Isotope effect on vibrational excitation induced collision of H2 and D2 molecules

Some effects of the rotational motion of a vibrating rotator on its collisionally induced vibrational excitation are studied for H2 and D2 molecules initially at rest colliding with a He atom. Whatever the collision energy, the vibrational energy transfers are almost equal, due to cancellation between two dynamically opposed effects (in terms of collisional adiabaticity): the rotational modulation effect on the vibrational coordinate favors the vibrational energy transfer in H2, the frequency effect in D2. Numerical results (derived from computed trajectories) are presented to confirm this interpretation.