Low-Energy Electron-Driven Processes in few Gas Phase Molecules

Ghosh, Soumya (2026) Low-Energy Electron-Driven Processes in few Gas Phase Molecules. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (Phd thesis of Soumya Ghosh (19RS092)) 19RS092.pdf - Submitted Version Restricted to Repository staff only Download (26MB)

Abstract

This doctoral thesis provides a kinematically complete investigation into the dissociation dynamics of several polyatomic molecules, such as methyl iodide (CH3I), carbonyl sulfide (OCS), 1-propanol, and difluoromethane (CH2F2), following interactions with low-energy electrons. The research focuses on the fundamental mechanisms of Dissociative Electron Attachment (DEA), where an incident electron (≤ 15 eV) is captured to form a Transient Negative Ion (TNI), and Ion-Pair Dissociation (IPD), a non-resonant process occurring at higher energies (≥ 15 eV) that produces correlated cation-anion pairs. These processes are critical for understanding chemical reactivity in fields such as atmospheric chemistry, astrochemistry, and radiation biology. A significant portion of this work involves the development and optimization of advanced experimental instrumentation. A Velocity Map Imaging (VMI) spectrometer was utilized to reconstruct the three-dimensional momentum of fragments, while a high-mass-resolution Time-of-Flight Mass Spectrometer (TOFMS) was employed to measure absolute dissociation cross-sections using the Relative Flow Technique (RFT). To address the technical challenge of probing near-threshold resonances, a custom Trochoidal Electron Monochromator (TEM) was designed and simulated, capable of producing a pulsed, magnetically collimated electron beam with an energy resolution of 20-50 meV. The experimental results revealed complex quantum behaviors across the targeted molecular systems: • Methyl Iodide (CH₃I): VMI studies of I⁻ fragments near 0 eV uncovered a pronounced forward-backward asymmetry, providing direct evidence for the coupling of partial waves (s-, p-, and d-waves) mediated by the molecule’s strong anisotropic dipolar field. Additionally, highmass- resolution measurements corrected previous literature by identifying CH⁻₂ as the dominant secondary fragment rather than CH⁻. • Carbonyl Sulfide (OCS): By combining experimental cross-sections with Multi-Configurational Time-Dependent Hartree (MCTDH) modeling, resonances were identified at 5.0, 6.8, and 10.2 eV. The study demonstrated that dissociation is heavily influenced by geometric distortion, where the TNI must undergo significant bending-facilitated by Renner-Teller vibronic coupling-rather than following simple axial recoil. • 1-Propanol: The study identified four fragment anions (H−,O−,OH−, and C₃H₇O⁻) and suggested that bond-cleavage mechanisms are driven by vibronically coupled Feshbach resonances, where the resonance energy is determined primarily by the neutral conjugate produced. • Difluoromethane (CH₂F₂): Analysis of CHF⁻ and F⁻ fragments revealed that a substantial portion of collision energy is redistributed into the internal vibrational and rotational excitation of the fragments. Finally, the research expanded into condensed-phase dynamics by implementing a supersonic gas jet expansion using an Amsterdam piezo valve. This setup successfully generated NO dimers and tetramers, providing a robust platform for investigating the transition from gas-phase molecules to weakly bound molecular aggregates and the behavior of solvated excess electrons. Collectively, this work establishes a benchmark for understanding the interplay between electronic structure and nuclear dynamics in molecular fragmentation.

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