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

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 (A3Π2 and X3Σ1− ). 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 A3Π and Σ−5 states of NH and PH are higher than the corresponding v′ = 2 vibrational levels of the A3Π state indicating that the crossings with higher electronic states would not affect laser cooling. Meanwhile, the extremely small vibrational branching loss ratios of the A3Π2 → a1Δ2 transition for NH and PH (NH: 1.81 × 10–8; PH: 1.08 × 10–6) indicate that the a1Δ2 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 A3Π2 → X3Σ1− transition, which feature highly diagonal vibrational branching ratio R00 (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.

Production of ultracold polyatomic molecules with strong polarity by laser cooling: A detailed theoretical study on CaNC and SrNC

“…The C 2v symmetry is used for the constrained PEC scanning, while the C s symmetry is used for the un-constrained PEC scanning. The selection of active space is very important for the icMRCI + Q calculations (Yu and Bian, 2012;Feller et al, 2014;El-Kork et al, 2017;Zeid et al, 2020;Li and Bian, 2021;Moussa et al, 2021). Here, the chosen active space includes 7 electrons in 13 orbitals for CaNC or SrNC.…”

A theoretical study on laser cooling feasibility of XH (X = As, Sb and Bi): effects of intersystem crossings and spin–orbit couplings

“…From our previous work, we can notice that the location of the crossing point of NH is between ν ′ = 3 and ν ′ = 4 and that of PH is between ν ′ = 2 and ν ′ = 3 of the A 3 Π state. 49 So the location of the crossing point moves down towards the ground vibrational level of the A 3 Π state with the increasing atomic number in the group VA hydrides. A similar trend of the crossing point shifting down a group can also be seen for the group IVA hydrides, in which there is a crossing between the B 2 Σ − and A 2 Δ states for each group member and the crossing point also moves down toward the ν ′ = 0 level of A 2 Δ.…”

“…[46][47][48] The crossing point for AsH is located nearby the n 0 = 2 vibrational level of the A 3 P state, that of SbH is close to n 0 = 1, and that of BiH is between the n 0 = and n 0 = 1 levels of A 3 P. From our previous work, we can notice that the location of the crossing point of NH is between n 0 = 3 and n 0 = 4 and that of PH is between n 0 = 2 and n 0 = 3 of the A 3 P state. 49 So the location of the crossing point moves down towards the ground vibrational level of the A 3 P state with the increasing atomic number in the group VA hydrides. A similar trend of the crossing point shifting down a group can also be seen for the group IVA hydrides, in which there is a crossing between the B 2 S À and A 2 D states for each group member and the crossing point also moves down toward the n 0 = 0 level of A 2 D. 13 Therefore, an empirical law of ''crossing point shifting down'' when going from Period 2 to Period 6 can be generalized, which may be applicable for hydrides of all groups in the periodic table.…”

Ab Initio Calculation of Ground- and Low-Excited-State Spectroscopic Data and Transition Properties of SBr+

“…Fast decaying intermediate states can flip the parity and break the cooling cycle. At the same time, the relative strength of the photon loss pathways is more directly related to the vibrational branching ratios than Franck–Condon factors in the laser cooling cycle. − For X 3 Σ 1 – (1)­1, the difference between the equilibrium bond length and the ground state is 0.0003 Å. This means that the Franck–Condon factor between the two states is large, but the adiabatic excitation energy is only 52 cm –1 and the strength of the transition between the two states is weak, so it is not a suitable choice.…”