Joseph L. Dehmer

Elastic electron scattering by CO2, OCS, and CS2 from 0 to 100 eV

The integrated elastic electron scattering cross sections for CO2, OCS, and CS2 were calculated from 0 to 100 eV using the continuum multiple‐scattering model with the Hara exchange approximation. For each molecule, a strong π‐type shape resonance occurs between ∼1–4 eV, followed by multiple, weakly resonant features at higher kinetic energy. The latter are only marginally observable in the integrated cross section, but, in some cases, should be clearly observable in vibrational excitation spectra. Agreement with available experimental information is good for all three cases, except for a gross underestimation of the background (nonresonant) scattering cross section in OCS resulting from our omission of dipole scattering in this work.

Vibrational effects in the shape-resonant photoionisation of CO2

Multiple scattering model calculations of vibrationally averaged asymmetry parameters (B) for the 1 pi g, 1 pi u, 3 sigma u and 4 sigma g levels of CO2 are compared to fixed-nuclei calculations. Vibrational branching ratios and vibrationally resolved asymmetry parameters beta nu for the 1 pi g and 1 pi u levels are also presented. The latter are compared to recent He I line source measurements. The resonant features of the vibrationally averaged beta is less pronounced than those calculated with fixed nuclei, while the resolved spectra show shape-resonant features which are not apparent in the unresolved spectra.

Spectral variation of molecular photoelectron angular distributions: valence shells of N2 and CO

Photoelectron angular distributions have been calculated, with the multiple-scattering method, for electrons ejected from the outer shells 2 sigma u, 1 pi u and 3 sigma g of N2 and 4 sigma , 1 pi and 5 sigma of CO from threshold to 10 Ryd electron kinetic energy. Clear evidence of the sigma f wave (l=3) shape resonance is seen in the 3 sigma g spectrum of N2 and in all three spectra of CO. Agreement with very recent wavelength-dependent measurements made at the Bonn synchrotron by Marr and Holmes (1979) is generally good, but more detailed measurements and further theoretical work are needed to eliminate the remaining discrepancies.

Vibrational branching ratios and photoelectron angular distributions in 5σ photoionisation of CO

Vibrationally resolved photoelectron angular distributions have been calculated for the 5 sigma photoionisation channel of CO using the multiple-scattering method. Vibrational branching ratios and vibrationally unresolved integrated cross sections and photoelectron angular distributions are also reported and compared with available measurements. Both angular distributions and branching ratios exhibit striking non-Franck-Condon behaviour caused primarily by the f-wave shape resonance in the sigma photoionisation continuum. Significant discrepancies between theory and experiment exist for the weaker vf=2,3 vibrational levels and interaction with nearby two-electron excitation is proposed as a likely cause.

Nuclear motion effects in the photoionisation of CO2

A calculation of the photoionisation cross section of CO2 reveals that the sigma u shape resonance at approximately 19 eV, accessed by the 1 pi g and 4 sigma g valence levels, is reduced and broadened by coupling to the ground-state vibrational motion. Off resonance the effect of nuclear motion is negligible. Results are compared with experimental branching ratios.

Electron scattering from HCN

The hybrid model of electron-polar molecule scattering has been used to calculate scattering of electrons from first-row diatomic molecules (LiF); large, strongly polar systems (CsCl); and systems showing pronounced vibrational effects (HCl, DCl). Initial results are reported for the extension to HCN, the first calculation on a polyatomic molecule and the first such calculation on a strongly polar system expected to display a characteristic shape resonance within a few eV of threshold. (GHT)

Theoretical prediction of shape-resonance-enhanced nuclear motion effects in molecular photionization

Shape resonances in molecular photoionization are predicted to induce strong non-Franck-Condon effects over a spectral range several times broader than the resonance half-width. This is manifested by large deviations from Franck-Condon vibrational intensity distributions and strong dependence of photoelectron angular distributions on the vibrational state of the residual ion. These effects are illustrated for the 3σg photoionization channel of N₂ in Figures 1 and 2 using the multiple-scattering model and the adiabatic nuclei approximation.

Closure formula for vibrational excitation in electron-polar molecule scattering

Application of the closure formula to electron-dipolar molecule scattering is straightforward if the nuclei are frozen at R = Re. Here it is applied to vibrationally inelastic scattering, focusing particularly on the R dependence of exact and Born approximation T matrix elements. This R-dependence must be the same for the two levels of approximation if the closure formula is to be applied.

Electron scattering from CsCl

A spectral study of electron-molecule scattering on cesium chloride is reported. A hybrid method of calculating the core potential is presented. At 5 and 20 eV, differential cross sections are compared between calculated and experimental with good agreement. (GHT)

Vibrational excitation in electron-DCl scattering

A preliminary report is given of the results of fixed-nuclei and adiabatic calculations of the e-HCl and e-DCl electron-molecule scattering systems. The calculations are performed in the rotational and vibrational adiabatic approximation, and the use of DCl rather than HCl allows calculation to substantially lower energy. (GHT)