Femtosecond time-resolved studies of desorption from a surface

Abstract

Desorption induced by pulses of visible light of 200-fs duration has been investigated for the model system of nitric oxide (NO) molecules absorbed on a Pd(lll) surface. The nature of the desorption process has been examined through characterization of the final-state energy distributions (rotational, vibrational, and translational) of the desorbed NO molecules.1 Important observations include the high desorption yield and its strong nonlinear dependence on laser intensity, as well as the high degree of vibrational excitation of the desorbed NO molecules. These results indicate that the desorption process cannot be either a conventional thermal or a photochemical mechanism. Recently, direct time-domain investigations of the desorption process have been undertaken. These measurements are accomplished by a correlation technique in which the total desorption yield is recorded as a function of the separation in time between two equivalent excitation pulses. In this manner, a subpicosecond relaxation time is inferred. This finding is compatible with an electron mechanism for desorption but appears to preclude a desorption process based on vibrational excitation of the substrate. The nature of the desorption process will be discussed in terms of calculations of the electronic and lattice temperature of the Pd substrate induced by the laser radiation.