The feasibility of laser cooling $M\mathrm{H}$ ($M=\mathrm{Be},\mathrm{Mg},\mathrm{Ca},\mathrm{Sr},\mathrm{and}\mathrm{Ba}$) is investigated using ab initio quantum chemistry. The ground state $X{}^2{\Sigma }^{+}$ and first excited state $A{}^2\Pi$ of $M\mathrm{H}$ species are calculated using the multireference configuration interaction (MRCI) level of theory. For $\mathrm{CaH}$, $\mathrm{SrH}$, and $\mathrm{BaH}$, the spin-orbit coupling effects are also taken into account in electronic structure calculations at the MRCI level. Calculated spectroscopic constants for ${}^9\mathrm{BeH}$, ${}^{24}\mathrm{MgH}$, ${}^{40}\mathrm{CaH}$, ${}^{86}\mathrm{SrH}$, and ${}^{138}\mathrm{BaH}$ show good agreement with available theoretical and experimental results. The permanent dipole moments and transition dipole moments (TDMs) of the $X{}^2{\Sigma }^{+}$ and $A{}^2\Pi$ states of $M\mathrm{H}$ species are also calculated at the MRCI level of theory, and they are in good agreement with previous theoretical results. With the potential energy curves and TDMs obtained, the highly diagonally distributed Franck-Condon factors $f_{00}\left({}^9\mathrm{BeH}:0.998{,}^{24}\mathrm{MgH}:0.954{,}^{40}\mathrm{CaH}:0.961{,}^{86}\mathrm{SrH}:0.978,\mathrm{and}{}^{138}\mathrm{BaH}:0.971\right)$ for the $A{}^2\Pi \left(v^{\prime }=0\right)\to X{}^2{\Sigma }^{+}\left(v=0\right)$ transition are determined. Radiative lifetime calculations of the $A{}^2\Pi \left(v^{\prime }=0\right)$ state (${}^9\mathrm{BeH}:82.0\mathrm{ns}$, ${}^{24}\mathrm{MgH}:48.6\mathrm{ns}$, ${}^{40}\mathrm{CaH}:33.3\mathrm{ns}$, ${}^{86}\mathrm{SrH}:33.2\mathrm{ns}$, and ${}^{138}\mathrm{BaH}$: 68.6 ns) are found to be short enough for rapid laser cooling. The proposed main cycling laser drives the $X{}^2{\Sigma }^{+}\left(v=0\right)\to A{}^2\Pi \left(v^{\prime }=0\right)$ transition at wavelength ${\lambda }_{00}\left({}^9\mathrm{BeH}:497.2\mathrm{nm}{,}^{24}\mathrm{MgH}:525.5\mathrm{nm}{,}^{40}\mathrm{CaH}:675.4\mathrm{nm}{,}^{86}\mathrm{SrH}:740.3\mathrm{nm},\mathrm{and}{}^{138}\mathrm{BaH}:952.6\mathrm{nm}\right)$. The results demonstrate the possibility of laser cooling $M\mathrm{H}$ species, and provide a promising theoretical reference for further theoretical and experimental research on alkaline-earth-metal monohydrides.

The feasibility of laser cooling BH and GaF is investigated using ab initio quantum chemistry. The ground state X ¹Σ⁺ and first two excited states ³Π and ¹Π of BH and GaF are calculated using the multireference configuration interaction (MRCI) level of theory. For GaF, the spin–orbit coupling effect is also taken into account in the electronic structure calculations at the MRCI level. Calculated spectroscopic constants for BH and GaF show good agreement with available theoretical and experimental results. The highly diagonal Franck–Condon factors (BH: f₀₀ = 0.9992, f₁₁ = 0.9908, f₂₂ = 0.9235; GaF: f₀₀ = 0.997, f₁₁ = 0.989, f₂₂ = 0.958) for the ¹Π (v′ = 0–2) → X ¹Σ⁺ (v = 0–2) transitions in BH and GaF are determined, which are found to be in good agreement with the theoretical and experimental data. Radiative lifetime calculations of the ¹Π (v′ = 0–2) state (BH: 131, 151, and 187 ns; GaF: 2.26, 2.36, and 2.48 ns) are found to be short enough for rapid laser cooling. The proposed laser cooling schemes that drive the ¹Π (v′ = 0) → X ¹Σ⁺ (v = 0) transition use just one laser wavelength λ₀₀ (BH: 436 nm, GaF: 209 nm). Though the cooling wavelength of GaF is deep in the UVC, a frequency quadrupled Ti:sapphire laser (189–235 nm) could be capable of generating useful quantities of light at this wavelength. The present results indicate that BH and GaF are two good choices of molecules for laser cooling.

We investigate the feasibility of laser cooling BBr and BCl using ab initio quantum chemistry. The multi-reference configuration interaction method (MRCI) is used to calculate the ground state X¹Σ⁺ and the low-lying excited state A¹Π, where Davidson modification with the Douglas–Kroll scalar relativistic correction is also taken into account. The calculated spectroscopic constants are in good agreement with available experimental values. The potential energy curves, permanent dipole moments (PDMs), transition dipole moments (TDMs) followed by Franck–Condon factors and radiative times for the transitions from the A¹Π state to the ground state X¹Σ⁺ are obtained as well. The determined Franck–Condon factors are highly diagonally distributed and the evaluated radiative lifetimes are of the order of nanoseconds. Furthermore, the a³Π → X¹Σ⁺ transitions of BBr and BCl are also strongly diagonal and the X¹Σ⁺ → A¹Π transitions perhaps can be followed by the X¹Σ⁺ → a³Π transitions to attain a lower Doppler temperature. Long-range behavior of BBr and BCl has also been studied, and a double well is found in the A¹Π state of BBr. The shallow long-range well might open up even more channels for laser cooling of BBr. The results demonstrate the possibility of laser cooling BBr and BCl, and provide a promising theoretical reference for further research on BBr and BCl.