TEMPERATURE-DEPENDENCE OF THE FREQUENCY-SHIFT OF ADSORBATE VIBRATIONS VIA INTERACTION WITH SUBSTRATE PHONONS - CO ON PT(111)

We analyse the effects of various anharmonic interactions between an adatom and a surface atom on the vibrational lineshape of the adsorbate, especially on the temperature dependence of the frequency shift. We find that, in the case where the surface atom vibrates perpendicularly to the surface, the anharmonic coupling produces a red shift in the frequency of the adsorbate vibration as the temperature increases, and varies in proportion to T at high temperatures. In contrast, surface vibrations parallel to the surface are more likely to give a blue shift to the adsorbate vibration. When applied to the Pt-CO stretch of CO on Pt(111) at low coverages, excellent qualitative agreement can be obtained between the experiment and our theory involving the perpendicular modes; quantitatively, the predicted shift is unable to account for the experiment unless the anharmonicity at the surface is much greater than a Morse potential would predict. © 1990.