Abstract

Current developments in high power subpicosecond lasers are making possible a new class of experiments in which matter may be pumped or probed with picosecond or even subpicosecond x-ray pulses. These experiments require accurate knowledge of the x-ray pulse duration. We have demonstrated a picosecond time scale, soft-x-ray continuum absorption spectroscopy technique and are using it to measure the temporal profile of laser-generated soft-x-rays. The technique is based on the rapid but permanent switching of the transmission of an x-ray absorbing medium by multiphoton ionization with a 200 fsec laser pulse. We use Kr gas as the absorbing medium and monitor the transmission on two ~90 eV transitions from the neutral ground state. When the Kr is ionized these absorption features vanish. The change in transmitted energy as a function of the time delay between the ionizing and x-ray pulses is then the integral of the x-ray pulse shape. In principle, the resolution of this technique is limited only by the ionizing pulse duration which can be much better than the current state-of-the-art for x-ray streak cameras [1]. An advantage of this method over the transient Stark shift method reported previously [2] is that it is useful for soft-x-ray pulse lengths ranging from the resolution limit, to the recombination time of the absorbing medium. This type of measurement will have applications to transient species spectroscopy and other aspects of high-intensity laser and plasma physics.

© 1992 The Author(s)