SUBPICOSECOND TRANSIENT INFRARED-SPECTROSCOPY OF ADSORBATES - VIBRATIONAL DYNAMICS OF CO/PT(111)

The vibrational dynamics of excited CO layers on Pt(111) were studied using infrared (IR) pump-probe methods. Resonant IR pulses of 0.7 ps duration strongly pumped the absorption line (nu almost-equal-to 2106 cm-1) of top-site CO. Weak probe pulses delayed a time t(D) after the pump were reflected from the CO-covered Pt(111) surface, and dispersed in a monochromator to determine the absorption spectrum of the vibrationally excited CO band, with time resolution < 1 ps and monochromator resolution < 1 cm-1. Transient spectra were obtained as a function of CO coverage, surface temperature, and laser fluence. Complex spectra for t(D) < 0 show features characteristic of a perturbed free induction decay, which are expected based on multiple-level density-matrix models. For t(D) greater-than-or-equal-to 0, the CO/Pt absorption exhibits a shift to lower frequency and an asymmetric broadening which are strongly dependent on fluence (1.3-15 mJ/cm2). Spectra return to equilibrium (unexcited) values within a few picoseconds. These transient spectral shifts and the time scale for relaxation do not depend (within experimental error) on coverage for 0.1 less-than-or-equal-to THETA-CO less-than-or-equal-to 0.5 ML or on temperature for 150 less-than-or-equal-to T(s) less-than-or-equal-to 300 K. A model for coupled anharmonic oscillators qualitatively explains the t(D) > 0 spectra in terms of a population-dependent decrease in frequency of the one-phonon band, as opposed to a transition involving a true CO(upsilon = 2) two-phonon bound state. The rapid relaxation time and its insensitivity to T(s) and THETA-CO are consistent with electron-hole pair generation as the dominant decay mechanism.