Fast electron penetration in laser-irradiated solids

<p>In this letter we present data comparing the yield of Cu K-alpha radiation for foils of differing thickness irradiated with a Ti:Sapphire laser generating 40 fs pulses at 800 nm and incident at 45 degrees. At tight focus, the yields for all thicknesses are similar, whilst away from best focus there are clear differences. We discuss the origin of these similarities and differences in terms of the penetration of fast electrons into the foil and the possible importance of refluxing of fast electrons as they reach the non-irradiated side of the foil.</p>