ENHANCED ANHARMONICITY IN THE INTERACTION OF LOW-Z ADSORBATES WITH METAL-SURFACES

Temperature-dependent surface extended x-ray-absorption fine-structure (SEXAFS) measurements on half a monolayer of O and N atoms on Ni(100) and Cu(100) surfaces indicate enhanced anharmonic contributions in the adsorbate-substrate pair potential, with respect to bulk values. From the temperature dependence of the SEXAFS amplitude and phase we calculate the thermal expansion coefficients. These coefficients are found to be enhanced independent of the system, establishing the observed anharmonicity as a general surface-related property. The implications of these findings for surface reconstructions are discussed. © 1990 The American Physical Society.