FEMTOSECOND TIME-RESOLVED SURFACE-REACTION - DESORPTION OF CO FROM CU(111) IN LESS-THAN 325 FSEC

The laser-induced desorption of CO molecules from a Cu(111) surface is time resolved with 100-fsec resolution. We find the desorption reaction is completed in less-than-or-equal-to 325 fsec after the 100-fsec, 2-eV photon energy pump pulse. The only mechanism consistent with this abrupt, efficient (sigma is similar to 10(-18) cm2) desorption is a novel one: The transient hot substrate electrons (T(electron,max) is similar to 3600 K, while T(lattice,max) is similar to 160 K) drive multiple electronic excitation-deexcitation cycles of the CO-Cu complex within the vibrational relaxation time. Such cycles leave the molecules vibrationally excited in the ground state potential which enhances the desorption on subsequent electronic excitations.