Experimental confirmation of ponderomotive-force electrons produced by an ultrarelativistic laser pulse on a solid target

First measurements of relativistic electrons produced by the interaction between a 400-fs, 1.06-mu m ultrahigh-intensity laser pulse (I > 10(19) W/cm(2)) and a solid target are presented. For electrons ejected along the laser propagation axis in the forward direction, the relation of the hot temperature T-h versus the laser intensity I is found to be in very good agreement with a J x B ponderomotive model described by S.C. Wilks et al. [Phys. Rev. Lett. 69, 1383 (1992)], where T-h = (gamma(osc) - 1)mc(2). Hot temperature of transverse electrons scales as T-h similar to I-1/3.