Understanding the dynamics of molecule-surface interaction with lasers and molecular beams

Understanding the atomic scale motion of chemical reactions occurring on metal surfaces has become an important topic of research, one that is motivated by its technological implications for heterogeneous catalysis. The importance of heterogeneous catalysis for humankind can hardly be overestimated. Its application range from pollution abatement to food industry. Unfortunately, even today, in most cases we are lacking detailed molecular level understanding of these processes. Studies carried out under ultra-high vacuum conditions, with clean surfaces and with molecules in well-defined internal state distributions provide a wealth of information based on which a detailed microscopic view of the process can be constructed. In my talk I will refer to recent studies of diatomics collisions with single crystal coinage metals studied by combination of molecular beams and laser spectroscopy, exemplifying how detailed studies of vibrational energy transfer, rotational and translational inelasticity can provide detailed insights into the dynamics of gas surface encounters. Special emphasis will be given on our recent observations of anomalously slow vibrational relaxation during molecular adsorption and desorption on a metal surface that suggest that vibrationally-promoted surface chemistry can occur via the most common mechanism of heterogeneous catalytic chemistry on metal surfaces that is of Langmuir and Hinshelwood (LH). In this study, using molecular beam and laser-based preparation and detection methods, we have directly detected adsorption and subsequent desorption of vibrationally excited CO molecules from a Au(111) surface. These experiments show that adsorption, translational thermalization, and desorption can occur without the equilibration of molecular vibration. These results suggest that vibrational promotion of LH chemistry is possible even on a metal substrate, where dissipation of excess vibrational energy of adsorbates is otherwise known to be very efficient. (author)