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

We have observed the photoassociative spectra of colliding ultracold ³⁹K and ⁸⁵Rb atoms to produce KRb* in all eight bound electronic states correlating with the ³⁹K (4s) + ⁸⁵RL(Sp_1/2 and 5p_3/2) asymptotes. These electronically excited KRb* ultracold molecules are detected after their radiative decay to the metastable triplet (a³∑⁺) state and (in some cases) the singlet (X¹∑⁺) ground state. The triplet (a³∑⁺) 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 C₆ values correlating to the K(4s) + Rb (Sp_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(1), 1(2)), with one set of similar C₆ values within the upper triad and a different set of similar C₆ 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 1¹Π ∼ 4(1) 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 a³∑⁺ (or X¹∑⁺) molecules; we do know from Kasahara et al. that levels of the 1¹Π ∼ 4(1) and 2¹Π ∼ 5(1) states in the intra-asymptote region are predissociated. A small fraction (≤1/3) of the triplet (a³∑ ⁺) ultracold molecules formed are trapped in the weak magnetic field of our magneto-optical trap (MOT).