ANALYTICAL POTENTIAL OF PHOTOELECTRON SPECTROSCOPY

Report for Analytical Chemists

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

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spectrometer measures binding energies of electrons in a molecule by deter mining 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 irans-1,3dichloropropene, it is possible to as sign peaks due to the ionization of chlorine "3p lone pair" electrons and to subdivide these into chlorine adjacent to a 7r-bond and chlorine separate from a π-bond. It is also possible to distinguish between C—C 7Γ and C—C σ orbitals as well as to distinguish between the iso mers. 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 elemen tary theory of the technique and its applicability to analyze in general will be given. The technique of electron spec troscopy 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. I t is thus concerned with the ejec tion of electrons from valence or bitals of molecules, in contrast to ESCA which is principally con PHOTOELECTRON

cerned with the electrons ejected from inner orbitals. In practical terms, PES differs from ESCA in that the source is different, samples are generally introduced as vapors, the interpretation of spectra is based upon molecular orbital the ory, and the chemical information obtained from the spectra is differ ent. These differences will be dealt with in the present paper. The points which are common to ESCA and PES, viz., the basic idea and the collection and counting of ejected electrons have been dealt with by Hercules in the accom panying 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 distribu tions of the orbiting electrons, data concerning these variables are of paramount importance in our at tempts to rationalize the many facets of molecular formation and reactivity. During the past 30 years, a great wealth of information about molecular orbitals has ac cumulated from purely theoretical considerations, but, except in the simplest cases MO theory cannot predict precisely the energies bind ing the electrons to the nucleus, nor can it always give with certainty the energetic ordering of the various or bitals. Although the energies of electrons in inner (iC-shell) orbitals could be measured by X-ray meth ods, and the energy of the most

loosely bound electron measured by various spectroscopic techniques, the energies of several of the elec trons in the valence shells of mole cules were not obtainable with cer tainty. Because these electrons are most directly involved in molecu lar bonding and hence most influ enced by substituents, measurement of their binding energies should pro vide invaluable information to the oretical chemists and a useful means of qualitative identification to the analyst. Photoelectron spectros copy provides this information by measuring the experimental param eter most closely related to the en ergy of a molecular orbital—the ionization potential (IP)— of the electron which occupies it. The "first ionization potential" (ΙΡΧ or Ii) of an atom or molecule is usu ally defined as the minimum en ergy needed to eject a single elec tron from the highest filled orbital level. Similarly the "second ioniza tion potential" is the energy needed to eject a single electron from the second highest filled orbital level of the neutral molecule. Second I P 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 ef fectively a distribution curve of the energies of electrons emitted from a substance by the action of mono chromatic radiation. The most en ergetic photoelectron which can be

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

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43 A


tron released from the ith highest occupied orbital is given in terms of the impacting photon frequency, v, by Ei = hv - Ii

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

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

mally presented. Bands a t lower "electron kinetic energy" (higher I P values) correspond to ionization from deeper molecular orbitals and involve the formation of molecular ions in configurationally excited

electronic state (X) :

electronic states (A, B , C, 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. T h e molecular ion m a y of course fragment after the electron has been ejected, but this cannot affect the energy imparted to the latter. T h e kinetic energy Eit of a photoelec-

+

M + hv^M

(X) + e-

(1)

Electrons released according to Equation 1 are therefore responsible for the "highest electron energy b a n d " in a photoelectron spectrum. This is usually referred to as the "first b a n d " (since it relates to the first I P ) and is the one a t t h e righthand end of t h e spectrum as nor44 A .


(2)

where h is Planck's constant and J4 is the relevant ionization potential. I n photoelectron spectroscopy, hv is known, Et is found from the spectrum, and thus the orbital ionization potentials are obtainable directly. Measurements of this t y p e using P E S 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 import a n t role in other branches of chemistry, including analytical chemistry. One of the analytically interesting aspects of P E S is t h a t some v a lence 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 t h a t the I P of a "chlorine 3p lone p a i r " electron is different in CH 3 C1 from t h a t in C 2 H 5 C1 b y 0.29 eV (1 eV = 96.3 Kjoule mol" 1 ). Since their introduction, the design of photoelectron spectrometers has been greatly improved, principally by Turner, who has reviewed the development (4). In all the instruments a beam of photons is directed into an ionization chamber containing the sample under examination. T h e kinetic energies of the ejected electrons are then sorted out by means of -an energy analyzer. A schematic representation of the commercially available P c r k i n - E l m e r / T u r n e r 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 t y p e s : (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 I. Resonance Emission Lines of Rare Gases and Hydrogen Suitable for Use in Photoelectron Spectroscopy À LINES

GAS

584

NEON ARGON KRYPTON XENON

304 (He+) 736 and 744 1048 and 1067 1165 and 1236 1296 and 1470

inert gases. This latter system is generally preferred since it is more convenient to use and provides a more copious flux. T h e helium resonance lamp is t h e one most commonly used. P u r ified helium is subjected to a microwave or an electric discharge, whereupon, if t h e gas pressure is correctly adjusted (needle valve) a characteristic peach-colored plasma is generated, and virtually the only ionizing radiation t o be r e leased has a wavelength of 584 Â (energy 21.22 e V ) . Bleeding small controlled amounts of suitable gases into the helium stream has the effect of suppressing the 594 Â emission and exciting other emission lines characteristic of the second gas. The emission lines obtainable in this w a y are summarized in T a ble 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 t h a n the other inert gas lines the helium lamp is doubly satisfactory. T h e possibility of developing a source producing a copious flux of monochromatic v a c u u m ultraviolet photons with energies rather higher t h a n 21 eV has a t t r a c t e d considerable attention. N o such source has yet been made, b u t nevertheless, modification of t h e discharge conditions in a conventional helium discharge lamp produces a small flux of 304 Â radiation (40.8 eV) in addition to the usual 584 Â radiation (5~7). Photoelectron spectra o b tained with such a source therefore contain bands due to ionizing processes incurred by the interaction of molecules with both 584 Â and 304 Â photons. T h e 304 Â produced 46 A ·

1215 (Lyman a)

HYDROGEN HELIUM

bands are much weaker t h a n the 584 À bands, but even so Brundle (7) has obtained a sufficiently high count rate to detect vibrational structure in the methane C 2s band ( I P 24 e V ) . High standards of purity seem to be i m p o r t a n t to obtain intense monochromatic radiation from rare gas resonance lamps. T h e 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 a n d target chamber sections. T h e 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 t u b e into the ionization chamber (Figure 2 ) . Sample Requirements. The sample is introduced into the ionization chamber in the vapor phase. T h e pressure needed to obtain a good spectrum varies from compound to compound, but is normally in the range 0.02 m m H g to 0.1 m m H g . Too high a sample pressure is to be avoided 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 Χ 10 - 4 m m H g , which


would happen if the pressure in the target chamber exceeded about 0.1 m m H g . Energy Analysis a n d 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. T h e en trance slit of the electrostatic a n a lyzer is so positioned t h a t only elec trons released along the electric vector of the photon beam (i.e., at 90° to the direction of photon propagation) or along p a t h s not differing significantly from this are collected. Once inside the analy zer, photoelectrons describe dif ferent p a t h s depending upon their kinetic energies and the voltage applied to the analyzer plates. During the course of a spectrum, the voltage applied to the selector plates is swept so t h a t progres sively more and more energetic electrons are brought to a focus in t u r n on the exit slit, through which t h e y emerge and fall on t h e first dynode of an electron multi plier placed behind the slit. Relationships of P e a k Ener gies a n d Shapes with Orbital Character. Koopmans' theorem (9) equates the ionization potential with the negative of the H a r t r e e Fock orbital energy and this im plies t h a t the photoelectron spec t r u m 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 (10). Nevertheless, present M O theory is of great help in the interpretation of photoelectron spectra (3) and it is certain t h a t values from t h e latter will lead to considerable re finements of the theory in the next few years. All molecular orbitals can in principle be revealed by E S C A, but P E S is concerned a t present only with orbitals for which electron binding energies are less t h a n 21 eV. I n practical terms this means t h a t the spectral range of P E S , ca. 15 eV, is not much greater t h a n just one of the broader bands in an X - r a y excited spectrum. However, the improvement in resolution, ca. 0.015 eV compared with 1-2 eV means t h a t vibrational fine struc-


ture is commonly observed and that the spectrum is appreciably effected by the subtle changes in the elec tron distribution in a molecule which occurs as different substituents are introduced or removed. Thus the nature of the spectral bands encountered must be con sidered. Ionization can produce a molec ular ion in excited vibrational and rotational states, Ev and Er re spectively. The total energy Ε of the ejected electron will then be Ε = hv ~ I - Ev - Er Ev and Er are much smaller than I, but in a high resolution spec trum, vibrational fine structure can be observed and helps to 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 de duced, for diatomic molecules, from consideration of (a) the Frank-Condon principle, that electronic transitions occur so rapidly that the ground state mole

Figure 3 . text) 48 A ·

cule and molecular ion formed on ionization have the same internuclear distance, re, and (6) the BornOppenheimer equation, which with the Frank-Condon principle indi cates the most likely transition from a ground state molecule to the molecular ion. The vibrational energies of the molecular ion are given by Ev = tv' + ^\hœ - (v\+

j h hxo>

where v' is the vibrational quantum number in the ionic state, h is Planck's constant, χ is the anharmonicity constant, and ω the vibra tional frequency. All of the pos sible transitions have an 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, 1/ = 0 and (2) the vertical ionization potential associated with the transi tion in which the internuclear dis tance of the ion is that of the parent molecule, v' J> 0. The mole cule is not greatly affected by the removal of a nonbonding or very weakly bonding electron and so the equilibrium internuclear separation

Some typical band shapes occurring in photoelectron spectra (see


in the molecular ion is almost iden tical 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 at lower energies correspond ing to less probable transitions to Vi, v2', and so forth (Type I, Fig ure 3). When an electron is re moved 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 cor responds to the transition to v„. The peak heights within a band are proportional to the probabilities of transitions (Franck-Condon fac tors) and hence the energy of the most intense peak corresponds to the vertical ionization potential. There will be other peaks in the band with intensities determined by the vibrational overlap integrals (Types II and I I I ) . There will also be changes in the force con stant, k, which is related to the fre quency, ω, by

where μ is the reduced mass. The vibrational frequency, ω, of the ion can be measured from the energy differences, AEV! between peaks in the photoelectron spectral band and can be compared with the corresponding vibrational fre quency of the parent molecule. A shift to lower frequency on ioniza tion (increase in re) indicates that the electron was ionized from a bonding orbital, and conversely a shift to higher frequency indicates ionization from an antibonding or bital. 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 indi cate that: (a) vibrational spacings are so small that higher resolu tion is needed to observe individual levels, (6) there is no "well" in the potential energy surface of the ionic state so that ionizing transi tions are into a continuum, or (c)


TABLE II. GAS A Kr Xe

Figure 4 . High resolution photoelectron spectrum of argon excited by the impact of the 584-A radiation

dissociation or predissociation of the ions is taking place and leading to broadening of vibrational peaks to such an extent that 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, 10). 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 to the interpretation of a spectrum is provided below. Calibration of Photoelectron Spectra. The 584 Â photoelectron spectra of the inert gases argon, krypton, and xenon are especially 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 y 2 (J = 1 ± V ? ) , and these two ionic states have 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 IP (eV) (Spin-orbit components) 15.759, 15.937 14.000, 14.665 12.130, 13.436

gases enables the deflecting voltages corresponding to the appearance of these peaks to be accurately measured (digital voltmeter) and thus an 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 II. 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 at 9.55 eV and 10.16 eV. All these "stan-

Figure 5. Photoelectron bands arising owing to the ionization of electrons occupying " n o n b o n d i n g " or weakly bonding orbitals (see ref. 8)



Figure 6. The photoelectron spectra of cis- and trans-l,3-dichloropropene excited by the impact of the 584-A radiation (see text) dards" are best used when introduced concurrently with the sample being examined. This is sometimes difficult, however, and then the target compound and the calibrations compound must be examined independently, the deflecting voltages corresponding to spectral features in each case being noted. The disadvantages of this method are small errors owing to field-shifts and surface potential charges are liable to occur. In the commercially available Perkin-Elmer/ Turner instrument, the analyzer potential difference measurer reads the approximate I P (eV) directly, but for accurate measurements, this has to be calibrated. "Chemical Shifts" of Weakly Bonded Electrons. The applicability of photoelectron spectroscopy to analysis stems largely from the observations that electrons ionized out of weakly bonding orbitals give rise to narrow spectral bands, and that the positions which these bands occupy in spectra are quite dependent on the chemical environment of the element around which the MO concerned is partially localized. The sort of weakly bonding orbitals accessible to ionization by 584 Â photons are the "2p" and "3p", "4p", and "5p" orbitals of fluoro, chloro, bromo, and iodo compounds, "oxygen lone pairs," certain types of 7r-orbitals 52 A ·

and, to a slightly lesser extent, "nitrogen lone p a i r s " (photoelectron spectroscopy reveals t h a t frequently nitrogen "lone p a i r s " are partially delocalized throughout t h e σ framework of molecules—e.g., for anilines and diazines, in accord with theoretical predictions—see Figure 5.) Phosphorus and sulfur compounds have not been exten sively studied to date by photo electron spectroscopy, b u t later work in this laboratory along these lines is planned. T h e present program of work is directed toward assessing t h e use of photoelectron methods in various analytical problems, especially in the first instance concerning agri culturally important chemicals. To conclude this introductory paper, we now report preliminary results (using 584 Â radiation) for two isomeric compounds which fall in this category—viz., cis- and trans- 1,3-dichloropropene, and consider the applicability of t h e technique to analytical chemistry in general. Photoelectron Spectra of cis- and trans-1,3-Dichloropropene

T h e spectra of these two molecules are shown in Figure 6. I n both cases the spectra were obtained by setting t h e analyzer voltage t o a constant value, and sweeping through a range of voltages a p plied to a repeller electrode a n d the rear of t h e ionization chamber. (The spectra were obtained on D r . D . W. Turner's a p p a r a t u s a t Oxford University.) Peaks due to low-energy photoelectrons (high I P orbitals) show up more clearly using this system (12). T h e spectra illustrate two points particularly relevant t o analytical studies: (a) T h e effects of structural isomerism are reflected to some extent in t h e binding energies of t h e valence shell electrons and (b) T h e position and number of peaks corresponding to what might loosely be termed "chlorine 3p lone pair electrons" indicate t h e presence of chlorine atoms in nonequivalent positions. These points are further discussed in t h e following brief analysis of t h e spectra, where t h e prominent features are related t o peaks and bands in t h e spectra of structurally related materials.


B a n d Centered at 1 0 eV. B y analogy with ethylene, this band relates to electron ejection from t h e ethylenic C—C -π orbital. I n ethylene itself, this orbital, t h e highest energy one (lowest I P ) is represented in t h e photoelectron spectrum by a sharply structured band with its adiabatic component a t I P 10.51 eV. T h e successive re sults of stabilization of this orbital owing to electron releasing effects from substituent groupings is re flected in t h e -n- ionization potentials of vinyl chloride and t h e 1,3-dichloropropenes. T h e π-orbital pho toelectron band h a s its adiabatic component a t 10.0 eV in vinyl chloride, and a t about 9.8 eV in t h e 1,3-dichloropropenes. B a n d Centered at ~ 1 1 . 2 eV. This band must be identified with electron ejection from a predomi n a n t l y nonbonding orbital largely localized around t h e chlorine atom of t h e —CH 2 C1 group. (In methyl chloride, t h e chlorine "lone p a i r " are the most easily ionizable electrons, the adiabatic I P being 11.28 eV.) Bands at ~ 1 1 . 8 a n d 1 3 . 1 e V . These two bands we also believe relate to orbitals derived principally from "CI 3 p " orbitals, but in this instance from the ones associated with t h e chlorine atom of t h e CHC1 = C H group. T h e reason for there being two bands is ex plicable if we t a k e into considera tion t h e fact t h a t one of t h e 3p orbitals of t h e chlorine substituent is coplanar with t h e ethylenic w orbital and one is not. One of t h e orbitals therefore overlaps more ef fectively with t h e 7Γ orbital t h a n t h e other, and loses more of its nonbonding character. This is clearly the 13.1-eV I P orbital since t h e photoelectron band a t this I P is much broader t h a n t h e 11.8-eV I P band. R e m a i n i n g Bands. T h e other bands in t h e spectra are associated with t h e C — H , C—C, and C—Cl σ orbitals, and are all broad, as is t o be expected from t h e bonding na ture of these orbitals. The 14-19 eV region of t h e t w o spectra is significantly different for the two isomers and thus might be termed a "finger-print region," as it could serve t o distinguish t h e iso-


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 at 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 MO 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 are part 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. But if the 40 eV photons of the He 304 Â discharge proves to be a viable source, it seems likely that it 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-

Figure 7. Photoelectron spectrum of a nitrogen-carbon dioxide mixture ( 5 2 % N2, 4 8 % C 0 2 ) 54 A ·


ties give some information about the relative numbers of atoms present, but so many factors determine the intensity that elemental analysis is impossible. I t may be possible to carry out quantitative determinations by making use of calibration curves. We are at present investigating this possibility by performing quantitative analyses of gas mixtures. Preliminary results show that it is possible to determine carbon dioxide-nitrogen mixtures containing 1-40% 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 CLC, 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 valuable "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 far 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


Figure 8. Photoelectron spectrum of air

infrared and nmr spectroscopy. Good spectra are obtained on milligram amounts of volatile substances—e.g., benzene, and even the spectrum of ferrocene has been obtained (4). I t should prove possible to modify the instrument to t a k e involatile samples and we are at present examining this possibility. The developments over the last decade in G L C and mass spectrometry suggest t h a t a considerable improvement in sensitivity and range of sample materials will be effected. Indeed, D r . Brundle of Bell Telephone Co. has already built an apparatus which is at least 20 times as sensitive as the model described above (7). However, because the operating pressure is about ten times greater t h a n t h a t used in a mass spectrometer, it is unlikely t h a t t h e technique will be as sensitive as mass spectrometry. I t should also be relatively easy to improve t h e sensitivity at the expense of resolution. Conclusion I t has been argued t h a t on t h e oretical and practical grounds photoelectron spectroscopy has considerable analytical potential. At t h e very least it should prove a valuable complimentary technique for identification and a t best it could be a quantitative tool. 56 A

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Acknowledgment

W e t h a n k Shell Agricultural R e search (Sittingbourne) for t h e provision of samples of cis- and trans1,3-dichloropropene and D r . D . W. Turner for allowing us to use facilities in his laboratory a t Oxford University. W e 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 . ) .

D. Betteridge graduated from t h e University of Birmingham with a B.Sc. in 1957 and a P h . D . in 1960 after studying under R. Belcher and T . S. West. H e was then suc cessively research fellow a t t h e Uni versity of Virginia (1960-61) with John H . Yoe, the University of Arizona (1961-62) with H e n r y Freiser, and t h e Atomic Energy R e search Establishment, Harwell (1962-63) with A. A. Smales. Since 1963 he has been a lecturer at the University College of Swan sea (Wales).

References

(1) D. M. Hercules, Anal. Chem., 42 (1), 20 A (1970). (2) L. I. Yin, I. Adler, and R. Lamothe, Appl. Spectrosc. 23, 41 (1969). (3) À. D . Baker, Accts. Chem. Res. (in press). (4) D. W. Turner, Physical Methods in Advanced Inorganic Chemistry, 1968, ρ 74; Advances in Mass Spectrometry, 1968, ρ 755. (5) I. Omura and H. Doi, Jap. J. Appl. Phys.,6,275 (1967). (6) W. C. Price, paper presented at the Royal Society Symposium on Photo electron Spectroscopy, London, Febru ary 1969 (to be published). (7) C R. Brundle, private communica tion. (8) A. D . Baker, C. R. Brundle, and D . W. Turner, Inter. J. Mass Spectrosc. & Ion Phys., 1, 443 (1968). (9) T. Koopmans, Physica, 1, 104 (1934). (10) D . W. Turner, A. D. Baker, C. Baker, and C. R. Brundle, "High Reso lution Molecular Photoelectron Spec troscopy," Wiley & Sons Ltd., in prepa ration. (11) C. Baker and D. W. Turner, Chem. Commun., 6, 460 (1969). (12) J. H. D . Eland and C. J. Danby, J. Sci. Instr., 1, 406 (1968).

ANALYTICAL CHEMISTRY, VOL. 4 2 , NO. 1 , JANUARY 1970

A. D. Baker is a graduate of I m perial College, London, having ob tained t h e B.Sc. degree in 1965. For his P h . D . he worked on photo electron spectroscopy under t h e su pervision of one of the pioneers in this field, D r . D . W . Turner, first a t Imperial College and then a t Ox ford University. H e is now a re search fellow at Swansea.

ANALYTICAL POTENTIAL OF PHOTOELECTRON SPECTROSCOPY

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