Ultrafast X-ray diffraction studies on Si(111) and DMABN crystals using Cu-Kα radiation

摘要

A novel laser-pumped X-ray source is used to investigate generation of shock waves in a semiconductor and conformational changes in a molecular crystal. Ultrashort Cu-Kα pulses are generated by focusing 130 fs laser pulses from the ATLAS titanium-sapphire laser of our institute on a slowly moving copper tape. Irradiating Si(111) surfaces with a few 100 mJ/cm~2 pulses at 800 nm we observe an increase in the integrated reflection on a relatively slow time scale of several 100 ps. This observation is explained by the increased geometrical structure factor generated by the shock wave propagating into a mosaic crystal. The work on conformational changes was performed with DMABN (dimethylaminobenzonitrile, sum formula C_9H_(10)N_2). A pump-probe experiment using the third harmonic of the titanium-sapphire laser (λ = 265 nm) as the pump yields indications of an increase of the 004 reflection in a time shorter than 10 ps. Such an increase is expected owing to photo-induced rotation of the two methyl groups around the major axis of the molecule.