White X-ray pulse emission of alkali halide aqueous solutions irradiated by focused femtosecond laser pulses: A spectroscopic study on electron temperatures as functions of laser intensity, solute concentration, and solute atomic number

Aqueous solutions of alkali halides were irradiated with focused femtosecond laser pulses (130 fs, 780 nm, 1 kHz) in air, and X-ray emission spectra were measured. The laser power density at the focus was estimated to be 2 PW/cm² when the laser intensity was 0.65 mJ/pulse. Broad X-ray emission spectra having tails to the higher energy were commonly observed, often accompanied by characteristic K and L lines of solutes. Electron temperatures (T_e) were obtained from the slopes of the broad X-ray spectra on the basis of the assumption of Maxwell-Boltzmann distribution of electron energy. The value of T_e was 1.90 keV in the case of distilled water irradiated with laser pulses of 0.53 mJ/pulse laser pulses, while two components of T_e (2.67 and 9.83 keV) were observed in the case of saturated cesium chloride aqueous solution (6.5 mol/dm³). These two components may be related to different electron acceleration processes such as inverse-bremsstrahlung and resonance absorption.