Study of the 4 1Σ+g ‘‘shelf’’ state of Na2 by optical–optical double resonance spectroscopy

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Abstract
Due to an avoided crossing, the 4 1Σ+g state of Na2 has an unusual potential energy curve with a ‘‘shelf’’ on its outer wall. By using optical–optical double resonance techniques, vibrational levels from v=0 to 130 have been observed and the shelf was found around v=48–60. From the experimental data we have successfully constructed a Rydberg–Klein–Rees (RKR) potential curve, which can reproduce measured data with a standard deviation of 0.064 cm−1. A set of Dunham coefficients was obtained for the lower part v=0–45 of that state. Several interesting effects related to the shelf, such as the unusual behavior of Bv and Dv, of the vibrational wave functions and the semiclassical vibrational periods, and of the rotational effective potential, are discussed.