Inelastic X-ray Scattering Spectroscopy of Small Molecules and Their Radiation Chemistry

Non-resonant inelastic x-ray scattering can provide information on various atomic-scale properties and phenomena by probing the spectrum of electronic excitations. The technique allows bulk-sensitive measurements and relatively freely tunable sample conditions even for excitations with energies in the soft x-ray range. Also non-dipolar excitations are accessible by virtue of the ability to impose finite momentum transfer in the scattering process. This thesis comprises of studies that apply non-resonant inelastic x-ray scattering to questions of chemistry and physics in gas and solid phase. More specifically, the works focus on selected cases of gas-phase samples at elevated temperatures and pressures as well as on a solid-state chemical reaction of an organic compound. The electronic excitation spectra from gas-phase molecules exhibit temperature dependence due to molecular vibrations, which affect both the intensities and energies of the electronic transitions. The spectra measured at varied sample temperature and with varied momentum transfer help to give insight to the vibrational effects when interpreted using spectrum simulations. Especially the vibrations that distort the molecular symmetry from that of the equilibrium geometry are demonstrated to be important. A classic example of topochemical reactions is the dimerization of crystalline cinnamic acid, which is usually induced by ultraviolet light illumation. The presented study with inelastic x-ray scattering shows the reaction take place also due to x-ray irradiation. The insitu measured time-resolved spectra allow to obtain reaction kinetics data, and the utilization of imaging method to follow also the spatial progress of the reaction. These novel experiments using non-resonant inelastic x-ray scattering and the applied analysis methods demonstrate the versatility of the technique and help to envision future studies.