Surface temperature dependence of rotational excitation of H2 scattered from Pd(111)

We have measured rotational excitation into rotational states J = 3, 4, and 5 for H-2 scattered from Pd(111) as a function of surface temperature and incident translational energy. Excitation is found to occur even when the incident H-2 translational energy is less than the energy level spacing between the initial and final rotational states. Thus, part of the excitation energy is coming from the surface, not from solely translational-rotational energy coupling. There is a strong surface temperature dependence to the rotational excitation that is well described by an Arrhenius-type expression. When fit to the Arrhenius equation, the apparent activation energy is less than the rotational energy level spacing and decreases as the translational energy of the incident molecules is increased. Based on inspection of the calculated H-2/Pd(111) potential energy surface, we attribute this lowered activation energy to an extension of the bond length when the molecule interacts with the surface. The stretching of the molecular bond increases the moment of inertia of the molecule, which then decreases the spacing between the rotational energy levels. We suggest that the final states of molecules which do not dissociate reflect features of the potential energy surface associated with open, dissociative pathways. (C) 1999 American Institute of Physics. [S0021-9606(99)70645-8].