TIME-RESOLVED MEASUREMENTS OF SUBSTRATE TO ADSORBATE ENERGY-TRANSFER

The use of ultrafast infrared pulses to characterize the transient response of visible and ultraviolet pulse-excited CO/Pt(111) is described. A 15 200 cm-1 (1.9 eV) pump pulse of 1.0 ps duration and incident energy density of ca. 16 mJ cm-2 excites electron-hole pairs in the platinum. The accompanying transient response leads to an overall heating of the system, as revealed by a decrease in infrared reflectivity of the platinum and a shift to lower frequency of the 2106 cm-1 CO-stretching mode. Unlike the transient response observed for resonant infrared pumping of the CO stretch, the shift associated with visible pumping persists for over 50 ps and is believed to reflect activation of the 60 cm-1 frustrated CO-translation mode which is anharmonically coupled to the CO stretch. By using a density matrix formalism for the IR response to the pump-induced CO(v = 0 --> 1) frequency shift, and incorporating a three-temperature model for the CO/Pt(111) surface (T(e), T(lat), and T(ads)), fitting of the measured spectral response as a function of time-delay places limits on the timescale for energy transfer from the substrate to the adsorbed CO. Similar results were obtained with 34 612 cm-1 (4.3 eV) pump pulses of < 1.2 ps duration.