Probing adsorbate-substrate coupling: The isotope effect in femtosecond laser-induced desorption

The coupling between adsorbed molecules and a substrate constitutes one of the most fundamental interactions in surface science. In this thesis, we present investigations of the nature of adsorbate-substrate energy flow in the model system of O2/Pd(111). Our studies have been conducted using excitation of the system by femtosecond laser pulses. Such radiation provides a distinctive tool to examine adsorbate-substrate coupling by producing substrate excitation in which the electronic and lattice degrees of freedom, each separately in approximate internal thermal equilibrium, can be driven significantly out of equilibrium with one another. One can thus induce high substrate electronic temperatures with a cold lattice, which facilitates the study of non-adiabatic coupling of substrate electronic excitation to adsorbate nuclear motion.; Particular attention has been focused on a comparative investigation of the behavior of the two oxygen isotopes 16O2 and 18O2. It is found that the lighter isotope of oxygen desorbs under femtosecond laser excitation with a probability of 1.8 +/- 0.3 times higher than the heavier species. This large isotope effect implies that the desorption process occurs with the adsorbate system significantly out of thermal equilibrium with both the electronic and lattice degrees of freedom of the substrate. A pronounced isotope effect is indeed a hallmark of the nonthermal processes associated with the conventional photochemistry that occurs for lower laser excitation powers. Despite this qualitative agreement with the behavior expected for a conventional photochemistry, the measurements reported here cannot be reconciled with the standard models of such photochemical processes. To understand the isotope results, along with other previous findings for femtosecond laser-induced processes, one turns to a quasi-equilibrium model that treats the distinct degrees of freedom as internally thermalized, while allowing their temperatures to couple over a finite time scale. Use of simple models based on such electronic coupling leads to semi-quantitative agreement with experiment for isotope effect without the introduction of any adjustable parameters.