Communication: A new spectroscopic window on hydroxyl radicals using UV plus VUV resonant ionization

A 1 + 1' multiphoton ionization (MPI) detection scheme for OH radicals is presented. The spectroscopic approach combines initial excitation on the well-characterized A(2)Sigma(+)-X-2 Pi band system with vacuum ultraviolet (VUV) ionization via autoionizing Rydberg states that converge on the OH+ A(3)Pi ion state. Jet-cooled MPI spectra on the (1,0) and (2,0) bands show anomalous rotational line intensities, while initial excitation on the (0,0) band does not lead to detectable OH+ ions. The onset of ionization with the (1,0) band is attributed to an energetic threshold; the combined UV + VUV photon energies are above the first member of the autoionizing (A(3)Pi)nd Rydberg series. Comparison of the OH 1 + 1' MPI signal with that from single photon VUV ionization of NO indicates that the cross section for photoionization from OH A(2)Sigma(+), v' = 1 is on the order of 10(-17) cm(2). (C) 2011 American Institute of Physics. [doi: 10.1063/1.3608061]