Observation of laser assisted photoelectric effect and femtosecond high order harmonic radiation

We report the first observation of laser-induced free-free transitions in the primary photoelectron spectra of gaseous helium ionized by ultrashort soft x-ray pulses. Measured transition amplitudes suggest that the Coulomb potential plays a significant role in determining the final electronic state. Additionally, we report the first direct measurements on the temporal duration of femtosecond high order harmonic radiation. The harmonic pulse duration is observed to exhibit strong dependencies upon both laser intensity and position of the generating medium relative to the laser focus. {copyright} {ital 1996 The American Physical Society.}