Logan P. Dempsey

Product branching between reactive and nonreactive pathways in the collisional quenching of OH AΣ+2 radicals by H2

The collisional quenching of OH radicals in their excited A 2Sigma+ electronic state by molecular hydrogen is examined to determine the partitioning between reactive and nonreactive pathways. This is achieved using a pump-probe laser technique to compare the population prepared in the excited OH A 2Sigma+ state with that produced in the OH X 2Pi ground state from nonreactive quenching. Only a small fraction of the products, less than 15%, arise from nonreactive quenching; reactive quenching is the dominant product channel. The branching between the product channels provides a new dynamical signature of the conical intersection region(s) that couple the excited state potential for OH A 2Sigma++H2 with OH X 2Pi+H2 and H2O+H products.

Fluorescence-dip infrared spectroscopy and predissociation dynamics of OH AΣ+2 (v=4) radicals

Fluorescence-dip infrared spectroscopy, an UV-IR double-resonance technique, is employed to characterize the line positions, linewidths, and corresponding lifetimes of highly predissociative rovibrational levels of the excited A (2)Sigma(+) electronic state of the OH radical. Various lines of the 4 <--2 overtone transition in the excited A (2)Sigma(+) state are observed, from which the rotational, centrifugal distortion, and spin-rotation constants for the A (2)Sigma(+) (v = 4) state are determined, along with the vibrational frequency for the overtone transition. Homogeneous linewidths of 0.23-0.31 cm(-1) full width at half maximum are extracted from the line profiles, demonstrating that the N = 0 to 7 rotational levels of the OH A (2)Sigma(+) (v = 4) state undergo rapid predissociation with lifetimes of < or =23 ps. The experimental linewidths are in near quantitative agreement with first-principles theoretical predictions.