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The K and Cs atom pair and its X 1 + and a 3 + electronic molecular states are studied by Fourier-transform spectroscopy with special emphasis on bound states close to the ground state asymptote (4s)K + (6s)Cs. Significant vibrational mixing due to hyperfine coupling was observed leading to improved potential energy curves for both states. The observed intensity pattern of the hyperfine structure for the last vibrational levels of X 1 + and a 3 + reveals deep insight into the multicomponent wave function of such levels with its influence for modeling the asymptotic behavior of the excited electronic states involved in the reported observations. Examples of derived properties of cold collisions are given and the importance of the present results for efficient production schemes for ultracold KCs is discussed.

The coupling of the X 1 + and a 3 + states of the atom pair Na + Cs and modelling cold collisions

The X 1 Σ + state of NaRb was studied by Fourier transform spectroscopy. An accurate potential energy curve was derived from more than 8800 transitions in isotopomers 23 Na 85 Rb and 23 Na 87 Rb. This potential reproduces the experimental observations within their uncertainties of 0.003 cm −1 to 0.007 cm −1 . The outer classical turning point of the last observed energy level (v ′′ = 76, J ′′ = 27) lies at ≈ 12.4 Å, leading to a energy of 4.5 cm −1 below the ground state asymptote.

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