Shift of Cl K_αand K_βLines in Laser Produced Dense Plasmas

Abstract

The features of chlorine K-lines have been investigated to develop X-ray probes for Compton scattering on warm dense plasmas. The shapes of these spectral lines have been studied at different laser energies by irradiation of thin saran foils by an intense ultra-short-pulse laser beam. The observed positions of Kα and K β lines undergo a red shift due to a laser induced plasma environment. With increasing intensities the shift changes direction and turns into a blue shift caused by a higher contribution of K-transitions occuring in further excited or ionized atoms. A theoretical approach based on the self-consistent ion sphere model is outlined to describe this inversion. Plasma polarization effects are taken into account as well as different ionization stages of the X-ray emitter within a warm dense plasma. Calculations using an improved ion sphere ansatz show good agreement with observations of the line shifts. ExperimentK α and K β lines serve as narrow keV X-ray sources which can be used, e.g., for diagnostics of dense plasmas [1][2][3]. Recently, X-ray spectroscopy has been performed at the EUROPA LASER FACILITY at the Lawrence Livermore National Laboratory. Thin saran foils (C 7 H 8 Cl 3 N) are irradiated by a terawatt ultra-short laser pulse. Hence neutral chlorine is excited or ionized. Filling of vacant energy levels in the lowest shell creates characteristic X-ray emission. The transitions 2p → 1s and 3p → 1s are referred to as K α and K β , respectively. EUROPA is a 100 fs Ti:Sapphire laser with a wavelength of 800 nm, a focal spot size of about 28 µm and an energy range from 100 mJ to 1.2 J. A graphite (HOPG) crystal in the mosaic focusing mode has been applied to disperse the scattered radiation onto a charged coupled device (CCD). The spectroscopic resolution was on the order of 10 −3 . Details are given in [4], see also [5]. K α and K β are measured at different laser energies, see Fig. 1. At low irradiation, the lines undergo a red shift. At increasing intensities blue satellites rise, resulting in an effective broadening of the line with respect to the limited spectral resolution. Shifts of the average line positions as function of laser energy are shown in Fig. 2. The position of the isolated line ∆E = 0 is extrapolated. Note that due to the chemical shift the K-lines at zero laser intensity might deviate from the free atom K-line position, see also [6]. Estimated errors of data are of the order of about 0.2 eV. That is particularly problematic for the measured shift of the K α line, since it is of the same order of magnitude as the estimated error. At low energies the average line positions are red shifted, but with increasing laser energies the shifts turn and the average line positions get shifted back to higher energies (blue shift). A possible interpretation of this inversion of the line shift with increasing laser energy can be given by many particle effects. In this work we focus on the influence of the plasma environment in the hot region created by the laser pulse. As well known, hot el...

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