Differential Cross Sections For Vibrational Excitation Research Articles

A study of electron impact excitation of molecular oxygen at a scattering angle of 180°

Excitation of molecular oxygen by electron impact has been observed at the backward scattering angle of 180°. For these studies a new solenoid system with a conical geometry has been designed to implement the angle-changing technique. Energy loss spectra have been measured to deduce differential cross-sections for vibrational excitation of the X 3Σ g − ground state and excitation of the a 1Δ g state at 180°. Excitation of the b 1Σ g + state has not been observed at 180° in agreement with the theoretically predicted selection rule Σ −←↦Σ + at that angle.

Resonances and Threshold Phenomena in Low-Energy Electron Collisions with Hydrogen Halides: New Experimental and Theoretical Results

Low-energy electron collisions with HCl and HBr and the deuterated compounds have been investigated by experimental and theoretical methods. New experimental results have been obtained on relative differential cross-sections for vibrational excitation and dissociative electron attachment. Measurements with high energy resolution for rotationally cooled molecules have revealed, in addition to shape resonance, threshold peaks and Wigner cusps, the existence of surprisingly sharp oscillatory structures in the elastic and v = 0 → 1 vibrational excitation cross-sections in a narrow range below the dissociative attachment threshold. The theoretical analysis is based on an improved nonlocal resonance model which has been constructed on the basis of ab initio fixed-nuclei scattering phase shifts for HCl and HBr and accurate ab initio calculations of the bound part of the HCl− and HBr− potential-energy functions. The high degree of agreement which has been obtained between experiment and theory for all channels indicates that the mechanisms responsible for the rich threshold structures in the collision cross-sections are completely understood.

Semiempirical calculations of inelastic electron-methylchloride scattering

Resonant R-matrix theory is used to describe inelastic electron-methylchloride scattering in a semiempirical way. Parameters of the theory are obtained by fitting differential vibrational excitation cross sections to experimental ones. Some specific features of the vibrational excitation cross sections are discussed, particularly the competition between the dipole and resonant contributions. The obtained parameters are used for calculations of the dissociative attachment from CH3Cl and CD3Cl with production of the Cl- negative ions. The results for a low-energy peak agree satisfactorily with the swarm analysis of Datskos et al. (1990) at temperatures of about 700-800 K. At lower temperatures the experimental cross sections in all measurements strongly exceed the theoretical ones.

Cross sections for resonant vibrational excitation ofN2by electron impact

We report the results of a theoretical study of resonant vibrational-excitation of ${\mathrm{N}}_{2}$ by low-energy electrons. The vibrational-excitation cross sections were calculated using ab initio fixed-nuclei resonance parameters in the complex-potential or boomerang model of Dub\'e and Herzenberg. The electronic resonance energy and decay width were extracted from several ab initio calculations of the $^{2}\ensuremath{\Pi}_{g}$ shape resonance of ${\mathrm{N}}_{2}^{\ensuremath{-}}$. The good agreement between the present cross sections and those obtained recently with the $R$-matrix technique indicates that the assumptions underlying the complex-potential model are valid for the case of ${{\mathrm{N}}_{2}}^{\ensuremath{-}}(^{2}\ensuremath{\Pi}_{g})$. The present results also show the sensitivity of the computed vibrational-excitation cross sections to the fixed-nuclei resonance parameters employed in the calculations. The differential vibrational-excitation cross sections at 90\ifmmode^\circ\else\textdegree\fi{} and the total integrated cross section which we have obtained agree resonably well with the available experimental data.