Laser-induced atomic fragment fluorescence spectroscopy: A facile technique for molecular spectroscopy of spin-forbidden states

Spectra of spin-forbidden and spin-allowed transitions in the mixed b$^3\Pi_u$ ~ A$^1\Sigma_u^+$ state of Na$_2$ 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$^1\Sigma_u^+$ state is excited, completely free of triplet excitation. At high laser power, photodissociation via the b$^3\Pi_u$ triplet state intermediate 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.