SUBPICOSECOND PROBING OF VIBRATIONAL-ENERGY TRANSFER AT SURFACES

Subpicosecond infrared pump-probe techniques are applied to the excited vibrational state dynamics of adsorbed CO on Pt(111). The CO (upsilon = 1) adlayer is probed as a function of IR pump fluence and CO coverage (0.1 less-than-or-equal-to THETA(CO) less-than-or-equal-to 0.5 ML). Spectral shifts in the transient CO vibrational spectrum are observed, indicative of a shift to lower frequency of the CO internal stretch mode as the degree of excitation of the adlayer is increased. The observed transient spectral response is discussed in terms of density matrix models which address the coherence time of the adlayer (upsilon = 1) excitation, and in terms of the excited state spectral characteristics of strongly coupled anharmonic oscillators. The two-to-three picosecond recovery time of the transient response is consistent with relaxation through electron-hole pair creation.