Sub-Doppler laser spectroscopy of the NaK molecule

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Abstract
Excitation of NaK molecules in a collimated molecular beam by a single mode tunable argon laser with sub‐Doppler resolution allows selective population of definite vibrational rotational levels (v′, J′) in the D 1Π‐state. The resultant fluorescence consists of a singlet spectrum in the 470–570 nm range and a triplet spectrum in the 625–715 nm range. The analysis of the triplet fluorescence, which consists of discrete lines and of a modulated continuum, allows the determination of the a 3Σ+ potential and yields very precise values for the dissociation energies in the a 3Σ+ and X 1Σ+ states, which are measured as De(3Σ+) =204±4 cm−1 and De(1Σ+) =5269±6 cm−1. For some excitation wavelengths a fluorescence spectrum with discrete lines and with a double‐modulated continuum is observed which is assigned to ’’ Condon internal diffraction’’ bands terminating close below and above the dissociation limit of the X 1Σ+ state. Its relevance for the determination of the upper and lower potential curves is discussed.