Direct Laser Cooling of Metal Monohydride Molecules

Preparation of ultracold dilute hydrogen gas can significantly improve the precision of hydrogen spectroscopy.  Fragmentation of laser coolable alkaline-earth metal monohydrides (e.g., BaH and CaH) provides a feasible scheme for this purpose. We have experimentally demonstrated optical cycling in BaH, and observed radiation-pressure force beam deflection as well as direct laser cooling. Here we present our latest data with CaH. A shorter lifetime and a larger optical cross section enable CaH to experience stronger light pressure with less laser power. We observed clear 1D Doppler and Sisyphus cooling profiles, and magnetic-field assisted dark state remixing. We used two different electronic excited states as the upper states, ΑΠ and ΒΣ, and acquired similar cooling profiles. Measurement of Franck-Condon factors (f₀₀, f₀₁, f₀₂) for both electronic excited states was also performed. Combined with calculated FCFs, it indicates that we can achieve more than 7×10⁴ photon scatters by adding a second vibrational repump laser, potentially enabling the loading and trapping of CaH in a radio-frequency MOT.