Trapping dynamics of isobutane, n-butane, and neopentane on Pt(111): Effects of molecular weight and structure

The molecular adsorption dynamics of isobutane, n-butane, and neopentane on Pt(111) was investigated using supersonic molecular beam techniques and stochastic trajectory calculations. Using the united atom approach, a single, pairwise-additive Morse Potential for methyl (methylene)- plantinum interactions quantitatively simulates the dependence of the initial trapping probability, α, on the initial translational energy, ET, and angle of incidence, θi, for each alkane. For both isobutane and n-butane, the dependence of α on ET and θi best scales with ETcos0.8θi, which is similar to that found previously for ethane and propane trapping on Pt(111). The initial trapping probability of neopentane exhibits a more pronounced dependence on θi, which scales according to ETcos1.3θi. The simulations suggest that the enhanced angular dependence of α for neopentane is related to its molecular weight. As the mass of the incident species is increased, momentum transfer to the surface becomes more efficient than the interconversion of incident parallel and normal momentum due to corrugation of the surface potential. The net effect is an increase in the trapping probability at glancing incidence compared to lighter molecules, and a resulting shift in the angular dependence of α towards normal energy scaling. The calculations also predict that collisional energy transfer to rotation is important in promoting adsorption. For each molecule, rotational excitation is determined to be the most effective energy transfer process that discriminates trapping from scattering. In addition, translational energy transfer to torsional vibration about the central C - C bond is highly efficient for n-butane, and greatly facilitates adsorption. Less excitation is predicted for C - C - C bending modes for all of these molecules. copyright 1999 American Institute of Physics