EXCITATION OF VIBRATIONAL MODES NEAR THRESHOLD IN C02 AND N20

The vibrational cross section by electron impact within 0.05 eV of threshold is measured in CO2 and N2O. A trochoidal monochromator is used as an electron source and SF6 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 CO2, 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. © 1969 The American Physical Society.