AVS 64th International Symposium & Exhibition | |
Surface Science Division | Wednesday Sessions |
Session SS+HC+NS-WeA |
Session: | Dynamical Processes at Surfaces |
Presenter: | Eric High, Tufts University |
Authors: | E. High, Tufts University E.K. Dombrowski, Tufts University A.L. Utz, Tufts University |
Correspondent: | Click to Email |
A modified King and Wells molecular beam reflectivity method was used to obtain thermal and quantum state resolved initial sticking probabilities (S0) for CH4 on nickel single crystals at elevated surface temperatures (500 to 1000K). We recorded the reactivity of a supersonic molecular beam of methane with and without laser excitation to v=1 of the v3 antisymmetric C-H stretching vibration. Square wave modulation of the laser source during CH4 deposition provided simultaneous real-time measurement of S0LaserOff and S0v3, resulting in a dramatic reduction in data acquisition time, a significant reduction in experimental error, and the opportunity to measure S as a function of accumulating surface coverage. On a Ni(111) surface, both laser-off and vibrational state-resolved measurements of S0 were essentially independent of surface temperature, within error, from 1000 to 800K and then decreased linearly as Tsurf dropped from 800 to 500K. These findings are consistent with the predictions of Reaction Path Hamiltonian calculations by Jackson and Guo over a wide range of incident kinetic energies (96 to 167 kJ/mol). The calculations suggest that at low incident energies, reactivity occurs predominantly at on-top sites, but as energy increases, reactions begin to occur at bridge sites as well. We will also present more recent measurements on the low step density Ni(997) surface that provide insight into the role of step-edge nickel atoms at these industrially relevant surface temperatures.