Graduate Studies

 

First Advisor

Martin Centurion

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Physics and Astronomy

Date of this Version

11-30-2023

Document Type

Dissertation

Comments

Copyright 2023, Sajib Kumar Saha

Abstract

Understanding natural light-induced phenomena requires a direct viewing of atomic motion during structural evolution, which, in turn, facilitates controlling and manipulating these light-induced processes. Ultrafast Electron Diffraction (UED) is a structure-sensitive technique that can probe electronic and nuclear dynamics at sub-angstrom spatial and femtosecond time scales. UED has become a vital tool for studying photo-induced molecular dynamics and underlying science. Organic ring systems are prevalent in biology, materials, and pharmaceuticals. Their synthesis and transformation are fundamental in synthetic chemistry, influencing various fields. We used UV photons to photo-excite different cyclic molecules and investigated their photo-dynamics using gas phase UED. The photochemical transformation of quadricyclane involves an ultrafast process in which a highly strained three-membered ring is converted into less strained five and six-membered rings. The isomerization of quadricyclane into norbornadiene has recently garnered considerable attention, particularly within its application as a Molecular Solar Energy Storage (MOST) system. The UV-induced reaction probed by MeV-UED displays simultaneous electronic and nuclear dynamics. Comparison of experimental data with simulations reveals a substantial structural change as the molecule crosses the conical intersection. The analysis also suggests the involvement of a dissociation channel in addition to QD to NB isomerization in photodynamics. Cis stilbene has been serving as a model system for exploring photoisomerization and photocyclization. While the dynamics of the first excited state have been extensively investigated, higher excited states remain less explored. We employed MeV-UED to examine the dynamics following ionization due to the absorption of two UV photons. The experimental data precisely captures the oscillations of the cis-stilbene cation and agrees well with theoretical predictions. Pentamethyl-cyclopentadiene (PMCP) is a small polyene molecule, and investigating its photoreaction can provide valuable insights into reaction mechanisms, which can be applied to studying other complex polyenes. Using our KeV-UED setup, we initiated the photoreaction by pumping with 266nm photon and probed by 90KeV electron beam. The preliminary analysis of this data shows a good pump-probe signal and structural change in the molecule.

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