Excellent Ultracold Molecular Candidates From Group VA Hydrides: Whether Do Nearby Electronic States Interfere?

Donghui Li; Wensheng Bian

doi:10.3389/fchem.2021.778292

Excellent Ultracold Molecular Candidates From Group VA Hydrides: Whether Do Nearby Electronic States Interfere?

Front Chem. 2021 Dec 16:9:778292. doi: 10.3389/fchem.2021.778292. eCollection 2021.

Authors

Donghui Li^{1

2}, Wensheng Bian^{1

2}

Affiliations

¹ Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing, China.
² School of Chemical Sciences, University of Chinese Academy of Sciences, Beijing, China.

Abstract

By means of highly accurate ab initio calculations, we identify two excellent ultracold molecular candidates from group VA hydrides. We find that NH and PH are suitable for the production of ultracold molecules, and the feasibility and advantage of two laser cooling schemes are demonstrated, which involve different spin-orbit states ( $A^{3} Π_{2}$ and $X^{3} Σ_{1}^{-}$ ). The internally contracted multireference configuration interaction method is applied in calculations of the six low-lying Λ-S states of NH and PH with the spin-orbit coupling effects included, and excellent agreement is achieved between the computed and experimental spectroscopic data. We find that the locations of crossing point between the $A^{3} Π$ and ${}^{5}{Σ^{-}}$ states of NH and PH are higher than the corresponding v' = 2 vibrational levels of the $A^{3} Π$ state indicating that the crossings with higher electronic states would not affect laser cooling. Meanwhile, the extremely small vibrational branching loss ratios of the $A^{3} Π_{2}$ → $a^{1} Δ_{2}$ transition for NH and PH (NH: 1.81 × 10^-8; PH: 1.08 × 10^-6) indicate that the $a^{1} Δ_{2}$ intermediate electronic state will not interfere with the laser cooling. Consequently, we construct feasible laser-cooling schemes for NH and PH using three lasers based on the $A^{3} Π_{2}$ → $X^{3} Σ_{1}^{-}$ transition, which feature highly diagonal vibrational branching ratio $R_{00}$ (NH: 0.9952; PH: 0.9977), the large number of scattered photons (NH: 1.04×10⁵; PH: 8.32×10⁶) and very short radiative lifetimes (NH: 474 ns; PH: 526 ns). Our work suggests that feasible laser-cooling schemes could be established for a molecular system with extra electronic states close to those chosen for laser-cooling.

Keywords: ab initio; electronic state crossing; group VA hydrides; molecular laser cooling; spin-orbit coupling; ultracold molecules.