Dynamics of structural phase transition studied by picosecond time-resolved X-ray diffraction

点击翻译按钮获取中文摘要

We investigated dynamics of a phase transition, which is induced by impulsive excitation, using a picosecond time-resolved X-ray diffraction technique. The pulsed X-rays were generated by focusing femtosecond laser beams onto metal targets at 10^17 W/cm^2 in a vacuum chamber, and extracted from a Be window. The X-rays were directed to a sample and the diffracted X-rays were detected by the L-N_2 cooled X-ray charge-coupled device. Time-resolved measurements were performed using a pump and probe technique. A 300-ps laser beam was irradiated on the sample at power density of 3.3 GW/cm^2 for excitation. The sample used was a silicon crystal (Si(111)). The time-resolved X-ray diffraction was obtained every 50 ps of time interval with a time-resolution of 10 ps. Lattice deformation corresponds to laser heating and acoustic-phonon propagation, which followed by the energy transfer from an electronic system to a lattice system, was directly observed. In order to increase the X-ray intensity at the sample position, an X-ray focusing system was constructed using a Johansson curved crystal. The X-rays were focused at a spot size with a diameter of 100・m and 400-times higher intensity. The newly constructed X-ray diffraction system can observe approximately 4 degrees at the spot.