THEORY OF THE VIBRATIONAL LINE-SHAPE OF AN ADATOM AT A BRIDGE SITE

The homogeneous vibrational line shape of an adatom on a surface is de termined by its interactions with other localized and delocalized mode s of the adsorbate and substrate. These interactions include anharmoni c bonds between the adatom and the surface, and also between the subst rate atoms themselves at the adsorption site. The line width and frequ ency shift of the perpendicular vibrational mode of an adatom at a bri dge site is calculated as a function of temperature, due to coupling t o phonon modes of a semi-infinite elastic continuum. A self energy for mulation is employed, using perturbative methods in a quantum field th eory. The effects of anharmonicity of both the adsorption bonds and of the interatomic bonds between substrate atoms at the bridge site, on the vibrational line shape, are compared. Energy relaxation by two-pho non emission is found to describe well the temperature-dependent line width in the O/Cu(110) system.