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 ${\mathrm{X}}^2\mathrm{\Pi }$ electronic state is first excited to the ${\mathrm{A}}^2{\mathrm{\Sigma }}^{+}$ state at around 281 nm, and subsequently ionized by 118 nm VUV radiation via the autoionizing (${\mathrm{A}}^3\Pi ,3\mathrm{d}$) Rydberg state. By tuning the VUV-free electron laser so that its bandwidth covers the entire A³$\Pi$ $(v$⁺ = 0, 3d) $\leftarrow$ A²$\mathrm{\Sigma }$⁺ $(v$″ = 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 ²Σ⁻ state used as resonant intermediate.

OH 自由基作为反应中间体所发挥的关键作用推动了状态分辨 OH 检测的先进方法。在这项工作中，我们利用大连相干光源产生的波长和带宽可调真空紫外 (VUV) 脉冲来修改先前报道的 1+1' UV + VUV 共振增强多光子电离方案 [JM Beames， F. Liu, MI Lester, C. Murray, J. Chem. 物理。134 , 241,102 (2011)；JM Beames, F. Liu, MI Lester, Mol。物理。112 , 897 (2014)]，其中 OH 在其地下${\mathrm{X}}^2\mathrm{\Pi }$ 电子态首先被激发到 ${\mathrm{一种}}^2{\mathrm{\Sigma }}^{+}$ 在大约 281 nm 处处于状态，随后通过自电离被 118 nm VUV 辐射电离（${\mathrm{一种}}^3\Pi ,3\mathrm{d}$) 里德堡状态。通过调谐无 VUV 电子激光器，使其带宽覆盖整个 A ³$\Pi$ $(v$⁺ = 0, 3d)$\leftarrow$一个²$\mathrm{\Sigma }$⁺ $(v$″ = 1) 波段，我们获得了增强的灵敏度和准确的相对强度，用于定量测定量子态分布。实验中观察到的相对线强度与模拟吸收强度一致，误差小于积分带强度的 1%。1+1' 方案也与方便的单色 2+1 方案进行了比较 [M. Collard, P. Kerwin, A. Hodgson, Chem. 物理。莱特。 179 , 422–428 (1991)]，由于用作共振中间体的 D ² Σ ^-状态的快速预解离而受到影响。