Infrared–Infrared double resonance spectroscopy of 39K2: The 1 3Δg state

Twenty-one vibrational levels, v = 8, 9, 11–27, 29, and 30, of the 39K2 1 3Δg state have been observed by perturbation facilitated infrared–infrared double resonance fluorescence excitation spectroscopy. The 1 3Δg state exhibits a case (a) coupling in the low-J limit and evolves rapidly to case (b) coupling as J increases. A spin–orbit constant, A = −0.365 cm−1, which is obtained from the observed term values, agrees well with the prediction from the value of the K atom 3d spin–orbit splitting. Molecular constants and the RKR potential curve agree well with theoretical results.