Femtosecond laser pulses were applied to study the energy deposition depth
and transfer to the lattice for Au, Ni, and Mo films of varying thickness.
The onset of melting, defined here as damage threshold, was detected by mea
suring changes in the scattering, reflection and transmission of the incide
nt light. Experiments were done in multi-shot mode and single-shot threshol
d fluences were extracted by taking incubation into account. Since melting
requires a well-defined energy density, we found the threshold depends on t
he film thickness whenever this is smaller than the range of electronic ene
rgy transport. The dependence of the threshold fluence on the pulse length
and film thickness can be well described by the two-temperature model, prov
ing that laser damage in metals is a purely thermal process even for femtos
econd pulses. The importance of electron-phonon coupling is reflected by th
e great difference in electron diffusion depths of noble and transition met
als.