Time-Resolved X-ray Diffraction Studies of Laser-Induced Dynamics in Solids

Abstract: X-ray diffraction is an invaluable tool in the field of structural dynamics. In the work described in this thesis, time-resolved X-ray diffraction experiments were carried out to investigate ultrafast lattice dynamics. Ultrashort laser pulses were used to induce non-thermal melting and large-amplitude strain waves, and X-rays were used to probe these phenomena. Non-thermal melting was studied in indium antimonide (InSb). It was found that the inertial model, which states that the motion of the atoms is determined by their initial vibrational energy at the time of laser irradiation, accurately describes the process of non-thermal melting. It was demonstrated that the model is valid over a large range of temperatures, from 35 to 500 K, when taking the zero-point energy into account at low temperatures. It was also shown how the process of non-thermal melting can be used as a timing monitor to determine the relative timing of laser and X-ray beams in pump/probe experiments. It was shown how the use of an opto-acoustic transducer could reduce the duration of an Xray pulse. The transducer was made of a thin gold film deposited on the surface of bulk InSb. Upon heating the thin gold film with an ultrashort laser pulse, a strain wave was generated in the semiconductor. This resulted in a modulated phonon spectrum and X-ray reflectivity. It was shown that a 100 ps long X-ray pulse can be transformed to a 20 ps pulse with an 8% efficiency. A large-amplitude strain wave was generated in graphite using an ultrashort laser pulse to elucidate the potential role of strain in phase transitions. The temporal evolution of the strain wave was mapped, and the pressure deduced. It was found that it was possible to induce a pressure and temperature corresponding to the region in the carbon phase diagram in which diamond can form.