||The main content of this thesis is divided into two parts which relate to the two fields of surface science: surface desorption and surface diffusion. In the first part, we have investigated the desorption of diatomic (CO) and polyatomic (NH3, ND3) molecules from Pt(111) surface using femtosecond laser. We find that the femtosecond laser-induced desorption of ammonia is very different from that of carbon monoxide from Pt( 111). The strong laser pulse width dependence of carbon monoxide desorption shows CO desorbs via a hot-electron driven process while NH3(ND3) desorbs by the conventional thermal process even under femtosecond laser irradiation. Little isotope effect between the desorption of NH3 and ND3 also supports our conclusion about ammonia desorption. As we critically examined femtosecond desorption of CO from Pt(111), we find the short-lived antibonding states at lower energy predicted by the “Adiabatic Model” of Jennison et.al. could be excited by the photoelectrons, and the excitation of these states would allow desorption to occur via DIMET mechanism. In the second part, we have studied the surface diffusion of H on W(100) surface. From the low energy electron diffraction (LEED) studies, it is known that the W(100) surface undergoes a reversible phase transition between a c(2x2) structure at low temperature and a (1x1) disordered structure at high temperature with the hydrogen present up to a coverage of 0.3 ML. Our diffusion data show that an anomalous reduction of diffusion occurs near the phase transition temperature(Tc). Combining the diffusion data with the LEED patterns, we conclude that the H diffusion near Tc is affected by phase transition of W(100) substrate which is predicted by previous theoretical studies.