Thermal CO desorption from photoexcited free metal-carbonyl clusters has been resolved in real time using two-color pump-probe photoelectron spectroscopy. Sequential energy dissipation steps between the initial photoexcitation and the final desorption event, e.g., electron relaxation and thermalization, have been resolved for ${\mathrm{Au}}_2(\mathrm{CO}{)}^{-}$ and ${\mathrm{Pt}}_2(\mathrm{CO}{)}_5^{-}$. The desorption rates for the two clusters differ considerably due to the different numbers of vibrational degrees of freedom. The unimolecular CO-desorption thresholds of ${\mathrm{Au}}_2(\mathrm{CO}{)}^{-}$ and ${\mathrm{Pt}}_2(\mathrm{CO}{)}_5^{-}$ have been approximated by means of a statistical Rice-Ramsperger-Kassel calculation using the experimentally derived desorption rate constants.

• Received 23 October 2001

DOI:https://doi.org/10.1103/PhysRevLett.88.076102