Excitation of Vibrational Modes near Threshold* in CO2andN2O

Abstract

The vibrational cross section by electron impact within 0.05 eV of threshold is measured in C${\mathrm{O}}_{2}$ and ${\mathrm{N}}_{2}$O. A trochoidal monochromator is used as an electron source and S${\mathrm{F}}_{6}$ gas is used as a scavenger of zero-energy electrons appearing as a result of vibrational excitation of the molecules. It is found that the cross section near the threshold of all fundamental modes is of the same order. This indicates that the Born theory using a dipole interaction is not applicable for optically forbidden modes near threshold in C${\mathrm{O}}_{2}$, but it appears that it is applicable for optically active modes, both at threshold and at high energies. The theory is not expected to hold near resonances.