The photoassociative spectroscopy, photoassociative molecule formation, and trapping of ultracold 39K85Rb

We have observed the photoassociative spectra of colliding ultracold K-39 and Rb-85 atoms to produce KRb* in all eight bound electronic states correlating with the K-39 (4s) + Rb-85(5p(1/2) and 5p(3/2)) asymptotes. These electronically excited KRb* ultracold molecules are detected after their radiative decay to the metastable triplet (a(3)Sigma(+)) state and (in some cases) the singlet (X(1)Sigma(+)) ground state. The triplet (a(3)Sigma(+)) ultracold molecules are detected by two-photon ionization at 602.5 nm to form KRb+, followed by time-of-flight-mass spectroscopy. We are able to assign a majority of the spectrum to three states (2(0(+)), 2(0(-)), 2(1)) in a lower triad of states with similar C-6 values correlating to the K(4s) + Rb (5p(1/2)) asymptote; and to five states in an upper triad of three states (3(0(+)), 3(0(-)), 3(1)) and a dyad of two states (4(l), 1(2)), with one set of similar C6 values within the upper triad and a different set of similar C-6 values within the dyad. We are also able to make connection with the short-range spectra of Kasahara et al. [J. Chem. Phys. 111, 8857 (1999)], identifying three of our levels as v = 61, 62 and 63 of the 11 H 4(l) state they observed. We also argue that ultracold photoassociation to levels between the K(4s) + Rb (5p(3/2)) and K(4s) + Rb (5p(1/2)) asymptotes may be weakly or strongly predissociated and therefore difficult to observe by ionization of a(3)Sigma(+) (or X(1)Sigma(+)) molecules; we do know from Kasahara et al. that levels of the 1(1) Pi similar to 4(1) and 2(1)Pi similar to 5(1) states in the intra-asymptote region are predissociated. A small fraction (less than or equal to1/3) of the triplet (a(3)Sigma(+)) ultracold molecules formed are trapped in the weak magnetic field of our magneto-optical trap (MOT).