ROTATIONAL COOLING IN ASSOCIATIVE DESORPTION

The energy distributions of H2 molecules desorbing from Cu surface are explored in the context of a model in which molecular rotations with the angular momentum vector along the surface normal (the helicopter mode) are assumed to be free, while rotations for which the angular momentum vector lies in the plane parallel to the surface (the cartwheel mode) are taken to be hindered. In addition, a coupling is introduced between the cartwheel types of rotations and translations away from the surface. Using the classical trajectory method, it is found that the cartwheel type of rotations are "cooled" below their thermal value implying that the desorbing molecules should be predominantly of the helicopter type. These results are more generally applicable and are in qualitative agreement with available data. © 1990 American Institute of Physics.