Isotope effects on intensities in the K-shell excitation spectra of CH4 and CD4 studied by 2.5 keV electron impact

The isotope effect on the intensity of the 1a1(K) » 3a1(3s) transition in methane has been examined by inner shell electron energy loss spectroscopy. The intensity ratio, IK»3s(CD4)/IK»3s(CH4) is 0.81 ± 0.08 which is at variance with a recent theoretical prediction by Bagus et al. It is shown that the measured ratio is compatible with a Herzberg-Teller vibronic coupling mechanism for this transition.