Rotational effects in dissociation of H2 on Pd(111):: Quantum and classical study

We study rotational effects in dissociation of H-2 on Pd(111) through six-dimensional quantum dynamical and classical trajectory calculations. The potential energy surface was obtained from density functional theory. Quantum dissociative adsorption and rotational excitation probabilities are compared with initial-rotational-state-selective measurements. At low energies, dynamic trapping plays an important role, promoting reaction. For low values of the rotational quantum number J, the trapping is mainly due to translation to rotation energy transfer. The decreasing role of trapping when J increases contributes to the decrease of the dissociation probability. For larger values of J trapping is the result of energy transfer to parallel translational motion. Because trapping due to energy transfer to parallel translational motion is only effective at very low energies, the change in trapping mechanism with J causes the minimum of the reaction probability versus collision energy curve to shift to lower energies with increasing J, as previously observed in experiments. Together with dynamic trapping, rotational hindering (for small values of J) and an adiabatic energy transfer from rotation to translation (for high values of J) produce the nonmonotonous dependence of P-diss on J that is observed in our calculations and experiments at low energies. Finally, we predict a nonmonotonous dependence of the quadrupole alignment A(0)((2)) on J as observed in associative desorption experiments on H-2/Pd(100). It is due to rotational hindering for small J and adiabatic energy transfer from rotation to translation for large J. (C) 2003 American Institute of Physics.