Subpicosecond transient infrared spectroscopy of adsorbates. Vibrational dynamics of CO/Pt(111)

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Abstract
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 (ν≊2106 cm−1 ) of top‐site CO. Weak probe pulses delayed a time tD 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. Transient spectra were obtained as a function of CO coverage, surface temperature, and laser fluence. Complex spectra for tDtD≥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≤ΘCO≤0.5 ML or on temperature for 150≤Ts≤300 K. A model for coupled anharmonic oscillators qualitatively explains the tD>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(v=2) two‐phonon bound state. The rapid relaxation time and its insensitivity to Ts and ΘCO are consistent with electron–hole pair generation as the dominant decay mechanism.