Solvation and ionisation of alkali metals in liquid ammonia: a path integral Monte Carlo study

Quantum path integral Monte Carlo calculations have been used to study the properties of the alkali atoms Li, Na and Cs immersed in liquid ammonia. The solvent has been treated using a pairwise additive intermolecular potential fitted to experimental data. The alkali-atom solvent potential consists of two parts; an ion-core-solvent interaction fitted to quantum chemical calculations and a valence-electron-solvent pseudopotential taken from the solid-state literature. Two distinct forms of pseudopotential have been employed, one having an attractive, and the other a repulsive core. In the latter case, the equilibrium structure of Na and Cs is found to be an ionised state consisting of a fully solvated ion core, plus a well-separated, compact solvated valence electron. In the case of Li, the equilibrium structure for both models appears to be a dipolar or excitonic atom. The relative merits of the two models are discussed and, where possible, contact is made with data on metal ammonia solutions.

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