Vibrational relaxation and pump-probe spectroscopies of adsorbates on solid surfaces

Recent experimental and theoretical progress in nonlinear pump-probe spectroscopies for vibrational relaxation of adsorbates on solid surfaces is reviewed. Among the extensive body of experimental works, typical examples of conventional infrared absorption spectra of hydrogen atoms on Si surfaces and carbon monoxide molecules chemisorbed on metal surfaces are presented to demonstrate how the mechanism of the vibrational relaxation and the corresponding relaxation time are inferred from the observed linewidth. The theoretical foundation of the pump-probe surface vibrational spectroscopy, which allows a direct determination of the relaxation time is presented on the basis of the density-matrix formulation. In the limit, where the time envelope of the pump and probe field is given by the delta-function, the analytical expressions are derived for the so-called transient absorption difference spectrum and sum-frequency-generation, which directly measure the energy (population) relaxation time T-1 and the total dephasing time T-2 of the vibrational excited states of adsorbates. Effects of nonlinear coherent interaction between pump and probe pulses are discussed for pulse durations shorter than the vibrational dephasing time. Experimental results of various transient adsorbate vibrational spectra in frequency and time domains are presented and discussed in comparison with the relaxation time estimated from the analysis of infrared absorption spectra of adsorbates. Vibrational dynamics of the adsorbate low-frequency mode, which plays an important role as an exchange partner in the dephasing process of high-frequency mode, is also reviewed in conjunction with transient vibrational response to ultrafast substrate heating by visible pulse irradiation.