A comparative study of electron and positron scattering from molecules. IV. CH3Cl, CH3Br, and CH3I molecules

An experimental study of electron and positron scattering from CH3Cl, CH3Br , and CH3I molecules has been carried out, and total cross sections (TCSs) for both projectiles were determined. Several strong structures due to reso nances in the TCSs have been observed for electron impact, while weak but n ot negligible structures have also been seen for positron impact. A strong variation for the dominant resonance peak seen at around 10 eV was found to depend on a type of halogen atoms, and a detailed study of this dependence on molecular species has been performed to understand the origin and natur e of these resonances. The continuum multiple-scattering method has been em ployed for the analysis of experimental results in addition to the evaluati on of the elastic cross section. For larger halogen atoms, TCSs tend to pos sess larger magnitudes at energies above 100 eV than for smaller halogen at oms suggesting that the halogen atoms in fact dominate the dynamics, and th eir magnitudes increase in the order of CH3I>CH3Br>CH3Cl. A comparative stu dy of CH4 was also performed to provide insight on the effects of molecular geometrical structure and electronic state. (C) 2001 American Institute of Physics.