Clusters tagged by alkali metals

Water clusters and mixed formic acid-water clusters produced in molecular beam have been tagged by one sodium atom and investigated by mass spectrometry and tunable UV laser photoionization. Two series of clusters Na-(HO) and Na-(HCOOH)(HO) have been detected at various photon energies, from their appearance threshold in the mass spectrum up to 0.5 eV above. The appearance energies of the mixed clusters, as a function of cluster size, show a flattening shifted toward higher energies with respect to the pure water cluster. This spectroscopic signature is an indication of a significant contribution from the acid molecule to the water cluster and may be useful to predict more carefully the structure of ionized and neutral clusters.