2D transverse laser cooling of a hexapole focused beam of cold BaF molecules

TL;DR

This paper demonstrates 2D transverse laser cooling of BaF molecules using a combination of buffer gas cooling, electrostatic focusing, and laser techniques, achieving significant photon scattering and benchmarking simulations for future experiments.

Contribution

It introduces a novel combination of methods for laser cooling of BaF molecules and provides experimental data to validate trajectory simulations for enhanced molecular beam flux.

Findings

Achieved a scattering rate of 6.1×10^5 s^-1 on the cooling transition.

Each molecule scatters approximately 400 photons during cooling.

Dark state leaks are less than 10%.

Abstract

A cryogenic buffer gas beam, an electrostatic hexapole lens, and 2D transverse Doppler laser cooling are combined to produce a bright beam of barium monofluoride ($^{138}$Ba$^{19}$F) molecules. Experimental results and trajectory simulations are used to study the laser cooling effect as a function of laser detuning, laser power, laser alignment, and interaction time. A scattering rate of 6.1(1.4) $\times 10^{5}$ s$^{-1}$ on the laser cooling transition is obtained; this is $14 \%$ of the expected maximum, which is attributed to limited control of the magnetic field used to remix dark states. Using 3 tuneable lasers with appropriate sidebands and detuning, each molecule scatters approximately 400 photons during 2D laser cooling, limited by the interaction time and scattering rate. Leaks to dark states are less than 10$\%$. The experimental results are used to benchmark the trajectory simulations to predict the achievable flux 3.5 m downstream for a planned $e$EDM experiment.