School of Mechanical Engineering, Purdue University, West Lafayette, Indiana 47907-2088, USA.
J Chem Phys. 2017 Mar 28;146(12):124311. doi: 10.1063/1.4978921.
We discuss the results of high-resolution, sub-Doppler two-photon-absorption laser-induced fluorescence (TPALIF) spectroscopy of nitric oxide at low pressure and room temperature. The measurements were performed using the single-longitudinal mode output of a diode-laser-seeded optical parametric generator (OPG) system with a measured frequency bandwidth of 220 MHz. The measurements were performed using a counter-propagating pump beam geometry, resulting in sub-Doppler TPALIF spectra of NO for various rotational transitions in the (0,0) vibrational band of the AΣ - XΠ electronic transition. The experimental results are compared with the results of a perturbative treatment of the rotational line strengths for the 20 different rotational branches of the XΠ(v″ = 0) → AΣ(v' = 0) two-photon absorption band. In the derivation of the expressions for the two-photon transition absorption strength, the closure relation is used for rotational states in the intermediate levels of the two-photon transition in analogy with the Placzek treatment of Raman transitions. The theoretical treatment of the effect of angular momentum coupling on the two-photon rotational line strengths features the use of irreducible spherical tensors and 3j symbols. The final results are expressed in terms of the Hund's case (a) coupling coefficients a and b for the XΠ(v″ = 0) rotational level wavefunctions, which are intermediate between Hund's case (a) and case (b). Considerable physical insight is provided by this final form of the equations for the rotational line strengths. Corrections to the two-photon absorption rotational line strength for higher order effects such as centrifugal stretching can be included in a straightforward fashion in the analysis by incorporating higher order terms in these coupling coefficients a and b, although these corrections are essentially negligible for J < 50. The theoretical calculations of relative line intensities are in good agreement both with our experiment and with published experimental results. In addition, the calculated line shapes and relative intensities for closely spaced main branch and satellite transitions are in excellent agreement with our experimental measurements.
我们讨论了在低压和室温下使用二极管激光种子光参量发生器 (OPG) 系统的单纵模输出进行的高分辨率、亚多普勒双光子吸收激光诱导荧光 (TPALIF) 光谱测量结果。该系统的测量带宽为 220MHz。测量采用反传泵浦光束几何结构，得到了 AΣ - XΠ 电子跃迁（0,0）振动带中各种转动跃迁的 NO 的亚多普勒 TPALIF 光谱。实验结果与 XΠ（v″=0）→AΣ（v'=0）双光子吸收带的 20 个不同转动支的转动线强度微扰处理结果进行了比较。在推导双光子跃迁吸收强度表达式时，中间能级的转动状态采用了类似于 Raman 跃迁的 Placzek 处理的封闭关系。双光子转动线强度中角动量耦合效应的理论处理采用了不可约球张量和 3j 符号。最终结果以 XΠ（v″=0）转动能级波函数的 Hund's case (a) 耦合系数 a 和 b 表示，这些系数介于 Hund's case (a) 和 case (b) 之间。这些方程的最终形式为转动线强度提供了相当大的物理洞察力。通过在这些耦合系数 a 和 b 中纳入高阶项，可以直接将诸如离心拉伸等高阶效应对双光子吸收转动线强度的修正纳入分析中，尽管对于 J<50，这些修正基本可以忽略不计。相对谱线强度的理论计算与我们的实验和已发表的实验结果非常吻合。此外，对于紧密间隔的主支和卫星跃迁，计算出的谱线形状和相对强度与我们的实验测量结果非常吻合。
