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Photodissociation Dynamics of Free Radicals Using High-n Rydberg Atom Time-of-Flight Spectroscopy

  • Author(s): Sun, Ge
  • Advisor(s): Zhang, Jingsong
  • et al.

Free radicals are open-shell systems that have one or more unpaired valence electrons, and are thus short-lived and highly reactive. Free radicals can be found as important reactive intermediates in a wide range of chemical environments, for example, combustion, atmospheric chemistry, plasma, and interstellar chemistry. The photodissociation studies of free radicals can provide spectroscopic and electronic excited state information of free radicals, probe excited-state potential energy surfaces (PESs), their interactions, and their influence on reaction dynamics, and provide benchmarks for theoretical studies of the open-shell systems. The photodissociation dynamics of free radicals were studied using the high-n Rydberg H-atom time-of-flight technique. Several systems were investigated in the dissertation. The hydroxyl radical (OH), which is a prototypical diatomic radical, has been examined via several rovibrational levels in the A2Σ+ state. The H + O(3PJ) product branching ratios and angular distributions from the A2Σ+ state were obtained. Sequential two-photon excitations to the 22П and B2Σ+ repulsive states were observed, leading to the H + O(1D) and H + O(1S) products, respectively. Ultraviolet (UV) photodissociation of a series of alkyl radicals, ethyl (C2H5), n-butyl (n-C4H9), i-butyl (i-C4H9), t-butyl (t-C4H9), and 1-pentyl (1-C5H11), via their Rydberg states were observed to have two main dissociation channels: (i) direct dissociation from the electronic excited state, and (ii) unimolecular dissociation after internal conversion from the electronic excited state to the ground state. 2-Propenyl radical (CH2CCH3), in the alkenyl category, undergoes unimolecular dissociation on the ground electronic state following the UV photoexcitation and internal conversion from the excited state. Vinoxy radical (CH2CHO) is a prototypical unsaturated alkoxy radical. After photoexcitation to the B ̃2A" state, the vinoxy radical undergoes unimolecular dissociation following internal conversion to the ground state (X ̃2A") and the first excited state (A ̃2A'). The UV photodissociation of thiomethoxyl radical (CH3S), a sulfur system, is similar to that of the ethyl radical and has two main channels: (i) direct dissociation on the repulsive PES, and (ii) unimolecular dissociation after internal conversion from the electronic excited state to the ground electronic state.

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