Density functional approach to the dynamic response at metal surfaces: van der Waals reference plane position and excitation of electron-hole pairs

Abstract

The time-dependent density functional approach is used to calculate the response of a metal surface to uniform external fields that vary exponentially in time. The frequency dependence of the dynamic image plane is evaluated for several jellium surfaces. This quantity is of interest since it directly enters the expression for the reference plane position of the atom-metal van der Waals interaction. Results are presented for He and H₂ interacting with simple and noble-metal surfaces for which previously only approximate forms of the dynamic image plane were available. Also, from the slope of the image plane at small imaginary frequencies, the power absorption of a metal due to excitation of electron-hole pairs is calculated. These results differ strongly from previous estimates in which the surface part of the self-consistent potential is approximated by the static limit of the induced Coulomb potential. In part, this discrepancy is caused by long-ranged surface fields present at finite frequencies. Moreover, exchange-correlation contributions to the effective potential are shown to play an important role in determining the correct position of the dynamic image plane and the transition rate for electron-hole pair excitations.