Repetitive ultrafast melting of InSb as an x-ray timing diagnostic

We have demonstrated the possibility of using repetitive ultrafast melting of InSb as a timing diagnostic in connection with visible-light pump/x-ray probe measurements at high-repetition-rate x-ray facilities. Although the sample was molten and regrown approximately 1x10(6) times, a distinct reduction in time-resolved x-ray reflectivity could be observed using a streak camera with a time resolution of 2.5 ps. The time-resolved x-ray reflectivity displayed this distinct decrease despite the fact that the average reflectivity of the sample had fallen to approximately 50% of its original value due to accumulated damage from the prolonged laser exposure. The topography of the laser-exposed sample was mapped using an optical microscope, a stylus profilometer, photoelectron microscopy, and a scanning tunneling microscope. Although the surface of the sample is not flat following prolonged exposure at laser fluences above 15 mJ/cm(2), the atomic scale structure regrows, and thus, regenerates the sample on a nanosecond timescale. In the fluence range between 15 and 25 mJ/cm(2), the laser power is sufficient to melt the sample, while regrowth occurs with a sufficiently good structure to allow the extraction of timing information via ultrafast time-resolved x-ray measurements. This can be applied for timing purposes at synchrotron radiation and x-ray free-electron laser facilities. It is also noteworthy that we were able to reproduce the fluence dependencies of melting depth and disordering time previously obtained in single-shot, nonthermal melting experiments with higher temporal resolution.