Direct Photoassociative Formation of Ultracold KRb Molecules in the Lowest Vibrational Levels of the Ground State

We report continuous direct photoassociative formation of ultracold KRb molecules in the lowest vibrational levels $(v\mathrm{"}=0-10)$ of the electronic ground state $(X^1{\mathrm{\Sigma }}^{+})$, starting from ${}^{39}$K and ${}^{85}$Rb atoms in a magneto-optical trap. The process exploits a newfound resonant coupling between the $2(1),v^{\mathrm{\prime }}=165$ and $4(1),v^{\mathrm{\prime }}=61$ levels, which exhibit an almost equal admixture of the uncoupled eigenstates. The production rate of the $X^1{\mathrm{\Sigma }}^{+}$ ($v\mathrm{"}$=0) level is estimated to be $5\times {10}^3$ molecules/sec.