Dynamics of molecular adsorption of ethane with platinum(111): a supersonic molecular beam study. Arumainayagam, Christopher R.; Schoofs, Gregory R.; McMaster, Mark C.; Madix, Robert J. Dep. Chem., Stanford Univ., Stanford, CA, USA. J. Phys. Chem. (1991), 95(3), 1041-7. CODEN: JPCHAX ISSN: 0022-3654. Journal written in English. CAN 114:89066 AN 1991:89066 CAPLUS (Copyright 2001 ACS)

The mol. interaction of C₂H₆ with Pt(111) was studied by using supersonic mol. beam techniques and temp.-programmed desorption (TPD). Ethane adsorbs molecularly on the terrace sites of Pt(111) at 95 K with a satn. coverage of .apprx.0.3 monolayer and desorbs at .apprx.132 K during TPD. Isothermal desorption expts. suggest that C₂H₆ desorbs from the second layer with first-order desorption kinetics. Dissocn. of molecularly adsorbed C₂H₆ is negligible upon subsequent heating. The trapping probability of C₂H₆ on the clean surface at normal incidence decreases from .apprx.0.91 to 0.13 as the incident translational energy (E_t) increases from 6 to 40 kJ/mol at a surface temp. of 95 K in apparent semiquant. agreement with a modified hard-cube model. However, over a range of incident angles, Q_i, the initial trapping probability scales with E_t(cos² Q_i) demonstrating the participation of momentum parallel to the surface in the trapping process and the necessity of more sophisticated theories, such as 3-dimensional trajectory calcns., to describe adequately trapping over a wide range of incident translational energies and incident angles. At all incident translational energies studied, the trapping probability of C₂H₆ at a surface temp. of 95 K increases continuously with C₂H₆ coverage up to monolayer satn., indicating that C₆H₆ traps more efficiently onto adsorbed C₂H₆ than onto a clean surface. This behavior is expected since the mass of adsorbed C₂H₆ is significantly less than that of a Pt atom, leading to increased energy transfer upon impact with the adsorbed species. Based on this principle, extrinsic precursor states are expected for mol. adsorption on all surfaces except at translational energies that preclude trapping into the second layer. Together with mol. beam expts. of C₂H₆ dissocn. on Pt(111) performed in the authors' lab., the exptl. results indicate that dissociative C₂H₆ adsorption on Pt(111) proceeds more effectively via direct collisional activation at high incident translational energies rather than via a precursor-medicated mechanism at low incident translational energies.