Thermodynamics and Kinetics of Two-Dimensional H2 Gas on Ag(111) Studied by Tip-Enhanced Raman Spectroscopy.

Surface diffusion is an essential elementary process of molecules on surfaces which undergo low-dimensional transport, aggregation, and catalytic reactions; however, evaluating the thermodynamics and kinetics of adsorbates rapidly diffusing on a surface as a two-dimensional gas (2DG) remains challenging even for direct observation by scanning tunneling microscopy. In this study, we analyze thermodynamic and kinetic processes of H2 physisorbed on Ag(111) using tip-enhanced Raman spectroscopy (TERS) at 7-14 K. The temperature evolution of the TERS intensity of the H2 rotational transition reveals that the concentration of the H2 2DG on the surface is maximum at 10-11 K, decreasing at lower temperatures due to molecular aggregation and as well at higher temperatures due to desorption from the surface. Based on Van't Hoff and Arrhenius equations, the potential energy diagram of the molecular aggregation and desorption are evaluated, demonstrating the utility of TERS for analyzing surface dynamics.