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 ( $1^4{A}_2^{\prime }$) of the sodium trimer. Upon excitation to the $2^4{E}^{\prime }$ 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 $1^4{A}_2^{\prime }$ state of ${\mathrm{Na}}_3$ is due to nonadditive effects.