Spectroscopy of the a3Σu+ state and the coupling to the X1Σg+ state of K2

We report on high resolution Fourier-transform spectroscopy of fluorescence to the a(3)Sigma(+)(u) state induced by two-photon or two-step excitation from the X-1 Sigma(+)(g) state to the 2(3)Pi(g) state in the molecule K-2. These spectroscopic data are combined with recent results of Feshbach resonances and two-color photoassociation spectra for deriving the potential curves of X-1 Sigma(+)(g) and a(3)Sigma(+)(u) up to the asymptote. The precise relative position of the triplet levels with respect to the singlet levels was achieved by including the excitation energies from the X-1 Sigma(+)(g) state to the 2(3)Pi(g) state and the frequencies of the fluorescence down to the a(3)Sigma(+)(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 S-2 ground state.