Threshold structures in the cross sections of low-energy electron scattering of methane

Abstract

The energy and angular dependences of the elastic and inelastic scattering of electrons from methane have been measured in the range of collision energies from 0.1 to 1.8 eV. Of special interest are the shapes of the excitation functions of the four possible vibrational modes of the molecule, because the elastic cross section shows a pronounced Ramsauer-Townsend minimum and this is compatible with the existence of a short-lived compound state of the electron-molecule system being detectable very close to the thresholds of the inelastic channels. The measurements show structures right at the thresholds, which have half widths of 0.1-0.3 eV and seem to be predominantly in the s and f-waves of the scattered electrons. In addition, a cusp structure has been found in the nu 2, nu 4 channel. The transmission functions of both the electron gun and the electron optics of the detector have been carefully controlled and therefore the peak shapes and half widths of the threshold structures should be more accurate than the results of Rohr (1980) who first reported the existence of the threshold peaks in methane at a 60 degrees scattering angle.