Time-Resolved Diffraction Studies of Structural Dynamics in Solids

Sammanfattning: Studies of the structural dynamics of solids can improve our understanding of atomic motion in materials, and may thus help in the manufacture of new devices or the development of materials with novel structures and properties. Ultrashort laser pulses, a few tens of femtoseconds long, can deliver high energies (mJ–kJ). This energy is absorbed by the electrons in a solid material, leading to a rapid increase in the electron temperature within the duration of the laser pulse. The energy will then be transferred to the crystal lattice, resulting in an increase in the lattice temperature, which triggers lattice motion such as vibrations (phonons) and disordering (melting of solids). The distance between neighboring atoms in solids is on the order of 10^(-10) m (Ångström). Since the wavelength of X-rays is in the range of nanometers to Ångströms, which is of the same order as the interatomic distances in solids, X-rays can be used to detect structural changes in solids. The structural dynamics in solids can then be monitored as a function of time by combining ultrashort laser pulses with X-ray techniques. This thesis focuses on the structural dynamics of solids on the time scale of femtoseconds to picoseconds. The studies described in this thesis were divided into two categories based on the laser excitation fluence: below the damage threshold of the sample, and above the damage threshold of the sample. The electron diffusion in a Ni film was studied at fluences below the damage threshold, using a Ni/InSb photo-acoustic transducer. An X-ray switch based on a Au/InSb photo-acoustic transducer was designed and tested as part of the commissioning of the FemtoMAX beamline at the MAX IV Laboratory in Lund. Using fluences above the damage threshold, pressure waves were generated in an Al/InSb photo-acoustic transducer due to the melting of the Al film. The pressure waves were probed and characterized in the InSb substrate. The pressure was in the region of the phase diagram where phase transition could occur. Pressure waves with a similar amplitude were also generated and characterized in graphite. Non-thermal melting was also studied in InSb at fluences above the damage threshold. The findings presented in this thesis contribute to both applications of physics for the manufacture of new devices, and to fundamental physics, by improving our understanding of hydrodynamic pressure waves and phase transitions. The study involving the Ni/InSb photo-acoustic transducer demonstrated an alternative method of characterizing the basic physical properties of a metallic film, while the Au/InSb switch provides a potential means of generating short X-ray pulses at storage rings. The characterization of the pressure waves in the Al/InSb photo-acoustic transducer and graphite extend the current knowledge on the generation of short pressure pulses in solids on the picosecond time scale, which attracts more attention to understand the phase transition process on the ultrafast time scale. The study of non-thermal melting provided new approaches for timing diagnostics at free-electron laser facilities.