o n P o t e n t i a l s . The adiabatic i o n i z a t i o n p o t e n t i a l of a molecule i s t h e o r e t i c a l l y defined as the energy required to remove an e l e c t r o n completely from the neutral molecule i n i t s ground state to form the corresponding molecule-ion also i n i t s ground v i b r a t i o n a l state. The i o n i z a t i o n p o t e n t i a l s obtained by the photoionization and spectroscopic methods are the adiabatic values. I o n i z a t i o n p o t e n t i a l s measured by e l e c t r o n impact studies do not n e c e s s a r i l y correspond to the adiabatic values, because of the p r o b a b i l i t y that the molecule-ions formed i n the i o n i z a t i o n process may be i n v i b r a t i o n a l l y excited states. Values obtained by the e l e c t r o n impact studies are u s u a l l y c a l l e d the v e r t i c a l i o n i z a t i o n p o t e n t i a l s . The d i f f e r e n c e between the adiabatic and v e r t i c a l i o n i z a t i o n p o t e n t i a l s can best be i l l u s t r a t e d with a diagram, ( f i g u r e 1), where AB i s the p o t e n t i a l energy curve f o r the normal e l e c t r o n i c state of the molecule, and AB* and AB** are two excited e l e c t r o n i c states, while AB + i s the p o t e n t i a l energy curve f o r the molecule-ion. It can be seen that the adiabatic i o n i z a t i o n p o t e n t i a l i s the d i f f e r e n c e i n energy between the lowest 9 10 v i b r a t i o n a l states i n AB and AB +. E l e c t r o n i c t r a n s i t i o n s induced by e l e c t r o n impact have to be v e r t i c a l , because they are governed by the Eranck-Condon p r i n c i p l e , which states that the equilibrium internuclear p o s i t i o n s of both the molecule and the molecule-ion w i l l not be changed by an e l e c t r o n i c transit-ion. Thus, the i o n i z i n g t r a n s i t i o n s w i l l only take place i n the shaded region oabo, and the i o n i z a t i o n p o t e n t i a l of the parent molecule corresponds to the distance of oa. It i s apparent from the f i g u r e , that the v e r t i c a l i o n i z a t i o n p o t e n t i a l w i l l i u s u a l l y be higher than the adiabatic i o n i z a t i o n p o t e n t i a l . In f a c t , the d i f f e r e n c e i s usually between 0.02 to 0.5 ev. 3 » Determination of I o n i z a t i o n P o t e n t i a l s , a. The Spectroscopic Measurements. One of the most accurate methods f o r the determination of i o n i z a t i o n p o t e n t i a l s i s the spectroscopic method. This involves a study of molecular spectra, and the f i t t i n g of the data obtained i n t o a Rydberg s e r i e s : -v = I - 0 I. 1 (n + a ) 2 where: I and a are constants s p e c i f i c to a p a r t i c u l a r molecule, 11 v = wavelength, of a p a r t i c u l a r s p e c t r a l l i n e , R = Rydberg constant, n = integer, (1, 2, 3 n). As n increases, the value of the wavelength, n, converges to the l i m i t set by I, which i s the i o n i z a t i o n p o t e n t i a l of the molecule. In spectroscopic measurements, the wavelength can be measured with great accuracy, and the uncertainty i n the value of the i o n i z a t i o n p o t e n t i a l derived from i t i s usually only a few parts i n a thousand. However, t h i s method cannot be applied to those molecules which give continuous or d i f f u s e d spectra, i n which i t i s d i f f i c u l t to measure sharp Rydberg convergence l i m i t s . The number of molecules that can be studied by t h i s method i s very small compared with other e x i s t i n g methods, even the spectra of simple molecules l i k e n i t r i c oxide, iodine etc. are so complex, that no s a t i s f a c t o r y r e s u l t s have been reported so f a r . b. The E l e c t r o n Impact Studies. The e s s e n t i a l s f o r making e l e c t r o n impact measurements are a beam of electrons of known energy emitted from a hot filament, passing through the gas under i n v e s t i g a t i o n , and a device f o r detecting the ions produced and f o r measuring t h e i r quantity. The instruments commonly used are c a l l e d mass spectrometers. E l e c t r o n impact studies have been highly developed, and are f a r more generally applicable as molecules with 12 strongly bound electrons can be investigated by t h i s method. The e l e c t r o n impact method uses an e l e c t r o n beam emitted from a hot filament. This e l e c t r o n beam w i l l not be monoenergetic i n character but w i l l possess an energy spread which w i l l be mainly Maxwell-Boltzmann i n nature owing to temperature of filament. In addition, f u r t h e r energy spread w i l l be imparted to the ele c t r o n beam by the v a r i a t i o n of temperature along the filament due to conduction of heat through the supporting leads. The d i f f i c u l t y of obtaining e l e c t r o n beams with s u f f i c i e n t l y low energy spread causes most of the information to be inaccurate. The diminution of the energy spread i n the el e c t r o n beams used i n the mass spectrometers u t i l i z i n g the el e c t r o n impact method i s at present under i n v e s t i g a t i o n i n t h i s laboratory. Success i n t h i s f i e l d of study w i l l g r e a t l y improve the accuracy of values of i o n i z a t i o n p o t e n t i a l s obtained by e l e c t r o n impact. The i o n i z a t i o n e f f i c i e n c y curves obtained from e l e c t r o n impact data are often d i f f i c u l t to i n t e r p r e t , and d i f f e r e n t workers use d i f f e r e n t methods f o r t h e i r i n t e r p r e t a t i o n s . As a r e s u l t , the values of a p a r t i c u l a r i o n i z a t i o n p o t e n t i a l reported by d i f f e r e n t workers show wide variance. 13 c. The Photon Impact Method. This method consists of a combination of both photoionization and mass spectrometry. A gas i s illu m i n a t e d by a beam of e n e r g e t i c a l l y homogenous u l t r a v i o l e t r a d i a t i o n , of gradually increasing energy, which can be measured with a high degree of accuracy. Successive stages of e x c i t a t i o n can be reached, u n t i l f i n a l l y , when the energy of the photon reaches-a c e r t a i n value, i o n i z a t i o n takes place. A mass spectrometer i s used to focus, and to d i f f e r e n t i a t e the p a r t i c u l a r i o n under i n v e s t i g a t i o n , and f i n a l l y to measure the ion current produced. The energy spread i n the photon beam i s considerably smaller than that i n the e l e c t r o n beam, and f o r t h i s reason the i o n i z a t i o n p o t e n t i a l s obtained by photon impact are more accurate than those measured by e l e c t r o n impact. The photon impact method i s f a r more generally applicable than the spectroscopic, as molecules with complex spectra which do not exhibit well defined Rydberg s e r i e s could not be studied by the spectroscopic method but could be studied using the photon impact techniques. This w i l l be demonstrated i n l a t e r chapters. 14 CHAPTER I I I . Experimental. 1. Introduction. The work described i n t h i s t h e s i s was made possible by the a v a i l a b i l i t y of a photoionization mass spectrometer, designed and completed i n I960 by C. A. McDowell and D. C. Frost i n t h i s laboratory. D e t a i l s of the theory and the e l e c t r o n i c setup w i l l be published elsewhere, and a b r i e f d e s c r i p t i o n i s given here. The machine was a combination of a monochromator and a mass spectrometer as shown i n f i g u r e 2. The two parts could be i s o l a t e d by a 2% inches diameter Crane wedge-type valve i n the e x i t arm. This valve was f i t t e d with an appropriate o i l - r i n g w e ll suited to high vacuum work. The wedge faces and seats were machined, and the faces covered with rubber d i s c s to e f f e c t good vacuum seals. Figure 2 i s a diagram of the main part of the instrument. U l t r a v i o l e t r a d i a t i o n was generated from the l i g h t source L, and passed through a narrow entrance s l i t to a grating G, s i t t i n g on a table T. By turning the arm A, monochromatic r a d i a t i o n of a c e r t a i n wavelength could be selected. The r e f r a c t e d monochromatic l i g h t passed through the e x i t s l i t to the i o n source S, and was c o l l e c t e d by a photo-electron m u l t i p l i e r P. 15 16 Gaseous or l i q u i d compounds under study were introduced i n t o the i o n source through a f i n e leak from the gas handling system. The ions, formed by the i n t e r a c t i o n of the molecules with the photon beam, t r a v e l l e d through several s l i t s i n t o the analyser. A f t e r d e f l e c t e d by the magnetic f i e l d H, the resolved i o n beam emerged from the s l i t i n the i o n e x i t i n t o the e l e c t r o n m u l t i p l i e r M, and was c o l l e c t e d by a screen electrode. The i o n current was measured by means of a v i b r a t i n g reed electrometer. This chapter itemises the major e s s e n t i a l components and outlines t h e i r s p e c i a l c h a r a c t e r i s t i c s . 2. The Monochromator. A 1-meter monochromator based on the Seya-Namioka mounting was constructed. The body of the monochromator was constructed i n a compact brass u n i t , with f i x e d entrance and e x i t s l i t s . The instrument was e s p e c i a l l y f i n e f o r the vacuum region because the source and e x i t s l i t s d i d not need to be moved to remain on the o p t i c a l a x i s . The only mechanical motion involved was a simple r o t a t i o n of the concave grating, which gre a t l y s i m p l i f i e d the vacuum sea l problems. a. The Light Source. The l i g h t source was an electrodeless discharge lamp which was a modified form of that described by 17 Z e l i k o f f et a l . J It was b a s i c a l l y a c a v i t y designed to resonate at 2450 mc. per sec. The i n s i d e of the c a v i t y was s i l v e r p l a t e d to obtain a high e f f i c i e n c y . Figure 4 shows a diagram of the l i g h t source. A stub e x c i t e r B, 12 cm. long, was fed at the 4 cm. point at E, from a Raytheon Microtherm unit. High voltage appeared across the gap between the 12-cm. stub end and the adjacent c a v i t y wall. A 4 cm. choke A, prevented e l e c t r i c a l leakage through the opening and ensured earthing of the ca v i t y and surface. A mixture of helium and a i r were fed i n at T, and a discharge could be maintained i n the c e n t r a l pyrex tube. A tuning stub D, could be adjusted to give maximum l i g h t i n t e n s i t y from the l i g h t source. b. The Grating System. In t h i s spectrometer, a 15,000 l i n e s per inch, 54 cm. f o c a l length concave grating with an active area of 25 x 30 mm. was used. The grating rotated about a v e r t i c a l axis through the centre of the grating which enable wavelengths between 5 0 0 $ and 2000.2 to be selected. The grating G, and the external d r i v i n g system are shown schematically i n f i g u r e 2. The grating G was clamped, i n a holder, i n p o s i t i o n on a table T i n s i d e the grating housing. The grating was rotated by moving 18 FIGURE 3 . G A S H A N D L I N G S Y S T E M T O I O N S O U R C E F I N E L E A K S T O R A G E B U L B T O P U M P 33= 0 T 3 S A M P L E T U B E FIGURE 4. L I G H T S O U R C E 19 the 12-inch, arm A which was connected to a spindle passing through a vacuum sea l to the grating table base. The arm A bore against a p r e c i s i o n micrometer screw Z which could e i t h e r be turned manually or by the v a r i a b l e speed motor U. D was a rubber-ringed gear wheel which helped to transmit a smooth drive to the micrometer. The readings could be read d i r e c t l y from the micrometer Z, and then converted i n t o wavelengths. c. Detecting and Recording System. A R.C.A. 1P21 m u l t i p l i e r photo-tube contained i n a l i g h t proof brass box was used as a photon detector. Measurement was ef f e c t e d by means of fluorescence. Sodium s a l i c y l a t e was used as a fluorescent material f o r i t s s t a b i l i t y , i t s superior response and f o r the constancy of i t s quantum e f f i c i e n c y i n the region between 850$ and 24-00$. The output of the 1P21 m u l t i p l i e r was amplified by a K e i t h l e y d.c. electrometer, capable of measuring -12 down to 10 amperes, and fed i n t o a Speedomax Model G recorder. d. The Energy Conversion Scale. The wavelength of a p a r t i c u l a r beam of monochromatic l i g h t depends on the p o s i t i o n of the d i f f r a c t i o n grating, and i s given by the Bragg Equation:-20 n X = 2d s i n <j> I I I . 1 where n i s the order of the l i n e , d i s the grating spacing, and <fy i s the angle of d i f f r a c t i o n f o r wavelength 7\ . The order n, can be roughly determined by spectrum a n a l y s i s . (Only n = 1 was used i n t h i s work.) The grating spacing d i s an accurately determined constant. A r e l a t i o n therefore e x i s t s between the wavelength and the angle of d i f f r a c t i o n f o r any wavelength i n the spectrum. The grating table used i n the present work was connected to a 12-inch arm A which bore against a p r e c i s i o n micrometer screw. The 584.2 l i n e i n the helium spectrum, and the 1215.2 oV- l i n e i n the hydrogen spectrum were focussed separately, and the micrometer readings were recorded f o r each l i n e . Using these two readings, a c a l i b r a t i o n curve was drawn, r e l a t i n g wavelength to micrometer reading, from which the wavelength of any other s p e c t r a l l i n e could be determined d i r e c t l y from the micrometer reading. The assumption that r a d i a t i o n i s absorbed or emitted i n energy un i t s (quanta) l e d to the d e r i v a t i o n of the famous equation:-B = — I I I . 2 where E i s the energy of the r a d i a t i o n i n e l e c t r o n v o l t s , h i s the Planck's constant, "A i s the wavelength of the 21 radiation and c is the velocity of light. This equation gives a linear relationship between the energy i n electron volts and wavelength of the radiation i n Angstrom units. A conversion table for wavelengths to electron volts based on this equation was published i n 1961 by (43') -1 Sampson^ '' and the conversion factor used was 1 cm. = 12397.8 + 0.5 x 10~ 8 ev. This table was used through-out this work for a l l energy conversion. 3. The Mass Spectrometer. (36 37") The mass spectrometer was a N i e r w ' " type instrument, using a 60° sector shaped magnetic f i e l d for mass analysis; the permitted radius of ion path was 15 cm. The source used yielded an ion beam nearly homogeneous i n energy. A calibrated potentiometer supplied the ion accelerating voltage, which was continuously variable over the range 580 to 2800 volts. The magnetic f i e l d was provided by a steel electromagnet, and enabled focussing over the range of mass numbers 1 to 300 with an ion accelerating voltage of 2500 volts. a. The Mass Spectrometer Equation. It w i l l be useful to write down the simple equations governing the motion of a charged particle i n a mass spectrometer. 22 If the charged p a r t i c l e of mass M(g), charge e(e.s.u.) and v e l o c i t y v(cm. per s e c ) , (very much l e s s than the v e l o c i t y of l i g h t ) i s sent i n t o a magnetic f i e l d of force H(e.s.u.), the equation of motion may be derived as follows. Since the force i s always at r i g h t angles to the d i r e c t i o n of motion of the p a r t i c l e , there i s no l i n e a r but a constant angular a c c e l e r a t i o n . From elementary mechanics, i t i s seen that the p a r t i c l e w i l l experience a c e n t r i f u g a l force, and f o r equilibrium t h i s must balance the force due to the magnetic f i e l d , i . e . ^ = Hev I I I . 3 where R i s the radius of curvature of the i o n beam. If i t i s assumed now that the charged p a r t i c l e acquires i t s v e l o c i t y by f a l l i n g through an e l e c t r o s t a t i c p o t e n t i a l d i f f e r e n c e V(e.s.u.), the p o t e n t i a l energy must be the same as the k i n e t i c energy of the p a r t i c l e a f t e r a c c e l e r a t i o n , i . e . M|5 = eV I I I . 4 If equations I I I . 3 and I I I . 4 are combined, elimi n a t i n g v, then M _ A 2 . 1 X 1 , 23 The equation I I I . 5 may he termed the mass spectrometer equation. In the mass spectrometer used i n the present work, the radius of curvature of the charged p a r t i c l e i s f i x e d , and when a n a l y s i s of the ions formed from a given molecule i s desired, e i t h e r the i o n a c c e l e r a t i n g voltage i s maintained constant, and the magnetic f i e l d strength v a r i e d continuously (magnet scanning), or the a c c e l e r a t i n g voltage i s v a r i e d keeping the magnetic f i e l d strength constant (voltage scanning). b. The Gas Handling System. Figure 3 (p« 18) shows a diagram of the gas handling system. Gaseous compounds under normal conditions were introduced i n t o the storage bulbs (2 f i v e - l i t r e and 1 t h r e e - l i t r e bulbs) by means of a gas sample tube. Taps T^ and remained closed and only T^ and T^ were i n open p o s i t i o n s . L i q u i d samples were placed i n the tube and degassed by repeated f r e e z i n g i n l i q u i d a i r , pumping off and melting. F i n a l l y , the vapor was introduced i n t o the storage bulbs. The system was evacuated by a rotary o i l pump and was c o n t r o l l e d by the tap T^. Gaseous samples, a f t e r i n i t i a l expansion i n t o the t h i r t e e n - l i t r e storage bulbs, entered the mass spectrometer i o n source v i a the tap T-., and a molecular 24-leak of m e t r o s i l sintered glass. The leak dimensions were such that a few tenths of a millimeter pressure -5 i n the storage bulbs produced a pressure of 3 x 10 ^ mm. Hg. i n the i o n source ( measured with an i o n i z a t i o n gauge). A l l the taps were l u b r i c a t e d with Apiezon M grease. c. The Ion Source. Figure 5 shows a diagram of the i o n source. The electrodes P 2, P^, P^ and P^ were made of s t a i n l e s s s t e e l chosen f o r i t s non-magnetic properties and corrosion resistance. The i o n i z a t i o n box C, and i t s associated members were al s o made of s t a i n l e s s s t e e l . A l l the spacers, i n s u l a t o r s and the sample i n l e t tube were made of Pyrex glass. Gases entered the i o n i z a t i o n box C through L, and d i f f u s e d i n t o the photon beam between S-^  and S 2. Any p o s i t i v e ions that were produced by photon impact were drawn out of the i o n i z a t i o n box by a small e l e c t r i c f i e l d between the i o n r e p e l l e r R and the f i r s t s l i t P^. The f i n a l i on ac c e l e r a t i n g voltage was connected to P^ and P^, and f r a c t i o n s of i t to the s p l i t p l a t e s P 2 and P^. The p o t e n t i a l s on each h a l f of P^ and P^ can be adjusted to a l i g n the ion beam i n the source. The i o n beam l e f t the i o n source through the e x i t 25 FIGURE 5. ON S O U R C E • L ^3 p 4 P 5 2 Above: (a) Cross-section showing e l e c t r o n impact. Below: (b) Cross-section perpendicular to (a) showing photon impact, L n R m m L F 3 B P 2 P3 Ll • 26 s l i t P,- i n t o the analyser. A tungsten filament mounted behind a s l i t provided a beam of electrons E which passes over the ion e x i t s l i t at r i g h t angles to the d i r e c t i o n of the photon beam B. The e l e c t r o n beam was received i n a trap of conventional type. This filament provided a means of obtaining e l e c t r o n impact r e s u l t s f o r comparison and f o r l o c a t i o n of the i o n beam. The i o n i n t e n s i t y r e s u l t i n g from e l e c t r o n impact was always l a r g e r than that from photon impact. d. The Analyser and the Electromagnet. The analyser tube was made from a 5 cm. diameter copper tube bent through 9 0 ° on a radius of curvature of 1 7 . 2 cm. It was f l a t t e n e d over the centre s e c t i o n to f i t between the 2 . 2 cm. pole gap of the analyser magnet. The complete unit was r i g i d l y locked to the framework of the instrument so that i t s p o s i t i o n could be f i x e d with respect with the magnet. The electromagnet comprised f i v e 1 0 , 0 0 0-turn c o i l s wound on a low carbon s t e e l core of 6" x 3" section. The machined pole pieces were made of the same material and had a gap of 2 . 2 cm. The f i e l d strength i n the pole gap was approximately 1700 gauss, and was va r i a b l e f o r the detection over the range of 27 mass numbers 1 to 300 with an i o n a c c e l e r a t i n g voltage of 2500 v o l t s . e. The E l e c t r o n M u l t i p l i e r . On emerging from the magnetic analyser, the resolved i on beam passed through the 2 mm. wide s l i t A, and f e l l on to the f i r s t p l a t e of the e l e c t r o n m u l t i p l i e r . A suppressor electrode B was maintained at negative 22)6 v o l t s to suppress any secondary electrons which arose from the i o n bombardment of the s l i t A. The e l e c t r o n m u l t i p l i e r was a s e n s i t i v e detector of p o s i t i v e ions. In t h i s detector M ( f i g u r e 2) the p o s i t i v e ions impinged upon the f i r s t p l a t e g i v i n g r i s e to secondary electrons. These were accelerated and focused on to the second p l a t e , g i v i n g r i s e to a second, more numerous, generation, and so on, through 8 stages, which r e s u l t e d i n a gain of about 10 . The pl a t e s were connected to successively higher p o s i t i v e p o t e n t i a l s . Since, i n the f i r s t instance, the p o s i t i v e ion current was converted i n t o an e l e c t r o n current, whereas i n succeeding instances, the e l e c t r o n current was simply m u l t i p l i e d , the conversion p l a t e s served an unique function. The plate s were made of 2% Be-Cu. These were ac t i v a t e d i n an oven at 400° C., i n an atmosphere of argon, to obtain a t h i n oxide surface 28 when exposed to a i r . The oxide layer was stable at atmospheric pressure and possessed a m u l t i p l i c a t i o n f a c t o r s i g n i f i c a n t l y greater than unity (about 4-). The p r i n c i p a l merits of the el e c t r o n m u l t i p l i e r were i t s extreme s e n s i t i v i t y and i t s f a s t response. The f i n a l e l e c t r o n current was c o l l e c t e d by a f i n e screen, and a n i c k e l wire d i r e c t e d the current to a v i b r a t i n g reed electrometer. f. The Vi b r a t i n g Reed Electrometer. The usefulness of a v i b r a t i n g reed electrometer i s i n the measurement of small d.c. voltages. The electrometer consisted of two parts: the head unit and the main a m p l i f i e r . The input d.c. p o t e n t i a l which arose from the passage of the el e c t r o n current through 12 a large r e s i s t o r of the order of 10 ohms, was converted to a.c. by applying i t through a se r i e s r e s i s t o r to a capacitor whose capacitance was p e r i o d i c a l l y varying with time. This a.c. s i g n a l then underwent several stages of a m p l i f i c a t i o n and the r e c t i f i e d output was displayed on a meter situat e d i n the rack u n i t . The input r e s i s t o r and capacitor were mounted i n small i n d i v i d u a l p l u g - i n u n i t s to f a c i l i t a t e changes of range and response time. The input capacity could also be va r i e d i n t e r n a l l y i n the range of 0 to 10 pf. Internal s e n s i t i v i t y controls enabled the a m p l i f i e r gain 29 to be adjusted i n the range 1 to 30,000. This was u s e f u l f o r s c a l i n g the output to reduce i o n i z a t i o n curves to equal s e n s i t i v i t y . 12 With an input resistence of 10 ohms and capacitance of 5 pf•, currents as low as 10" amperes could be measured against a random background noise of —17 5 x 10 ' amperes. The time constant i n t h i s range was about f i v e seconds. The a m p l i f i e r output could be fed into a chart recorder which enabled automatic recording of appearance p o t e n t i a l curves. 4-. The Vacuum System. The vacuum system i s very important i n photoionization work, because oxygen absorbs r a d i a t i o n strongly i n the region below 2000$. This system followed conventional l i n e s f o r mass spectrometers, and can be divided i n t o four sections. a. The Analyser Tube. The analyser tube was pumped from the source end of the tube by a 2-inch a l l - m e t a l MC:F-60 f r a c t i o n a t i n g o i l d i f f u s i o n pump f i t t e d with c o l d b a f f l e s , and backed by a duo-seal vacuum pump. The ultimate vacuum _7 of these pumps was 5 x 10 ' mm. Hg., and the net pumping speed was between 20 to 30 l i t r e s per second. 30 A R.C.A.-194-9 i o n i z a t i o n gauge, mounted on the top of the cold trap, was used to measure the pressure i n t h i s region. -p. The Monochromator. The monochromator was pumped by a 6-inch a l l - m e t a l MCF-700 f r a c t i o n a t i n g o i l d i f f u s i o n pump f i t t e d with c o l d b a f f l e s , and backed by a large duo-seal vacuum pump. A l l the f r a c t i o n a t i n g d i f f u s i o n pumps used i n t h i s instrument were f i t t e d with safety-r e l a y s coupled to the mains power and water-cooling system. Whenever the power- or water-cooling-supply cut off a c c i d e n t a l l y , the d i f f u s i o n pumps switched off immediately, and remained o f f u n t i l a push-button was pressed manually to r e s t a r t them. c The Gas Handling System. This system has been described before on page 23 of t h i s chapter. I t was not f i t t e d with a d i f f u s i o n pump, and was evacuated s o l e l y by a duo-seal vacuum pump. d. The Light Source. D i f f e r e n t i a l pumping was employed at P ( f i g u r e 4-, page 18) to minimise the gas pressure between the lamp e x i t and monochromator entrance s l i t S. I t was important to evacuate t h i s region, because the l i g h t i n t e n s i t y 31 diminished i f the u l t r a v i o l e t r a d i a t i o n was allowed to ionize the gas p a r t i c l e s here. Also, i f gas p a r t i c l e s from the l i g h t source were allowed to d i f f u s e d i n t o the ion source, complications would a r i s e . The pumping was f a c i l i t a t e d by a 1 0 0 - l i t r e per minute r o t a r y pump. 5 . Operation of the Machine. A l l the ro t a r y o i l pumps were run continuously f o r s i x months, a f t e r which time they were overhauled; the same operating time applied to the two f r a c t i o n a t i n g o i l d i f f u s i o n pumps i n the analyser tube s e c t i o n and i n the monochromator. A l l e l e c t r i c a l c i r c u i t s were switched on about an hour before the commencement of a run, which took about three to four hours to perform. The e l e c t r i c a l c i r c u i t s were not l e f t on permanently but switched off on completion of a run. When the ion and photon beams became very low, both the ro t a r y and the d i f f u s i o n pumps were stopped, and a i r was introduced i n t o the system. The grating was taken out, dismounted, and sprayed with pure toluene. The photoelectron m u l t i p l i e r was cleaned with methanol, and coated with a f r e s h layer of sodium s a l i c y l a t e . The p l a t e s i n the e l e c t r o n m u l t i p l i e r were f i r s t cleaned with Brasso, then dipped in t o pure toluene, 32 and f i n a l l y a c t i v a t e d i n an oven f o r one minute. A f t e r t h i s operation, both the ion and the photon beams were found to be g r e a t l y improved. 6. Experimental Techniques. a. Samples. In general, i t was highly desirable to use very pure samples f o r photoionization studies. I f the sample contained an impurity, the i o n current would be markedly af f e c t e d , and as a r e s u l t , the photoionization e f f i c i e n c y curve would be d i f f i c u l t to i n t e r p r e t . The n i t r i c oxide, ammonia and nitromethane were supplied by Matheson of Canada Co. Ltd. and were not further p u r i f i e d . The nitrogen dioxide was prepared by r e a c t i n g oxygen with n i t r i c oxide, and p u r i f i e d by low temperature d i s t i l l a t i o n . The benzene and a n i l i n e were C P . grade chemicals, and were f u r t h e r p u r i f i e d by repeated f r a c t i o n a l d i s t i l l a t i o n . b. Experimental Procedure. The preparation f o r making photoionization measurements consisted of:-i . s e l e c t i o n of sample, i i . p u r i f i c a t i o n of sample, i i i . i n t r o d u c t i o n of sample i n t o the gas 33 handling system, and subsequently i n t o the mass spectrometer, i v . warmup of the l i g h t source and the e l e c t r o n i c equipment f o r a period of an hour. When a l l these preparations had been completed, the intense s p e c t r a l l i n e of helium at 584$ was focused. The magnetic f i e l d was adjusted to bring the ions to be studied to focus at the c o l l e c t o r with an i o n acc e l e r a t i n g voltage at about 1000 to 2000 v o l t s . Sometimes, i t was also necessary to adjust the ion acc e l e r a t i n g voltage to give the maximum io n current. A i r was next introduced into the l i g h t source, maintaining an a i r to helium r a t i o of 1 to 4. This increased the number of s p e c t r a l l i n e s i n the spectrum, enabling more points on the photoionization e f f i c i e n c y curve to be obtained. The spectrum of the l i g h t source was then scanned manually. On reaching the top of a peak, the scanner was stopped f o r a period of f i v e to ten minutes which allowed ample time f o r the i o n current i n the recorder to integrate. This procedure was repeated at the top and bottom of each peak of the spectrum. The wavelength at the top of a peak was noted, and the r e l a t i v e height between the top and bottom of the peak was measured using the recorder trace. Using t h i s method, the ion and photon currents were measured simultaneously. 34 c. Photoionization E f f i c i e n c y Curve. In the determination of appearance p o t e n t i a l s using the photoionization mass spectrometer, curves were drawn, from the experimental data, r e l a t i n g the number of ions of a given kind per number of incident photons, produced by photon impact, to the energy of the i o n i z i n g photon beam. The i o n i z a t i o n y i e l d f o r a given molecule at a c e r t a i n wavelength i s defined by the equations:-Y i e l d = number of primary ion- p a i r s number of incident photons r e l a t i v e i on current r e l a t i v e photon current The curve r e l a t i n g the photoionization y i e l d to the energy of the i o n i z i n g photon beam i s r e f e r r e d to as the photoionization e f f i c i e n c y curve. The i o n i z a t i o n p o t e n t i a l s were interpretated from t h i s curves. 7 . Sonrces of Err o r . One of the major possible sources of error was due to the presence of impurity i n the sample. A l l the samples used i n t h i s work were ei t h e r pure chemicals or well c a r e f u l l y p u r i f i e d i n t h i s laboratory, and the r e s u l t s presented i n t h i s t h e s i s exhibited good 35 r e p r o d u c i b i l i t y among several runs. The second source of error was i n the i n t e r p r e t a t i o n of the photoionization e f f i c i e n c y curves. In the determination of the threshold i o n i z a t i o n p o t e n t i a l s , the r e s u l t s could be determined with high degree of accuracy, however, i n the determination of the second i o n i z a t i o n p o t e n t i a l s of most molecules, i t was d i f f i c u l t to determine the energy of maximum slope of ' the curve, and only estimated p o s i t i o n s were obtained from the experimental curves i n spi t e of the f a c t that the band width of the photon beam used was only 0 . 0 0 5 ev. The t h i r d source of error was caused by the v a r i a t i o n of the gas pressure during a run. In the earl y phase of t h i s work, a t h r e e - l i t r e storage bulb f o r the supply of gaseous molecules was used. It was found that a f t e r about four hours of operation, (the normal period of a complete run), the gas pressure dropped to nearly a h a l f i t s o r i g i n a l value, and asoa r e s u l t the i o n current dropped by about one-half while the photon current remained constant. Two f i v e - l i t r e gas bulbs were therefore added to the supply system making a t o t a l capacity of t h i r t e e n l i t r e s . The gas pressure dropped f o r the same period of time was smaller, and the loss i n ion current was n e g l i g i b l e . 36 The threshold i o n i z a t i o n p o t e n t i a l s reported i n t h i s t h e s i s represented the average of at l e a s t f i v e runs. The d i f f e r e n c e between each run was u s u a l l y not more than 0 . 0 5 ev. Taking other f a c t o r s i n t o considera-t i o n , the error f o r t h i s i o n i z a t i o n p o t e n t i a l was estimated as about eight parts i n a thousand. 37 CHAPTER IV. TJie I o n i z a t i o n and D i s s o c i a t i o n of Ammonia. The ammonia molecule i s known (14) to have a pyramidal symmetry C 3v» and i t s molecular o r b i t a l formula i s : -NH3: ( l a 1 ) 2 ( 2 a 1 ) 2 ( l e ) / ' " ( 3 a 1 ) 2 ; 1 A 1 IV. 1 The molecular o r b i t a l s are l i s t e d i n the order of increasing energy. The e l e c t r o n i c structure of the ground state of the ammonia molecule can be compared with that of the nitrogen atom, which has a co n f i g u r a t i o n 2 2 5 of Is , 2s , 2p . When a nitrogen atom and three hydrogen atoms combine to form an ammonia molecule, the two Is electrons of the nitrogen atom occupy the (la-^) o r b i t a l , the innermost o r b i t a l of the ammonia molecule. The two (2a-^) o r b i t a l electrons are bonding electrons to a c e r t a i n extent, and the electrons occupying t h i s o r b i t a l s are the 2s electrons of the nitrogen atom, and one of the 2p electrons of the nitrogen atom, and the three electrons of the hydrogen atoms form the four ( l e ) o r b i t a l s i n ammonia. These are strongly bonding o r b i t a l s . The two remaining 2p electrons of the nitrogen atom form the non-bonding o r b i t a l of O a - ^ ) , which i s an o r b i t a l c o n s i s t i n g a p a i r of unshared electrons and i s l o c a l i z e d l a r g e l y on the nitrogen atom of the ammonia. 38 Figure 6 shows the photoionization e f f i c i e n c y curve f o r the ammonia molecule. The appearance p o t e n t i a l , observed at the point of i n i t i a l onset of i o n i z a t i o n of 10.14 + 0.04 ev., obviously r e f e r s to the energy required to remove an el e c t r o n from the non-bonding (3a^) o r b i t a l to form an NH^+ i n i t s 1^ ground energy state, assuming that the symmetry i s retained i n the ion. Immediately following the i n i t i a l appearance of ions at 10.14 ev., the i o n i n t e n s i t y r i s e s s t e a d i l y with increasing photon energy up to about 11.5 ev. "feetween 11.5 ev. and about 13 ev., snaa the i o n i n t e n s i t y remains f a i r l y constant. At t h i s l a t t e r energy, an inner or second adiabatic i o n i z a t i o n p o t e n t i a l i s indi c a t e d , since the curve begins to r i s e . The photon energy at the point of maximum slope, which occurs at about 14 .7 ev., should correspond to the second v e r t i c a l i o n i z a t i o n p o t e n t i a l . This second i o n i z a t i o n p o t e n t i a l i s i d e n t i f i e d as being due to the formation of the f i r s t excited state of the NH^+ ion, i . e . the E state, by the removal of an e l e c t r o n from the ( l e ) degenerate o r b i t a l of the ammonia molecule. The i o n i n t e n s i t y curve continues to r i s e to about 15 ev. a f t e r which energy i t s t a r t s to decline. This may be explained as being caused by the d i s s o c i a t i o n 3 9 FIGURE 6. PHOTOIONIZATION EFFICIENCY CURVES FOR A M M O N I A 12 13 15 16 17 18 E PHOTON ENERGY 40 of the NH^+ ion, since on the same diagram, the photoionization e f f i c i e n c y curve of NH^ ions appear at 15*09 ev., and r i s e s t e a d i l y as photon energy increases. 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 ammonia at 10.14 ev. i s i n good agreement with the r e s u l t s obtained by other workers using the photoionization methods. Inn ( 2 1 ) i n 1953 and W a t a n a b e i n 1 9 5 9 , measured the absorption and photoionization c o e f f i c i e n t s of ammonia using u l t r a v i o l e t r a d i a t i o n , and determined 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 ammonia as 1 0 . 1 3 + 6 . 0 2 ev. and 1 0 . 1 5 + 0 . 0 2 ev. r e s p e c t i v e l y . Walker and W e i s s l e r ^ ^ ' i n 1955 measured the photoionization e f f i c i e n c y and cross s e c t i o n of ammonia, and 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 to be 1 0 . 0 7 + 0 . 0 5 ev. A l l these workers used photoionization instruments of d i f f e r e n t design, and used d i f f e r e n t methods f o r the i n t e r p r e t a t i o n of the photoionization e f f i c i e n c y curves. The good agreement between our r e s u l t s and t h e i r s suggests that the method used i n the present work f o r obtaining i o n i z a t i o n p o t e n t i a l s i s accurate and r e l i a b l e . The values 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 ammonia, obtained by e l e c t r o n impact and using mass spectrometry, are a l l s l i g h t l y higher than those C29") obtained by photoionization. Mann, H u s t r u l i d and Tate v " i n 1 9 4 0 , using the el e c t r o n impact method, found the 41 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 ammonia to be 1 0 . 5 + 0 . 1 ev, Later, Frost and M c D o w e l l w / i n 1 9 5 8 , and Morrison and (51*) Nicholson ' i n 1 9 5 2 , using a modified and much more s e n s i t i v e mass spectrometer, 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 ammonia to be 10.40 + 0 . 0 2 ev. and 10.42 + 0 . 0 5 ev. r e s p e c t i v e l y . The f a c t that the el e c t r o n impact method tends to measure the v e r t i c a l process and not n e c e s s a r i l y the minimum energy required f o r i o n i z a t i o n , accounts f o r the i o n i z a t i o n p o t e n t i a l s obtained by t h i s method being generally larger (by 0 . 0 2 to 0 . 5 ev.) than the adiabatic p o t e n t i a l s . A t h e o r e t i c a l c a l c u l a t i o n on the o r b i t a l (7) energies of ammonia was undertaken by Duncan v' J i n 1957y and he 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 ammonia to be 9 * 9 4 ev., which i s considerably lower than a l l the reported experimental values. No photoionization r e s u l t s f o r the second i o n i z a t i o n p o t e n t i a l of ammonia have been reported so f a r . Sun and W e i s s l e r ^ ^ measured the absorption maxima i n the extreme u l t r a v i o l e t continuous spectrum of ammonia and found the second i o n i z a t i o n p o t e n t i a l (7) of .ammonia to be 1 7 . 1 + 0 . 5 ev. Duncan w ' c a l c u l a t e d t h e o r e t i c a l l y the second i o n i z a t i o n p o t e n t i a l of ammonia to be 16 . 2 0 ev., and McDowell^ 2^ used the el e c t r o n impact method and found the second i o n i z a t i o n 42 potential of ammonia to be 1 5 . 3 1 ev. Our present result i s 14.7 ev. A l l these values on the second, ionization potential of ammonia d i f f e r greatly from one another, and i t i s d i f f i c u l t to say which i s nearest to the correct value. However, more work in this particular region has been planned and w i l l be carried out i n the near future, with the intention of obtaining more accurate results which may help to explain the differences between previously reported values for the second ionization potential of ammonia. The following table summarises the reported values of the threshold ionization potential of ammonia: Table 1: The Fi r s t Ionization Potential of Ammonia. I.P. (ev.) WORKERS METHODS YEAR 10.14 + 0.04 Present result Photoionization 1962 1Q.13 + 0 . 0 2 Inn Photoionization 1953 1 0 . 1 5 + 0 . 0 2 Watanabe Photoionization 1959 1 0 . 0 7 + 0 . 0 5 Walker, Weissler Photoionization 1955 1 0 . 5 + 0 . 1 Mann, Hustrulid, Tate Electron Impact 1940 10.42 + 0 . 0 5 Morrison, Nicholson Electron Impact 1952 10.40 + 0 . 0 2 Frost, McDowell Electron Impact 1958 9.94 Duncan Theoretical 1957 43 The D i s s o c i a t i o n of Ammonia. Figure 6 shows the photoionization e f f i c i e n c y curve f o r NH2. The NH 2 + i o n has an appearance p o t e n t i a l of 15.09 + 0.05 ev., and so i t must a r i s e from t r a n s i t i o n 2 2 + inv o l v i n g the A-^  or the E energy states of NH^ ion. (Assuming the l a t t e r to have the symmetry Cj v«) Recent studies of the photochemical decomposition of ammonia^) give a mechanism f o r the NH 2 + i o n formation which requires a primary d i s s o c i a t i o n and i o n i z a t i o n process:-NH-, + hv = NH 0 + H IV. 2 3 ^ WH2 + hv = NH 2 + + e IV. 3 In the i n i t i a t i o n step, the ammonia molecule i s di s s o c i a t e d i n t o a r a d i c a l NH 2 and a hydrogen atom. In the second step, the NH 2 + ion i s formed from the NH 2 r a d i c a l s . The NH 2 + i o n w i l l be predominent, since the i o n i z a t i o n p o t e n t i a l of NH 2 i s much les s than that of the hydrogen. The NH 2 + i o n can also be formed by the d i s s o c i a t i o n of the NH,+ ions i n the E state. 5 NR"5+ + hv = M 2 + + H IV. 4 Both these mechanisms w i l l be operative i n the formation of NH 2 + ions at the expense of the NH^+ ions. They help to explain the decline of the photoionization 44 e f f i c i e n c y curve fo r ammonia molecule at about 15 ev. The energy required to remove the f i r s t hydrogen, i . e . D(NrLp-H), i s known^ 5 0^ to be 104 Kcal. (4 . 5 2 ev.). The appearance p o t e n t i a l of NIL,* ion, or V(NH 2 +), has been obtained from f i g u r e 6 as 15.09 ev. Hence, using the following equation:-V(NH 2 +) = 1>(NH2-H) + I(NH 2) + K.E. + E.A. IV. 5 where: K.E. i s the k i n e t i c energy of NH 2 + ion, E.A. i s the el e c t r o n a f f i n i t y of NH2. The i o n i z a t i o n p o t e n t i a l of NH 2 i s le s s than that of the hydrogen, and the formation of NH 2 + ions should have no k i n e t i c energy. The elec t r o n a f f i n i t y of NH 2 i s very small. Therefore, the i o n i z a t i o n p o t e n t i a l of NH 2 i s equal to or les s than 10 .57 ev. 4-5 CHAPTER V. Photoionization of Nitrogen Dioxide. The ionization potential of nitrogen dioxide has been studied extensively by many authors. Electron impact studies have led to the values of 11 ev. by Stueckelberg and Smyth^48^ and 13 .98 + 0.12 ev. by (40 (25*) C o l l i n and Lossing. ' Kandel, ' using an indirect electron impact method to study the ionization and dissociation of nitromethane, obtained a value of 9 . 9 1 ev. Ultraviolet spectroscopic studies by Price and Simpson^"4"0^ led to a value of 12.3 ev. for the f i r s t ionization potential based on the extrapolation of two probable Rydberg bands i n the far u.v. absorption spectrum of nitrogen dioxide. More recently, Weissler, Sampson, Ogawa and Cook^ 2^ using a combination of a photoionization monochromator and a mass spectrometer obtained a value of 11.3 ev. for the f i r s t ionization potential. Further photoionization studies by Nakayama, (54-*) Kitamura and Watanabe w ' have yielded a value of 9 . 7 8 + 0 . 0 5 ev. for the f i r s t ionization potential of nitrogen dioxide, and suggested the occurrence of a second ionization potential at 11.62 ev. There appear to be remarkable disagreements Frost, D.C., Mak, D.S.H. and McDowell, C.A. Can. J. Chem. June, 1962 (In Press). 46 between the previously reported values 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 t h i s compound. We, therefore, using a combination of a vacuum monochromator and a mass spectrometer, have measured the photoionization e f f i c i e n c y as a f u n c t i o n of energy i n the range of 9 to 14 e l e c t r o n v o l t s . In a d d i t i o n , some e f f o r t has been made to explain the apparent discrepancies. The e l e c t r o n i c structure of nitrogen dioxide C-53 4 6 5 5 ^ has been considered by many a u t h o r s . » i t consists of four inner and seventeen valency electrons, and i s bent i n the ground state. Recent work has confirmed the e a r l i e r view that the unpaired e l e c t r o n can be regarded as being l o c a l i z e d mainly on the nitrogen atom, with the formation of the most loosely bound (4a^) molecular o r b i t a l . I t has been known f o r some time that t r i a t o m i c molecules which possess four inner and sixteen valency electrons have a l i n e a r structure. Carbon dioxide i s one such example. When a l ? t h valency e l e c t r o n i s added, as i n the case of nitrogen dioxide, a new symmetry i s necessary, and the molecule becomes bent. The value of 140° f o r the 0N0 angle has been deduced by Pauling from entropy measurements. The ground state of the N0 2 + ion, on the other hand, i s l i n e a r , because i t i s i s o e l e c t r o n i c with carbon dioxide. Thus, nitrogen dioxide changes symmetry on i o n i z a t i o n . 47 Figure 1 shows the photoionization e f f i c i e n c y curve f o r the nitrogen dioxide molecule. The appearance p o t e n t i a l , observed at the point of i n i t i a l onset of i o n i z a t i o n of 9*80 + 0.05 ev. r e f e r s to the removal of the 1 7 t h valency e l e c t r o n from the (4a-^) molecular o r b i t a l to form a JK^*1" i o n i n i t s ground state. This threshold i o n i z a t i o n p o t e n t i a l at 9*80 ev. i s i n good agreement with the adiabatic value obtained by Nakayama et a l . ^ 5 4 ) ( 9 . 7 8 + 0 . 0 2 ev.) For the f i r s t v o l t a f t e r the threshold energy, the i o n e f f i c i e n c y remains f a i r l y constant and remarkably low i n comparison with that at higher energies. The low i o n e f f i c i e n c y shows that the i o n i z a t i o n p r o b a b i l i t y i n t h i s region i s very small, and thus i t w i l l not be easy to detect i t with instruments of low s e n s i t i v i t y . It should be pointed out that Weissler et a l . ^ 2 ^ d i d not investigate the region below 11 ev. i n much d e t a i l , and f o r t h i s reason missed 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 t h i s compound. Their reported value at 1 1 . 3 ev. should be the second i o n i z a t i o n p o t e n t i a l . (The present r e s u l t f o r the second i o n i z a t i o n p o t e n t i a l of nitrogen dioxide i s about 1 1 . 1 ev.) The value of 1 2 . 3 ev., reported by P r i c e and Simpson, based on the u l t r a v i o l e t spectroscopic data may be due to a higher i o n i z a t i o n p o t e n t i a l of t h i s 10 II 12 13 14 EV. PHOTON ENERGY. PHOTOIONISATION EFFICIENCY CURVE FOR N O j . 49 molecule, or i t could be due to the oxygen i n the sample, because the i o n i z a t i o n p o t e n t i a l of oxygen (12.21 + 0.04 e v . ) ^ 1 0 ) i s closed to t h i s value. They stated that the ele c t r o n removed i n the i o n i z a t i o n process was a non-bonding e l e c t r o n of the oxygen rather than the 'odd' el e c t r o n l o c a l i z e d mainly on the nitrogen atom. Kandel, > ' using an i n d i r e c t e l e c t r o n impact method to study nitromethane, obtained an appearance p o t e n t i a l of 12.47 ev. f o r the N0 2 + ion, and taking the d i s s o c i a t i o n energy of the (CH^-N02) bond as 2 . 5 6 ev. he deduced a value of 9 » 9 1 ev. 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 nitrogen dioxide. Our present r e s u l t of 9.80 ev. i s i n good agreement with t h i s value. Figure 8 shows the e l e c t r o n impact i o n i z a t i o n e f f i c i e n c y curves f o r N0 2 + and K r + , which we obtained using a conventional mass spectrometer. C l e a r l y the curved p o r t i o n of the N0 2 + i o n i z a t i o n e f f i c i e n c y curve extends over a much greater energy range than that of K r + , and i t i s d i f f i c u l t to decide the threshold energy f o r N0 2 + formation. It would seem therefore that i n the case of nitrogen dioxide there i s l i k e l y to be considerable undertainty as to the i n t e r p r e t a t i o n of 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 from e l e c t r o n impact studies, lie f i n d t h i s value to be about 10 .5 ev., E L E C T R O N E N E R G Y ( U N C O R R . ) 51 using the i n i t i a l upward break method, and, since i t l i e s between the f i r s t and second adiabatic i o n i z a t i o n p o t e n t i a l s , i t should decrease as the i o n current detector s e n s i t i v i t y i s increased. The following table summarises the reported values of 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 nitrogen dioxide:-Table 2: 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 Nitrogen Dioxide • I.P. (ev. ) WORKERS METHODS TEAR 9.80 + 0.05 Present r e s u l t Photoionization 1962 9.28 + 0.02 Nakayama et a l . Photoionization 1959 11.5 W e i s s l e r e t a l . Photoionization 1959 12.3 P r i c e , Simpson Spectroscopic 1941 11 Steuckelberg, Smyth E l e c t r o n Impact 1950 15.98 + 0.12 C o l l i n , Lossing E l e c t r o n Impact 1958 9.91 Kandel E l e c t r o n Impact 1955 The i o n i n t e n s i t y i n our photoionization e f f i c i e n c y curve ( f i g u r e 7) begins to r i s e a f t e r 10.5 ev. i n d i c a t i n g an inner adiabatic i o n i z a t i o n p o t e n t i a l . The photon energy at the point of greatest slope i n t h i s region, about 11.1 ev., should equal the corresponding v e r t i c a l i o n i z a t i o n p o t e n t i a l , provided i o n i z a t i o n i s 52 occurring only by t h i s second process. The e l e c t r o n impact measurements of Steuckelberg and Smyth** """^  and a l s o C o l l i n and Lossing^ ' seem to be associated with our v e r t i c a l i o n i z a t i o n p o t e n t i a l at 11.1 ev. At photon energies greater than 12 ev., the i o n i z a t i o n e f f i c i e n c y f o r ~$Q>^ shows a s l i g h t decrease. Why t h i s should be so i s not c l e a r , unless p r e - i o n i z a t i o n i s responsible f o r the maximum. The i o n i z a t i o n e f f i c i e n c y increases at about 12.4- ev. can be c o r r e l a t e d with P r i c e and Simpson's^*"^ Sydberg s e r i e s l i m i t at 12 .5 ev. i f the l a t t e r i o n i z a t i o n p o t e n t i a l i s not due to the i o n i z a t i o n of oxygen molecules i n the sample as has been previously stated. 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 ( v e r t i c a l ) at 12 .7 ev. would correspond to the photon energy where the photoionization curve has the greatest slope. We f i n d no evidence to support the proposed i o n i z a t i o n p o t e n t i a l which Nakayama et a l . ' claimed to have observed at 11.62 ev. In view of the marked increase i n i o n current which we observed j u s t below 11 ev., i t may well be that the r i s e i n t o t a l i o n i z a t i o n seen by Nakayama et a l . at 10 . 8 5 ev. i s only p a r t i a l l y due to formation of the i o n p a i r NO"1" and 0~ ( t h e o r e t i c a l l y formed at a minimum energy of 10 . 9 8 ev.). We attempted to c l a r i f y t h i s point by studying the formation of N0 + 53 i o n f r o m n i t r o g e n d i o x i d e by p h o t o i o n i z a t i o n . I t was e x t r e m e l y d i f f i c u l t , however, t o o b t a i n s a t i s f a c t o r y r e s u l t s . E ven w i t h v e r y p u r e n i t r o g e n d i o x i d e as a s t a r t i n g m a t e r i a l t h e r e seemed t o be more N 0 + i o n s p r e s e n t t h a n would have been e x p e c t e d f r o m t h e p r o c e s s under d i s c u s s i o n . T h i s e x t r a n i t r i c o x i d e c o u l d p r o b a b l y have b e e n p r o d u c e d by t h e p h o t o l y s i s of t h e n i t r o g e n d i o x i d e i n t h e i o n chamber. L i g h t o f w a v e l e n g t h l e s s t h a n 4-0002 i s w e l l known t o cause t h e f o l l o w i n g r e a c t i o n : -2 N 0 2 + hv = 2N0 + 0 2 V. 1 I t i s p o s s i b l e t h a t t h e o c c u r r e n c e of t h i s r e a c t i o n c a u s e d our e x p e r i m e n t s t o be i n d e f i n i t e on t h i s p a r t i c u l a r p o i n t . C l o s e r e x a m i n a t i o n of t h e b a s i s on w h i c h (340 Nakayama e t a l . ' e v a l u a t e d t h e s e cond i o n i z a t i o n p o t e n t i a l of 11.62 ev. makes i t a p p a r e n t t h a t i t may not be so w e l l e s t a b l i s h e d as i t might a t f i r s t s i g h t a p p e a r . Those a u t h o r s a s s i g n e d s i x a b s o r p t i o n bands as R y d b e r g bands and f i t t e d them t o t h e R y d b e r g s e r i e s : -V M = 93695 7 V. 2 m ( n + 0 . 2 i r w i t h n = 3 ( ? ) , 4 , 5, 6, 7, 8 I n T a b l e 2 of t h e i r p a p e r t h e y compare t h e c a l c u l a t e d and o b s e r v e d p o s i t i o n s , of t h e bands. The agreements f o r bands w i t h 54-n = 4- to 8 are f a i r l y good but i n the case of the band with n = 3 a discrepancy of 6334- cm""1" was found between the observed and the c a l c u l a t e d frequencies. Presumably f o r t h i s reason the authors l a b e l l e d t h i s band (n = 3)» as being d o u b t f u l l y assigned. None of the t h e o r e t i c a l values f o r the i o n i z a t i o n p o t e n t i a l s agree well with the experimental r e s u l t s . In t h i s connection we may note that McEwen^ 2 8^ has pointed out that the N0 2 + can be formed i n s i n g l e t and t r i p l e t excited states. It i s important to r e a l i z e that both the r a d i c a l (NC^) ground state and the three lowest s i n g l e t configurations of the p o s i t i v e i o n are strongly s t a b i l i z e d by i n t e r a c t i o n with low energy doubly excited configurations.: The corresponding 3 3 in t e r a c t i o n s with the B 2 and A 2 t r i p l e t configurations of the i o n are much smaller. The d i f f i c u l t y i n adequately taking these f a c t o r s i n t o account i n the c a l c u l a t i o n s makes the t h e o r e t i c a l r e s u l t s l e s s s a t i s f a c t o r y than might have been expected. 55 CHAPTER VI. The F i r s t Ionization Potentials of Nitromethane and  Ni t r i c Oxide. (1) Photoionization of Nitromethane. Literature concerning the ionization of nitromethane i s very limited, and practic a l l y no photoionization or spectroscopic measurements have been reported for comparison. Kandel v y J using an electron impact method, found the ionization potential of nitromethane to be 11.54- + 0 . 0 9 ev. McEwen,^8^ using a self-consistent molecular orbital determination, found the theoretical ionization potential of nitromethane as 15«51 ev. McEwen has also pointed out that nitromethane has a very polar Ti electron structure. It has a high dipole moment, and the nitrogen atom has a positive charge of almost plus one. The effect of hyperconjugation with the methyl group w i l l undoubtedly increase the ionization potential i n comparison with nitrogen dioxide. This agrees with the experimental results of this work. We found the ionization potentials of nitrogen dioxide and nitromethane to be 9*80 ev. and 10.83 ev. respectively. The photoionization efficiency curve for the 56 nitromethane molecule i s shown i n figure 9» The appearance potential, observed at the point of i n i t i a l onset of ionization, of 10.83 + 0 . 0 3 ev., refers to the removal of a u electron from the most loosely bound molecular orbital of the molecule i n i t s ground state, to form the CH^I*"^* ion also i n i t s ground state. The threshold ionization potential at 10.83 ev. compares favourably with the v e r t i c a l ionization potential C25*) measured by Kandel. " The v e r t i c a l ionization potential w i l l usually be higher than the adiabatic ionization potential by 0.02 to 0.5 ev. as has been previously stated. The photoionization efficiency curve rises rapidly after the threshold energy, showing a high ionization transition probability. However, the ion intensity remains f a i r l y constant from 11.8 to 12 ev., and from 13 to 13•5 ev. These can either indicate the appearance of fragment ions, or explain as being due to the excitations of the parent molecule to higher vibrational energy levels of the positive ion. (23"") It i s interest to point out that Kandel, v '' while using electron impact techniques, has been abled to obtain the appearance potentials of JSO^ and CH^+ ions from the nitromethane molecule. These, he found to be 12.47 + 0.02 ev. and 13*58 + 0.06 ev. respectively. IONS PER PHOTON (ARB. UNITS). 58 Assuming that h i s v e r t i c a l i o n i z a t i o n p o t e n t i a l of the nitromethane molecule at 11.34 ev. corresponds to our ad i a b a t i c i o n i z a t i o n p o t e n t i a l at 10.83 ev. , and that a l l other appearance p o t e n t i a l s of the fragment ions are s h i f t e d accordingly, the appearance p o t e n t i a l s of the fragment ions, N 0 2 + and CH^ +, should be equal to 11.96 ev. and 13*07 ev. r e s p e c t i v e l y . The appearance of N 0 2 + and CH^ + ions at these energies, would help to explain the breads i n the photoionization e f f i c i e n c y curve at 12 ev. and 13 ev. We have attempted to c l a r i f y t h i s point by studying the formation of the fragment ions, N 0 2 + and CHj +, from nitromethane by photoionization. It was extremely d i f f i c u l t , however, to obtain s a t i s f a c t o r y r e s u l t s . Even with very pure nitromethane as a s t a r t i n g material, the i o n currents due to both the N 0 2 + and GH^+ fragment ions were exceedingly small, and the photoionization e f f i c i e n c y curves of these two fragment ions could not be s a t i s f a c t o r i l y i n terpreted. It would also appear however that had i t been possible to obtain points i n the regions shown by the dotted curves i n f i g u r e 9 that three breaks i n the i o n i z a t i o n e f f i c i e n c y curve might have been detected as shown. These three breaks could then be explained as being due to e x c i t a t i o n s of the parent molecule to 59 higher v i b r a t i o n a l energy l e v e l s of the p o s i t i v e ion. We .are i n the process of b u i l d i n g a new 17-stage e l e c t r o n m u l t i p l i e r f o r the i o n i n t e n s i t y measurement, to replace the 8-stage m u l t i p l i e r at present i n use. It i s hoped -that t h i s improvement i n i o n i n t e n s i t y measurement w i l l help to d i f f e r e n t i a t e between the possible explanations f o r the shape of the i o n i z a t i o n e f f i c i e n c y curve. 60 ( 2 ) Pilotoionization of N i t r i c Oxide. I o n i z a t i o n of n i t r i c oxide has been considered important i n the c a l c u l a t i o n of transport properties of a i r at high temperatures. Various upper atmospheric phenomena have been a t t r i b u t e d to the presence of n i t r i c oxide. The D-layer i n the upper atmosphere was believed to be formed p r i m a r i l y by the process of photoionization of n i t r i c oxide by Lyman alpha r a d i a t i o n , and the photoionization study of t h i s molecule w i l l provide fu r t h e r information concerning the D-layer formation. Unlike nitromethane, the i o n i z a t i o n p o t e n t i a l of n i t r i c oxide has been studied extensively by many authors. E l e c t r o n impact studies have l e d to the values of 9.4 + 0 . 2 ev. by Hagstrum^ 1 1^ and 9.24 + 0.04 ev. by Clarke. C l o u t i e r and S c h i f f , ^ using the ret a r d i n g p o t e n t i a l method, obtained a value of 9 ? 2 5 + 0 . 0 2 ev. 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 of n i t r i c oxide. Unfortunately, the spectrum of n i t r i c oxide i s very complex; no Rydberg s e r i e s convergence has been studied with any degree of success, and consequently there are no d i r e c t spectroscopic measurements a v a i l a b l e f o r (54) comparison. Further work by Walker and W e i s s l e r , w ' using a combination of a photoionization monochromator and a mass spectrometer, y i e l d e d a value of 9 * 2 0 + 0 . 0 3 ev, 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 . Photoionization 61 (59*) studies c a r r i e d out "by Watanabe, Marmo and Inn, v"^ y have y i e l d e d a value of 9 * 2 3 + 0 . 0 3 ev. 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 the n i t r i c oxide. The t h e o r e t i c a l c a l c u l a t i o n of t h i s molecule f o r i t s i o n i z a t i o n p o t e n t i a l agrees w e l l with the experimental r e s u l t s . Y a m a z a k i , u s i n g a s e l f - c o n s i s t e n t f i e l d molecular o r b i t a l method, obtained a value of 9«14 ev. The e l e c t r o n i c structure of n i t r i c oxide has been considered by several authors. (^3) when a ground state oxygen atom ( P) and a ground state nitrogen atom C^S) approach each other, s i x d i f f e r e n t i n t e r a c t i o n s are p o s s i b l e , corresponding to spectroscopic states of 2 2 4 4 6" " 6 £ , u , ' 2 , ~\ , 2 and n of the n i t r i c oxide p molecule. Only experimental evidence f o r the T\ and h. ~\ states i s a v a i l a b l e , and the other states are probably repulsive states. Mulliken has shown that the e l e c t r o n i c structure of n i t r i c oxide i n the ground state can be w r i t t e n i n terms of molecular o r b i t a l s : -NO: K K ( r g 2 s ) 2 ( r u 2 s ) 2 ( ( T g 2 p ) 2 ( T T u 2 p ) 4 ( T ^ 2 p ) ; 2 T \ g 71. 1 the molecular o r b i t a l s are i n order of decreasing binding energy from l e f t to r i g h t . The photoionization e f f i c i e n c y curve f o r the n i t r i c oxide molecule i s shown i n f i g u r e 10. The appearance p o t e n t i a l , S9 63 observed at the point of i n i t i a l onset of i o n i z a t i o n of 9.28 + 0.04- ev. r e f e r s to the removal of an antibonding e l e c t r o n from the (jr 2 p ) molecular o r b i t a l . These S threshold i o n i z a t i o n p o t e n t i a l i s i n good agreement (59) with the adiabatic values obtained by Watanabe et a l . ' It a l s o agrees with the v e r t i c a l i o n i z a t i o n p o t e n t i a l (2) determined from e l e c t r o n impact studies by C l a r k e v y and by S c h i f f et a l . ^ The following table summarises the reported values of 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 n i t r i c oxide:-Table 3 : The I o n i z a t i o n P o t e n t i a l of N i t r i c Oxide. I.P. (ev.) WORKERS METHODS YEAR 9.28 + 0.04 Present r e s u l t Photoionization 1962 9 . 2 3 + 0 . 0 2 Watanabe,Marmo,Inn Photoionization 1953 9 , 2 0 + 0 . 0 3 Walker, Weissler Photoionization 1955 9.4 + 0 . 2 Hagstrum E l e c t r o n Impact 1951 9 . 2 5 + 0 . 0 2 C l o u t i e r , S c h i f f E l e c t r o n Impact 1959 9.24 + 0.04 Clarke E l e c t r o n Impact 1954 9.14 Yamazaki Th e o r e t i c a l 1959 A f t e r the threshold i o n i z a t i o n p o t e n t i a l of 9.28 ev., the ion e f f i c i e n c y was found to r i s e to s i x increments. The average energy separation between each 64 step i s about 0.3 ev. That these increments are not due to experimental errors, was v e r i f i e d by the f a c t that many determinations of the i o n i z a t i o n e f f i c i e n c y curve produced i d e n t i c a l r e s u l t s . Instrumental errors were also r u l e d out since the study of other molecules i n the same energy range using the same instrument showed no s i m i l a r increments i n the i o n i z a t i o n e f f i c i e n c y curves. These steps c l e a r l y indicated that the i o n i z a t i o n of n i t r i c oxide does not r e s u l t i n ions i n the lowest l e v e l of the ground state, but also i n ions i n the higher v i b r a t i o n a l l e v e l s (v = 1, 2, 3» 4, 5 )• No v i b r a t i o n a l steps were observed i n the photoionization e f f i c i e n c y curves a f t e r a photon energy of 11 ev. This can be explained by the f a c t that the v i b r a t i o n a l l e v e l s of the N0 + i o n at a higher p o t e n t i a l energy would be cl o s e r to one another; the differ e n c e of photoionization t r a n s i t i o n p r o b a b i l i t y between two adjacent v i b r a t i o n a l l e v e l s would be small, and the detection of v i b r a t i o n a l structure i n the photoionization e f f i c i e n c y curve would be d i f f i c u l t . Each point on the curve a f t e r a photon energy of 11 ev. appears to represent the t r a n s i t i o n p r o b a b i l i t y of a sum of a few v i b r a t i o n a l l e v e l s of the N0 + ion, and consequently a smooth curve would be expected instead of the v i b r a t i o n a l steps. The presence of the peak at 12 ev. i s at present i n e x p l i c a b l e , unless 65 p r e - i o n i z a t i o n i s responsible f o r t h i s maximum. Support f o r the appearance of v i b r a t i o n a l s t r u c -ture i n tne photoionization e f f i c i e n c y curve between the energies of 9*3 and 11 ev. i s found i n tne work of Miescher and B a e r , ^ ^ and Tanaka. ^ 1) Miescher and Baer, while studying a high current discharge i n n i t r i c oxide, found a system of bands i n the emission spectrum. Tanaka l a t e r confirmed t h i s system of bands was due to the X 1 X + state of the n i t r i c oxide. The (0,0), (0,1), (0,2), (0,5) members of the Miescher-Baer emission bands agree with the v i b r a t i o n a l increments of our present r e s u l t . Furthermore, Watanabe et a l . using photoionization techniques, and Herzeler, Inghram (2.5) and Nicholson, y ' using photoionization and mass spectrometric methods, have separately reported the detection, i n the same region, of these s i x v i b r a t i o n a l t r a n s i t i o n s i n the n i t r i c oxide molecule. 66 CHAPTER VII. Photoionization of Aromatic Compounds. (1) Photoionization of Benzene Molecule. The structure of the benzene molecule has been extensively studied by p h y s i c a l and chemical means. Accurate values of the i o n i z a t i o n p o t e n t i a l of t h i s molecule are p a r t i c u l a r l y important i n connection with the previous empirical methods of determining molecular properties of conjugated systems. The i o n i z a t i o n p o t e n t i a l of the benzene molecule has been evaluated by many authors. E l e c t r o n impact studies have le d to the values of 9 * 8 + 0.1 ev. by H u s t r u l i d , Kusch and T a t e / 2 0 ) 9 . 4 5 ev. by H o n i g / 1 ^ 9 . 5 8 ev. by M c D o w e l l / 2 ^ 9 . 5 2 ev. by Morrison and N i c h o l s o n / 5 2 ^ and 9 . 3 8 ev. by Wacks and Dibeler. U l t r a v i o l e t spectroscopic studies by P r i c e and Wood^^) have y i e l d e d a value of 9«24 ev. as 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 the benzene molecule. These workers predicted the occurrence of a second i o n i z a t i o n p o t e n t i a l at 11.7 ev. Photoionization studies by Watanabe^ 0) (12) gave a value of 9 . 2 4 ev. Hedge and Matsen^ J and Hush and Pople^- 1^) obtained values of 9«37 ev. and 9 . 8 7 ev. r e s p e c t i v e l y using molecular o r b i t a l c a l c u l a t i o n s . Our present r e s u l t s show that 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 67 of the benzene molecule to be 9.4-6 + 0.03 ev., and the second i o n i z a t i o n p o t e n t i a l to be 11.6 ev. The following table summarises 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 the benzene molecule Table 4: The Io n i z a t i o n P o t e n t i a l of Benzene. I.P. (ev.) WORKERS METHODS YEAR 9.46 + 0.03 Present r e s u l t Photoionization 1962 9-24 P r i c e , Wood Spectroscopic 1948 9.24 Watanabe Photoionization 1954 9.8 + 0.1 Hustrulid,Kusch,Tate E l e c t r o n Impact 1938 9.43 Honig E l e c t r o n Impact 1948 9.52 + 0.08 Morrison, Nicholson E l e c t r o n Impact 1952 9.58 McDowell E l e c t r o n Impact 1954 9.46 + 0.1 Omura, Baba, Higasi E l e c t r o n Impact 1955 9-38 Wacks, Dibeler E l e c t r o n Impact 1959 9.37 Hedge, Matsen Th e o r e t i c a l 1958 9.87 Hush, Pople Theo r e t i c a l 1955 The photoionization e f f i c i e n c y curve f o r the benzene molecule i s shown i n f i g u r e 11. The appearance p o t e n t i a l of the benzene parent ion at 9.46 + 0.03 ev. corresponds to the removal of the TT e l e c t r o n above and below the plane of the benzene r i n g . The appearance 68 PHOTOIONIZATION EFFICIENCY CURVE FOR B E N Z E N E 1 0 12 13 14 15 PHOTON ENERGY 69 p o t e n t i a l at 9*46 ev. i s higher than the spectroscopic i o n i z a t i o n p o t e n t i a l obtained by Price and Wood, and the adiabatic value found by Watanabe. However, i t i s i n good agreement with the i o n i z a t i o n p o t e n t i a l s given by Omura, Baba and H i g a s i ; by Honig; and by Wacks and Dibeler. The i o n e f f i c i e n c y i n the photoionization e f f i c i e n c y curve r i s e s afte.r the threshold energy of 9.46 ev., and the curve e x h i b i t s a pronounced change i n slope atria photon energy of 10.5 ev. From previous work i t has been demonstrated that a sudden change i n the slope can be associated with an energy l e v e l i n the ground state of the parent ion. Fox and Hickam^ 8) have also reported a change i n slope at 10.4 ev. i n t h e i r i o n i z a t i o n e f f i c i e n c y curve obtained by e l e c t r o n impact studies of benzene. In view of the f a c t that both instruments used are of completely d i f f e r e n t design, the change of slope observed i n the same energy range can be taken as evidence that t h i s phenomenon i s not instrumental i n o r i g i n . However, the lack of information concerning the known energy l e v e l s of the ground states of both the molecule and i t s parent i o n makes a straightforward i n t e r p r e t a t i o n of the curve i n t h i s region somewhat d i f f i c u l t . The i o n i n t e n s i t y f u r t h e r increases to about 70 11 ev., and then shows a s l i g h t decrease. The reason for the existance of a peak at 11 ev. i s not cl e a r . A study has been made by Hustr u l i d , Krush and T a t e ^ 2 0 ^ of the appearance of the i o n i c fragments of benzene under electron impact. The lowest appearance p o t e n t i a l of the fragment ions was fo r the C^H^+ ion at 4 .7 ev. above the i o n i z a t i o n p o t e n t i a l of the benzene molecule. The s l i g h t decrease of our curve a f t e r 11 ev. cannot be reconc i l e d with the appearance of any fragment ions. The photoionization e f f i c i e n c y curve begins to r i s e at the photon energy of about 11.6-ev. In view of the shape of the curve, and the high t r a n s i t i o n p r o b a b i l i t y of the i o n i z a t i o n process, a second i o n i z a t i o n p o t e n t i a l of the benzene molecule i s indicated. The second i o n i z a t i o n p o t e n t i a l at 11.6 ev. confirms the Rydberg convergence of the spectroscopic measurements (41) at 11.7 + 0.3 ev. by Price and Wood,v J w n o suggested that t h i s process could be due to the removal of a CT electron. 71 (2) Photoionization and D i s s o c i a t i o n of A n i l i n e . A n i l i n e i s an important chemical i n the dye industry and i s widely used i n many other f i e l d s i n Chemistry. Very l i t t l e information has been reported concerning the i o n i z a t i o n p o t e n t i a l of t h i s molecule and i t s fragment ions. Watanabe and M o l t l , ^ 1 ^ i n 1 9 5 7 , reported 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 the a n i l i n e molecule as 7«70 + 0 . 0 1 ev. No new measurements have been reported since. We used a combination of a vacuum monochromator and mass spectrometer, and studied the photoionization of t h i s molecule. In add i t i o n , some e f f o r t has been made to explain the occurrence of the fragment ions. The photoionization e f f i c i e n c y curves f o r both the parent and fragment ions of the a n i l i n e molecule are shown i n f i g u r e 12. It i s seen that the threshold energy ( 7 » 6 7 + 0 . 0 5 ev.) agrees with the adiabatic value obtained-by Watanabe and M o l t l . The appearance p o t e n t i a l of the a n i l i n e molecule at 7 . 6 7 ev. corresponds to the removal of the e l e c t r o n l a r g e l y l o c a l i z e d on the benzene r i n g . The appearance p o t e n t i a l of the a n i l i n e molecule i s much lower than that of the ammonia. (Our present r e s u l t f o r the i o n i z a t i o n p o t e n t i a l of ammonia i s 10.14-ev.). Watanabe has pointed out that the ph o t o i o n i z a t i o n 72 PHOTOIONIZATION EFFICIENCY CURVES FOR A N I L I N E PHOTON ENERGY 73 I.P. of a n i l i n e shows that s u b s t i t u t i o n of the hydrogen atom i n ammonia by the phenyl r a d i c a l has a marked e f f e c t i n lowering the i o n i z a t i o n p o t e n t i a l . The appearance p o t e n t i a l of the a n i l i n e molecule i s also lower than that of benzene/ (our present r e s u l t f o r the i o n i z a t i o n p o t e n t i a l of benzene i s 9'46 ev.) According to P r i c e ^ 2 ) the lowering i n the i o n i z a t i o n p o t e n t i a l on s u b s t i t u t i n g a NIL-, group i n benzene i s due to a large s t a b i l i z a t i o n of the molecular ion, and t h i s s t a b i l i z a t i o n i s produced by charge t r a n s f e r e f f e c t . The photoionization e f f i c i e n c y curve shows a marked change of slope at about 9 ev. This p o s i t i v e l y i n d i c a t e s the appearance of fragment ions, because the appearance p o t e n t i a l of the a n i l i n e fragment ions i s about 9 ev. The cause of the change of slope with higher photon energies, about 10 ev., i s not c l e a r at t h i s stage. From the photoionization e f f i c i e n c y curve (fi g u r e 12) the appearance p o t e n t i a l of the a n i l i n e fragment ions i s 9*20 + 0 . 0 5 ev. Mass ana l y s i s using the mass spectrometer, i n d i c a t e s the a n i l i n e fragment ion to be of mass one l e s s than the parent molecule, and t h i s proves the formation of the fragment ions i s caused by the removal of a hydrogen atom from the parent molecule. The fragment i o n i n t e n s i t y s t a r t s to r i s e . 74 after the threshold energy at 9.20 ev., and a break i n the energy curve i s observed: at about 11 ev. This indicates an inner ionization potential, and the second ionization potential of the fragment ion i s estimated at about 11.5 ev.( Two different processes are seen to be responsible for the formation of the fragment ion C^H^N* from aniline. We would expect at least two processes, since i t i s possible to remove a hydrogem atom from either the NH2 group or from the benzene ring to give either C 6H 5NH + or C 6H 4RH 2 + ions. The theoretical calculation of the dissociation energy of benzene, i.e. D(C^H^-H), by D i e t z , w ' gave a value of 4.0 ev., assuming that the CgH^+ ion containing six carbons i s produced by the removal of the hydrogen i n atomic form and that the ion i t s e l f i s ring shaped. The rupture of a ring and the subsequent loss of a hydrogen atom does not occur. This can be attributed to the well-known sta b i l i z a t i o n of fused ring aromatic compounds. The NH2 group i s known to exhibit a positive mesomeric effect, and the electronically excited states may be visualized as involving the polar resonance forms:-0 - % ^  <E>-»^H 7 5 This resonance structure would make the rupture of the (C-H) bonds i n the phenyl r i n g easier than the breaking of the same bond i n the benzene molecule. The d i s s o c i a t i o n energy f o r the rupture of the (C-H) bond i n the a n i l i n e molecule would be expected to be l e s s than that f o r the benzene molecule. The energy required to remove the f i r s t hydrogen r a d i c a l from ammonia, i . e . D ( N H 2 ~ H ) , i s known to be 4 . 5 2 e v . ^ ° ) Considering the mesomeric e f f e c t of the N H 2 group previously discussed, the energy required to break the (N-H) bond i n a n i l i n e would be expected to be larger than that required to break the same bond i n ammonia. The diff e r e n c e i n the energy needed to rupture the (C-H) bond with the removal of a hydrogen atom to form the C^H^N^* ion, and to rupture the (N-H) bond to form the C£-Hr-NH+ i o n from the a n i l i n e molecule 6 5 would be estimated to be close to 2 ev. Our present r e s u l t s show that the appearance p o t e n t i a l of the a n i l i n e fragment i o n i s 9 . 2 0 ev. and the inner i o n i z a t i o n p o t e n t i a l of the same fragment i o n i s estimated at 1 1 . 5 ev. The di f f e r e n c e between these two p o t e n t i a l s i s about 2 ev. From a l l these considerations, i t i s possible to say that the appearance p o t e n t i a l of the fragment 76 i o n corresponds to the rupture of the (C-H) bond i n the phenyl r i n g , and the removal of a hydrogen r a d i c a l to form the C^ H^ NHg"1" fragment ion. The inner i o n i z a t i o n p o t e n t i a l of about 2 v o l t s above the threshold energy, would r e f e r to the rupture of the (N-H) bond i n the NH2 group, and the removal of a hydrogen r a d i c a l to form the C&H^NH+ fragment ion. The i o n i z a t i o n p o t e n t i a l of the fragment ion can be obtained from the follwoing equation:-V(C 6H 4NH 2 +) = I(C 6H 4NH 2) + D^H^NH^H) + K.E. + E.A VIII. 1 V(C 6H 5NH + ) = I(C 6H 5NH ) + DtCgH^NH-H ) + K.E. + E.A VIII.2 where: V(X +) = appearance p o t e n t i a l of X, I(X) = i o n i z a t i o n p o t e n t i a l of X, K.E. = k i n e t i c energy, and E.A. = e l e c t r o n a f f i n i t y . The fragment ions should be formed without k i n e t i c energy, and the e l e c t r o n a f f i n i t y f o r these fragment ions i s small. The i o n i z a t i o n p o t e n t i a l of the fragment ions, from the previously considerations, should be equal f o r both processes, and should be i n the order of 5 ev. 77 CHAPTER V I I I . The Conclusion. In our photoionization studies, we have succeeded i n obtaining r e l i a b l e r e s u l t s f o r the i o n i z a t i o n p o t e n t i a l s of s i x compounds, namely, ammonia, nitrogen dioxide, nitromethane, n i t r i c oxide, benzene and a n i l i n e , and also the appearance p o t e n t i a l s of some of t h e i r fragment ions between the photon energy of 7 and 14 e l e c t r o n v o l t s . Although reproducaable r e s u l t s have been obtained between 7 and 14 e l e c t r o n v o l t s , the prospect a f t e r the l a t t e r energy l i m i t i s rather dim. The discharge lamp provided an intense spectrum both i n i n t e n s i t y and i n the number of s p e c t r a l l i n e s between 7 and 14 e l e c t r o n v o l t s , however, the photon i n t e n s i t y drops off r a p i d l y f o r higher energies. Presumably f o r t h i s reason, many s p e c t r a l l i n e s i n t h i s higher energy region are missing from the spectrum. Reaeach w i l l be c a r r i e d out i n t h i s laboratory f o r b u i l d i n g a new l i g h t source and i t w i l l be a part of my future work. Watanabe has reported the a c c i d e n t a l discovery that a small amount of platinum vapor deposited on the surface of a grating has an e f f e c t that greatly increases the i n t e n s i t y of the monochromatic l i g h t , and he gives convincing evidence f o r t h i s i n l a t e r 7 8 experiments. Thus, further reaeach pointing i n t h i s d i r e c t i o n should not be delayed. However, a grating usually contains more than 1 5 , 0 0 0 l i n e s per inch, and the space between two adjacent l i n e s i s exceedingly small. The method of coating a t h i n layer of platinum without a f f e c t i n g the functioning of the grating i s another problem we w i l l have to face. The i o n i z a t i o n t r a n s i t i o n p r o b a b i l i t y r e s u l t i n g from photon impact i s comparatively much smaller than that from e l e c t r o n impact; and the t r a n s i t i o n p r o b a b i l i t y f o r fragment i o n formation i s comparatively smaller than that f o r the parent ions i n both methods. Presumahly f o r these reasons, we have been unable to obtain the i o n i z a t i o n p o t e n t i a l s of some fragment ions which have been determined by e l e c t r o n impact studies. To remedy t h i s , a 17-stage e l e c t r o n m u l t i p l i e r has been constructed to be set up i n t h i s instrument f o r the measurement of ion i n t e n s i t y . This w i l l replace the 8-stage e l e c t r o n m u l t i p l i e r we have been using, undoubtedly, the improvement i n the i o n s e a s i s t i v i t y w i l l help us i n the study of both the parent as well as the fragment ions. The s i x molecules under i n v e s t i g a t i o n are of considerable importance i n both industry and i n chemistry. They are simple molecules and the e l e c t r o n i c structures 79 are comparatively easy to explain. The i o n i z a t i o n p o t e n t i a l s of both the parent and fragment ions of these molecules are rather low. Most of them are within our working photon energy range of 7 to 14-e l e c t r o n v o l t s . Some of these molecules have been studied by many workers using other approaches, however the reported values of the. i o n i z a t i o n p o t e n t i a l i n the l i t e r a t u r e often showed marked disagreements among themselves. We present new photoionization measurements, and help to explain the apparent discrepancies. 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