Femtosecond Pump/Probe Spectroscopy of CO on Ru{101̄0} from Experimental and Theoretical Perspectives

The effect of an 800 nm pump pulse on the C−O vibrational stretch of carbon monoxide adsorbed on Ru{101̄0} is followed in real time with vis−IR sum frequency spectroscopy. The observed transient red shift can be modeled by the thermal effect of the laser, acting upon one of the adsorbate phonon modes. We find that the temperature of the mode could couple to the substrate electron heat bath with a time constant of 5−10 ps, while also assuming coupling to the lattice heat bath. We also consider the effect of the degree of electronic coupling (and other rapid effects) upon the theoretical form of such spectra, and from this conclude that the observed red shift in this case must indeed be due to thermal effects.