Relative vibrational intensities for the B1Σ+reverse arrowX1Σ+ transition in carbon monoxide

In the electron impact spectrum of carbon monoxide two vibrational peaks (v′=0, 1) are found in excitation of the B1Σ+ state. (Higher vibrational states are predissociated.) According to the Franck-Condon principle, relative intensities of vibrational progressions in a single electronic transition should not change with scattering angle and this has been verified, in past research, for many such progressions. However, the intensity ratio for the two peaks, associated with the B1Σ+ state, changes markedly with angle. For example, with incident electrons of 400 eV energy the ratio changes by a factor of ten in passing from a scattering angle of 0 to 20°. This is the typical behavior of excitations associated with different electronic transitions. Conceivably, strong interaction of two electronic states (due to breakdown of the Born-Oppenheimer approximation) could be involved. If we accept the assignment of the two peaks as due to transitions to the v′=0 and v′=1 vibrational levels of B1Σ+ (as spectroscopists do) then breakdown of the Born-Oppenheimer approximation is the only acceptable alternative.