Reflectivity of laser-produced plasmas generated by a high intensity ultrashort pulse*

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Abstract
Solid state density aluminum and carbon plasmas were generated by 350 fs KrF laser pulses at intensities of 1017 W cm−2. Reflectivity, x-ray emission, and spectra of the backscattered radiation were measured as a function of the laser polarization, angle of incidence, and intensity (I). For p-polarized light, the absorption (A) is almost intensity independent for 1015 W cm−2<I<2×1017 W cm−2, and can reach values up to A=0.8. For constant laser intensity, p-polarized light is up to a factor of 15 more efficient in generating x rays than s-polarized light. A polarization and angle of incidence-dependent absorption mechanism, such as resonance absorption or vacuum heating, is consequently important. The spectral line shape and positions of the reflected radiation from the carbon plasma showed a substantial intensity dependence. It is demonstrated that the backscattered spectrum is influenced by the plasma expansion (Doppler effect) and the temporal development of the electron density.