C. M. Truesdale

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.

Photoionization cross sections and photoelectron asymmetries of the valence orbitals of NO

Partial photoionization cross sections and photoelectron asymmetry parameters were measured for the valence orbitals of NO over the photon energy range 16–31 eV by photoelectron spectroscopy, using synchrotron radiation. The results are compared with previous measurements and with theoretical curves based on the multiple‐scattering model. Maxima observed in the partial cross sections of the 2π, 5σ, and 4σ orbitals are attributed to continuum shape resonances. The multiple‐scattering model calculations predict the observed photoelectron asymmetries of the 5σ and 1π orbitals very well, but the calculated asymmetry curves are shifted strongly from the measured asymmetries produced by the 2π and 4σ orbitals. The π molecular orbitals are observed to produce qualitatively different photoelectron asymmetry curves than do the σ orbitals. We discuss the double‐angle‐time‐of‐flight method for the measurement of photoelectron angular distributions. This technique offers the advantages of increased electron collectio...

PHOTOELECTRON ANGULAR DISTRIBUTIONS OF H2O

The partial cross section and asymmetry parameters of the 2B1, 2A1, and 2B2 ionic states of H2O+ have been measured at photon energies of 18–31 eV, by time‐of‐flight photoelectron spectroscopy using synchrotron radiation. The atomic character of the molecular orbitals is discussed in the interpretation of similarities and differences between the measurements for these states. Other synchrotron radiation data, resonance line measurements, and (e, 2e) dipole results are included. A multiple‐scattering Xα (MSXα) calculation and a ground‐state inversion method calculation are in good agreement with our cross section measurements. There is excellent agreement between experimental asymmetry parameters and those computed from the MSXα method. Stieltjes–Tchebycheff imaging model calculations suggest the presence of resonant channels. This may be the origin of weak resonance structure present in the measurements.

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.

PHOTOIONIZATION MASS SPECTROSCOPY OF Ne DIMERS

The photoionization efficiency curve of Ne2 as a function of wavelength has been obtained between 560 and 620 A with a synchrotron radiation photon source. Detailed considerations of the autoionization mechanisms and symmetry of the excited states of Ne2 have led to the partial assignment of the excited electronic states of Ne2 which are responsible for the structure observed in its photoionization efficiency curve, and demonstrated the importance of Rydberg states with a B 2Πg core. The lower bound to the dissociation energy of Ne+2 was determined to be 1.24±0.08 eV (28.6±1.8 kcal/mol).

Photoelectron angular distributions of the N2O outer valence orbitals in the 19–31 eV photon energy range

Photoelectron asymmetry parameters, partial cross sections, and branching ratios for the X, A, B, and C states of N2O+ were measured using synchrotron radiation in the photon energy range 19–31 eV. Vibrationally averaged data are reported for all four states, as well as vibrationally resolved data for the A and C states. The data are compared with a multiple scattering calculation, (e, 2e) dipole measurements, and similar data on CO2. The N2O and CO2 results show remarkable state‐by‐state similarity in their asymmetry parameters.

Photoelectron asymmetries and two-electron satellites near the 3p → 3d giant-resonance region in atomic Mn

Abstract The partial cross-sections and photoelectron angular distributions for several lines in atomic Mn have been measured at photon energies between 50 and 72eV. The intensities of the 3 d correlation satellites at 24–26 eV binding energy behave similarly to the mainline intensity near the 3 p → 3 d giant resonance, but show an enhancement near the 3 p threshold which is not present for the main line. A configuration-interaction analysis is applied to help identify the origins of the satellites. The 3 p /3 d branching ratio from 55–72eV and the shape of the 3 d cross section in the resonance region are in good agreement with many-body perturbation-theory calculations.

Photoelectron angular distributions from H2 and D2

Abstract The photoelectron asymmetry parameters of H 2 and D 2 have been measured using synchrotron radiation over the photon energy range 19–27 eV. The results are compared with previous measurements and several theoretical calculations. A few of the theoretical calculations are in good agreement with experiment, but most of them predict asymmetries which are too large. Essentially identical β values were measured for H 2 and D 2 . This result is discussed in terms of differences which could arise due to vibrational and rotational structure.

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.