Present (•) experimental total cross sections (TCSs) for positron scattering from iodomethane
Figure 3. Present (•) experimental total cross sections (TCSs) for positron scattering from iodomethane. The errors represent the statistical component (±1σ) of the overall uncertainties only. Also plotted are the present measured TCSs corrected for the forward-angle scattering effect () at selected energies (as discussed in the text, the corrected data indicate upper bound estimates for the forward-angle corrections). Additionally shown are the present elastic integral cross sections calculated combining our SMC approach with the Born dipole approximation to improve the description of higher partial waves (see section 3). The present theoretical results rescaled by the squared ratio of the experimental to theoretical permanent dipole moment are also given (see section 4). Our results are compared to the earlier data by Kimura et al [4]. See the legend in the figure for more details. The threshold energies for positronium (Ps) formation and the first ionization in CH3I [8] are indicated by the black arrows labelled 'Ps' and 'IP', respectively.
Abstract
We report experimental total cross sections (TCSs) and calculated elastic integral cross sections (ICSs) for positron collisions with iodomethane (methyl iodide, CH3I). The experimental TCSs were obtained with a linear transmission technique, for energies from 0.1 up to 50 eV. The present TCS data agree well with those previously reported (Kimura et al 2001 J. Chem. Phys. 115 7442) at higher energies (above 7 eV), but significant discrepancies are found at the lower energies. The present ICS computations were performed with the Schwinger multichannel method (SMC) and the Born dipole approximation in the incident energy range from 0.1 eV up to 10 eV. Iodomethane poses a great challenge to theoretical descriptions of the collisions dynamics. In addition to the neglect of inelastic channels, the main difficulty found in the SMC approach is related to numerical limitations that prevent a thorough description of correlation–polarization effects. Although our ICS calculations do not compare well with the present TCS data, the results are encouraging, as iodomethane would challenge all the presently available computational approaches.
