Irradiation of a Ru(001) surface covered with CO using intense femtosecond
laser pulses (800 nm, 130 fs) leads to desorption of CO with a nonlinear de
pendence of the yield on the absorbed fluence (100-380 J/m(2)). Two-pulse c
orrelation measurements reveal a response time of 20 ps (FWHM). The lack of
an isotope effect together with the strong rise of the phonon temperature
(2500 K) and the specific electronic structure of the adsorbate-substrate s
ystem strongly indicate that coupling to phonons is dominant. The experimen
tal findings can be well reproduced within a friction-coupled heat bath mod
el. Yet, pronounced dynamical cooling in desorption, found in the fluence-d
ependence of the translational energy, and in a non-Arrhenius behavior of t
he desorption probability reflect pronounced deviations from thermal equili
brium during desorption taking place on such a short time scale. (C) 2000 A
merican Institute of Physics. [S0021-9606(00)70122-X].