The adsorption dynamics of molecular methane, propane, and neopentane on Pd(111): Theory and experiment

The molecular adsorption dynamics of methane, propane, and neopentane on Pd(111) was studied using supersonic molecular beam techniques and stochastic trajectory simulations. The sticking probability of the alkanes was measured as a function of incident energy, incident angle, and alkane coverage. In each case, the trapping probability decreased with increasing incident translational energy, which is expected for nonactivated molecular trapping. Nonnormal energy scaling was observed for the trapping probability of all of these alkanes, indicating corrugation of the gas-surface interaction potentials. The modified Kisliuk model was employed to describe the self-coverage dependence of the alkane on Pd(111). The trapping probability of each of these alkanes on Pd(111) is higher than that on Pt(111), consistent with the lower mass of palladium. Using the Morse potential for the methyl (methylene)-platinum two body potential obtained from the stochastic trajectory analyses of alkanes trapping on Pt(111), the trapping probabilities of alkanes on Pd(111) were predicted nearly exactly. Energy transfer calculations for alkane trapping on Pd(111) clearly indicate that excitation of the lattice vibrations and molecular cartwheeling rotations are the principal mechanisms for determining trapping.