Fluorescence-Dip Infrared Spectroscopy and Predissociation Dynamics of OH A (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 electronic state of the OH radical. Various lines of the 4←2 overtone transition in the excited A state are observed, from which the rotational, centrifugal distortion, and spin-rotation constants for the A (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 (v = 4) state undergo rapid predissociation with lifetimes of ⩽ 23 ps. The experimental linewidths are in near quantitative agreement with first-principles theoretical predictions