15min:

JOSEPH M. BEAMES, FANG LIU AND MARSHA I. LESTER, Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6323.

OH radicals are of significant atmospheric interest as a dominant oxidizing agent in day-time tropospheric chemistry. In this study, a 1+1' multiphoton ionization (MPI) scheme is employed to record rotationally-resolved spectra of OH radicals via the A^{2 +} resonant intermediate state. UV excitation is used to prepare OH A^{2 +}(v=1, J , F_i), which is subsequently ionized by a second photon of fixed frequency VUV (118.3 nm), generated by tripling the 355 nm output of a Nd:YAG laser. The mass-selected OH⁺ ion signal from 1+1' MPI is detected using time-of flight mass spectrometry and compared with the laser-induced fluorescence (LIF) signal arising from OH A^{2 +}-X² (1,0) excitation. The MPI signal is observed over a range of UV+VUV total energies, corresponding to various OH A^{2 +} (v=1, J , F_i) intermediate states, with relative intensities that differ considerably from LIF. The ion signal is enhanced relative to LIF at combined UV + VUV photon energies consistent with an autoionizing 3 d Rydberg state that converges on the OH⁺ A³ ion core; direct ionization into OH⁺ X^{3 -} is forbidden in a one-photon electric-dipole transition from OH A^{2 +}. The MPI intensities have been quantified relative to LIF over the OH A^{2 +}-X² (1,0) region such that this scheme is now applicable for quantitative state-selective detection of OH X² .