Matsika, Spiridoula2025-07-252025-07-252024-12https://scholarshare.temple.edu/handle/20.500.12613/11183The interaction of light with matter is a fundamental phenomenon that drives many natural and technological processes, from vision perception to energy generation through photosynthesis. Among the diverse manifestations of light-matter interactions, photochemistry explores how light initiates or influences chemical reactions, unveiling mechanisms essential to nature and industry. Within photochemistry, photoionization, the process where photons eject electrons to create ions, plays a pivotal role in processes ranging from atmospheric chemistry to astrophysics. It governs phenomena like DNA damage from UV radiation, the formation of Earth's ionosphere, and the chemical evolution of interstellar clouds. As laser technologies advanced, photoionization in intense laser fields, known as strong-field ionization (SFI), emerged as a powerful tool for probing molecular dynamics. SFI enables the creation of highly charged molecular states and reveals fundamental molecular properties such as bond strength, charge distribution, and dissociation dynamics. These insights are particularly significant in understanding molecular behavior under extreme conditions, such as high-energy environments or space. To investigate the ultrafast dynamics initiated by SFI, this dissertation employs trajectory surface hopping (TSH), a molecular dynamics method that captures non-adiabatic transitions between electronic states. By combining TSH with different quantum chemistry methods, this work addresses key challenges in modeling the excited-state dynamics of molecular cations. Chapter 3 explores conformer-specific dissociation dynamics in dimethyl methylphosphonate (DMMP), revealing how molecular conformations influence fragmentation pathways. Chapters 4 and 5 focus on state-resolved dissociation of formaldehyde dications, identifying the role of specific electronic states in fragmentation, and uncovering a metastable excited state with implications for space chemistry. Chapter 6 extends this analysis to formaldehyde tetracation, highlighting the quantum contributions to Coulomb explosion imaging. Finally, Chapter 7 presents a computational advancement: the integration of Newton-X with TeraChem for GPU-accelerated TSH simulations. By bridging computational techniques with experimental observations, this work advances the understanding of ultrafast molecular processes and their broader implications in photochemistry and quantum molecular dynamics. The findings aim to illuminate fundamental questions and pave the way for future explorations of molecular behavior in extreme environments.192 pagesengIN COPYRIGHT- This Rights Statement can be used for an Item that is in copyright. Using this statement implies that the organization making this Item available has determined that the Item is in copyright and either is the rights-holder, has obtained permission from the rights-holder(s) to make their Work(s) available, or makes the Item available under an exception or limitation to copyright (including Fair Use) that entitles it to make the Item available.http://rightsstatements.org/vocab/InC/1.0/Computational chemistryComputational physicsElectronic structure theoryNon-adiabatic dynamicsQuantum chemistryStrong-field ionizationSurface hoppingChemistryText2025-06-16Singh_temple_0225E_15997.pdf