Precision spectroscopy of the $A^2\Pi$ $\leftarrow$ $X^2\Sigma^+$ transition in BaF

TL;DR

This study performs high-resolution spectroscopy on BaF molecules, precisely measuring hyperfine structures and transition frequencies to improve molecular constants, aiding future laser cooling and electron dipole moment experiments.

Contribution

The paper provides highly accurate measurements of BaF transition frequencies and molecular constants, enhancing previous data and supporting future precision experiments.

Findings

Hyperfine structures fully resolved in both states

Transition frequencies measured at sub-MHz accuracy

Improved molecular constants for low-J states

Abstract

High-resolution spectroscopy on the $A^2\Pi$ - $X^2\Sigma^+$ electronic system of $^{138}$Ba$^{19}$F is performed using a cold molecular beam produced by a buffer gas source. The hyperfine structure in both $X^2\Sigma^+$ ground and $A^2\Pi$ excited states is fully resolved and absolute transition frequencies of individual components are measured at the sub-MHz level making use of frequency-comb laser calibration. Sets of molecular constants for the $X^2\Sigma^+$($v=0,1$) and $A^2\Pi$($v=0,1$) levels are determined, with improved accuracy for the $T_{v',v''}$ band origins and spin-orbit interaction constants for the $A^2\Pi$ excited states, that represent the presently measured highly accurate transitions for low-$J$ states as well as previously determined transition frequencies in Fourier-transform emission studies for rotational levels as high as $J \geq 100$. The extracted molecular constants reproduce the measured transition frequencies at the experimental absolute accuracy of 1 MHz. The work is of relevance for future laser cooling schemes, and is performed in the context of a measurement of the electron dipole moment for which BaF is a target system.