  Optical spectroscopy has been the primary tool to study the electronic
structure of molecules. However the strict spin selection rule has severely
limited its ability to access states of different spin multiplicities. Here we
propose a new strategy to activate spin-forbidden transitions in molecules by
introducing spatially highly inhomogeneous plasmonic field. The giant
enhancement of the magnetic field strength resulted from the curl of the
inhomogeneous vector potential makes the transition between states of different
spin multiplicities naturally feasible. The dramatic effect of the
inhomogeneity of the plasmonic field on the spin and symmetry selection rules
is well illustrated by first principles calculations of C60. Remarkably, the
intensity of singlet-triplet transitions can even be stronger than that of
singlet-singlet transitions when the plasmon spatial distribution is comparable
with the molecular size. This approach offers a powerful means to completely
map out all excited states of molecules and to actively control their
photochemical processes. The same concept can also be applied to study nano and
biological systems.
