Knowledge Bank

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$2\Sigma +$ resonant intermediate state. nderline{\textbf{134}}, 241102 (2011).} UV excitation is used to prepare OH A$2\Sigma +$(v=1, \textit{J}, F), 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$2\Sigma +$-X$2\Pi$(1,0) excitation. The MPI signal is observed over a range of UV+VUV total energies, corresponding to various OH A$2\Sigma +$ (v=1, \textit{J}, F) 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\textit{d} Rydberg state that converges on the OH$+$ A$3\Pi$ ion core; nderline{\textbf{110}}, 345 (1999).} direct ionization into OH$+$ X$3\Sigma -$ is forbidden in a one-photon electric-dipole transition from OH A$2\Sigma +$. The MPI intensities have been quantified relative to LIF over the OH A$2\Sigma +$-X$2\Pi$(1,0) region such that this scheme is now applicable for quantitative state-selective detection of OH X$2\Pi$.