  We study optical gain in a gas of cold 39K atoms. The gain is observed during
operation of a conventional magneto-optical trap without the need for
additional fields. Measurements of transmission spectra from a weak probe show
that the gain is due to stimulated Raman scattering between hyperfine ground
states. The experimental results are reproduced by a simplified six-level
model, which also helps explain why such gain is not observed in similar
experiments with rubidium or cesium.
