Molecular spectral parameters are fundamental data for IR radiation calculation. Currently, there is no such gas spectral database as the high resolution transmission molecular absorption database (HITRAN database) in China. In this paper, supported by the National Numerical Windtunnel (NNW) project, we developed the calculation codes of high-temperature air spectral parameters to support the calculation of target IR radiation. To be specific, NO is one of the products of chemical reactions in the high-temperature air flow field of hypersonic vehicles, and the radiation generated by its vibration-rotation levels is in the typical band of infrared detectors. Based on the molecular potential and permanent dipole moment of split NO calculated by the ab initio method, we calculated the line intensity of NO at 8000 K, and the theoretical calculation results were in a good agreement with the values from the HITRAN database at 300 K and 3000 K. Moreover, we calculated the absorption coefficient of NO at the X 2Π1/2 state at 296 K and 2000 K by employing the narrow-band model. It turns out that without the help of experimental spectral constants, the proposed method can obtain more line positions than the HITRAN database in both low and high vibration levels, which can provide the high-temperature spectral data of NO for the calculation of target radiation characteristics in the NNW project.
Ab initio是基于量子力学基本原理直接求解薛定谔方程的量子化学计算方法,包括Hatree-Fock方法(HF)、完全活性空间自洽场(CASSCF)方法[4-5]以及多参考组态相互作用方法(MRCI)[6-7]。计算NO分子势能数据的步骤如下:首先,采用HF方法对轨道进行初步预测;然后,采用CASSCF方法进行态平均计算;最后,基于CASSCF方法获得的参考波函数,使用MRCI和Davidson修正[8](MRCI+Q)得到NO分子基态的能量。根据上述过程,本文使用MOLPRO软件计算获得了NO分子基态X2Π以及分裂后的X2Π1/2和X2Π3/2态的势能曲线和永久偶极矩。
Permanent dipole moment curve of NOFig. 2基于获得的势能曲线和永久偶极矩,本文计算得到了从末态能级到初态能级的自发辐射爱因斯坦系数,其与HITRAN数据库的数据符合得很好;此外,本文还计算还得到了HITRAN数据库中没有的谱线的爱因斯坦系数,并获得了线强度。从获得的转动态之间的爱因斯坦系数可以推断,目前的永久偶极矩分裂的结果是合理的。爱因斯坦系数以及在HITRAN数据库中提取的值见表2和表3。此外,本文还计算了永久偶极矩平衡键长处的分裂值,它可以为实验测量提供参考。
表 2. 计算得到的X2Π1/2态1-0带转动跃迁的爱因斯坦系数与HITRAN数据库中数据的比较
Table 2. Computational Einstein coefficient of 1-0 band rotational transition at X2Π1/2 state & Einstein coefficient in HITRAN database
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杨全顺, 江涛, 李辉, 高铁锁. Ab Initio 方法研究NO分子红外光谱参数[J]. 光学学报, 2021, 41(2): 0230001. Quanshun Yang, Tao Jiang, Hui Li, Tiesuo Gao. Study on IR Spectral Parameters of NO by Ab Initio Method[J]. Acta Optica Sinica, 2021, 41(2): 0230001.