Structural dynamics study of small molecules and nanostructures using time-resolved X-ray solution scattering

Abstract

The work presented in this thesis addresses structural dynamics studies of two different systems using time-resolved X-ray solution scattering. Diiodomethane, $CH_2I_2$, in a polar solvent undergoes a unique photoinduced reaction whereby $I_2^−$ and $I_3^−$ are produced from its photodissociation, unlike for other iodine-containing haloalkanes. While previous studies proposed that homolysis, heterolysis, or solvolysis of iso-$CH_2I$-I, which is a major intermediate of the photodissociation, can account for the formation of $I_2^−$ and $I_3^−$, there has been no consensus on its mechanism and no clue for the reason why those negative ionic species are not observed in the photodissociation of other iodine-containing chemicals in the same polar solvent, for example, $CHI_3, C_2H_4I_2, C2F4I2, I_3^−, and I_2$. Here, using time-resolved X-ray solution scattering, we revisit the photodissociation mechanism of $CH_2I_2$ in methanol and determine the structures of all transient species and photoproducts involved in its photodissociation, and reveal that $I_2^−$ and $I_3^−$ are formed via heterolysis of iso-$CH_2I$−I in the photodissociation of $CH_2I_2$ in methanol. In addition, we demonstrate that the high polarity of iso-$CH_2I$−I is responsible for the unique photochemistry of $CH_2I_2$. We report the successful synthesis of Au nanoparticles from liquid solutions by synchrotron X-ray irradiation. By use of X-ray scattering methods, the structural relaxations in condensed matter, and in particular in nanoscale systems are directly accessible. Gold particles were fabricated by the X-ray irradiation of gold (III) terpyridine complex aqueous solution. The structure and size distribution of the prepared particles were evaluated by transmission electron microscopy and time-resolved X-ray solution scattering. Following the laser pulse, shape and angular shift of the Bragg reflection from crystalline gold nanoparticles are resolved stroboscopically using 100 ps X-ray pulses from a synchrotron. We observed the lattice expansion and relaxation of AuNPs heated by intense femtosecond laser pulse are measured by time-resolved X-ray solution scattering.