  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.
