Knowledge Bank

Ammonia solvated alkali ions were produced in a continuous nozzle expansion of $NH3$/Ar equipped with a thermionic ion emitter. Clusters produced in this way are very arm compared to those traditionally formed in molecular beams and serve as models for ion solvation in solution. We excited the $V2$ umbrella mode of ammonia, that is very sensitive to intermolecular interactions, to predissociate the cluster. $Cs+(NH3)N$ cluster ions with N up to 45 have been produced and spectra have been recorded for $N=6$ to 13 in the $CO2$ laser region. Large frequency shifts in absorption peaks have been observed upon increasing from $N=6$ through $N=9$. For $N=10$ through $N=13$, these shifts were much smaller. A first solvation shell size of ten ammonias for the oesium ion was consistent with the experimental observations. $Na+(NH3)M$ cluster ions with M upto 50 have been produced and spectra for $M=6$ to 12 have been recorded. The spectral shifts are somewhat less sensitive to cluster size than in $Cs3+(NH3)M$. No predissociation was observed for from $1020-1090cm-1$. A change in peak intensity was observed upon increasing from $M=6$ to $M=9$. Substantial frequency shifts in the absorption peaks were observed when M was increased from $M=9$. A sharp feature at $1085cm-1$ remained constant in frequency for the entire range of cluster size studied. The apparent onset of depletion at $M=7$ implied a first solvent shell size of six ammonias for the sodium ion. Ammonia molecules in a bulk-like environment were observed for $N>10$. The dependence of solvent molecule vibrations on cluster ion size and structure will be discussed.