Laser cooling of the alkaline-earth-metal monohydrides: Insights from an ab initio theory study

The feasibility of laser cooling MH (M = Be, Mg, Ca, Sr, and Ba) is investigated using ab initio quantum chemistry. The ground state X (2)Sigma(+) and first excited state A (2)Pi of MH species are calculated using the multireference configuration interaction (MRCI) level of theory. For CaH, SrH, and BaH, the spin-orbit coupling effects are also taken into account in electronic structure calculations at the MRCI level. Calculated spectroscopic constants for (BeH)-Be-9, (MgH)-Mg-24, (CaH)-Ca-40, (SrH)-Sr-86, and (BaH)-Ba-138 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 MH 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 infinity ((BeH)-Be-9 : 0.998,(MgH)-Mg-24 : 0.954,(CaH)-Ca-40 : 0.961,(SrH)-Sr-86 : 0.978, and (BaH)-Ba-138 : 0.971) for the A (2)Pi (v' = 0) -> X (2)Sigma(+) (v = 0) transition are determined. Radiative lifetime calculations of theA (2)Pi (v' = 0) state (9BeH : 82.0ns, (MgH)-Mg-24 : 48.6 ns, (CaH)-Ca-40 : 33.3ns, (SrH)-Sr-86 : 33.2 ns, and (BaH)-Ba-138: 68.6 ns) are found to be short enough for rapid laser cooling. The proposed main cycling laser drives the X (2)Sigma(+) (v = 0) -> A (2)Pi (v' = 0) transition atwavelength. lambda infinity ((BeH)-Be-9 : 497.2 nm, (MgH)-Mg-24 : 525.5nm, (CaH)-Ca-40 : 675.4nm, (SrH)-Sr-86 : 740.3nm, and (BaH)-Ba-138 : 952.6 nm). The results demonstrate the possibility of laser cooling MH species, and provide a promising theoretical reference for further theoretical and experimental research on alkaline-earth-metal monohydrides.