  Approaching energy coupling in laser-irradiated metals, we point out the role
of electron-electron collision as an efficient control factor for ultrafast
optical absorption. The high degree of laser-induced electron-ion
nonequilibrium drives a complex absorption pattern with consequences on the
transient optical properties. Consequently, high electronic temperatures
determine largely the collision frequency and establish a transition between
absorptive regimes in solid and plasma phases. In particular, taking into
account umklapp electron-electron collisions, we performed hydrodynamic
simulations of the laser-matter interaction to calculate laser energy
deposition during the electron-ion nonequilibrium stage and subsequent matter
transformation phases. We observe strong correlations between optical and
thermodynamic properties according to the experimental situations. A suitable
connection between solid and plasma regimes is chosen in accordance with models
that describe the behavior in extreme, asymptotic regimes. The proposed
approach describes as well situations encountered in pump-probe types of
experiments, where the state of matter is probed after initial excitation.
Comparison with experimental measurements shows simulation results which are
sufficiently accurate to interpret the observed material behavior. A numerical
probe is proposed to analyze the transient optical properties of matter exposed
to ultrashort pulsed laser irradiation at moderate and high intensities.
Various thermodynamic states are assigned to the observed optical variation.
Qualitative indications of the amount of energy coupled in the irradiated
targets are obtained.
  Keywords: ultrafast absorption ; umklapp electron-electron collision ;
collisional absorption ; laser-matter interaction
