Photochemistry and Photo-Induced Ultrafast Dynamics at Metal Surfaces

Abstract

Abstract This account describes excitation mechanisms and ultrafast nuclear dynamics at metal surfaces induced upon photon irradiation. After reviewing possible excitation mechanisms for photochemistry of adsorbates on metal surfaces, we discuss the ultra-violet photochemistry of saturated hydrocarbons, i.e., methane and cyclohexane, on metal surfaces. Although these alkanes in the gas phase do not absorb photons at ≈6 eV, CH bond dissociation takes place upon 6.4-eV photon irradiation on metal surfaces. Hybridization between CH antibonding orbitals of the alkanes and metal bands, forming a new band across the Fermi level, is proposed to be responsible for both the photochemistry and CH vibrational mode softening. Since electronic relaxation at metal surfaces takes place very rapidly, most of excited adsorbates do not proceed to dissociation and/or desorption, but rather they are vibrationally excited. To probe photo-induced nuclear motions in real time, we developed femtosecond (fs) time-resolved second harmonic generation spectroscopy on metal surfaces. This method was used to explore the dynamics of photo-excited coherent vibration at Pt(111) surfaces covered with alkali-metal atoms. Irradiation of fs pump-laser pulses induced coherent vibrational motions of the stretching vibration of alkali atoms with respect to the metal surface and the Rayleigh modes of the Pt surface, which manifest themselves in modulations of second harmonic intensity of probe pulses. We also demonstrated that selective excitation of a phonon mode can be achieved by using tailored light pulse trains.