  We demonstrate laser cooling of barium monofluoride (138BaF) molecules. We
use serrodynes to synthesize time-sequenced optical spectra that can be
precisely tailored to the hyperfine structure of this heaviest non-radioactive
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 towards using intense beams of barium
monofluoride for precision measurement applications, and will be useful for
cooling other molecular species with complex level structure.
