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arXiv:2307.08312 · Physical Review A

Absorption spectroscopy for laser cooling and high-fidelity detection of barium monofluoride molecules

Marian Rockenhäuser, Felix Kogel, Einius Pultinevicius, Tim Langen

Published Dec 14, 2023• 20 citations
arXivHTMLPDF

Abstract

Currently, there is great interest in laser cooling of barium monofluoride (BaF) molecules for precision tests of fundamental symmetries. We use high-resolution absorption spectroscopy to characterize several as yet imprecisely known transition frequencies required to realize such cooling. We extract an improved set of molecular constants for the bosonic $^{138}\mathrm{Ba}^{19}\mathrm{F}$ and $^{136}\mathrm{Ba}^{19}\mathrm{F}$ isotopologues, confirm the existence of a significant hyperfine splitting in the excited state of the laser cooling cycle, and investigate the effects of this splitting on the achievable cooling forces. As a direct application of our spectroscopic insights, we experimentally demonstrate nearly background-free fluorescence imaging of a BaF molecular beam in a glass cell vacuum chamber. We expect such high-fidelity detection to be useful for various types of precision measurement scenarios.

DOI
10.1103/physreva.108.062812

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We demonstrate laser cooling of barium monofluoride ( BaF 138 ) molecules. We use serrodynes to synthesize time-sequenced optical spectra that can be precisely tailored to the hyperfine structure of this heaviest nonradioactive alkaline earth monofluoride. By optimizing these optical spectra, we realize strong Sisyphus cooling forces that efficiently collimate a molecular beam. Our technique is an important step toward using intense beams of barium monofluoride for precision measurement applications, and will be useful for cooling other molecular species with complex level structure. Published by the American Physical Society 2024
Abstract Direct laser cooling of molecules has made significant progress in recent years. However, the selective cooling and manipulation of generic molecular samples based on their isotopic composition, which is ubiquitous in atomic laser cooling, has not yet been achieved. Here, we demonstrate such isotopologue-selective laser cooling and near-arbitrary control over the isotopic composition of a molecular beam, using barium monofluoride (BaF) as an example. The manipulation of the rare and previously uncooled 136 BaF is achieved in the presence of several isotopologues of significantly higher natural abundance. Our results enable intense molecular beams, high fidelity detection and manipulation of select low-abundance isotopologues or isotopic mixtures. Such beams are a first step towards isotopologue-selective molecular trapping and will be useful for applications in trace gas analysis, cold collisions and precision tests of fundamental symmetries.
We reported a detailed experimental study of the cold collision of barium monofluoride (BaF) with buffer gas and the high-resolution spectroscopy relevant with direct laser cooling. BaF molecules are efficiently produced with laser ablation and buffer-gas cooled in a cryogenic apparatus. The laser cooling relevant transition $|X\phantom{\rule{0.16em}{0ex}}^{2}\mathrm{\ensuremath{\Sigma}},v=0,N=1\ensuremath{\rangle}$ to $|A\phantom{\rule{0.16em}{0ex}}^{2}\mathrm{\ensuremath{\Pi}},{v}^{\ensuremath{'}}=0,{J}^{\ensuremath{'}}=1/2\ensuremath{\rangle}$ is identified. The collision cross section with buffer gas is measured to be $1.4(7)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}\phantom{\rule{4.pt}{0ex}}{\text{cm}}^{\ensuremath{-}2}$, which is very suitable for buffer-gas cooling. Both rotational and vibrational temperatures are effectively cooled, and a large amount of molecules are quenched into the desired states. Our study provides an important benchmark for further laser cooling of the BaF molecule.
We theoretically investigate the laser cooling of fermionic barium monofluoride (137BaF) molecules, which are promising candidates for precision studies of weak parity violation and nuclear anapole moments. This molecular species features two nuclear spins, resulting in a hyperfine structure that is considerably more complicated than the one found in the usual laser-cooled diatomics. We use optical Bloch equations and rate equations to show that optical cycling, sub-Doppler cooling and bichromatic forces can all be realized under realistically achievable experimental conditions.