The fluorescence of a forbidden transition from the ground state
(5I, J''=4) to the 18 249.275-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ (J=6) level in Nd i was directly observed by atomic-beam laser spectroscopy. This transition was found to be caused by the second-order hyperfine perturbation from the close-lying 18 249.784-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ (J'=5) level. Thus the fluorescence peaks were observed only for odd isotopes of
143Nd and
145Nd, and no peaks for even isotopes. Both the relative frequencies and the relative intensities of all the observed forbidden transitions agree well with the theoretical calculation.
Forbidden transitions between energy levels typically involve violation of selection rules imposed by symmetry and/or conservation laws. A nanomagnet tunneling between up and down states violates angular momentum conservation because of broken rotational symmetry. Here we report observations of highly forbidden transitions between spin states in a Ni_{4} single-molecule magnet in which a single photon can induce the spin to change by several times ℏ, nearly reversing the direction of the spin. These observations are understood as tunneling-assisted transitions that lift the standard Δm=±1 selection rule for single-photon transitions. These transitions are observed at low applied fields, where tunneling is dominated by the molecule's intrinsic anisotropy and the field acts as a perturbation. Such transitions can be exploited to create macroscopic superposition states that are not typically accessible through single-photon Δm=±1 transitions.