Resonances in electron scattering by molecules

The main features of resonance scattering of electrons by molecules are described and resonances are determined on the basis of the theory of collisions in a two-body system, as well as resonances emerging as a result of collisions in a few-body system. Regularities in the emergence of such resonances and their characteristics are analyzed. The results of calculations of these resonant processes occurring during collisions of electrons with diatomic molecules, made on the basis of the quantum theory of scattering in a few-body system, are presented. The results of calculating the cross sections of resonant processes of electron collisions with molecules are compared with the available experimental data and with the results of calculations based on other approximations.     

Search for low-lying resonances in electron scattering by atomic hydrogen

Measurements of the transmission of an energy-selected electron beam through atomic hydrogen fail to indicate any resonance below 9·5-eV electron energy. The negative result contradicts recent suggestions by Rudkjøbing and by Van Rensbergenthat some diffuse interstellar absorption bands might be attributable to preionizing transitions in the hydrogen negative ion. A bulk of reliable theoretical data, however, justifies our finding.     

Giant resonances in cold electron scattering by CS(2)

Experimental data are presented for the scattering of cold electrons by CS2, for both integral and backward scattering, between a few meV and a few hundred meV impact energy. Giant resonances with cross sections in excess of 50 A(2) are observed below 100 meV, associated with the transient formation of CS(-)(2) at 15 meV and with the bend and symmetric stretch of CS(2) at thresholds of 49 and 82 meV, respectively. The resonance at 49 meV is 2 orders of magnitude greater in cross section than a dipole impulsive model predicts. These structures are superimposed on a sharp rise in the scattering cross section at low energy, which may be attributed to virtual state scattering.     

Calculation of electron hydrogen scattering resonances by the coordinate-rotation method

Energies and widths of severalS andP type resonance ine-H scattering are determined in a calculation involving only square-integrable functions of the integral transform type obtained by the generator coordinate method. Several resonances are apparently calculated for the first time. Advantages and disadvantages of the coordinate-rotation method are critically examined and discussed. A restricted perturbation scheme for the calculation of resonances is proposed and applied to the first¹S resonance of the He⁺ —e system.     

Extended molecules and geometric scattering resonances in optical lattices

We develop a theory describing neutral atom scattering at low energies in an optical lattice. We show that, for a repulsive interaction, as the microscopic scattering length increases the effective scattering amplitude approaches a limiting value which depends only on the lattice parameters. In the case of attractive interaction a geometric resonance occurs before reaching this limit. Close to the resonance, the effective interaction becomes repulsive and supports a weakly bound state, which can extend over several lattice sites.     

Eikonal expansion in electron scattering: I. Elastic scattering

A systematic eikonal expansion for the scattering of high-energy electrons from nuclei is derived which starts from the iterated Dirac equation. The resulting scattering amplitude is written in an impact parameter representation depending on eikonal phases which are proportional to inverse powers of the energy. The first two correction terms to the leading Glauber-Baker amplitude are calculated. For a Coulomb potential they agree with a sinθ-expansion of the relativistic Coulomb scattering amplitude. In the case of scattering from an extended charge distribution at sufficiently high energies numerical partial wave calculations are accurately reproduced.     

Differential cross sections for electron scattering by polar molecules. Elastic scattering and vibrational excitation

By use of the effective range theory for electron scattering by polar molecules differential cross sections for elastic scattering by HCN and LiF molecules and for vibrational excitation of water molecules are calculated.     

Inelastic scattering of slow electrons from adsorbed molecules

The inelastic scattering of slow electrons from molecules adsorbed on a metal surface is studied. Previous calculations are improved by including the Coulomb interaction between the incident electrons and the induced charge density that they give rise to in the surface region of the metal.     

Shape resonances of oriented molecules: ab initio theory and experiment on hydrocarbon molecules

We report ab initio calculations of the x-ray absorption cross section for the near edge x-ray absorption fine structure of C2H6, and C2H4, and C2H2 at the C K-edge, based on a full multiple scattering formalism. The angular dependence of the electric dipole transition in the calculations as well as the angular dependent experiments for the oriented molecules give a good opportunity to compare both. The resonance can be assigned to a sigma(*) shape resonance. The multiple scattering formalism and the experiment agree well and thereby support the existence of such features in the spectra.     

Positron and electron scattering from alkane molecules

Total cross-sections (TCSs) for 0.2–1000 eV positrons and 0.4–1000 eV electrons colliding with normal-octane and cyclo-octane molecules have been studied using a relative measurement method. The TCS curves for positron and electron vary smoothly and compare well with other alkane molecules, in order of increasing carbon number. For positron scattering, weak humps at 1.5–2.5 eV for both normal- and cyclo-octane were observed. In the energy range lower than 2.2 eV, positron TCSs are roughly equal to or larger than electron TCSs. For electron scattering, a resonance peak at 8 eV and a shoulder at 25.0 eV were observed for both molecules. Over all the energy range, the TCS values for normal-octane are larger than those of cyclo-octane. The positron and electron TCS data for normal- and cyclo-octane molecules are briefly compared with those for normal- and cyclo-hexane.     

Intramolecular electron scattering and electron transfer following autoionization in dissociating molecules

Resonant Auger decay of core-excited molecules during ultrafast dissociation leads to a Doppler shift of the emitted electrons depending on the direction of the electron emission relative to the dissociation axis. We have investigated this process by angle-resolved electron-fragment ion coincidence spectroscopy. Electron energy spectra for selected emission angles for the electron relative to the molecular axis reveal the occurrence of intermolecular electron scattering and electron transfer following the primary emission. These processes amount to approximately 25% of the resonant atomic Auger intensity emitted in the studied transition.     

Inelastic scattering of slow electrons by molecules adsorbed on a small metal sphere

The differential cross section for inelastic scattering of slow electrons by dipole excitation of molecules adsorbed on a small metal sphere is calculated. The significance of the exact hard core wave functions in calculating the cross section is shown by comparison with the corresponding plane-wave result. A strong radius dependence is obtained for the cross section at low energies. We conclude from our results that the scattering process studied provides a promising testing ground for the quantum dynamics of slow electrons near a small metal sphere.