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 Title: The Rise Of The Exciton In Solid Ammonia Author(s): Cassidy, Andrew Contributor(s): Field, David; Dawes, Anita; James, Rachel Subject(s): Dynamics and kinetics Abstract: We study the dynamics of a system searching for minutes to hours to establish a population of nuclei, which can then go on to create a phase change, following the rise of an exciton in thin films of solid ammonia with deposition temperatures T$_{d}$ = 48 K, 50 K and 52 K. This behaviour is tracked by following the growth of the exciton, using vacuum ultraviolet absorption spectra of ices at 194.4 nm in the \~{A}$^{1}$A$_{2}$" $\leftarrow$ \~{X}$^{1}$A' band. Absorbance is observed to increase through an order of magnitude between Td = 48K to 52K, through greater flexing of the solid state structure, as the size of crystallites expands from an average of 10$\pm$ 2 unit cells at 48K to 34$\pm$ 8 at 52K. Time delays, associated with nucleation, are encountered before the appearance of exciton absorption, varying between 7870 seconds at 48K to 120 seconds at 52K, with rates of subsequent exciton absorbance growth between 1.49 x 10$^{-6}$ s$^{-1}$ and 1.19 x 10$^{-4}$ s$^{-1}$. Activation energies of 21.7$\pm$ 0.2 kJ mol$^{-1}$ for the nucleation process and 22.8$\pm$ 0.2 kJ mol$^{-1}$ for the phase change are derived, corresponding to the breaking of two to three hydrogen bonds. Results demonstrate a new means to track nucleation and recrystallization rates in polymorphic systems. Issue Date: 2021-06-22 Publisher: International Symposium on Molecular Spectroscopy Genre: Conference Paper / Presentation Type: Text Language: English URI: http://hdl.handle.net/2142/111162 Date Available in IDEALS: 2021-09-24
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