Spectroscopic and transition properties of LiCl<sup>–</sup> anion
Authors: Guo Rui , Tan Han , Yuan Qin-Yue , Zhang Qing , Wan Ming-Jie ,   Faculty of Science, Yibin University, Yibin 644007, China
Published: February 2022

<sec>The electronic structure of the X<sup>2</sup>Σ<sup>+</sup>, A<sup>2</sup>Π, B<sup>2</sup>Σ<sup>+</sup>, 3<sup>2</sup>Σ<sup>+</sup>, and 2<sup>2</sup>Π state of LiCl<sup>–</sup> anion are performed at an MRCI+Q level. Davison correction, core-valence correction and spin-orbit coupling effect are also considered. The ground state X<sup>2</sup>Σ<sup>+</sup> of LiCl<sup>–</sup> anion correlates with the lowest dissociation channel Li(<sup>2</sup>S<sub>g</sub>) + Cl<sup>–</sup>(<sup>1</sup>S<sub>g</sub>); the A<sup>2</sup>∏ state and B<sup>2</sup>Σ<sup>+</sup> state correlate with the second dissociation channel Li(<sup>2</sup>P<sub>u</sub>) + Cl<sup>–</sup>(<sup>1</sup>S<sub>g</sub>); the 3<sup>2</sup>Σ<sup>+</sup> state and 2<sup>2</sup>Π state correlate with the third dissociation channel Li<sup>–</sup>(<sup>1</sup>S<sub>g</sub>) + Cl<sup>–</sup>(<sup>2</sup>P<sub>u</sub>).</sec><sec>Spectroscopic parameters are calculated by solving the radial Schröedinger equation. The equilibrium internuclear distance <i>R</i><sub>e</sub> of the ground state X<sup>2</sup>Σ<sup>+</sup> is 2.1352 Å, which is a little bigger than the experimental datum, with an error being 0.5%. It is a deep potential well, and the dissociation energy <i>D</i><sub>e</sub> is 1.886 eV. These values are in good agreement with experimental data. The A<sup>2</sup>∏ state is at 13431.93 cm<sup>–1</sup> above the X<sup>2</sup>Σ<sup>+</sup> state. The <i>R</i><sub>e</sub> is 2.1198 Å, which is only 0.0154 Å smaller than that of the X<sup>2</sup>Σ<sup>+</sup> state. The values of energy level <i>G</i><sub>ν</sub> and rotational constant <i>B</i><i><sub>ν</sub></i> of five Λ-S states are also calculated. The values are in good agreement with available theoretical ones. The electronic structures of the excited states are also reported. The SOC effect weakly influences the spectroscopic parameters for the <inline-formula><tex-math id="M1">\begin{document}$ {\text{X}}{}^2\Sigma _{1/2}^ + $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20211688_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20211688_M1.png"/></alternatives></inline-formula>, <inline-formula><tex-math id="M2">\begin{document}$ {\text{A}}{}^2{\Pi _{1/2}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20211688_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20211688_M2.png"/></alternatives></inline-formula>, <inline-formula><tex-math id="M3">\begin{document}$ {\text{A}}{}^2{\Pi _{3/2}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20211688_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20211688_M3.png"/></alternatives></inline-formula>, and <inline-formula><tex-math id="M4">\begin{document}$ {\text{B}}{}^2\Sigma _{1/2}^ + $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20211688_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20211688_M4.png"/></alternatives></inline-formula> state. From the analysis of the SO matrix, it can be seen that the SOC effect plays a little role in realizing the A<sup>2</sup>Π <inline-formula><tex-math id="Z-20220220164508">\begin{document}$\leftrightarrow $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20211688_Z-20220220164508.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20211688_Z-20220220164508.png"/></alternatives></inline-formula> X<sup>2</sup>Σ<sup>+</sup> transition, so, it can be ignored.</sec><sec>The scheme of laser cooling of LiCl<sup>–</sup> anion has constructed at a spin – free level. The A<sup>2</sup>∏(<i>ν</i><i>′</i>) <inline-formula><tex-math id="Z-20220220164513">\begin{document}$\leftrightarrow $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20211688_Z-20220220164513.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20211688_Z-20220220164513.png"/></alternatives></inline-formula> X<sup>2</sup>Σ<sup>+</sup>(<inline-formula><tex-math id="Z-20220220164757">\begin{document}$v'' $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20211688_Z-20220220164757.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20211688_Z-20220220164757.png"/></alternatives></inline-formula>) transition has a highly diagonally distributed Franck-Condon factor <i>f</i><sub>00</sub> = 0.9898, the calculated branching ratio of the diagonal term <i>R</i><sub>00</sub> is 0.9893, and spontaneous radiative lifetime of A<sup>2</sup>∏ is 35.45 ns. A main pump laser and two repumping lasers for driving the A<sup>2</sup>∏(<i>ν</i><i>′</i>) <inline-formula><tex-math id="Z-20220220164518">\begin{document}$\leftrightarrow $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20211688_Z-20220220164518.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20211688_Z-20220220164518.png"/></alternatives></inline-formula> X<sup>2</sup>Σ<sup>+</sup>(<inline-formula><tex-math id="Z-20220220164751">\begin{document}$v'' $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20211688_Z-20220220164751.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20211688_Z-20220220164751.png"/></alternatives></inline-formula>) transitions are required. The laser wavelengths are 744.10, 774.30 and 772.42 nm, respectively. Owing to the summation of <i>R</i><sub>00</sub>, <i>R</i><sub>01</sub>, and <i>R</i><sub>02</sub> being closer to 1, the A<sup>2</sup>∏(<i>ν</i><i>′</i>) <inline-formula><tex-math id="Z-20220220164522">\begin{document}$\leftrightarrow $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20211688_Z-20220220164522.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20211688_Z-20220220164522.png"/></alternatives></inline-formula> X<sup>2</sup>Σ<sup>+</sup>(<inline-formula><tex-math id="Z-20220220164743">\begin{document}$v'' $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20211688_Z-20220220164743.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20211688_Z-20220220164743.png"/></alternatives></inline-formula>) transition is a quasicycling transition. These results imply that the LiCl<sup>–</sup> anion is a candidate for laser cooling.</sec>