Continuum Multiple Scattering Xα Calculations Research Articles

A generic π* shape resonance observed in energy-dependent photoelectron angular distributions from two-colour, resonant multiphoton ionization of difluorobenzene isomers

We present new evidence for the existence of a near threshold π* shape resonance as a common feature in the photoionization of each isomer of difluorobenzene. Experimentally, this is revealed by significant changes in the anisotropy of the photoelectron angular distributions (PADs) following the ionization of the optically aligned S1 state of these molecules at varying photon energies. Continuum multiple scattering Xα calculations reproduce this behaviour well, and allow the visualisation of the continuum shape resonances. The resonances are unusually narrow in energy (<1 eV), but nevertheless appear to extend right down to the ionization thresholds--exactly the low energy range typically accessed in laser-based resonance enhanced multiphoton ionization (REMPI) schemes. The anticipation of such pronounced energy dependence in the PADs and cross-sections sought for other molecules, and an ability to accurately predict such features, should be important for the reliable application and interpretation of experiments involving REMPI probing of those molecules.

Overlapping spheres continuum multiple-scattering Xα calculations of elastic cross sections in electron-molecule scattering

A recently developed parameter free continuum overlapping spheres multiple-scattering method for calculating low-energy elastic scattering cross sections in polyatomic molecules is described. Results on nitrogen, ethylene, halogenosubstituted ethylenes, 1,3,5-trifluorobenzene are presented and compared to experiment.

Electron attachment to the fluoro-, bromo-, and iodomethanes studied by means of electron transmission spectroscopy and Xα calculations

The electron transmission spectra of the fluoro-, bromo-, and iodomethanes (except CH2F2 ) and of some monohaloalkanes are reported. The resonance energies for electron attachment to the halomethanes have also been evaluated by means of bound and continuum multiple scattering Xα calculations, which closely reproduce the low-energy experimental data. A large electron affinity increase is observed on going from the fluorine to the heavier halogen derivatives, while within each series of halides the electron affinity increases with increasing number of halogen atoms. The ground anion state is found to be stable in CHBr3, CBr4, and in all the iodomethanes except CH3I. The fragment anion production, as a function of the electron impact energy, has been measured for some of the compounds under investigation, in order to obtain experimental information on anion states very close to zero energy or slightly stable, which are not accessible in electron transmission.

Absolute partial photoionization cross sections of ethylene

Absolute partial photoionization cross sections for ionization out of the first four valence orbitals to the X 2B 3u, A 2B 3g, B 2A g and C 2B 2u states of the C 2H 4 + ion are presented as a function of photon energy over the energy range from 12 to 26 eV. The experimental results have been compared to previously published relative partial cross sections for the first two bands at 18, 21 and 24 eV. Comparison of the experimental data with continuum multiple scattering Xα calculations provides evidence for extensive autoionization to the X 2B 3u state and confirms the predicted shape resonances in ionization to the A 2B 3g and B 2A g states. Identification of possible transitions for the autoionizing resonances have been made using multiple scattering transition state calculations on Rydberg excited states.

Temporary anion states in the chloromethanes and in monochloroalkanes

The resonance energies for electron attachment to the chloromethanes are evaluated by means of bound and continuum multiple scattering Xα calculations. The results closely reproduce the experimental electron transmission spectroscopy data and confirm their previous assignment. Electron transmission and dissociative attachment spectra of monochloroalkanes are also reported, in order to obtain information on the effects of branching at the substituted carbon atom and of alkyl chain length on the resonance and chlorine anion production energies.

The d and f resonances in the core level absorption spectra of TeF 6

The post-edge resonant features observed in the Te p and d core level absorption spectra of TeF 6 have been characterized with stabilized bound state and continuum multiple scattering Xα calculations. The nature of the resonances above the Te 4p edge is similar to that observed in SF 6 and can be attributed to excitations into quasibound channels with substantial kd character. The localization of the continuum channel is due to strong interactions between the Te d and F p and d orbitals. The origin of the resonances above the Te 4d edge is different. It is better described as a manifestation of the atomic-like d→ kf delayed onset as a consequence of the Te centrifugal potential.