VIBRATIONAL-EXCITATION IN UV-PHOTOEMISSION OF ADSORBATES

By careful preparation of layered physisorbate systems, i.e. molecular monolayers on rare gas spacer layers, new insight can be gained into lineshape analysis of adsorbate photoemission, and even vibrational finestructure can be resolved in favorable cases. The most remarkable result is the failure of the conventional approach to describe physisorbate photoemission spectra by convolution of vibrational spectra of isolated molecules with some broadening function. Instead it is found that the lineshape may best be described by a superposition of two harmonic oscillators if the photohole delocalizes on the timescale of the vibrational motion. Other major contributions to the observed "solid state" broadening are variations of the effective vertical potential, but not lifetime broadening.