Vibrational relaxation and energy gap law breakdown in the X ³Σ^- state of NH and ND in solid argon

The lifetimes of the v = 1 levels in the ground electronic states of NH and ND were measured. It is shown that the rate of vibrational relaxation in NH is faster by at least a factor of 150, contrary to the predictions of the energy gap law. This is due to the localized rotational mode being the energy acceptor and to the larger amplitude of center of mass motion in the hydride. The absolute rates are considerably slower than the corresponding rates in the excited A ³Π state owing to the lower polarizability and lower barrier to free rotation in the ground electronic state.