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 (Te) 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 Te 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 Te (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.
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