Experimental study of the interaction of subpicosecond laser pulses with solid targets of varying initial scale lengths

We have studied experimentally the angular and energy distribution of the suprathermal electrons produced during the interaction of a 120 fs, 50 mJ, 800 nm, $P$-polarized laser pulse on ${\mathrm{SiO}}_2$ targets. A sharply collimated jet of electrons is observed in the laser specular reflection direction, in the plane of incidence, superimposed to an angularly uniform electron distribution. Electron energies are $\approx 20$ keV for a laser intensity of $4\times {10}^{16}$ W cm${}^{-2}$ and $45°$ incidence angle. The electron jet is weaker and angularly broadened with the introduction of a laser prepulse controlling the electron density gradient scale length. Laser absorption and $K\alpha$ line intensity measurements show a maximum for a prepulse delay of $\approx 6$ ps with an electron energy rising to $\approx 180$ keV. Gradient scale length measurements at this prepulse delay fit the laser absorption peak scaling obtained from standard resonant absorption theory.