Spectroscopy of the a^3Σ_u^+ state and the coupling to the X^1Σ_g^+ state of K_2

We report on high resolution Fourier-transform spectroscopy of fluorescence to the a^3Σ_u^+ state excited by two-photon or two-step excitation from the X^1Σ_g^+ state to the 2^3Π_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^1Σ_g^+ and a^3Σ_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^1Σ_g^+ state to the 2^3Π_g state and down to the a^3Σ_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.