Dynamics with a rotational transition state

Rotationally inelastic collisions between hydrogen molecules and polar molecules, especially water, are of great importance in assessing the thermodynamic properties of interstellar gases. We show here on a simple model that momentum exchange is efficiently mediated by a relative transition state. It acts as a threshold between an inner region and an outer region around the polar molecule. The geometric properties of classical dynamics at threshold are discussed and a capture type of theory is described.