LITERATURE CITED
(3) Orion Research Inc.. Cambridge, Mass., Instruction Sheet for Double-
(1) A. F. Isbeil, Jr.. R . L. Pecsok, R . H. Davies, and J. H. Purnell, Anal. Chern., 45,2363 (1973). (2) Orion Research Inc.. Cambridge, Mass., Instruction Manual for Sulfide ion-Selective Electrode Model 94-16 (1974).
RECEIVEDfor review October 13, 1975. Accepted December 8,1975.
Junction Reference Electrode Model 90-02 (1974).
Atomic Subshell Cross Sections for Qualitative Analysis of Photoelectron Spectra James M. Delaney’ and J. Wayne Rabalais’ Depatiment of Chemistry, University of Houston, Houston, Texas 77004
T o use electron spectroscopy for qualitative analysis, it is important to consider both the energies and relative intensities of the bands in an electron spectrum. Considerable attention has been paid to photoelectron energies; Siegbahn et al. ( I ) have compiled a very useful table of the binding energies for the various subshells of the elements 1 to 104. As an aid to analytical chemists, we present the photoionization cross sections of the elements in graphic form. This graph serves two purposes. First, it provides a quick determination of which orbital of an element will have the largest photoionization cross section. Second, it can be used along with binding energies in confirming elemental analysis of an unknown. In performing a qualitative analysis of the bands in an electron spectrum, one naturally uses Siegbahn’s table to effect an identification of the various elements present in the sample. Often times, there are peaks which are energetically close together or peaks that could correspond to two or more elements. In these cases, we have found it particularly useful to use the relative cross sections in conjunction with the binding energies to obtain elemental assignments. Assignments must be consistent with regards to relative band intensities as well as binding energies. Our experiences indicate that it is usually sufficient to know only the approximate relative intensities to be expected for the bands in order t o make a qualitative analysis. We have found that a plot of the elemental subshell cross sections vs. atomic number is very useful for quick analysis of spectra. The purpose of this communication is to present this plot for use in analyzing electron spectra. The theoretical photoionization cross sections per electron for each subshell of the elements are presented in Figure 1. The cross sections were obtained from the compilation of Scofield ( 2 ) , who used relativistic Hartree-FockSlater wave functions to calculate the cross section for each atomic subshell a t excitation energies from 1.0 to 1500 keV. The plot in Figure 1 consists of the cross section per electron for the lowest energy (most intense) spin-orbital component a t an excitation energy of 1500 eV (approximating that of A1 K a a t 1486 eV). Actually, interpolation of Scofield’s values shows that the relative subshell cross sections a t 1486 or 1254 eV (Mg KD) are very close to those a t 1500 eV. The cross sections in the figure are in barns and each is divided by the number of electrons in that particular spin orbit component. We propose that the table in Ref. 2 be used as follows. After analyzing the energetic positions of peaks in an electron spectrum, one has a good idea of the elemental composition of the sample. In order to confirm this analysis, the approximate relative intensities of the peaks should be Present address: PPG Industries, Pittsburgh, Pa. 920
ANALYTICAL CHEMISTRY, VOL. 48, NO. 6, M A Y 1976
compared to the curves in Figure 1. Agreement between energies and intensities provides a cogent elemental analysis. Many of the cross sections vary by as much as four orders of magnitude, making it extremely difficult to observe photoelectron bands from some particular subshells of an element. The figure is also useful in attempting to detect minute quantities of an element in an unknown sample. For example, if one is looking for trace amounts of Eu in a sample, from Figure 1 it is obvious that the best bands to look for are those from the 3d orbitals. These are the ones of highest intensity. The 3p orbitals provide cross sections which are almost as high as those of the 3d orbitals and would also be useful; however, the 4s, 4p, 4d, 4f, and all other occupied orbitals have cross sections that are lower than those of the 3d orbitals by an order of magnitude; consequently, they would be more difficult to observe. Several notes of caution should be mentioned in using this figure. 1) The cross sections plotted are “per electron” and must be multiplied by the electron occupancies of the orbitals in their respective molecules and ions. The plot was made in this manner in order to allow one to estimate the intensities of valence bands, of elements and ions, which may have varied occupancies. For example, in Mo there are 6 electrons occupying the 4d and 5s valence bands, in Moo2 there are only 2 electrons in these bands, and in Moo3 the valence bands of Mo are completely empty. In using the figure for fully occupied core orbitals, one would simply multiply the intensity from the curve by 2 for s bands, by 4 for p3j2 bands, by 6 for d5j2 bands, and by 8 for f7j2 bands. 2) For any given spectrometer one may have to correct the experimental intensities for systematic errors such as electron kinetic energy dependence of the analyzer, photoelectron angular distributions, escape depths of photoelectrons, etc. Most instruments have transmission functions varying with E-l and the escape depth function is roughly E+0.7.Thus, with these instruments, the observed peak intensity relations do resemble cross sections. In contrast, instruments with transmission functions varying as E+’ exert a strong monotonic bias which distorts the observed peak intensities. In most instruments the angle between the photon source, sample, and electron acceptance slit is go’, while the intensities in the figure represent total cross sections, Le., cross sections for ejection of electrons in all directions. Our studies ( 3 ) have shown that the ratio of differential cross section to total cross section, u/utot, will vary by a t most a factor of 2 for the extreme cases of the angular distribution parameter p (i.e., -1 to +2) a t 90’. Therefore, angular distributions can produce some alteration of intensities, although these will remain constant for a given acceptance angle. 3) Scofield’s cross sections are only order of magnitude
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approximations for intensities of photoionization bands. Intensities of photoionization bands should be obtained by integrating the area of the band and all of its satellite structures. Since peak intensities and widths are a function of the lifetime of the hole-state, the chemical state of the element, contamination, phonon broadening, satellite structure, excitation energy, etc., these variables can also contiibute to uncertainties. 4) The use of ESCA for quantitative analysis and the handling of systematic errors has recently been discussed (4-9).
(2) (3) (4) (5)
(6) (7) (8) (9)
"ESCA, Atomic, Molecular, and Solid State Structure Studied by Means of Spectroscopy", Almqvist and Wiksells, Uppsala. Sweden, 1967. J. H. Scofield, Lawrence Livermore Laboratory Report UCRL-51326 (1973). J. T. Huang, J. W. Rabalais, and F. 0. Ellison, J. Electron. Spectrosc., 6, 85 (1975). W. J. Carter, G. K. Schweitzer, and T. A. Carlson, J. Electron. Spectrosc., 5, 827 (1974). V. I. Nefedov, N. P. Sergushin, I. M. Band, and M. B. Trzhaskovskaya, J. Electron. Spectrosc., 2, 383 (1973). C. A. Tolman. W. M. Riggs, W. J. Linn, C. M. King, and R. C. Wendt, Inorg. Chem., 12, 2770 (1973). C. D. Wagner, Anal. Chem., 44, 1050 (1972). R. S. Swingle, Anal. Chem., 47, 21 (1975). P. C. Kemeny, J. G. Jenkin, J. Liesegang, and R. C. G. Leckey, Phys. Rev., 9, 5307 (1974).
LITERATURE CITED (1) K. Siegbahn, C. Nordling, A. Fahlman. R. Nordberg, K. Hamrin, J. Hedman, G. Johansson. T. Bergmark, S. E. Karlsson, I. Lindgren, 8. Lindberg,
RECEIVEDfor review October 3, 1975. Accepted December 8, 1975. Supported by the U S . Army Research Office.
Spectroelectrochemical Cell for Anaerobic Transfer of Biological Samples for Low Temperature Electron Paramagnetic Resonance Studies James L. Anderson' Department of Chemistry, The Ohio State University, Columbus, Ohio 43210
An electrochemical cell is reported, suitable for generation, optical monitoring, and anaerobic transfer of multiple aliquots of concentrated enzyme or other biological samples for analysis by low-temperature electron paramagnetic resonance (EPR) or other external physical techniques. Results are reported for low-temperature EPR measurements on samples of cytochrome c oxidase in varying redox states. Present address, D e p a r t m e n t of Chemistry, S t a t e University, Fargo, N.D. 58102.
North D a k o t a
Cytochrome c oxidase is the oxygen-reducing terminal enzyme of the respiratory chain. Cytochrome c oxidase has four active one-electron metal centers: two heme iron and two copper components. In the fully oxidized state, where Fe(II1) and Cu(I1) should exhibit EPR resonances, only 30-40% of the heme and copper components are detectable ( I ). Low spin Fe(I1) and Cu(1) are also EPR-undetectable. Little evidence has been found for any high-spin Fe(I1). One of the coppers is believed to be EPR-undetectable (I, ANALYTICAL CHEMISTRY, VOL. 48, NO. 6, MAY 1976
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