Generation of hard x rays by femtosecond laser pulse interaction with solid targets in atmosphere

X ray radiation as high as 50 keV, including Kα of Ba and Mo, have been observed from a solid target during the interaction of low energy ∼0.65 mJ, 1 kHz 40 femtosecond laser pulses focused in air at atmospheric pressure. Energetic electrons generating such x rays are possibly produced when the field strength in laser pulse wake exceeds the runaway threshold in air. Two dimensional particle-in-cell simulations that include optical field ionization of air and elastic collisions support this mechanism.