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

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) + 85Rb(5$p_{1/2}$ and 5$p_{3/2})$ 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([see full textsee full textsee full textsee full textsee full textsee full textsee full text])) in a lower triad of states with similar C6 values correlating to the K(4s) + Rb (5$p_{1/2})$ asymptote; and to five states in an upper triad of three states (3(0$^{+})$, 3(0$^{-})$, 3([see full textsee full textsee full textsee full textsee full textsee full textsee full text])) and a dyad of two states (4([see full textsee full textsee full textsee full textsee full textsee full textsee full text]), 1([see full textsee full textsee full textsee full textsee full textsee full textsee full text])), 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 1$^{1}\Pi \sim $ 4([see full textsee full textsee full textsee full textsee full textsee full textsee full text]) state they observed. We also argue that ultracold photoassociation to levels between the K(4s) + Rb (5$p_{3/2})$ and K(4s) + Rb (5$p_{1/2})$ 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 \sim $ 4([see full textsee full textsee full textsee full textsee full textsee full textsee full text]) and 2$^{1}\Pi \sim $ 5([see full textsee full textsee full textsee full textsee full textsee full textsee full text]) states in the intra-asymptote region are predissociated. A small fraction ($\le $1/3) of the triplet (a$^{3}\Sigma ^{+})$ ultracold molecules formed are trapped in the weak magnetic field of our magneto-optical trap (MOT).