Ultracold atoms at temperatures close to the recoil limit have been achieved by extending Doppler cooling to forbidden transitions. A cloud of ${}^{40}\mathrm{Ca}$ atoms has been cooled and trapped to a temperature as low as $6\mu \mathrm{K}$ by operating a magnetooptical trap on the spin-forbidden intercombination transition. Quenching the long-lived excited state with an additional laser enhanced the scattering rate by a factor of 15, while a high selectivity in velocity was preserved. With this method, more than $10\%$ of precooled atoms from a standard magnetooptical trap have been transferred to the ultracold trap. Monte Carlo simulations of the cooling process are in good agreement with the experiments.

• Received 18 May 2001

DOI:https://doi.org/10.1103/PhysRevLett.87.123002