The intensity of He atoms specularly reflected from a partially adsorbate-covered metal surface is determined by the tiny but existing corrugation of repulsive potential in the adsorbate-free area of the surface, since the He beam is diffusively scattered from the adsorbate-induced sigularities in scattering potential. The asymptotic decay constant of the local electron density giving the repulsive potential for scattered He is known to be proportional to the square root of the work function [1]. Since the corrugation amplitude sensed by He beam is a function of the He-scattered depth at a relevant kinetic energy, we can measure the local work function at the adsorbate-free area. In this study, we have found that the local work function at the adsorbate-free Pt(111) area is reduced by a small amount of adsorbed hydrocarbon. This result is consistent with the delocalization of the work function reduction observed with STM on the Cs-adsorbed Pt(111) surface [2].
On the clean Pt(111) surface, the surface reflectivity of scattered He beam decreases as increasing the kinetic energy of He, since the He molecules with higher kinetic energies are scattered at the deeper position of the surface, where the corrugation amplitude for the He molecules is larger. The Pt(111) surface with the hydrocarbon coverage of 0.009 is prepared by introducing the supersonic CH4 beams at the normal component of kinetic energy of 265 meV. And we have found that the decrease in surface reflectivity of He with respect to the kinetic energy becomes slower on the surface with adsorbates, where macroscopic work function is observed to be reduced. With increasing the coverage, the decrease in the reflectivity becomes further smaller. The results indicate the delocalization of work function reduction also in the case of Pt(111) covered with hydrocarbon adsorption.
[1] J. Harris and A. Liebsch, Phys. Rev. Lett. 49 (1982) 341.
[2] A. Sinsarp et al., Jpn. J. Appl. Phys. 42 (2003) 4882.
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