We report on high resolution Fourier-transform spectroscopy of fluorescence to the a^3Sigma_u^+ state excited by two-photon or two-step excitation from the X^1Sigma_g^+ state to the 2^3Pi_g state in the molecule K_2. These spectroscopic data are comb
ined with recent results of Feshbach resonances and two-color photoassociation spectra for deriving the potential curves of X^1Sigma_g^+ and a^3Sigma_u^+ up to the asymptote. The precise relative position of the triplet levels with respect of the singlet levels was achieved by including the excitation energies from the X^1Sigma_g^+ state to the 2^3Pi_g state and down to the a^3Sigma_u^+ state in the simultaneous fit of both potentials. The derived precise potential curves allow for reliable modeling of cold collisions of pairs of potassium atoms in their ^2S ground state.
A comprehensive study of the electronic states at the 4s+5s asymptote in KRb is presented. Abundant spectroscopic data on the astate state were collected by Fourier-transform spectroscopy which allow to determine an accurate experimental potential en
ergy curve up to 14.8 AA . The existing data set (C. Amiot et al. J. Chem. Phys. 112, 7068 (2000)) on the ground state Xstate was extended by several additional levels lying close to the atomic asymptote. In a coupled channels fitting routine complete molecular potentials for both electronic states were fitted. Along with the line frequencies of the molecular transitions, recently published positions of Feshbach resonances in $^{40}$K and $^{87}$Rb mixtures (F. Ferlaino et al. Phys. Rev. A 74, 039903 (2006)) were included in the fit. This makes the derived potential curves capable for an accurate description of observed cold collision features so far. Predictions of scattering lengths and Feshbach resonances in other isotopic combinations are reported.
