Energy transfer plays a key role in the sticking and scattering of atoms and molecules incident on solid surfaces. It is generally known that trapping of gas species, the initial step in adsorption, is mediated by phonon excitations. However, adsorption of H atom on metal surfaces raises an interesting and fundamental question concerning the mechanism of energy transfer because phonon excitation is expected to be inefficient due to the very small mass and the short interaction time. To gain insight into H-metal surface interactions, we have measured the sticking probability of H atom on Cu(111) and Pt(111) at 85 K for various incidence angles.
An effusive thermal H atom beam was produced by prolysis of H2 molecules in a hot (~1940 K) W capillary tube, and the beam flux on the sample surface was estimated to be 3.1×1013 atoms /cm2 s. The absolute coverage of H atom was calibrated by comparing the TPD signal with that of the saturated Si(100):H(2x1) phase.
The sticking probability of H atom on Cu(111) was measured to be 0.37 at normal incidence and monotonically increases to reach 0.52 at grazing angles, whereas it varies from 0.73 to ~ 1.0 on Pt(111). One can expect less efficient phonon loss by H atom on Pt(111) because of the lower Debye temperature of Pt(230 K) compared with that of Cu(111) (310 K). The results contrary this expectation implies that phonon loss is not the major energy transfer mechanism. Bound state resonance can also leads to trapping of H atom, in which the coupling is determined by the surface potential corrugation. H atoms adsorb on fcc hollow sites with a comparable adsorption energy of ~2.4 eV on both surfaces, and therefore the surface corrugation is similar for the two cases. Thus, we suggest that electron-hole pair excitations[1] play a significant role in the sticking of H atom on metal surfaces and the larger sticking probability of H atom observed on Pt(111) could be due to more efficient e-h pair excitations because of the large density of states of Pt(111) at the Fermi level. We will also briefly discuss the effect of incidence angle.
[1] H. Nienhaus, H. S. Bergh, B. Gergen, A. Majumdar, W. H. Weinberg and E. W. McFarland, Phys. Rev. Lett. 82, 446 (1999).
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