Vibrational branching fractions for laser cooling of nonlinear strontium-containing molecules

The vibrational branching fractions from the lowest excited electronic state for ${\text{SrOCH}}_3$, ${\text{SrNH}}_2$, and $\text{SrSH}$ are measured at the $<0.1\%$ level. Spectra are obtained by driving the $\tilde{X}\text{−}\tilde{A}$ transitions and dispersing the fluorescence on a grating spectrometer. We also perform ab initio calculations for the energies of vibrational levels relevant for laser cooling, as well as branching fractions to support the interpretations of all molecular spectra. Symmetry-group analysis is applied in conjunction with our data to study rotational closure in these molecules. These analyses indicate favorable prospects for laser cooling ${\text{SrNH}}_2$ and other similar alkaline-earth-metal(-like) amides for future beyond the Standard Model physics searches using polyatomic molecules with long-lived parity doublets.