PICOSECOND MEASUREMENT OF SUBSTRATE-TO-ADSORBATE ENERGY-TRANSFER - THE FRUSTRATED TRANSLATION OF CO PT(111)/

The transient infrared response of CO/Pt(111) following picosecond vis ible excitation is reported. A spectrally broad decrease in reflectivi ty correlates with heating of the Pt lattice, and an observed shift in the CO(upsilon = 0 --> 1) transition is interpreted as heating of the 60 cm-1 in-plane frustrated translational mode. A phenomenological th ree temperature model that assumes the adsorbate vibrational temperatu re T(ads) exclusively couples to either the electronic temperature T(e ) (with a time constant tau(e)) or to the lattice temperature T(lat) ( with a time constant tau(lat)) describes the temporal response Of the adsorbate vibrations. The lattice phonon temperature T(lat)(z,t) and m easured temperature dependence of the optical constants predict the ob served spectrally broad reflectivity change. Density matrix methods mo del the infrared response Of the transiently heated molecule. Limits o f tau(e) = 2 +/- 1 ps or tau(lat) < 1 ps are established by comparsion of predicted spectra and the data.