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| Home > Publications database > An atomic perspective of the photodissociation and geminate recombination of triiodide in condensed phases |
| Book/Report/Dissertation / PhD Thesis | PUBDB-2016-03652 |
; ;
2016
Verlag Deutsches Elektronen-Synchrotron
Hamburg
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Please use a persistent id in citations: doi:10.3204/PUBDB-2016-03652
Report No.: DESY-THESIS-2016-028
Abstract: The thesis presents progress made towards a thorough understanding of the photodissociation and geminate recombination of triiodide anion (I$_3^-$) in solution and solid state using novel time-resolved spectroscopic and structural methods that have matured in the past decade. An isolated I$_3^-$ has only three degrees of freedom, but in the condensed phase, the case of an open quantum system, its chemistry is transformed because other degrees of freedom from the surroundings (the bath) need to be fully taken into account. This system is a textbook example for understanding dissociation and recombination processes in condensed phases, but unresolved issues about the reaction pathways remain. To probe the issues, firstly, mid-UV pulse shaper-based closed-loop adaptive control as well as open-loop power and chirp control schemes were used in conjunction with single-color pump-probe detection of the yield of the photoproduct diiodide (I$_2^{-\cdot}$) to study the above reaction in ethanol solution. The experiments revealed a strong pump-chirp dependence of the I$_2^{-\cdot}$-yield (as much as 40% change). Subsequently, two possible mechanisms involving additional reaction channels were postulated in order to explain such effect. Secondly, pump-supercontinuum-probe spectroscopy and ultrafast electron diffraction were performed separately on solid state triiodide compound $n$-(C$_4$H$_9$)$_4$NI$_3$ (TBAT). This system was chosen to provide a well-defined lattice for the bath and to avail atomic resolution of the condensed phase reaction dynamics. In the optical experiment, coherent oscillations were observed within a probe delay of 1 ps that bear strong resemblance to the stretching modes of ground-state I$_3^-$ and I$_2^{-\cdot}$ fragment, which makes it the first to reliably distinguish the two species in a single measurement. In addition, the spectroscopic signature of a novel intermediate, the tetraiodide anion (I$_4^{-\cdot}$), was identified and its origin is attributed to intermolecular interaction of the iodine radical (I$^{\cdot}$) fragment and a neighboring I$_3^-$ chain. Moreover, a number of phonon modes of as low as $\sim$ 10 cm$^{-1}$ were extracted from the time series data of both optical and electron diffraction experiments using advanced frequency analysis (short-time Fourier transform and continuous wavelet transform) and deconvolution schemes, some with good agreement in frequency, which helped pin down the reaction-coupled modes. In the optical experiment, it was found that the libration modes exhibit a delayed appearance in comparison to the stretching modes, and their frequencies show measureable temperature dependence. These findings reveal the molecular details of the sizeable impact of the photo-induced reaction on the crystal lattice and bring insights to dissociative reactions in organic molecular solids, which have rarely been studied due mainly to their incomplete reversibility. The results can, therefore, serve as the experimental basis for further theoretical developments in the lattice effects on a chemical reaction (promotion, quenching and alteration). Together, these two case studies demonstrate the potential of a comparative and multimodal approach to exploring the broad parameter space (chirp, fluence, polarization, temperature, physical state, etc.) that affects the course of a reaction. Future studies are conceived to examine the dynamical details using shorter optical pulses in targeted spectral regions derived from present experiments, as well as to better resolve the fast molecular modes and confirm the time delay between internal and external modes using shorter and brighter electron bunches.
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