Molecular Dynamics Simulations of a Silver Atom in Water: Evidence for a Dipolar Excitonic State

The properties of a silver atom in bulk water were studied for the first time by molecular dynamics simulations using two complementary mixed quantum-classical approaches. The first one consists of treating by quantum mechanics one electron only, which interacts with a classical silver cation and solvent through one-electron pseudopotentials. The second one is Car-Parrinello molecular dynamics that treats all the valence electrons quantum-mechanically. Very good agreement is obtained between these two methods, and the calculated absorption spectrum of the solvated silver atom agrees very well with experimental data. Both simulations reveal that the silver atom is in the critical region for the appearance of a dipolar excitonic state and exhibits a dipole moment of $\sim 2\mathrm{D}$ with large fluctuations of $\pm 1\mathrm{D}$. The structure of the solvation shell is also analyzed.

Figure 1

Absorption spectrum: calculated (solid line); experimental (dashed line). The three components are separately shown (dotted, dash-dotted, and dash-dotted–dotted lines) corresponding to $s$-$p$ transitions.

Figure 2

Instantaneous configuration from QCMD calculation. The ${\mathrm{A}\mathrm{g}}^{+}$ cation (the central sphere) is localized inside the electronic cloud representing the excess electron.

Figure 3

${\mathrm{A}\mathrm{g}}^{+}/\mathrm{\text{water}}$ (top panel) and electron/water (bottom panel) radial distribution function calculated during QCMD (upper curves) and CPMD (lower curves) simulations. In each case the solid line refers to oxygen atoms and the dashed line to hydrogen atoms.

Figure 4

Infrared spectrum calculated by Fourier transformation of the electron-${\mathrm{A}\mathrm{g}}^{+}$ distance in CPMD.