DYNAMICS OF MOLECULAR ADSORPTION OF ETHANE WITH PT(111) - A SUPERSONIC MOLECULAR-BEAM STUDY

The molecular interaction of ethane with Pt(111) was investigated by using supersonic molecular beam techniques and temperature-programmed desorption (TPD). Ethane adsorbs molecularly on the terrace sites of Pt(111) at 95 K with a saturation coverage of ca. 0.3 monolayer and desorbs at ca. 132 K during TPD. Isothermal desorption experiments suggest that ethane desorbs from the second layer with first-order desorption kinetics. Dissociation of molecularly adsorbed ethane is negligible upon subsequent heating. The trapping probability of ethane on the clean surface at normal incidence decreases from approximately 0.91 to 0.13 as the incident translational energy, E-tau, increases from 6 to 40 kJ/mol at a surface temperature of 95 K in apparent semiquantitative agreement with a modified hard-cube model. However, over a range of incident angles, theta-i, the initial trapping probability scales with E-tau cos0.6 theta-i rather than with E-tau cos2 theta-i, demonstrating the participation of momentum parallel to the surface in the trapping process and the necessity of more sophisticated theories, such as three-dimensional trajectory calculations, to adequately describe trapping over a wide range of incident translational energies and incident angles. At all incident translational energies studied the trapping probability of ethane at a surface temperature of 95 K increases continuously with ethane coverage up to monolayer saturation, indicating that ethane traps more efficiently onto adsorbed ethane than onto a clean surface. This behavior is expected since the mass of adsorbed ethane is significantly less than that of a platinum atom, leading to increased energy transfer upon impact with the adsorbed species. Based on this principle, extrinsic precursor states are expected for molecular adsorption on all surfaces except at translational energies that preclude trapping into the second layer. Together with molecular beam experiments of ethane dissociation on Pt(111) performed in our laboratory, the results presented herein indicate that dissociative ethane adsorption on Pt(111) proceeds more effectively via direct collisional activation at high incident translational energies rather than via a precursor-mediated mechanism at low incident translational energies.