Picosecond Time-Resolved Pump-Probe XUV Absorption L-Edge Spectroscopy
We present what we believe to be the first measurements of broadband XUV absorption spectroscopy with picosecond time-scale resolution; figure 1 shows a schematic view of the experimental set-up. A 10-Hz, 60 m J, 100-fs Ti: Sapphire laser system is used to create a quasi-continuum of XUV radiation from a gold plasma (30 Å < λ < 300 Å) to act as a probe source. Part of the 100-fs laser beam also provides an ultrashort pump pulse to heat a sample onto which the XUV probe source is focused by a novel X-ray optic. By relaying the XUV source to the sample, we avoid problems with hot electrons, plasma and hard X-rays which would disturb the sample in a proximity geometry. A soft-X-ray spectrometer coupled to multichannel plates and a CCD camera collects spatially-resolved spectra of the sample’s absorption at different times during the laser heating, set by a variable delay between the pump laser and the probe XUV source. A unique jitter-free x-ray streak camera1 developed at the Center for Ultrafast Optical Science is used to obtain temporally-resolved XUV spectra of the probe source to measure its duration (not shown in Fig.1). The temporal duration of the probe is typically on the order of 20 ps but can be made shorter or longer by varying the laser parameters.2 This broadband high-time-resolution absorption spectroscopy technique offers a very interesting and flexible alternative to synchrotrons for experiments such as time-resolved EXAFS, chemical and physical dynamics, and ion characterization in laser ablation plumes.