Low vibrational levels of the ionic–covalent coupled state A0+ of NaI and its potential function

The A0+ state of NaI was investigated towards the minimum of its potential using laser-induced fluorescence and Λ-type double resonance spectroscopy via the covalent C0+ state, because significantly different potential functions were proposed in the literature due to missing spectroscopic data. The vibrational interval νadia = 135–282 measured previously by Schaefer et al. was extended to vibrational levels as low as νadia = 40. Transitions to lower levels are masked by the very strong C0+–B1 transition. An improved potential curve for A0+ was derived. The uncertainty by the unknown lowest vibrational levels is discussed. A comparison of the new curve with others reported in the literature is given.