Relativistic coupled-cluster calculations of RaOH pertinent to spectroscopic detection and laser cooling.

A relativistic coupled-cluster study of the low-lying electronic states in the radium monohydroxide molecule (RaOH), a radioactive polyatomic molecule of interest to laser cooling and to the search of new physics beyond the Standard Model, is reported. The level positions of the A ² Π _1/2 and C ² Σ states have been computed with an accuracy of around 200 cm ^-1 to facilitate spectroscopic observation of RaOH using laser induced fluorescence spectroscopy, thereby exploiting the systematic convergence of electron-correlation and basis-set effects in relativistic coupled-cluster calculations. The energy level for the B ² Δ _3/2 state has also been calculated accurately to conclude that the B ² Δ _3/2 state lies above the A ² Π _1/2 state. This confirms X ² Σ ↔ A ² Π _1/2 as a promising optical cycling transition for laser cooling RaOH.