UC Riverside

The photodissociation dynamics of small free radicals—from simple diatomic radicals like SH to complex polyatomic hydrocarbons such as C4H7—were investigated using the high-n Rydberg H-atom time-of-flight technique, a sensitive method for detecting photofragments. This approach enables accurate measurement of energy distributions and provides valuable insights into the angular distributions of fragments formed during photodissociation, illuminating the breakup processes of unstable radicals. Such studies uncover spectroscopic and excited-state properties of radicals, examine potential energy surfaces and their dynamics, and establish benchmarks for theoretical models of open-shell systems. The predissociation dynamics of the mercapto radical (SH/SD) were analyzed across various rovibrational levels of the A²Σ⁺ state. Predissociative lifetimes and vibrational energy levels of the A²Σ⁺ state were derived from the photofragment yield spectra of H/D + S(3PJ=2,1,0) products, offering insights into the radical's behavior at the onset of predissociation. Sequential two-photon excitations to the repulsive 2²Π and B²Σ⁺ states were observed, producing H/D + S(¹D) and H/D + S(¹S), respectively, with nonadiabatic crossings involved. Branching ratios and angular distributions of the H/D + S(3PJ=2,1,0) products from the A²Σ⁺ state were obtained to interpret the roles of different interstate interactions in predissociation. The isomer-dependent photodissociation dynamics of three C4H7 radicals—2-methylallyl, 1-methylallyl, and 2-buten-2-yl—were examined at wavelengths of 226–246 nm. These C4H7 radicals demonstrated two distinct H-loss dissociation pathways: (i) a predominant unimolecular dissociation following internal conversion from the electronically excited state to the ground state, and (ii) a minor, direct dissociation pathway from the excited Rydberg state(s). These findings reveal the varied dissociation dynamics of different radicals and highlight the intricate interplay between electronic states, including nonadiabatic interactions and conical intersections, which are essential for understanding the complexities of radical photochemistry.