Negative Ion Resonance States Research Articles

New interpretation algorithm for molecular negative ion resonance states

AbstractA new interpretation algorithm for molecular negative ion resonance states is proposed. This algorithm may be used when there exists a set of resonance states where the energetic distance between the resonances coincides with the corresponding distance between ionization energies. This set arises by a mechanism of electron‐excited Feschbach resonance. In this mechanism, during the collision of electrons with a molecule, one electron from an occupied molecular orbital (MO) is promoted to an unoccupied MO and the striking electron is captured to the same MO. An assumed MO, excited in the set of resonances, is determined by quantum chemical calculations. This assumption is checked by the construction of correlation diagrams for electron states. The algorithm was applied to the monothiocarbonates and perfluoroalkenes and gave satisfactory results.

Resonant electron-stimulated desorption of O − ions from oriented O 2 on graphite

We present measurements of the yield of O − ions in the resonant dissociative attachment reaction e −+O 2→O 2 −→O+O − in ζ-phase (upright) O 2 physisorbed on graphite. The yield of O − as a function of the electron impact energy shows three resonances at 8, 10, and 13 eV. Angular distribution measurements were performed on the O − ions desorbed from each resonance by rotating the substrate between the electron source and the ion detector. In each case we find that the desorbed O − ions are confined to an angular range close to the surface normal. Two important factors contributing to these angular profiles are: (i) the cross-section for capture of the incoming electron into the negative-ion resonance state, and (ii) the probability that the desorbing O − ion in emitted in the direction of the detector. In the present case, the narrow O − emission profile dominates the angular distributions.

Selective resonance population in electron scattering by adsorbed molecules.

We have investigated vibrational excitation of physisorbed ${\mathrm{O}}_{2}$ by electron impact in the 4--14-eV range, where a single broad resonance is observed in gas-phase studies. We show that by orienting the molecule in two different alignments on the surface of graphite it is possible to selectively populate two different negative-ion resonance states in this energy range, the $^{4}\mathrm{\ensuremath{\Sigma}}_{\mathit{u}}$ state (at \ensuremath{\sim}8.5 eV) and the $^{2}\mathrm{\ensuremath{\Pi}}_{\mathit{u}}$ state (at \ensuremath{\sim}6 eV), each of which leads to molecular vibrational excitation. Calculations show that this selective resonance population arises from the distinctive differential electron-capture cross sections of the two molecular states, into which electrons are ``fed'' by the surrounding surface structure.

The scattering of slow electrons by polyatomic molecules. A model study for CH4, H2O and H2S

Cross sections have been calculated for the scattering of slow electrons by water and methane molecules. A simple model is used: scattering is from a rigid molecule, exchange and polarisation effects are included in an approximate way, exchange by orthogonalising the scattered electron function to bound orbitals of the same symmetry and polarisation by using a potential correct at large distances but with an ad hoc short range cut-off parameter; the short range static interaction is, however, obtained from SCF target wavefunctions. Total, momentum-transfer and differential cross sections are given for CH4. In this body-fixed approximation total cross sections are undefined for molecules with a permanent dipole moment such as H2O and H2S; results are given here for momentum-transfer and differential cross sections of H2O. Possible negative-ion resonance states in H2O and H2S are discussed with reference to recent dissociative attachment and vibrational excitation experiments.

Absolute total electron scattering cross section of CO, CO 2 and OCS in the low energy region

Absolute total cross sections for electron scattering from CO, CO 2 and OCS molecules at low energies have been measured. An increase of the cross sections due to the formation of negative ion resonance states was observed.

Compound-negative-ion resonance states and threshold-electron excitation spectra of N-heterocyclic molecules: Pyridine, pyridazine, pyrimidine, pyrazine, and sym-triazine

The threshold-electron excitation spectra of the N-heterocyclic molecules pyridine, pyridazine, pyrimidine, pyrazine, and sym-triazine have been studied with a newly constructed electron spectrometer. These are presented, discussed, and compared with photoabsorption and, whenever possible, with electron impact spectral data. Optically forbidden and n→ π* transitions have been observed. Two compound negative ion resonance (CNIR) states (``shape'' resonances) have been detected below the first excited electronic states of all N-heterocyclic molecules studied, except sym-triazine for which only one CNIR state has been observed. These CNIR states are discussed in terms of the effect of a perturbation introduced by the more electronegative nitrogen atom(s) in the benzene ring on the two lowest (degenerate) antibonding orbitals of benzene.