Precision spectroscopy of the A2Π←X2Σ+ transition in BaF

High-resolution spectroscopy on the A2Π−X2Σ+ electronic system of 138Ba19F is performed using a cold molecular beam produced by a buffer gas source. The hyperfine structure in both X2Σ+ ground and A2Π 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 X2Σ+(v=0,1) and A2Π(v=0,1) levels are determined, with improved accuracy for the Tv′,v″ band origins and spin-orbit interaction constants for the A2Π 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≥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.