Picosecond measurement of substrate-to-adsorbate energy transfer: The frustrated translation of CO/Pt(111)

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Abstract
The transient infrared response of CO/Pt(111) following picosecond visible excitation is reported. A spectrally broad decrease in reflectivity correlates with heating of the Pt lattice, and an observed shift in the CO(v=0→1) transition is interpreted as heating of the 60 cm−1 in-plane frustrated translational mode. A phenomenological three temperature model that assumes the adsorbate vibrational temperature Tads exclusively couples to either the electronic temperature Te (with a time constant τe) or to the lattice temperature Tlat (with a time constant τlat) describes the temporal response of the adsorbate vibrations. The lattice phonon temperature Tlat(z,t) and measured temperature dependence of the optical constants predict the observed spectrally broad reflectivity change. Density matrix methods model the infrared response of the transiently heated molecule. Limits of τe=2±1 ps or τlat<1 ps are established by comparison of predicted spectra and the data.