Auger Lines in X-Ray Photoelectron Spectrometry C . D. Wagner Shell Decelopment Company, E m e r y d l e , Calif, 94608 A study has been made of Auger electrons detectable from all elements, using AI Ka photon irradiation in an X-ray photoelectron spectrometer. Auger electrons from forty-four of the elements are readily observable, many in high intensity. There are three series, the KLL for elements boron through magnesium, the LMM for sulfur through selenium, and the MNN for ruthenium through neodymium. Strong chemical effects are noted on the shapes, intensities, and locations of some of the Auger lines.
IN A N EARLIER PAPER (I), a survey has been made of the sensitivities of the elements as detected by a Varian Induced Electron Emission (X-ray Photoelectron) Spectrometer. This spectrometer is an electrostatic type, utilizing a variable retarding voltage on the emitted electrons (2). The earlier paper related exclusively to the detection of the photoelectrons ejected by direct photon (A1 K a ) irradiation. However, the Auger electrons emitted upon subsequent relaxation of the excited ions are also detected. In this paper are assembled the results of the survey as pertains to the Auger electrons. There are extensive data on Auger electrons emitted under bombardment with electron beams having energies in the kiloelectronvolt range. There is little information on Auger electrons induced by irradiation with photons in this energy range. There are differences to be expected, both in the phenomena and in the experimental recording of the spectra. First, an electron beam as ordinarily used is highly destructive t o insulating samples, and sample materials with organic moieties ordinarily cannot be successfully examined by this technique. Most such materials can be successfully examined under photon radiation. Second, cross-sections for ionization of the deeper core levels relative to the shallower levels are relatively much greater under photon irradiation, so Auger electrons resulting from deep core level ionization should be enhanced in intensity relative to those resulting from ionization of the shallower levels. Third, absence of a bombarding electron beam permits recording of a direct spectral distribution curve rather than a differential curve. Fourth, resolution, especially for high kinetic energy electrons, is, in general, better in the Varian instrument; instrumental line widths are 0.5-1.0 eV. For any practical uses of Auger spectra, the lines must have significant intensity. This imposes the following rather special requirements for Auger lines of interest. First, the initial ionization must be one of high probability, i.e., it involves emission of a photoelectron that itself is prominent in the electron spectrum. For this reason, with elements through 2 = 12 (Mg), the Auger electron must result from initial ionization in the 1s shell. For elements through 2 = 34 (Se) it would have to involve the 2p shell, for elements through 2 = 65 (Tb) it would be most likely the 3d, and for the remaining elements, the 4d or 4f. Second, no Auger line can exceed the excitation photon energy (1487 eV for A1 K X-rays) and, moreover, the primary ionization must not require more energy than the photons can provide. Third, (1) C. D. Wagner, ANAL.CHEM.,44,1050 (1972). (2) N. H. Weichert and J. C. Helmer, Aduarz. X - R a y A t d . , 13, 406 (1970).
an Auger transition will not easily be observed in this instrument if its kinetic energy is less than 100 eV. Therefore, any Auger line involving an initial ionization of .'
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Figure 2. LMM Augers Figure 1. KLL spectra
and by the maximum retarding energy of the spectrometer. While the spectrometer is capable of detecting electrons of kinetic energy up to 2000 volts, the excitation photons possess only 1487 eV. However, the Auger spectrum is not dependent upon the monochromaticity of the exciting photons. Therefore, it is feasible t o use the more diffuse, higher energy X-ray photons for creation of the vacancies (e.g., chromium a t 5415 eV) and thus make accessible Auger electrons with kinetic energies between 1487 and 2000 volts. This will be done in the future. This will certainly make accessible KLL Auger lines from AI, Si, and P, and L M M Auger lines for Br, Kr, Rb, Sr, and Y . If the M N N Auger lines become more easily identifiable beyond the rare earths, it would be energetically possible t o detect them for most of the remaining elements. EXPERIMENTAL
As a general procedure, compounds were powdered in a n agate mortar to
