15min:

LOGAN P. DEMPSEY, ERIKA L. DERRO, ILANA B. POLLACK AND MARSHA I. LESTER, Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6323.

The OH radical plays a major role in atmospheric and combustion environments, where it is generally detected by Laser Induced Fluorescence (LIF) using the A ^{2 +}-X ² band system. Excited vibrational levels of the OH A ^{2 +} (v \geq 3) state have been difficult to study by LIF due to rapid predissociation, which results in a fluorescence quantum yield that is essentially zero. In the present work, the OH A ^{2 +} (v=4) state is characterized using a Fluorescence Depletion Infrared (FDIR) spectroscopy technique. A UV laser promotes OH radicals from the v''=1 level of the ground X ² state to the v'=2 level of the excited A ^{2 +} state. An IR laser further excites the OH radicals on an overtone transition to the v=4 level of the A ^{2 +} state. Fluorescence is collected from OH A ^{2 +} (v'=2) exclusively, which is depleted when the IR laser is resonant with a 4 2 overtone transition. The IR frequencies of various P- and R-branch lines of the overtone transition are measured, and spectroscopic constants for the A ^{2 +} (v=4) state are extracted using combination differences. The rotational, centrifugal distortion, and spin-rotation constants are determined, along with the vibrational frequency for the overtone transition. These constants are compared with calculated values from Dunham coefficients for the OH A ^{2 +} state and a merged fit of experimental data from previous studies.