P.A. Heimann

Core‐level photoelectron and Auger shape‐resonance phenomena in CO, CO2, CF4, and OCS

Cross sections and angular‐distribution asymmetry parameters were measured directly for C(KVV) Auger electrons and C 1s photoelectrons from CO, CO2, CF4, and OCS, O(KVV) Auger electrons and O 1s photoelectrons from CO and CO2, and S(LVV) Auger electrons and S 2p photoelectrons from OCS using synchrotron radiation. The measurements were made in the photon‐energy ranges 270–350, 545–680, and 160–190 eV, respectively. Shape resonances were observed in all of these molecular systems. The cross‐section results are compared with previous experimental data obtained by electron energy‐loss measurements, electron–ion coincidence results, and photoabsorption studies. The asymmetry‐parameter results are the first of their kind for these molecular core levels. The present results are compared with available theoretical predictions obtained from Stieltjes–Tchebycheff imaging techniques, Hartree‐Fock static‐exchange calculations, and the multiple‐scattering method.

Nitrogen K‐shell photoemission and Auger emission from N2 and NO

The first gas‐phase photoelectron measurements near the nitrogen K edges of N2 and NO are reported. Shape‐resonance behavior is exhibited in the cross sections for both N 1s photoemission and N KVV Auger emission. The measured cross sections agree well with absorption and electron‐energy‐loss results, and with Stieltjes–Tchebycheff moment‐theory calculations (for N2) except for a small energy shift. The measured asymmetry parameters for N 1s photoemission also exhibit changes in the shape‐resonance region, but not as pronounced as those predicted by the multiple‐scattering method (MSM). Comparison of all of the results with the MSM calculations indicates that the shape‐resonance effects predicted by the MSM are higher in energy, and are narrower and more intense than those actually observed.

Resonance effects on the inner-valence levels of SF6 in the photon-energy range 52–72 eV

Abstract We report the photoionization partial cross section and asymmetry parameter in the 52–72 eV photon-energy range for the inner-valence orbitais (3t1u, 2eg, and 4a1g) in gaseous SF6. These results, combined with those for the (inner valence)/(outer valence) branching ratio, indicate resonant enhancement of the inner-valence levels at ≈ 59 eV photon energy which we associate with the 3t1u → eg shape resonance predicted by MSM Xα calculations.

Photoelectron asymmetry parameter for the S 2p level of SO2

Abstract We present a measurement of the asymmetry parameter (β) for S 2p core-level photoionization of SO 2 in the vicinity of several continuum resonances. Results are contrasted with β(2p) for the central atoms of the molecules SF 6 and SiF 4 . Atomic calculations for β(S 2p) appear to reproduce qualitatively the molecular curve for SO 2 aside from a shift to lower β values for the molecular case.

GAS-PHASE PHOTOEMISSION WITH SOFT X-RAYS: CROSS SECTIONS AND ANGULAR DISTRIBUTIONS

A summary is presented of typical gas-phase photoemission studies based on synchrotron radiation in the 50-5000 eV range, using beam lines at the Stanford Synchrotron Radiation Laboratory. Three topics are addressed: atomic inner-shell photoelectron cross sections and asymmetries, correlation peaks in rare gases, and core-level shape resonances in molecules. Photoelectron cross-section a(nZ) and asymmetry-parameter a(n0 studies in mercury vapor at photon energies up to 270 eV (up to 600 eV for a4f) extend coverage of these parameters to n<5 and 5<3. Comparison with Dirac-Slater and relativistic random-phase approximation calculations reveals systematic discrepancies. For example, distinct Cooper minima in a(n iZ,) are observed but not predicted, while predicted a(n9) values are typically too high. Correlation satellites have been studied for the K shells of helium (hv = 68-90 eV), neon (hv = 870-960 eV) and argon (hv = 3200-3320 eV). In helium the n=2 satellite peak was shown to have mainly 2p character at threshold, and its asymmetry was measured through the autoionizing resonance region. Tentative evidence was obtained that the neon satellites are less intense near threshold than in the high-energy limit, and that their intensities stay constant or decrease with increasing energy near threshold. A new satellite was observed in argon at 24.6 eV which appears to increase in intensity with energy. Molecular core-level shape resonances were observed for the first time by photoemission, yielding a(hv) and a(hv) for core levels from 180 eV binding energy (S 2p in SF6 and OCS) through C is in CO, CO2 and CF4, N ls in N2 and NO, and 0 is in CO and CO2 to 2490 eV (S ls in SF6). Several conclusions can be drawn about the photoelectron and Auger cross sections and asymmetry parameters.