  Only bosonic molecular species have been directly laser cooled to date,
primarily due to an abundance of bosonic isotopes in nature and to their
simpler hyperfine structure. Fermionic molecules provide new opportunities for
ultracold chemistry, quantum simulation, and precision measurements. Here we
report direct laser cooling of a fermionic molecular isotopologue, calcium
monodeuteride (CaD). With a nuclear spin I = 1, only 5 hyperfine states need to
be addressed for rotational closure in optical cycling. These hyperfine states
are unresolved for typical experimental linewidths. We present a method for
efficiently producing alkaline-earth metal hydrides and deuterides. We
demonstrate rotational closure and show magnetically assisted Sisyphus cooling
in one dimension for a beam of CaD molecules. Our results indicate that the
experimental complexity for laser cooling CaD is similar to that of calcium
monohydride (CaH). Laser cooling of CaD is a promising first step for
production of ultracold and trapped atomic deuterium.
