Structure and Dynamics of Adsorbates on Metal Surfaces Investigated with Nonlinear Optical Spectroscopy

Metal surfaces are a playground for heterogeneous reactions including catalysis and electrochemistry. They also serve as a template for thin film growth and an electrode in various devices. Thus, metal surfaces are important in both fundamental and applied sciences. This review presents two topics regarding the structure and dynamics of adsorbates on metal surfaces probed with sum frequency generation (SFG) spectroscopy. First, the directional orientation of water molecules in the ice crystalline thin film grown on a Pt(111) surface is described. Heterodyne detection of SFG makes it possible to determine the direction of water at the metal surface: they are preferentially oriented such that one of hydrogen atoms is directed toward the metal surface. This directional configuration propagates in the bulk of ice crystalline film through hydrogen bond network. Second, the ultrafast dynamics in the early stage of photo-stimulated desorption of CO on Cu(100) is described. Here the heterodyne detection of SFG is employed in pump-and-probe measurements. The phase and amplitude of SFG optical field obtained with this method are used for retrieving the perturbed free induction decay of CO stretch vibration polarization. This allows us to probe adsorbate dynamics leading to desorption induced by irradiation of an intense pump pulse. The ultrafast dynamics of adsorbates are the manifesta-tion of coupling between hot electrons in metal and frustrated motions of CO at the surface, which provide a clue for understanding nonadiabatic processes accompanying adsorbate motions, which are ubiquitous in metal and at its surface.