The specular reflectivity of plasma mirrors formed by subpicosecond pulses from a titanium:sapphire laser has been measured for different angles of incidence and for two different pulse lengths as a function of the laser intensity. Laser pulses with energies up to 250 mJ and pulse durations of 90 and 500 fs were focused onto a fused silica substrate. For angles of incidence between 6° and 45° the specular reflectivity increases to values of about 80% for intensities above a certain threshold intensity. The threshold intensity varies with the pulse length but is nearly independent of the angle of incidence. For very high intensities the specular reflectivity drops again to values of only a few percent.
We have studied the ionization and dissociation of molecules, exposed to intense short-pulse radiation at 780 nm and its second harmonic at 390 nm. We report the observation of a substantial enhancement in and ion production when the two colours are superposed. We have measured the ionization yield for argon under similar experimental conditions for comparison. A strong dependence of ion yield on the relative directions of the polarization of the two fields is also observed. In order to gain more insight into the dynamics, kinetic-energy spectra have been measured at different laser intensities for both wavelengths, both separately and with them superposed. For 390 nm radiation, these are the first kinetic-energy spectra reported for intense sub-picosecond induced dissociation. The experimental results are discussed and compared with the existing theoretical calculations.
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