Kortyna Andrew, Doney Kirstin, Nesbitt D J
JILA, National Institute of Standards and Technology and University of Colorado, Boulder, Colorado 80309, USA.
J Chem Phys. 2020 Apr 7;152(13):134305. doi: 10.1063/5.0002165.
Direct laser absorption of a slit supersonic discharge expansion provides the first high-resolution spectroscopic results on the symmetric CH stretch excitation (ν) of the bromomethyl (CHBr) radical in the ground electronic state. Narrowband (<1 MHz) mid-infrared radiation is produced by difference-frequency generation of two visible laser beams, with the open shell halohydrocarbon radical generated by electron dissociative attachment of CHBr in a discharge and rapidly cooled to T = 18 ± 1 K in the subsequent slit-jet supersonic expansion. A rovibrational structure in the radical spectrum is fully resolved, as well as additional splittings due to spin-rotation effects and Br/Br isotopologues in natural abundance. Spectroscopic constants and band origins are determined by fitting the transition frequencies to a non-rigid Watson Hamiltonian, yielding results consistent with a vibrationally averaged planar radical and an unpaired electron in the out-of-plane p orbital. Additionally, extensive satellite band structure from a vibrational hot band is observed and analyzed. The hot band data is compared to CFOUR/VPT2 (CCSD(T)cc-pVQZ) ab initio anharmonic predictions of the vibration rotation alpha matrix, which permits unambiguous assignment to CH symmetric-stretch excitation built on the singly excited CH out-of-plane bending mode (ν + ν ← ν). Longitudinal cooling of the Doppler width in the slit-jet expansion geometry also reveals partially resolved hyperfine structure on transitions out of the lowest angular momentum states in excellent agreement with predictions based on microwave studies. High level ab initio MOLPRO calculations [CCSD(T)-f12b/VnZ-f12 (n = 3, 4, CBS)] are also performed with explicitly correlated f12 electron methods for the out-of-plane CH bending mode over the halogen series CHX (X = F, Cl, Br, I), which clearly reveals a non-planar geometry for X = F (with a ΔE ≈ 0.3 kcal/mol barrier) and yet planar equilibrium geometries for X = Cl, Br, and I. Finally, a detailed Boltzmann analysis of the transition intensities provides support for negligible collisional equilibration of the entangled H atom nuclear spin states on the few hundred microsecond time scale and high collision densities of a slit supersonic expansion.
对狭缝超声速放电膨胀进行直接激光吸收，首次获得了处于基电子态的溴甲基（CHBr）自由基对称CH伸缩激发（ν）的高分辨率光谱结果。通过两束可见激光的差频产生窄带（<1 MHz）中红外辐射，在放电中通过CHBr的电子解离附着产生开壳层卤代烃自由基，并在随后的狭缝射流超声速膨胀中迅速冷却至T = 18 ± 1 K。自由基光谱中的振转结构得到了完全分辨，同时还分辨出了由于自旋 - 旋转效应和天然丰度下的Br/Br同位素异构体引起的额外分裂。通过将跃迁频率拟合到非刚性沃森哈密顿量来确定光谱常数和谱带起源，得到的结果与振动平均平面自由基以及平面外p轨道中的未配对电子一致。此外，还观察并分析了来自振动热带的广泛卫星带结构。将热带数据与振动转动α矩阵的CFOUR/VPT2（CCSD(T)cc - pVQZ）从头算非谐预测进行比较，这使得能够明确归属基于单激发CH平面外弯曲模式（ν + ν ← ν）的CH对称伸缩激发。狭缝射流膨胀几何结构中多普勒宽度的纵向冷却还揭示了从最低角动量态跃迁的部分分辨超精细结构，与基于微波研究的预测非常吻合。还使用明确相关的f12电子方法对卤素系列CHX（X = F、Cl、Br、I）的平面外CH弯曲模式进行了高水平的从头算MOLPRO计算[CCSD(T)-f12b/VnZ - f12（n = 3、4、CBS）]，这清楚地揭示了X = F时的非平面几何结构（具有ΔE ≈ 0.3 kcal/mol的势垒），而X = Cl、Br和I时为平面平衡几何结构。最后，对跃迁强度进行详细的玻尔兹曼分析，支持了在几百微秒时间尺度和狭缝超声速膨胀的高碰撞密度下，纠缠的H原子核自旋态碰撞平衡可忽略不计的观点。
