Electron scattering by the hydrocarbons C4H6,C5H8 , and C6H10

Abstract

We report calculated elastic integral and differential cross sections for electron collisions with the hydrocarbons 1,3-butadiene (${\mathrm{C}}_{4}{\mathrm{H}}_{6}$), 2-methyl-1,3-butadiene (${\mathrm{C}}_{5}{\mathrm{H}}_{8}$), and 2,3-dimethyl-1,3-butadiene (${\mathrm{C}}_{6}{\mathrm{H}}_{10}$) for impact energies up to 15 eV. Our calculations were performed with the Schwinger Multichannel Method with pseudopotentials, in the static-exchange and static-exchange plus polarization approximations. These molecules differ for the presence of one methyl group, in the case of ${\mathrm{C}}_{5}{\mathrm{H}}_{8}$, and two methyl groups, in the case of ${\mathrm{C}}_{6}{\mathrm{H}}_{10}$ in substitution of one and two hydrogen atoms in ${\mathrm{C}}_{4}{\mathrm{H}}_{6}$, respectively (methylation effect). For the polar molecule 2-methyl-1,3-butadiene, we included the Born closure procedure in order to account for the long-range potential. We found two ${\ensuremath{\pi}}^{*}$ shape resonances in the integral cross section of each one of the molecules studied. The present results are also compared with the experimental values for the resonances positions and with total cross sections available in the literature. In particular, we show that the minimum in the total cross section of ${\mathrm{C}}_{5}{\mathrm{H}}_{8}$ located at around 1.6 eV and assigned by the authors as a Ramsauer-Townsend minimum is, actually, a valley between the two ${\ensuremath{\pi}}^{*}$ shape resonances. Also for the ${\mathrm{C}}_{5}{\mathrm{H}}_{8}$ molecule, the enhancement in the total cross section below 1.6 eV is the tail of the low-lying shape resonance and not an effect due to its permanent dipole moment, as suggested by the authors. We discuss the influence of the methylation effect in the shape and magnitude of the elastic cross sections and also in the location of the ${\ensuremath{\pi}}^{*}$ shape resonances of these hydrocarbons.