NaCl polymers

Abstract

The equilibrium configurations of NaCl polymers containing from one to seven molecules were determined by numerically minimizing the total energy. A rigid ion model was used to describe the two particle interactions. The binding energy per molecule and the interatomic distances were found to increase monotonically as molecules were added to the polymer, even though the pentamer and heptamer possess permanent dipole moments. Once the equilibrium configuration was established, the second derivatives of the total enery with respect to atomic displacements were evaluated numerically and the dynamical matrix diagonalized to give the vibration frequencies of each polymer. The eigenvectors were also evaluated and used to calculate the strength of the infrared absorption for each mode of vibration. Experimental far infrared absorption spectra of NaCl in various stages of aggregation are also reported. The samples were prepared at liquid helium temperature by the simultaneous condensation of argon and NaCl vapor onto a transparent substrate. Although NaCl vapor contains only monomers and dimers, larger polymers could be formed by using a relatively high concentration of NaCl in the argon matrix. An assignment of absorption lines to specific polymers is made by comparing the experimental and calculated absorption spectra.