Optimization of femtosecond laser plasma Kα sources

Since the night when Conrad RÄontgen ¯rst saw his hand's bones on a °uorescence screen [71] x rays have been one of the most important tools for revealing the inner structure of matter. Beside x-ray imaging for medical diagnosis, the study of x-ray di®raction patterns in chemistry and physics has become a major application of x rays, because it allows the structure of crystalline solids with atomic spatial resolution to be analyzed. Processes which are determined by atomic motion, like chemical reactions and phase transitions, happen on a timescale comparable to the vibrational period of the atoms, which is typically 10¡13 s or 100 fs [76]. The ideal tool to investigate such a process would simultaneously o®er a temporal resolution smaller than this time scale and the ability to resolve the interatomic distances. Until recently, no such tool existed and the investigator had to choose between the femtosecond time resolution of optical spectroscopy with femtosecond lasers and the spatial resolving power of x-ray studies. The experimental method which came closest to the `ideal tool' was time-resolved x-ray di®raction using a synchrotron radiation source. It was especially used to study biologically important processes like the photoinduced trans-cis isomerization of a part of the protein rhodopsin which forms the light detecting process in the human eye [64] or the binding of oxygen and carbon monoxide to myoglobin [94]. These experiments had a nanosecond time resolution which proved to be accurate for the processes under investigation, but all-optical experiments indicate that the initial structural changes, which precede the studied ones, happen much faster, within several ten or hundred femtoseconds [34, 35]. Moreover, optical measurements have revealed that femtosecond time scales are also relevant to processes in solid state physics, like the atomic rearrangement during transitions from one solid phase to another [84] and the disordering of semiconductors after strong electronic excitation by a laser pulse [85]. State-of-the-art synchrotron/streak camera setups achieve a time-resolution of about 1 ps [52]. LOK