Vibrational excitation in electron-CH₄collisions: exchange interaction effects

The vibrational excitations of all four normal modes of the methane molecule in collisions with low-energy electrons (Ecoll<15 eV) are computed within the adiabatic approximation for the nuclear motion and solving the quantum dynamics for the scattered electron. The non-local exchange forces are treated by a separable expansion and by a semiclassical local model and their effects on the vibrationally inelastic cross sections are examined. By comparing the (ν1 + ν3) and the (ν2 + ν4) sums of the four vibrationally inelastic modes as two sets of `stretching' and `bending' modes, with existing experiments on the latter, we are able to show that both approaches produce rather good accord with the available data, while presenting differences in them. The local model for treating exchange, which affords a substantial reduction of computational time, is shown to reasonably well describe vibrational inelasticity, at least in the simpler situation of a non-linear target with off-centre light nuclei as in the present case.