Analytical potential of photoelectron spectroscopy - American

ANALYTICAL POTENTIAL OF PHOTOELECTRON SPECTROSCOPY D. Betteridge and A. D. Baker Chemistry Department, University College of Swansea, Singleton Park, Swansea, U.K.

spectrometer measures binding energies of electrons in a molecule by determining the kinetic energies of these electrons when they are ejected from the molecule by the impact of (usually) monochromatic radiation. The energy of radiation is such that electrons from several valence shell molecular orbitals can be ejected. The spectrum of a molecule reflects first the elements present in the molecule and second the bonding configurations of these elements relative to each other. For example in the spectra of cis- and trans-1,3dichloropropene, i t is possible t o assign peaks due to the ionization of chlorine “3p lone pair” electrons and t o subdivide these into chlorine adjacent to a X-bond and chlorine separate from a a-bond. It is also possible t o distinguish between C-C x and C-C u orbitals as well as to distinguish between the isomers. It is clear that qualitative information of a kind not readily given by other single techniques can be obtained from the method. A preliminary survey of the elementary theory of the technique and its applicability to analyze in general will be given. The technique of electron spectroscopy has been described in the previous paper (1). Photoelectron spectroscopy (PES) is that branch of electron spectroscopy in which the ionizing process is brought about by vacuum ultraviolet radiation. It is thus concerned with the ejection of electrons from valence orbitals of molecules, in contrast t o ESCA which is principally conAPHOTOELECTRON

cerned with the electrons ejected from inner orbitals. I n practical terms, P E S differs from ESCA in that the source is different, samples are generally introduced as vapors, the interpretation of spectra is based upon molecular orbital theory, and the chemical information obtained from the spectra is different. These differences will be dealt with in the present paper. The points which are common to E S A and PES, wix., the basic idea and the collection and counting of ejected electrons have been dealt with by Hercules in the accompanying paper (1). Some of the parameters affecting ESCA as an analytical technique have also been discussed recently (2). Theory and Operation

Because many of the properties of a molecule are dictated by the energies and the spatial distributions of the orbiting electrons, data concerning these variables are of paramount importance in our attempts to rationalize the many facets of molecular formation and reactivity. During the past 30 years, a great wealth of information about molecular orbitals has accumulated from purely theoretical considerations, but, except in the simplest cases h40 theory cannot predict precisely the energies binding the electrons to the nucleus, nor can it always give with certainty the energetic ordering of the various orbitals. Although the energies of electrons in inner (K-shell) orbitals could be measured by X-ray methods, and the energy of the most

loosely bound electron measured by various spectroscopic techniques, the energies of several of the electrons in the valence shells of molecules were not obtainable with certainty. Because these electrons are most directly involved in molecular bonding and hence most influenced by substituents, measurement of their binding energies should provide invaluable information to theoretical chemists and a useful means of qualitative identification to the analyst. Photoelectron spectroscopy provides this information by measuring the experimental parameter most closely related to the energy of a molecular orbital-the ionization potential (1P)- of the electron which occupies it. The “first ionization potential” (IP1 or 11) of an atom or molecule is usually defined as the minimum energy needed t o eject a single electron from the highest filled orbital level. Similarly the “second ionization potential” is the energy needed to eject a single electron from the second highest filled orbital level of the neutral molecule. Second IP processes should not be confused with processes which result in the formation of doubly charged ions, and which are equivalent to the processes frequently tabulated in the standard textbooks of inorganic chemistry under the heading of “second ionization potentials.” A photoelectron spectrum is effectively a distribution curve of the energies of electrons emitted from a substance by the action of monochromatic radiation. The most energetic photoelectron which can be

ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970

0

43 A

Report for Analytical Chemists

POSSIBLE IONIC CONFIGURATIONS

MOLECULAR CONFIGURATION

tron released from the ith highest occupied orbital is given in terms of the impacting photon frequency, Vl by

ENERGY OF PHOTOELECTRON

E,

w y

/

/

/

X

/

can interact Figure 1. Three possible ways in which a photon of frequency, with a hypothetical molecule AB having three ionization potentials less than hv

TO PUMPS-

U

hv

I n

i l

PIit

‘

I

Photorlrctronr

Figure 2. Schematic illustration of a photoelectron spectrometer

released as a result of photoionization is one ejected from the highest energy orbital of the molecule under examination. Such a process simultaneously involves the formation of a molecular ion in its ground electronic state (2) :

M

+ hv + M + (2)+ e-

(1) Electrons released according t o Equation 1 are therefore responsible for the “highest electron energy band” in a photoelectron spectrum. This is usually referred t o as the “first band” (since it relates t o the first IP) and is the one a t the righthand end of the spectrum as nor44 A

mally presented. Bands a t lower “electron kinetic energy” (higher IP values) correspond t o ionization from deeper molecular orbitals and involve the formation of molecular ions in configurationally excited electronic states 13, 6, etc.). This is schematically illustrated in Figure 1, where a hypothetical molecule having only three orbitals from which ionization can occur on the impact of hv is considered. The molecular ion may of course fragment after the electron has been ejected, but this cannot affect the energy imparted t o the latter. The kinetic energy Ei, of a photoelec-

(A,


=

hv - I ,

(2) where h is Planck’s constant and Ii is the relevant ionization potential. I n photoelectron spectroscopy, hv is known, E , is found from the spectrum, and thus the orbital ionization potentials are obtainable directly. Measurements of this type using PES have had a far-reaching effect on theoretical chemistry and molecular physics [for review and bibliography see reference ( 3 )] since their introduction in 1962 and can be expected in the next few years to play an increasingly important role in other branches of chemistry, including analytical chemistry. One of the analytically interesting aspects of PES is t h a t some valence shell ionization potentials show a marked substituent dependence. This is particularly apparent in the cases of electrons ejected from orbitals which are essentially localized in particular atomic sites within a molecule. A simple illustration of this is that the IP of a “chlorine 3p lone pair” electron is different in CH&1 from that in CaH5C1 by 0.29 eV (1eV = 96.3 Kj oule mol-l) . Since their introduction, the design of photoelectron spectrometers has been greatly improved, p:incipally by Turner, who has reviewed the development ( 4 ) . I n all the instruments a beam of photons is directed into an ionization chamber containing the sample under examination, The kinetic energies of the ejected electrons are then sorted out by means of an energy analyzer. A schematic representation of the commercially available Perkin-Elmer/Turner spectrometer is shown in Figure 2. Ionizing Sources. The ideal source for photoelectron spectroscopy should generate monochromatic radiation of sufficient energy to eject electrons from deep atomic and molecular orbitals. There are two basic types: (1) a continuous source in which one required line is selected by a vacuum monochromator, (2) a lamp producing one of the resonance emission lines of the

TABLE 1. Resonance Emission Lines of Rare Gases and Hydrogen Suitable for Use in Photoelectron Spectroscopy GAS

A LINES

HYDROGEN HELIUM

1215 (Lyman a) 584 304 (He+) 736 and 744 1048 and 1067 1165 and 1236 1296 and 1470

NEON ARGON KRYPTON XENON inert gases. This latter system is generally preferred since it is more convenient to use and provides a more copious flux. The helium resonance lamp is the one most commonly used. Purified helium is subjected to a microwave or an electric discharge, whereupon, if the gas pressure is correctly adjusted (needle valve) a characteristic peach-colored plasma is generated, and virtually the only ionizing radiation t o be released has a wavelength of 584 A (energy 21.22 eV). Bleeding small controlled amounts of suitable gases into the helium stream has the effect of suppressing the 594 A emission and exciting other emission lines characteristic of the second gas. The emission lines obtainable in this way are summarized in Table I. As can be seen, it is only helium which under these conditions emits a singlet resonance emission line and as this is also more energetic than the other inert gas lines the helium lamp is doubly satisfactory. The possibility of developing a source producing a copious flux of monochromatic vacuum ultraviolet photons with energies rather higher than 21 eV has attracted considerable attention. No such source has yet been made, but nevertheless, modification of the discharge conditions in a conventional helium discharge lamp produces a small flux of 304 A radiation (40.8 eV) in addition t o the usual 584 A radiation ( 5 - 7 ) . Photoelectron spectra obtained with such a source therefore contain bands due t o ionizing processes incurred by the interaction of molecules with both 584 A and 304 A photons. The 304 A produced 46A

bands are much weaker than the 584 A bands, but even so Brundle ( 7 ) has obtained a sufficiently high count rate to detect vibrational structure in the methane C 2s band (IP 24 eV) . High standards of purity seem to be important to obtain intense monochromatic radiation from rare gas resonance lamps. The presence of minimal amounts of water vapor, nitrogen, and oxygen in the light source has the effect of cutting the intensity of the wanted rare gas resonance lines, and exciting, in addition, other lines which can cause confusion in the interpretation of spectra (8). To prevent self-absorption, light source gas must be prevented from mixing with ionization chamber gas. This is done by having two lengths of aligned capillary tubing separating the light source and target chamber sections. The majority of light source gas is pumped away when it emerges from the first capillary tube. Photons, of course, pass unimpeded down the second capillary tube into the ionization chamber (Figure 2 ) . Sample Requirements. The sample is introduced into the ionization chamber in the vapor phase. The pressure needed to obtain a good spectrum varies from compound to compound, but is normally in the range 0.02 mm H g to 0.1 mm Hg. Too high a sample pressure is t o be aroided as electron-molecule collisions can broaden spectral peaks, and furthermore, the electron multiplier used in the detector system cannot operate and can be seriously damaged if the pressure around it rises appreciably above 1 x lk4 mm Hg, which


would happen if the pressure in the target chamber exceeded about 0.1 mm Hg. Energy Analysis and Detection of Photoelectrons. I n the first commercially available instrument the ejected electrons traverse an arc of 127' 17' under the influence of an electrostatic field. The entrance slit of the electrostatic analyzer is so positioned t h a t only electrons released along the electric vector of the photon beam (ie., a t 90" to the direction of photon propagation) or along paths not differing significantly from this are collected. Once inside the analyzer, photoelectrons describe different paths depending upon their kinetic energies and the voltage applied to the analyzer plates. During the course of a spectrum, the voltage applied t o the selector plates is swept so that progressively more and inore energetic electrons are brought to a focus in turn on the exit slit, through which they emerge and fall on the first dynode of an electron multiplier placed behind the slit. Relationships of Peak Energies and Shapes with Orbital Character. Koopmans' theorem (9) equates the ionization potential with the negative of the HartreeFock orbital energy and this implies that the photoelectron spectrum of a molecule is its molecular orbital energy level diagram laid on edge. This is a reasonable firstorder approximation, but it can have serious limitations ( 1 0 ) . Nevertheless, present 340 theory is of great help in the interpretation of photoelectron spectra (3) and it is certain that values from the latter will lead to considerable refinements of the theory in the next few years. All molecular orbitals can in principle be revealed by ESCA, but PES is concerned a t present only with orbitals for which electron binding energies are less than 21 eV. I n practical terms this means that the spectral range of PES,ca. 15 eV, is not much greater than just one of the broader bands in an X-ray excited spectrum. However, the improvement in resolution, ca. 0.015 eV compared with 1-2 eV means that vibrational fine struc-

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47A

ture is commonly observed and t h a t the spectrum is appreciably effected by the subtle changes in the electron distribution in a molecule which occurs as different substituents are introduced or removed. Thus the nature of the spectral bands encountered must be considered. Ionization can produce a molecular ion in excited vibrational and rotational states, E , and E , respectively. The total energy E of the ejected electron will then be

E

=

hv - I - E , - E,

E , and E , are much smaller than I, but in a high resolution spectrum, vibrational fine structure can be observed and helps t o identify the nature of the orbital from which the electron has been ejected. Thus, Figure 3 shows typical band shapes arising from the ejections of (1) a nonbonding, (2) a moderately strongly bonding or antibonding, and (3) a strongly bonding or antibonding electron. The observed shapes can be deduced, for diatomic molecules, from consideration of ( a ) the Frank-Condon principle, that electronic transitions occur so rapidly that the ground state mole-

cule and molecular ion formed on ionization have the same internuclear distance, re, and ( b ) the BornOppenheimer equation, which with the Frank-Condon principle indicates the most likely transition from a ground state molecule to the molecular ion. The vibrational energies of the molecular ion are given by

E , = (v’

+ i)hw

- (VI-+

i)

hxw

where v’ is the vibrational quantum number in the ionic state, h is Planck’s constant, x is the anharmonicity constant, and w the vibrational frequency. All of the possible transitions have a n associated ionization potential, but only two are usually referred to: (1) the adiabatic ionization potential which corresponds to the transition from the ground state of the molecule to the ground state of the ion, v’ = 0 and (2) the vertical ionization potential associated with the transition in which the internuclear distance of the ion is t h a t of the parent molecule, v’ 3 0. The molecule is not greatly affected by the removal of a nonbonding or very weakly bonding electron and so the equilibrium internuclear separation

II.

19.

INCREASING BINDING ENERGY Figure 3. text) 48 A

Some typical band shapes occurring in photoelectron spectra (see


in the molecular ion is almost identical with that in the molecule with the result that the adiabatic and vertical ionization potentials are coincident. For such a case the spectrum shows one sharp peak with perhaps one or two smaller peaks a t lower energies corresponding to less probable transitions to vl’, v2’, and so forth (Type I, Figure 3 ) . When an electron is removed from a bonding or antibonding orbital the internuclear distance of the molecular ion is greater or less, respectively, than that in the parent molecule. The vertical ionization energy is greater than the adiabatic one and corresponds to the transition to v,. The peak heights within a band are proportional to the probabilities of transitions (Franck-Condon factors) and hence the energy of the most intense peak corresponds t o the vertical ionization potential. There will be other peaks in the band with intensities determined by the vibrational overlap integrals (Types I1 and 111). There will also be changes in the force constant, k, which is related t o the frequency, 0 1 by where p is the reduced mass. The vibrational frequency, W , of the ion can be measured from the energy differences, AE,, between peaks in the photoelectron spectral band and can be compared with the corresponding vibrational frequency of the parent molecule. A shift to lower frequency on ionization (increase in r,) indicates t h a t the electron was ionized from a bonding orbital, and conversely a shift to higher frequency indicates ionization from an antibonding orbital. Ionization from very strongly bonding or antibonding orbitals may be characterized by broad bands showing no fine structure (Type IV, Figure 3 ) . Lack of fine structure in a band can also indicate that: ( a ) vibrational spacings are so small that higher resolution is needed to observe individual levels, ( b ) there is no “well” in the potential energy surface of the ionic state so that ionizing transitions are into a continuum, or ( c )

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TABLE II.

15937 15759 IONIZATION POTENTIAL, eV. Figure 4. High resolution photoelectron spectrum of argon excited by the impact of the 5 8 4 4 radiation

dissociation or predissociation of the ions is taking place and leading to broadening of vibrational peaks t o such an extent t h a t individual levels are indistinguishable. Merging of fine structures into a continuum (Type V, Figure 3 ) , may be caused by dissociation or by a crossing of potential energy curves, one of which is associated with a repulsive state (3, IO). Excitation of more than one mode of vibration in a given ionization process can lead to bands (Type VI, Figure 3) containing progressions in the frequencies of all the vibrations excited, and also combination bands are possible (10). Further refinements in identification of orbital levels arise from the observation of splitting of apparently degenerate levels by the Jahn-Teller effect ( 3 , 11). An example of the application of these observations t o the interpretation of a spectrum is provided below. Calibration of Photoelectron Spectra. The 584 A photoelectron spectra of the inert gases argon, krypton, and xenon are especiaIIy simple because in each case there is only one orbital from which ionization can take place. The spectra do not consist of single lines, however, since the removal of an electron can leave an ion in a state with a total angular momentum of either 3/2 or l / 2 (J = 1 f l/z), and these two ionic states h a r e different energies. A high-resolution spectrum of argon is shown in Figure 4. The sharpness of the peaks in the spectra of all the inert 50 A

Ionization Potentials of Rare Gases, A, Kr, and Xe

GAS

IP (eV) (Spin-orbit components)

A Kr Xe

15.759,15.937 14.000,14.665 12.130,13.436

gases enables the deflecting voltages corresponding t o the appearance of these peaks to be accurately measured (digital voltmeter) and thus a n ionization energy scale (linear) can be set up since the ionization energies of the inert gases are already well established and are given in Table 11. Other useful standards

are the atmospheric gases nitrogen and oxygen (both give spectra containing a large number of sharp peaks). Methyl iodide is useful for compounds with low ionization energies since the first band in its spectrum is a well-defined spin-orbit doublet with components a t 9.55 eV and 10.16 eV. All these %an-

.

I I

I

I

I

I

I

I

15

1

I

IO

I I I 1 1 1 1 1 1 1

15

15

IO

IO

15

20

?7 n

1

1

1

15

1

1

1

1

1

1

15

IO

IO

I NOlZATlON POTENTIAL

(ev)

Figure 5. Photoelectron bands arising owing to the ionization of electrons occupying "nonbonding" or weakly bonding orbitals (see ref. 8)


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and, to a slightly lesser extent, “nitrogen lone pairs” (photoelectron spectroscopy reveals t h a t frequently nitrogen “lone pairs” are partially delocalized throughout the u framework of molecules-e.g., for anilines and diazines, in accord with theoretical predictions-see Figure 5.) Phosphorus and sulfur compounds have not been extensively studied to date by photoelectron spectroscopy, but later work in this laboratory along these lines is planned. The present program of work is directed toward assessing the use of photoelectron methods in various analytical problems, especially in 18 17 16 15 14 13 12 II 10 ev the first instance concerning agriIONIZATION POTNTL culturally important chemicals. Figure 6. The photoelectron spectra T o conclude this introductory of cis- and trans-l,3-dichloropropene paper, we now report preliminary excited by the impact of the 5 8 4 4 raresults (using 584 A radiation) for diation (see text) two isomeric compounds which fall in this category-viz., cis- and dards” are best used when introtrans-l,3-dichloropropene,and conduced concurrently with the sample sider the applicability of the techbeing examined. This is sometimes nique to analytical chemistry in difficult, however, and then the targeneral. get compound and the calibrations compound must be examined indePhotoelectron Spectra of cis- and trans-1,3-Dichloropropene pendently, the deflecting voltages The spectra of these two molecorresponding t o spectral features cules are shown in Figure 6. I n in each case being noted. The disboth cases the spectra were obtained advantages of this method are by setting the analyzer voltage to small errors owing to field-shifts a constant value, and sweeping and surface potential charges are through a range of voltages apliable t o occur. I n the commerplied to a repeller electrode and cially available Perkin-Elmer/ the rear of the ionization chamber. Turner instrument, the analyzer (The spectra were obtained on Dr. potential difference measurer reads D. W. Turner’s apparatus a t Oxthe approximate IP (eV) directly, ford University.) Peaks due to but for accurate measurements, this low-energy photoelectrons (high has to be calibrated. IP orbitals) show up more clearly “Chemical Shifts” of Weakly using this system ( 1 2 ) . The spectra Bonded Electrons. The applicaillustrate two points particularly bility of photoelectron spectroscopy relevant t o analytical studies: ( a ) t o analysis stems largely from the The effects of structural isomerism observations t h a t electrons ionized are reflected to some extent in the out of weakly bonding orbitals give binding energies of the valence rise t o narrow spectral bands, and shell electrons and ( b ) The position that the positions which these and number of peaks corresponding bands occupy in spectra are quite to what might loosely be termed dependent on the chemical environ“chlorine 3p lone pair electrons” ment of the element around which indicate the presence of chlorine the &IO concerned is partially atoms in nonequivalent positions. localized. The sort of weakly These points are further disbonding orbitals accessible t o ioncussed in the following brief analyization by 584 A photons are the sis of the spectra, where the promi4 [ 2 p ” and “3p”, “4p”, and “5p” nent features are related to peaks orbitals of fluoro, chloro, bromo, and bands in the spectra of strucand iodo compounds, “oxygen lone turally related materials. pairs,” certain types of r-orbitals 52 A


Band Centered at 10 eV. B y analogy with ethylene, this band relates to electron ejection from the ethylenic C-C T orbital, I n ethylene itself, this orbital, the highest energy one (lowest IP) is represented in the photoelectron spectrum by a sharply structured band with its adiabatic component a t IP 10.51 eV. The successive results of stabilization of this orbital owing to electron releasing effects from substituent groupings is reflected in the r ionization potentials of vinyl chloride and the 1,3-dichloropropenes. The r-orbital photoelectron band has its adiabatic component a t 10.0 eV in vinyl chloride, and a t about 9.8 eV in the 1,3-dichloropropenes. Band Centered at -11.2 eV. This band must be identified with electron ejection from a predominantly nonbonding orbital largely localized around the chlorine atom of the -CH2C1 group. ( I n methyl chloride, the chlorine “lone pair” are the most easily ionizable electrons, the adiabatic IP being 11.28 eV.) Bands at -11.8 and 13.1 eV. These two bands we also believe relate to orbitals derived principally from “C1 3p” orbitals, but in this instance from the ones associated with the chlorine atom of the CHCl = C H group. The reason for there being two bands is explicable if we take into consideration the fact that one of the 3p orbitals of the chlorine substituent is coplanar with the ethylenic r orbital and one is not. One of the orbitals therefore overlaps more effectively with the T orbital than the other, and loses more of its nonbonding character. This is clearly the 13.1-eV IP orbital since the photoelectron band a t this IP is much broader than the 11.8-eV IP band. Remaining Bands. The other bands in the spectra are associated with the C-H, C-C, and C-Cl u orbitals, and are all broad, as is to be expected from the bonding nature of these orbitals. The 14-19 eV region of the two spectra is significantly different for the two isomers and thus might be termed a “finger-print region,” as it could serve to distinguish the isomers.

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TWO SHORT COURSES AT 159th ACS NATIONAL MEETING IN HOUSTON Meeting dates Feb. 22-27, 1970 Polymer Chemistry-Feb. 20-22. Presented by Dr. John K. Stille and Dr. Michael E. Freeburner: . fee $80. This new course provides an introduction t o polymer chemistry f o r organic chemists. Major emphasis i s on methods o f preparation, mechanisms, and kinetics. Detailed consideration i s given t o conformations and chain dimension, as well as configurations, tacticity, and order. Other topics such as structure-property relationships are given survey coverage. While t h e course i s pri. marily theoretical, techniques and applied aspects are included. A knowledge o f basic organic and physical chemistry i s sufficient background.

Intermediate NMR Spectroscopy-Feb. 26-27. Presented by Dr. Joseph B. Lambert and Dr. Gerhard Binsch: fee $75; required text, F. A. Bovey, “Nuclear Magnetic Resonance Spectroscopy~”Academic Press, 1969r $16.50. This

is designed for chemists who have already had practical experience in N M R , or training approximately equivalent to t h e ACS Short Course of LeRoy Johnson and Dr. Roy Bible. The object of the present course is to introduce t h e industrial, government, or academic chemist t o t h e current methods, techniques, a n d theories of NMR that may be used in solving strut. tural and kinetic problems i n chemistry. No particular academic degree i s a prerequisite. The emphasis w i l l be o n fundamentals and applications. Computer programs in current practice will be used i n simulation d u r i n g t h e course. Lectures will be supplemented with several Droblem.discussion sessions.

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Summary. The spectra illustrate the possibility of picking out atoms in different chemical environments, and of distinguishing between compounds having a close structural resemblance. The 1,3dichloropropenes, of course, are relatively simple molecules in comparison to most pesticides, and as may be expected, the spectra of pesticides of higher molecular weight give corespondingly more complex spectra. Many overlapping bands may be present, and exact analysis of such spectra is clearly not possible a t the present time. Nevertheless, just as one can obtain a lot of useful information from an infrared spectrum without performing a full normal coordinate analysis, we believe that a photoelectron spectrum will be of analytical value without the backing of a rigorous M O treatment. It seems probable that the presence of elements and molecular groupings can be detected. However, the identification of the atoms present in a molecule is not so certain as with ESCA. This is because valence shell electrons are frequently highly delocalized throughout the molecular framework. Thus the sulfur 3p electrons in thiophenes are delocalized and a r e p a r t of ;the aromatic system.

This is very apparent from the photoelectron spectra of thiophenes, which lack any sharp bands assignable to a sulfur lone pair. B u t if the 40 eV photons of the He 304 k discharge proves to be a viable source, it seems likely that i t would serve to excite electrons sufficiently localized to enable elemental identification. This would represent a good compromise between ESCA and PES by combining the advantages of identification of the former with the high resolution of the latter. We intend to examine the spectra of a variety of pesticides, herbicides, and model compounds to ascertain whether it will be possible to interpret spectra in a manner comparable with procedures used in infrared and nmr spectroscopy. General Comments Regarding Analytical Potential of PES

Quantitative. When X-rays are used as the exciting source, the relative peak intensities in the spectrum have often been found to be directly proportional to the relative number of atoms in the molecule, so that an elemental analysis can be obtained directly from the spectrum ( 1 ) . This ideal situation does not exist when photons are used for excitation. Peak intensi-

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Figure 7. Photoelectron spectrum of a nitrogen-carbon dioxide mixture ( 5 2 % Nz, 48% C O J 54A


ties give some information about the relative numbers of atoms present, but so many factors determine the intensity that elemental analysis is impossible. It may be possible t o carry out quantitative determinations by making use of calibration curves. We are a t present inyestigating this possibility by performing quantitative analyses of gas mixtures. Preliminary results show that i t is possible to determine carbon dioxide-nitrogen mixtures containing 1 4 0 % of the minor constituent with reasonable accuracy and precision. It should be possible quantitatively to resolve and determine mixed gases since their spectra are well-defined and contain intense bands (Figures 7 and 8). Spectra of organic molecules are much more complex, and although spectrum of a mixture appears to be the sum of the spectra of its components, it would seem best to carry out prior separation by TLC or GLC. The problems involved in a prior separation by GLC are very comparable with those involved in running a mass spectrum in conjunction with GLC, and as these problems are being solved it is reasonable to expect that a photoelectron spectrometer could be profitably hooked up with a GLC unit. Qualitative. It is clear from the spectra already obtained that the photoelectron spectrum is a valuahle “fingerprint” of a molecule. More importantly, as we have tried to show above, it gives information which is different and complimentary to that obtained by other techniques. When more experience in interpreting spectra is gained, the technique should prove invaluable in identifying products of reactions, indicating the extent and nature of substitution or coordination, and in predicting centers of reactivity within a molecule. Sensitivity and Sampling Problems. Because the work reported so f a r has been completely nonanalytical and has been confined, in the main, to volatile substances, it is difficult to assess the limits of sensitivity. At present it is reasonable to compare the sensitivity with

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ANAL.YTICAL CHEMIS7-RY, VOL. 4.2, NO. 1, JANUARY 1970

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1st. bond

D. Betteridge graduated from the University of Birmingham with a B.Sc. in 1957 and a Ph.D. in 1960 after studying under R. Belcher and T. S.West. He w&s then successively research fellow a t the University of Virginia (1960-61) with John H. Yoe, the University of Arizona (1961-62) with Henry Freiser, and the Atomic Energy Research Establishment, hment, Harwell Ha1 (1962-63) with A. A. Smales. Sm Since 1963 he has,s been a lect lecturer c . n a t the University College of* Swansea (Wales). ~~

Figure f 1. Photoelectron spectrum of air

infrared and nmr speictroscopy. Good spectra are obtainecIon milligram amounts of vola,tile substances--e.g., benzene, an(i even the spectrum of ferrocene has1 been obtamed (4). It should p rove possible to modify the inst1,ument to take involatile samples aiod we are a t present examining tllis possibility. The developmentsI over the last decade in GLC and nlass spectrometry suggest that a considerable improvement in sensi tivity and range of sample materia Is will be effected. Indeed, Dr. B rundle of Bell Telephone Co. hacI already built an apparatus which is a t least 20 times as sensitive as 1;he model described above (7). However, because the operating PIressure is about ten times greater than that used in a mass spectrom eter, i t is unlikely that the techniqil e will be as sensitive as mass specItrometry. It should also be relati. rely easy to improve the sensitivii;y a t the expense of resolution. Conclusion

It has been argued that on theoretical and practical grounds photoelectron spectroscopy has considerable analytical potential. At the very least it should prove a valuable complimentary technique for identification and a t hest it

Acknowledgment

We thank Shell Agricultural Research (Sittingbourne) for the provision of samples of cis- and trans1,3-diehloropropene and Dr. D. W. Turner for allowing us to use facilities in his laboratory a t Oxford University. We also also indebted to the Agricultural Research Council for providing funds to purchase a photoelectron spectrometer, and for the provision of a Research Fellowship (to A. D. B.).

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References (1) D. M.Hercules, Anal. Chem., 4a (l), 206 (1970). (2) L. I. Yin, I. Adler, and R. Lamothe, Appl. Speckosc. 23,41 (1969). (3) A. D. Baker. Accts. Chem. Res. (in

y."".,,. (4) D. W. Turner, Physical Methods in Advanced Inorganlo Chemktrti, 1968, p 74; Advances wz Mass Spectrometry, 1968, p 755. (5) I. Omum and E. Doi, Jap. J . Appl. Phya., 6,275 (1967).

W.C. Price, paper presented at the Royal Society Symposium on Photoelectron Spectroscopy, London, February 1969 (to be published). (7) C. R. Brundle, private communication. (8) A. D. Baker, C. R. Brundle, and D. W.Turner, Intel. J . Mass Spectrosc. (6)

& Ion Phys., 1,443 (1968). (9) T.Koopmans, Physics 1,104 (1934). (10) D. W. Turner, A. D. Baker, C. Baker, and C. R. Brundle, "High Resolution Molecular Photoelectron Spectroscopy," Wiley & Sons Ltd., in prepa-

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(11) C. Baker and D. W. Turner, Chem. Commun. 6 460 (1969). ( 1 1 ) T u ir ' W I ~ ~ , I r T nanhv I

A. D. Baker is a graduate of Imperial College, London, having ohtained the B.Sc. degree in 1965. For his Ph.D. he worked on photoelectron spectroscopy under the supervision of one of the pioneers in this field, Dr. D. W. Turner, first a t Imperial College and then a t Oxford University. H e is now a re. . ..

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Analytical potential of photoelectron spectroscopy - American

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