Time-resolved X-ray spectroscopy of optical-field-ionized plasmas

Abstract

Abstract:The time-dependent soft X-ray emission of helium and nitrogen plasmas generated by optical-field ionization is reported. The experiments were carried out by focusing pulses of the high-power Ti:sapphire laser of the Lund Institute ofTeclmology (A.= 796 nm, pulse duration 150 fs, pulse energy 150 mJ) to a 50 J.l.m diameter spot close to a nozzle, using He and N2 as target gases. The emission on He+, N4+ and N3+ resonance lines was recorded by means of a flat-field grating spectrometer coupled to an X-ray streak camera. A pronounced difference in the temporal shape of the emission of the Lyman-a. line of hydrogen-like helium and of the 2p-3d resonance lines of lithium-like and beryllium-like nitrogen was observed. The helium line exhibited an initial spike followed by a slow revival of the emission, whereas the nitrogen lines showed a slow decay after a fast initial rise. These observations are explained with the help of simulations.PACS: 52.25.Nr., 52.50.Jm.