Proof of the Resonant Acceleration Mechanism for Fast Electrons in Gaseous Laser Targets

We report the observation of fast-electron emission (10-20 keV) from gaseous targets irradiated by a C${\mathrm{O}}_{2}$ laser pulse. The electrons are emitted in a direction 30\ifmmode^\circ\else\textdegree\fi{} away from the electric vector, but in the plane of optical polarization. This is the first direct experimental observation of the resonant absorption effect, which has been proposed as being responsible for the efficiennt laser energy deposition in a plasma. In contrast, the parametric decay instability is not consistent with the observations.