TIME-RESOLVED PROBES OF SURFACE DYNAMICS

The excited vibrational state dynamics of CO adsorbed on Pt(111) are explored using subpicosecond infrared pump-probe techniques. The response for a Co(v = 1) adlayer for coverages of 0.1, 0.3, and 0.5 monolayers is measured as a function of time delay and probe wavelength. The observed transient CO vibrational spectra reveal a shift to lower frequency of the CO internal stretch mode as the degree of excitation of the adlayer is increased. These spectra are discussed in terms of density matrix models which address the coherence time of the adlayer (v = 1) excitation and 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. The relationship of the measured T1 decay to the observed lineshape is addressed.