Abstract

In contrast to atoms, laser cooling of molecules is very difficult because of the lack of a closed two-level scheme for recycling the population. The molecular photoassociation of cold atoms opens a promising alternative for the formation of cold molecules. Photoassociation of cold cesium atoms has permitted the first observation of translationally cold ground state molecules.¹ The photoassociative spectra are obtained by the usual trap-loss method as well as by pulsed laser photoionization of Cs₂ molecules into ${\text{Cs}}_2^{+}$ ions. The two spectra present markedly different features: while the 1_g and $0_{\text{u}}^{+}$ and $0_{\text{g}}^{-}$ vibrational progressions are present in the trap-loss spectrum, the ${\text{Cs}}_2^{+}$ ion spectrum presents only $0_{\text{g}}^{-}$ and 1_u vibrational progressions. The $0_{\text{g}}^{-}$ and 1_u long-range states below the dissociation limit 6s²S_1/2 + 6p²P_3/2 present Condon points at intermediate distances; such configurations offer rather efficient channels for the creation of triplet or singlet ground state molecules after spontaneous decay. Temperatures of the molecular cloud, T_m, at about 300 μK are measured for a trapped atomic sample at T_a ~ 200 μK, and as low as T_m ~ 85 μK, with 30% uncertainty, for T_a ~ 30 μK.² Different schemes to obtain formation of cold molecules through photoassociation will be discussed.

© 1999 Optical Society of America