The possibility of laser cooling of thallium chloride (TlCl) molecules has been investigated based on high-level ab initio calculations with the consideration of the core-valence and the spin-orbit coupling (SOC) effects. The potential energy curves of the 13 Λ-S states as well as the 24 Ω states split from them via SOC are obtained by multi-reference configuration interaction plus the Davidson correction. We show that the aΠ0+3-X1Σ0+ transition of TlCl is a possible candidate for laser cooling, which features highly diagonal Franck-Condon factors and no intermediate interacting electronic states. Based on our calculations, we propose an optical cycling scheme by utilizing four lasers at wavelengths around 320 nm with more than 104 cycles for photon absorption/emission and a sub-microkelvin temperature limit.
The possibility of laser cooling of thallium chloride (TlCl) molecules has been investigated based on high-level ab initio calculations with the consideration of the core-valence and the spin-orbit coupling (SOC) effects. The potential energy curves of the 13 Λ-S states as well as the 24 Ω states split from them via SOC are obtained by multi-reference configuration interaction plus the Davidson correction. We show that the aΠ0+3-X1Σ0+ transition of TlCl is a possible candidate for laser cooling, which features highly diagonal Franck-Condon factors and no intermediate interacting electronic states. Based on our calculations, we propose an optical cycling scheme by utilizing four lasers at wavelengths around 320 nm with more than 104 cycles for photon absorption/emission and a sub-microkelvin temperature limit.
Following the interest in the experimental realization of laser cooling for thallium fluoride (TlF), determining the potential of thallium chloride (TlCl) as a candidate for laser cooling experiments has recently received attention from a theoretical perspective [Yuan et al., J. Chem. Phys. 149, 094306 (2018)]. From these ab initio electronic structure calculations, it appeared that the cooling process, which would proceed from transitions between a3Π0+ and X1Σ0+ states, had as a potential bottleneck the long lifetime (6.04 µs) of the excited state a3Π0+, that would make it very difficult to experimentally control the slowing zone. In this work, we revisit the electronic structure of TlCl by employing four-component Multireference Configuration Interaction (MRCI) and Polarization Propagator (PP) calculations and investigate the effect of such approaches on the computed transition dipole moments between a3Π0+ and a3Π1 excited states of TlCl and TlF (the latter serving as a benchmark between theory and experiment). Whenever possible, MRCI and PP results have been cross-validated by four-component equation of motion coupled-cluster calculations. We find from these different correlated approaches that a coherent picture emerges in which the results of TlF are extremely close to the experimental values, whereas for TlCl the four-component calculations now predict a significantly shorter lifetime (between 109 and 175 ns) for the a3Π0+ than prior estimates. As a consequence, TlCl would exhibit rather different, more favorable cooling dynamics. By numerically calculating the rate equation, we provide evidence that TlCl may have similar cooling capabilities to TlF. Our analysis also indicates the potential advantages of boosting stimulated radiation in optical cycles to improve cooling efficiency.
The potential energy curves for four Λ-S and eight Ω states of the AlD molecule are calculated by using the valence internally contracted multireference configuration interaction approach with the Davidson modification (icMRCI+Q). The core-valence correlation and relativistic corrections are included. The extrapolation of potential energies to the complete basis set limit is made. Transition dipole moments (TDMs) between the six bound Ω states are also calculated by the icMRCI approach. The calculated spectroscopic constants are in good agreement with the available experimental data and theoretical values. Based on the obtained potential energy curves and transition dipole moments, highly diagonal Franck-Condon factors and vibrational branching ratios are determined for the A1π1(υ'=0,1)→X1Σ+0+(υ'') transition, short spontaneous radiative lifetime and narrow radiative width for the excited state A1π1 (υ'=0, 1) are also predicted. On this account, the optical scheme of the laser cooling for AlD molecule is constructed with A1π1(υ')↔X1Σ+0+(υ'') as the close-loop transition. The proposed laser drives X1Σ+0+(υ'')→A1π1(υ') transition by using three wavelengths (main laser beam λ00=420.96 nm; two repumping laser beams λ10=443.00 nm and λ21=448.94 nm). The Doppler temperature (TDoppler=44.94 μK) and recoiltemperature(TRecoil=3.73 μK) for the A1π1 (υ'=0)X1Σ+0+ (υ''=0) transition are also calculated. These results can provide theoretical support for the experimental study of laser-cooled AlD molecules.
By employing ab initio quantum chemistry method, we investigate the feasibility of laser cooling InF molecule. Four low-lying electronic states (X1Σ+, C1Π, 3Π and 23Π) of InF have been calculated using the multi-reference configuration interaction (MRCI) method. The spin-orbit coupling effects are also taken into account in the electronic structure computation at the MRCI level. The highly diagonal Franck-Condon factors for C1Π → X1Σ+ transitions are estimated. The radiative lifetime of the C1Π (v′ = 0) state is about 2.22 ns, which is found to be enough short for rapid laser cooling. Though the cooling wavelength of InF is located in the short-wavelength ultraviolet light (UVC), a frequency quadrupled Ti: sapphire laser (189–235 nm) could be capable of generating laser transition wavelength of InF. Furthermore, the C1Π → X1Σ+ transitions perhaps can be followed by the B3Π1 → X1Σ+0+ transitions to attain a lower Doppler temperature. Meanwhile, for achieving quasi-closed transition cycle of InF molecule, we investigate the hyperfine structure of the lowest state X1Σ+. Overall, the present results indicate the possibility of laser cooling InF molecules.
The potential energy curves and transition dipole moments of seven Λ-S states and six Ω states of LuH+ cation are calculated by using the multireference configuration interaction method plus Davidson correction with the pseudopotential electron basis set, and the spectroscopic parameters of the bound states are obtained. The spin–orbit coupling effect is considered in the calculations. The Franck–Condon factors (FCFs) and the Einstein spontaneous emission coefficients are obtained. The results indicate that the A2Δ3/2(ν′ = 0) ↔ X2Σ+1/2(ν″ = 0) and B2Π1/2(ν′ = 0) ↔ X2Σ+1/2 (ν″ = 0) transitions provide highly diagonally distributed FCFs. Spontaneous radiative lifetimes of A2Δ3/2 (ν′ = 0) and B2Π1/2 (ν′ = 0) excited states are both obtained. The cooling schemes of LuH+ cation are obtained using the ab initio method. For the B2Π1/2 ↔ X2Σ+1/2 transition, the influence of the intermediate state on the construction of quasi-closed cyclic transitions can be neglected. A main pump laser and two repumping lasers are required to construct the A2Δ3/2 ↔ X2Σ+1/2 and B2Π1/2 ↔ X2Σ+1/2 quasi-cycling transitions for laser cooling of LuH+ molecular cation. The results in this work provide reliable theoretical guidance for further experiments of laser cooling of LuH+ cation.
Ab initio calculations for three low-lying electronic states (X(2)Σ(+), A(2)Π, and 2(2)Π) of MgCl and MgBr molecules, including spin-orbit coupling, are performed using multi-reference configuration interaction plus Davidson correction method. The calculations involve all-electronic basis sets and Douglas-Kroll scalar relativistic correction. Spectroscopic parameters well agree with available theoretical and experimental data. Highly diagonally distributed Franck-Condon factors f00 for A(2)Π3/2,1/2 (υ' = 0) → X(2)Σ(+) 1/2 (υ″ = 0) are determined for both MgCl and MgBr molecules. Suitable radiative lifetimes τ of A(2)Π3/2,1/2 (υ' = 0) states for rapid laser cooling are also obtained. The proposed laser drives A(2)Π3/2 (υ' = 0) → X(2)Σ(+) 1/2 (υ″ = 0) transition by using three wavelengths (main pump laser λ00; two repumping lasers λ10 and λ21). These results indicate the probability of laser cooling MgCl and MgBr molecules.
A spin-orbit coupling electronic structure study of the AlZn molecule is conducted to investigate the molecular properties of the low-lying electronic states and their feasibility toward direct laser cooling. This study uses the complete active-space self-consistent field level of theory, followed by the multireference configuration interaction method with Davidson correction (+Q). The potential energy and dipole moment curves and the spectroscopic constants are computed for the low-lying doublet and quartet electronic states in the 2S+1Λ± and Ω(±) representations. The transition dipole moments, the Franck-Condon factors, the Einstein coefficient, the radiative lifetimes, the vibrational branching ratio, and the slowing distance are determined between the lowest spin-orbit bound electronic states. These results show that the molecule AlZn has a high potential for laser cooling through the X2Π1/2 → (2)2Π1/2 transition by utilizing four lasers at a wavelength in the ultraviolet region, reaching a sub-microkelvin temperature limit.