  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 138Ba19F and
136Ba19F 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.
