Tracking hot-electron induced adsorbate dynamics using surface nonlinear optical spectroscopy

　Hot carriers at metal surfaces can drive non-thermal adsorbate reactions. Characterizing non-equilibrium statistics among various degrees of freedom on an ultrafast time scale is crucial to understand and develop hot carrier-driven chemistry. In a series of works, we demonstrate multidimensional vibrational dynamics of carbon monoxide (CO) on metal surfaces along with hot-carrier induced desorption studied using time-resolved vibrational sum-frequency generation with phase-sensitive detection. Instantaneous frequency and amplitude of the CO internal stretching mode are tracked through a sub-picosecond time resolution, which is shorter than the vibrational dephasing time span. These experimental results, in combination with numerical analysis based on Langevin simulations, enable the extraction of nonequilibrium distributions of the external vibrational modes of desorbing molecules. Superstatistical distributions are generated with mode dependent frictional couplings within a few hundred femtoseconds of hot-electron excitation, and energy flow from hot electrons and intermode anharmonic coupling plays a crucial role in the subsequent evolution of non-Boltzmann distributions. (see commentary in Physics by APS)