  Among the various attempts to understand collisionless absorption of intense
ultrashort laser pulses a variety of models has been invented to describe the
laser beam target interaction. In terms of basic physics collisionless
absorption is understood now as the interplay of the oscillating laser field
with the space charge field produced in the plasma. A first approach to this
idea is realized in Brunel's model the essence of which consists in the
formation of an oscillating charge cloud in the vacuum in front of the target.
The investigation of statistical ensembles of orbits shows that the absorption
process is localized at the ion-vacuum interface and in the skin layer: Single
electrons enter into resonance with the laser field thereby undergoing a phase
shift which causes orbit crossing and braking of Brunel's laminar flow. This
anharmonic resonance acts like an attractor for the electrons and leads to the
formation of a Maxwellian tail in the electron energy spectrum. Most remarkable
results of our investigations are the Brunel-like hot electron distribution at
the relativistic threshold; the minimum of absorption at $I\lambda^2 \cong
(0.3-1.2)\times 10^{21}$ W/cm$^2\mu$m$^2$, in the plasma target with the
electron density of $n_e \lambda^2\sim 10^{23}$cm$^{-3}\mu$m$^2;$ the drastic
reduction of the number of hot electrons in this domain and their reappearance
in the highly relativistic domain; strong coupling of the fast electron jets
with the return current through Cherenkov emission of plasmons. The hot
electron energy scaling shows a strong dependence on intensity in the
moderately relativistic domain $I\lambda^2 \cong (10^{18} - 10^{20})$
W/cm$^2\mu$m$^2$, a scaling in vague accordance with current published
estimates in the range $I\lambda^2 \cong (0.14-3.5)\times 10^{21}$
W/cm$^2\mu$m$^2$, and a distinct power increase beyond $I=3.5\times 10^{21}$
W/cm$^2\mu$m$^2$.
