A UV+VUV MULTIPHOTON IONIZATION SCHEME FOR OH RADICALS

This research was supported by the Office of Basic Science of the Department of Energy.Author Institution: Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6323OH radicals are of significant atmospheric interest as a dominant oxidizing agent in day-time tropospheric chemistry. In this study, a 1+1$^\prime$ 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$_i$), 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$^\prime$ 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$_i$) 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\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$^2\Pi$