Electron simulated desorption

The good agreement between electrochemical and UHV data, documented in Figure 4, is a very important result, because it proves for the first time that the microscopic information which one obtains with surface science techniques in the simulation studies is indeed very relevant to interfacial electrochemistry. As an example of such microscopic information, Figure 5 shows a structural model of the inner layer for bromide specific adsorption at a halide coverage of 0.25 on Ag 110 which has been deduced from thermal desorption and low energy electron diffraction measurements /12/. Qualitatively similar models have been obtained for H2O / Br / Cu( 110) /18/and also for H2O/CI /Ag 110. ... 

The last years have witnessed tremendous progress in the theoretical description of surfaces and processes on surfaces. A variety of surface properties can now be described from first principles, i.e. without invoking any empirical parameters [1], In particular, whole potential energy surfaces (PES) can nowadays be mapped out by total energy calculations based on ab initio electronic structure theory. This development has also motivated new efforts in the dynamical treatment of adsorption/desorption processes in the last decade such as the development of efficient schemes for high-dimensional quantum dynamical simulations [2, 3]. 

For any dynamical simulation, a continuous representation of the PES is mandatory since the potential and the gradients are needed for arbitrary configurations. One can in fact perform ab initio molecular dynamics simulations in which the forces necessary to integrate the classical equations of motion are determined in each step by an electronic structure calculations. There have been few examples for such an approach [35-37], However, in spite of the fact that electronic structure calculations can nowadays be performed very efficiently, still there is a significant numerical effort associated with ab initio calculations. This effort is so large that in the ab initio dynamics simulations addressing molecular adsorption and desorption at surfaces the number of calculated trajectories has been well below 100, a number that is much too low to extract any reliable reaction probabilities. 

The adsorption behavior of diazines in X and Y zeolites has been studied by infrared spectroscopy (IR), temperature-programmed desorption (TPD), and simulation techniques. The studies showed that the interaction is determined by a donation of electron density from the nitrogen atoms of the probe molecules to the Lewis-acidic cations. The individual nature of the adsorption strongly depends on the Si/Al ratio of the zeolites, the kind of extraframework cation, and the positions of heteroatoms in the probe molecules. 

John C. Tully is Arthnr T. Kemp Professor of Chemistry, Physics, and Applied Physics in the Department of Chemistry at Yale University. Tully is a leading theorist studying the dynamics of gas surface interactions. He develops theoretical and computational methods to address fundamental problems and then works with experimentalists to integrate theory with observation. Energy exchange and redistribution, adsorption and desorption, and dissociation and recombination are among surface phenomena he has elucidated. He uses mixed quantum-classical dynamics, which allow the extension of conventional molecular dynamics simulation methods to processes involving electronic transitions or quantum atomic motion. He is a member of the National Academy of Sciences. 

---