Electric deflection of middle-size ammonia clusters containing (E(-), Na(+)) pairs

Effective polarizabilities of Na(NH3)n (n = 8-27) clusters were measured by electric deflection as a function of the particle size. A significant field-induced shift of the beam intensity profile without the occurrence of broadening revealed that the clusters behave as liquidlike polar objects in the conditions of the experiment (cluster temperatures were estimated in the range of 110-145 K). Most of the cluster polarity is attributed to the spontaneous promotion of the alkali atom valence electron to a diffuse state stabilized by the cluster solvent field, with the consequent formation of (e(-), Na(+)) pairs. The average modulus of the dipole of Na-NH3 clusters, μ0, was determined using the Langevin-Debye theory, and the data was compared with previous measurements obtained for Na-H2O clusters. Sodium-doped ammonia clusters exhibit much larger μ0 values and a step size dependence which is not present when the solvent is water. This evidence suggests that while the (e(-), Na(+)) structure is rather compact in Na(H2O)n clusters and remains almost unchanged during the solvation process, in Na(NH3)n the unpaired electron abandons the proximity of the Na(+) ion and gradually extends and occupies new solvent shells.