We have performed a detailed quantitative study of the intense electron pulse produced by nonlinear absorption during ultrafast laser-solid interaction at near normal incidence. The resulting Ea x-ray lines have been investigated by time-integrated spectroscopy in the 1-4 keV range and by Monte Carlo simulations of hot electron energy penetration in Al-Si02 and Al-CaF2 targets. Calibration of the observed electron fluence and Ea line intensities was provided by direct monoenergetic electron beam interaction with the same target. Optimum conditions for hot electron production were obtained by setting the prepulse energy fluence close to the target damage threshold. Results indicate that Ea lines were produced by a distribution function of hot electrons which carry 12% of the incident laser energy with a characteristic temperature of about 8 keV. Spectrally and spatially resolved Ea emission measurements using a cooled charge-coupled-device detector demonstrate the scaling capabilities of this x-ray source to energies in excess of 6 keV.
A frequency-domain interferometer for probing the variations of the dielectric constant of a plasma with sub-100- fs temporal resolution and lambda/2000 phase resolution is described. Imaging the plasma on the entrance slit of a spectrograph provides spatial resolution along a diameter of the focal spot. The technique is used to map out the expansion of the critical density surface of a femtosecond laser-produced plasma with subnanometer spatial resolution along the laser axis.
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