Saturated absorption spectroscopy of buffer-gas-cooled Barium monofluoride molecules

TL;DR

This paper demonstrates high-resolution Doppler-free saturated absorption spectroscopy of buffer-gas-cooled BaF molecules at 4K, revealing hyperfine structures crucial for laser cooling applications.

Contribution

It provides the first high-resolution spectra of cooled BaF molecules, accurately measuring hyperfine splittings relevant for laser cooling.

Findings

Achieved 19 MHz spectral resolution in cryogenic conditions

Resolved hyperfine transitions of BaF molecules

Measured hyperfine splitting of 18 MHz in the excited state

Abstract

We report an experimental investigation on the Doppler-free saturated absorption spectroscopy of buffer-gas-cooled Barium monofluoride (BaF) molecules in a 4~K cryogenic cell. The obtained spectra with a resolution of 19~MHz, much smaller than previously observed in absorption spectroscopy, clearly resolve the hyperfine transitions. Moreover, we use these high-resolution spectra to fit the hyperfine splittings of excited $A(v=0)$ state and find the hyperfine splitting of the laser-cooling-relevant $A^2\Pi_{1/2}(v=0, J=1/2, +)$ state is about 18 MHz, much higher than the previous theoretically predicted value. This provides important missing information for laser cooling of BaF molecules.