A free electron laser-based 1+1′ Resonance-Enhanced Multiphoton Ionization scheme for rotationally resolved detection of OH radicals with correct relative intensities

The key role played by the OH radical as a reactive intermediate motivates advanced methods for state-resolved OH detection. In this work, we take advantage of the wavelength- and bandwidth-tunable vacuum ultraviolet (VUV) pulses produced at the Dalian Coherent Light Source to modify a previously reported 1+1′ UV + VUV Resonance-Enhanced Multiphoton Ionization scheme [J. M. Beames, F. Liu, M. I. Lester, C. Murray, J. Chem. Phys. 134, 241,102 (2011); J. M. Beames, F. Liu, M. I. Lester, Mol. Phys. 112, 897 (2014)], in which OH in its ground XΠ electronic state is first excited to the AΣ state at around 281 nm, and subsequently ionized by 118 nm VUV radiation via the autoionizing (AΠ,3d) Rydberg state. By tuning the VUV-free electron laser so that its bandwidth covers the entire AΠ (v = 0, 3d) ← AΣ (v″ = 1) band, we obtain enhanced sensitivity and accurate relative intensities for quantitative determination of quantum state distributions. The relative line intensities observed in the experiment agree with the simulated absorption intensities to within an error of <1% of the integrated band intensity. The 1+1′ scheme is also compared to a convenient one-color 2+1 scheme [M. Collard, P. Kerwin, A. Hodgson, Chem. Phys. Lett. 179, 422–428 (1991)], which suffers due to rapid predissociation of the D Σ state used as resonant intermediate.