Abstract

The most promising aspects of ultrafast investigations of chemical systems are the possibility of resolving reactions on the time-scale of bond-formation and bond-breaking and of understanding the microscopic dynamical processes that drive reactions. For molecules adsorbed on solid surfaces, the relevant time-scale is 100 fsec: this time is short enough to resolve molecular bond changes as well as the transfer of energy between the electronic and vibrational degrees of freedom in the substrate and substrate-adsorbate complex. We have used 100 fsec time-resolved second-harmonic generation to probe the departure of adsorbed CO molecules from a Cu(111) surface after the surface is irradiated with a 100 fsec, 2 eV laser pump pulse. The desorption event is >90% completed within the first 325 fsec after the pump pulse. In contrast to techniques that measure the properties of desorbed molecules,[1] SHG allows us to probe the adsorbed molecules and in principle to directly measure the reaction time. Here, the reaction time is so short that we can identify the dynamical processes that drive this reaction. All conventional mechanisms (thermal and direct photochemical) are ruled out and a novel mechanism is proposed. We believe hot electrons, produced by 100 fsec excitation of the substrate, excite the adsorbate electronic state several times during the CO-metal stretch period and cooperatively pump the CO-metal stretch into high enough states to desorb the CO.

© 1992 The Author(s)