Photodetachment Spectroscopy of Negative Cluster Ions

Abstract

Negative ion photoelectron spectra have been obtained for a variety of gas-phase cluster anions using visible photons. The negative cluster ions were generated by injecting relatively low energy electrons directly into the high density portion of an expanding supersonic jet. The spectra of NO-(N2O)n=1,2, H-(NH3)n=1,2, NH2(NH3)n=1,2, NO-(Ar)1 NO-(Kr)1 NO-(Xe)1, O2 -(Ar)1 (N2O)2, and (CS2)2 - reveal that they are simple ion-molecule complexes in which the excess negative charges are largely localized on sub-ions within the larger cluster anions. In addition to information on the bonding of cluster ions, the spectra also provide electron affinities and ion-solvent dissociation energies as a function of cluster size. The spectra of (CO2)2 -, (SO2)2 -, and (NO)2 -, on the other hand, indicate that these species are more complicated cases, and that they are not well described as simple ion-molecule complexes. Also, in the case of NH4 -, evidence is found not only for the ion-molecule complex, H-(NH3)1, but also for a higher energy isomer of tetrahedral geometry. Other systems studied include negative cluster ions of water and alkali metal cluster anions. Even though H2O- is unstable, clusters of water are able to bind an electron to form (H2O)n -. The spectra of (H2O)n - =2,6,7,10–17,19 and Ar(H2O)n - =2,6,7 provide the vertical detachment energies for these species. The alkali metals are the simplest of metals. The spectra of Na- 2–5, K- 2–8, Rb- 2–4, and Cs- 2,3 yield electron affinities as a function of cluster size as well as the electronic state splittings for neutral alkali metal clusters.