Relaxation Dynamics in Image Potential States at Solid Interfaces

Abstract

The interface between metals and molecules remains an area of highly active research. The abrupt change in electronic environment between the electron gas of a metal and the covalent bonds of a molecule give rise to new surface states such as charge transfer excitons, metal-induced gap states (MIGS), and deep trap sites [1,2]. The behavior of electrons at these interfaces dominates both charge injection and charge collection in the growing number of molecular electronic devices. We have studied the dynamics and energetics of these interfacial states, specifically focusing on two particular states, the image potential state (IPS) and molecular based lowest unoccupied molecular orbitals (LUMOs).The image potential is the attractive potential that a charge feels near the surface of a conductor. An electron at the surface of metal induces a polarization in the metal, which exposes some of the underlying positive ionic cores. The actual shape of this polarization is quite complex inside the metal. Outside the metal, however, the potential is exactly that of a fictitious positive point charge placed inside the metal at a distance from the surface equal to that of the electron. This results in the classical 1/z distance-dependent form of the image potential. Quantum mechanically, this image potential can be solved analogously to a one-dimensional hydrogen atom, leading to a Rydberg-like series of bound states that converge to the vacuum level of the system.Electrons from inside the metal can be either directly or indirectly excited into these states upon exposure to ultraviolet (UV) light. For bare metal surfaces, the average distance from the surface of an electron in the first IPS (n ¼ 1, where n is the Rydberg quantum number) is on the order of 3 A . Energetically, the states are bound by 0.9-0.5 eV below the vacuum level, and decay via direct tunneling in under 50 fs. In the presence of a nonmetallic adsorbate, the IPS is partially screened by electrons in the adsorbate. The IPS can also hybridize with other molecular states at the interface, altering the energy of the state and changing the overlap of the IPS with the metal surface. In systems with strong electron-nuclear coupling such as organic molecules, the presence of an excess external charge can also lead to an induced nuclear motion that can further change the wave function and energy of the IPS. This makes