by perturbation facilitated optical-optical double resonance spectroscopy

have been studied by perturbation facilitated optical–optical double resonance spectroscopy for the first time. Molecular constants of those states arederived and compared with theoretical calculations. Varying the trial vibrational quantum numbersof these upper states, absolute vibrational quantum number assignments are obtained by comparisonbetween the observed resolved fluorescence intensities and the calculated Franck–Condon factors.The Rydberg–Klein–Rees potential curves are constructed and compared with theoreticalcalculations. Finally, the observation of the lowest ‘‘dark’’ vibrational levels of the b