Sub-Doppler cooling of sodium atoms in gray molasses

We report on the realization of sub-Doppler laser cooling of sodium atoms in gray molasses using the D-1 optical transition (3s S-2(1/2) -> 3p P-2(1/2)) at 589.8 nm. The technique is applied to samples containing 3 x 10(9) atoms, previously cooled to 350 mu K in a magneto-optical trap, and it leads to temperatures as low as 9 mu K and phase-space densities in the range of 10(-4). The capture efficiency of the gray molasses is larger than 2/3, and we observe no density-dependent heating for densities up to 10(11) cm(-3).