Optical excitation of metals with intense femtosecond laser pulses can create extreme non-equilibrium
conditions in the solid. The electronic system reaches several thousand degrees Kelvin on a
sub-picosecond time scale while the lattice (phonon bath) stays fairly cold. Such photoexcited hot
electrons may transiently attach to unoccupied adsorbate levels and this change in the electronic
structure may induce vibrational motions of the adsorbate substrate bond (see diagram below).
For high excitation densities with femtosecond pulses, multiple excitation/deexcitation cycles can
occur and may eventually lead to desorption of adsorbate molecules or reactions with co-adsorbed species.
After 1-2 ps the hot electron distribution equilibrates with the lattice and the heated phonon bath may
induce chemical reactions by thermal activation. Time-resolved experiments with a pair of two excitation
pulses allow distinction between phonon or electron driven mechanisms and provide insight into the
rate and pathways of energy flow in surface chemistry.