Spectra of spin-forbidden and spin-allowed transitions in the mixed b Π3 u ∼A 1 u+ state of Na2 are measured separately by two-photon excitation using a single tunable dye laser. The two-photon excitation produces Na (3p) by photodissociation, which is easily and sensitively detected by atomic fluorescence. At low laser power, only the A 1 u+ state is excited, completely free of triplet excitation. At high laser power, photodissociation via the intermediate b Π3 u triplet state becomes much more likely, effectively "switching" the observations from singlet spectroscopy to triplet spectroscopy with only minor apparatus changes. This technique of perturbation-assisted laser-induced atomic fragment fluorescence may therefore be especially useful as a general vehicle for investigating perturbation-related physics pertinent to the spin-forbidden states, as well as for studying allowed and forbidden states of other molecules.
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