Inelastic Multiphonon Helium
Scattering from a Stepped Ni(977) Surface


Figure 1

Energy transfer from a gas phase atom or molecule to a solid surface has been the subject of much study because of its importance in energy accommodation, sticking and other gas-surface phenomena. The details of energy exchange, including angular dependencies and energy and momentum partitioning, can be precisely examined with the use of molecular beam-surface scattering experiments.

We have measured the multiphonon energy exchange between a neutral He atom and a stepped Ni(977) surface in order to examine how the presence of a regular array of atomic-scale steps on a surface modifies energy exchange in the classical multiphonon scattering regime. At elevated
Figure 2
substrate temperatures, we compare the multiphonon scattering with the predictions of a classical theory that has previously been used by others for assessing energy exchange involving a smooth surface (Figure 1). When the temperature and angular dependence of the scattered multiphonon intensity is compared to this theory we found a significant discrepancy between the predictions and our experimental data, i.e., the momentum partitioning shows behavior deviating from the predictions of the theory (Figure 2). Because this theory has been used previously under similar conditions to successfully describe the details of the multiphonon energy exchange for helium scattered from a flat Cu(001) surface, we attribute the differences we see to be due to the difference between a smooth and stepped surface. Specifically, changes in the vibrational modes and associated surface density of states due to the presence of extended surface defects have a fundamental impact on the details of the energy exchange mechanism. The presence of step-localized optical phonon modes may account for this discrepancy.

Previous work has shown that the dominant phonons in multiphonon energy exchange are the low energy Rayleigh modes. On this stepped surface, however, two other low energy modes have been measured and identified as phonons localized at the step. These are optical modes with behavior away from the Brillioun zone center very similar to the surface Rayleigh mode. Near the zone center (at small parallel momentum transfer), though, the energy of each observed mode deviates from the Rayleigh mode, so that at DK = 0, where the Rayleigh mode goes to zero, the step modes still have energies of about 5 meV and 7 meV. These phonon modes are therefore easily accessible during inelastic collisions of a 60 meV He atom and the surface. In addition, the step localized optical modes have appropriate polarizations for collisionally induced energy transfer, with nonzero projections along the surface normal. Thus, significant energy transfer can even occur during collisions with little or no parallel momentum transfer, a condition unique to this stepped surface.



72. "Inelastic Multiphonon Helium Scattering from a Stepped Ni(977) Surface"

    D.J. Gaspar, A. T. Hanbicki, and S.J. Sibener, J. Chem. Phys. 109 6947-6955 (1998) Abstract



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