Enhanced infrared absorption of CO on smooth iron ultrathin films in correlation to their crystalline quality

We studied the infrared absorption lines for the stretching mode of CO on ultrathin Fe(1 0 0) prepared as smooth as possible on MgO(0 0 1) by various methods. For films that gave different atomic force microscopic pictures but similar LEED patterns, the signal strength and the CO-line shapes were investigated in normal transmission and also in reflection at grazing incidence of p-polarized light. The comparison of both types of spectra for various degrees of film smoothness made obvious that asymmetric absorption lines may occur only in the susceptibility component parallel to the surface. The oscillator strength of these asymmetric lines is correlated to the number of holes in the film. The usual reflectance change that arises due to CO vibrations perpendicular to the film may be considerably modified by the adsorbate induced changes in the parallel susceptibility.