Very low energy electron scattering in nitromethane, nitroethane, and nitrobenzene

Abstract

Absolute total integral and total backward scattering cross sections are reported for CH 3 NO 2 , C 2 H 5 NO 2 , and C 6 H 5 NO 2 at electron-impact energies from 30 meV to several eV, with some additional data at higher energies. These experimental data extend the range of species studied in low-energy electron scattering to include target molecules with dipole moments large enough to support dipole-bound states. Data provide a further test of the validity of the Born point-dipole approximation for the calculation of rotationally inelastic scattering cross sections for polar molecules, which are of importance in modeling the chemical and physical characteristics of industrial and natural plasmas. For CH 3 NO 2 and C 2 H 5 NO 2 , large cross sections are found, monotonically increasing with decreasing electron energy in qualitative and to some degree quantitative agreement with theory based on the Born model. As in earlier studies other polar species, the Born model is found to underestimate significantly backward-scattering cross sections. C 6 H 5 NO 2 shows a powerful dip in the total integral scattering cross sections <200 meV, a feature not found in the backward-scattering cross section and totally at variance with any simple theory. The suggestion is made that this behavior, in common with ClO 2 and Cl 2 O, may arise through interference between direct Born-type rotational excitation and an indirect channel involving temporary negative ion states in the continuum.