GENERATION OF COHERENT, FEMTOSECOND, X-RAY PULSES IN THE WATER WINDOW

We report experimental and theoretical results on high-harmonic genera tion with 25-fs laser excitation pulses. The shortest wavelength we ob serve, at 2.7 mn, is well within the ''water window'' region of X-ray transmission. In the case of all the noble gases, we obtain excellent agreement between theoretical predictions for the highest harmonic pho ton energy generated and our experimental observations. We also obtain excellent agreement between theory and experiment for the highest pho ton energy generated as a function of laser pulsewidth between 25 and 100 fs. Finally, we observe that the individual harmonic peaks near th e cutoff are well resolved for positively chirped pump pulses, but are unresolved in the case of negatively chirped excitation pulses.