T.A. Ferrett

THE RELATIONSHIP BETWEEN SHAPE RESONANCES AND BOND LENGTHS

A discussion is presented on the general nature of shape resonances in small molecules and how they may relate to molecular bond lengths. Criteria for assigning photoabsorption features as shape resonances are described, and the usefulness of photoemission experiments to such assignments is highlighted. Based on these criteria, all unambiguously identified K‐shell shape‐resonance features in molecules containing B, C, N, O, and F are examined in an attempt to identify empirically a shape‐resonance energy/bond length relationship. Although the available data are insufficient to establish a quantitative correlation, they indicate the influence of other variables besides the bond length to the energy position of a shape resonance.

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.

Reply to the ‘‘Comment on ‘The relationship between shape resonances and bond lengths’ ’’

The relationship between shape−resonance energies and bond lengths within agiven molecule is explored further.It is contended that some shape−resonance assignments presented by Stohr and Hitchcock (ref.1) are not convincing.(AIP)

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.