Photoemission, Fluorescence, and Photochemical Studies of Molecular Adsorbates on Metal and Semiconductor Surfaces

摘要

Three different pathways have been studied for the relaxation of an optically excited adsorbate/surface system. The first of these, photoemission, is characterized via the molecular coverage dependence, and time response of the total photoyield. The observed increase in the yield for the initial adsorbed monolayer is attributed to a lowering of the work function of the Ag(111) surface while the monotonic decrease in the photoyield upon further adsorption is interpreted as attenuation of the metal photoelectrons by the multilayers. Evidence of the structural phase transition for pyridine on Ag(111) is seen in both the coverage dependence and photoyield thermal desorption spectra. Long (approx. = 100 ns) photoyield decay times are observed for multilayer films. Models which account for the effects of trapping and diffusion of electrons in the molecular film under the influence of the image potential are discussed. Radiative emission as a relaxation mechanism for an excited molecule is difficult to observe when adsorbed on a non-insulator surface because of damping by efficient energy transfer to the solid. The first detection of fluorescence from a molecular monolayer on such a surface is reported for the system of tetracene/Si(111). The observed changes in the fluorescence spectrum as a function of coverage are interpreted in terms of aggregate formation. Photochemistry is a third path by which an excited adsorbate may relax, although its reactivity is also expected to be severely limited by competition with nonradiative energy transfer. Photodecomposition for a variety of aromatic molecules adsorbed on roughened Ag surfaces is reported using continuous radiation in the near-uv/visible. (ERA citation 12:028812)