Photoionization of Na(${\mathrm{NH}}_3$${)}_{\mathit{n}}$ and Na(${\mathrm{H}}_2$O${)}_{\mathit{n}}$ clusters: A step towards the liquid phase?

Ionization potentials (IP’s) for Na(${\mathrm{H}}_2$O${)}_{\mathit{n}}$ and Na(${\mathrm{NH}}_3$ ${)}_{\mathit{n}}$ are reported for the first time up to n=20. As expected, for Na(${\mathrm{NH}}_3$ ${)}_{\mathit{n}}$ the IP decreases more or less monotonically with decreasing (n+1${)}^{\mathrm{-}1/3}$ to a limit consistent with the bulk value of 1.45 eV. In contrast, for Na(${\mathrm{H}}_2$O${)}_{\mathit{n}}$ we find the IP to be constant at 3.17 eV for n≥4, close to the bulk value. For comparison we solve a one-electron Schrödinger equation in a dielectrically screened Coulomb potential. While agreement with the experiment is reasonable for small n, this one-center model fails for larger clusters, thus giving some first indication for the existence of two-center localized states in these microclusters.