Spin Polarized Alkali Clusters: Observation of Quartet States of the Sodium Trimer

Using helium cluster-isolation spectroscopy we have prepared and characterized the lowest quartet state ( 1⁴A₂^') of the sodium trimer. Upon excitation to the 2⁴E^' electronic state, laser induced fluorescence excitation and emission spectra have been obtained. With the aid of ab initio calculations, the spectra have been vibronically assigned and new insights on the nonadditivity of van der Waals interactions have been obtained. It is found that approximately 80% of the interaction energy in the 1⁴A₂^' state of Na₃ is due to nonadditive effects.