State selective time-of-flight measurements on surface reaction products.

In recent years enormous progress has been made in the understanding of the dynamical processes which govern adsorption and desorption reactions on surfaces. In the case of recombinative desorption of molecules from surfaces one can distinguish between two extreme cases: Recombination of thermalized atoms on the surface (Langmuir-Hinshelwood reaction) and recombination of impinging atoms with thermalized atoms (Eley-Rideal reaction). The final state of the nascent molecule (rotational- , vibrational-, and translational state) may be significantly different for these two reaction types due to the different energetics involved. Reactions of the latter type play an important role for various technological processes, e.g. etching in semiconductor Industry, production of synthetic diamond, hydrogen storage in metals, plasma-wall interaction. The main objective of this proposal is to study simultaneously all degrees of freedom of the molecules following an Eley-Rideal reaction. In particular we will focus on the model system hydrogen-aluminium. Hydrogen atoms produced in a highly efficient source will be aimed at hydrogen precovered aluminium surfaces. The hydrogen molecules originating from the Eley-Rideal reaction are then analyzed in a REMPI spectrometer. The REMPI (resonance enhanced multi photon ionization) spectroscopy allows the state selective detection (rotation, vibration) of hydrogen molecules via a resonant excitation and ionization process. In a specifically designed time-of-flight spectrometer the translational energy of the state selected hydrogen molecules can be measured. The main topics of this project will be as follows: Elucidation of the dynamics of Eley-Rideal reactions in general Determination of the energy partitioning into the degrees of freedom of the nascent molecule and into surface phonon excitation Influence of the surface structure on the Eley-Rideal reaction products Influence of surface coadsorbates on the Eley-Rideal reaction products

The fundamental understanding of the microscopic processes taking place when molecules impinge on a surface (adsorption) or leave a surface (desorption), is an important issue of surface science. In this project we have utilised a special laser-spectroscopic technique to tackle this problem. With this technique, called REMPI, (resonance enhanced multi photon ionisation) one can determine the population of the individual rotational and vibrational states of reacting and desorbing molecules. The goal of this project was the measurement of the flight time of these state selected molecules, in addition to the population of the rotational and vibrational states, in order to get additional information on the translational energy. This has been done by connecting a time of flight (TOF) spectrometer to a REMPI spectrometer, resulting in the new REMPI-TOF spectrometer. With this instrument we have studied the system hydrogen (deuterium) on various modified vanadium surfaces. This system is of technological importance in the context of energy storage, since vanadium is able to absorb huge amounts of hydrogen in the bulk. The most important findings can be summarised as follows: On a vanadium surface, either covered by oxygen or sulphur (these are the surfaces typically used for applications), the adsorption probability for molecular hydrogen is very small. The reason for the small sticking coefficient of molecular hydrogen is a high activation barrier for dissociative adsorption. In this case one can increase the adsorption probability if energy is channelled either into the translational, rotational or vibrational degree of freedom. If this assumption is true, it should also have some implications for the associative desorption processes, due to the validity of detailed balancing. This was exactly the question put forward in this project: Is the population of the individual degrees of freedom for the desorbing molecules as expected for equilibrated molecules or are there significant deviations? In fact, we have found that the translational energy distribution and the vibrational state distribution is hyper-thermal, but that the population of the rotational states is sub-thermal. These findings are of great importance for the microscopic understanding of the reaction processes taking place on a surface. The quantitative values obtained in this project are essential input parameters for the theoretical description of adsorption and desorption.