Ultrafast reaction dynamics of the associative desorption of C+O from Ru(001)

Femtosecond laser excitation of Ru(001) covered with atomic carbon and oxygen results in the recombinative desorption of CO molecules from the surface. A nonlinear fluence dependence of the reaction yield is found for excitation wavelengths of 400 and 800 nm. The higher reaction yield obtained for the 400-nm as compared to the 800-nm excitation can be attributed to the different optical penetration depths in Ru(001). As a consequence, higher surface temperatures rather than significant contributions of non-thermalized hot electrons cause the stronger laser-induced desorption with 400-nm pulses. Furthermore, the reaction mechanism of the CO recombination is investigated with two-pulse correlation measurements which exhibit a FWHM of ∼20 ps. However, despite this relatively wide two-pulse correlation (which usually is taken as an indication for a purely phonon-mediated energy flow), all experimental data are only consistently reproduced in the framework of frictional coupling between the substrate and the adsorbate, if both phonon and electronic contributions are included