Observation of a spin-protected high-energy isomer of Al4N− cluster

Photoelectron spectroscopy experiments combined with first-principles calculations based on generalized gradient approximation to the density functional theory show that an Al4N− anion cluster lying 0.97 eV above its ground-state structure can exist not because it is protected by an energy barrier but because it has a different spin multiplicity. Its existence is established by comparing the calculated vertical electron detachment energies with the experimental photo-detachment spectra. The electron affinity of Al3N, on the other hand, is anomalously low showing the characteristic of a closed-shell molecule and providing the signature of a monovalent Al.