Electronic structure and ultrafast dynamics in molecular films on metal surfaces.

摘要

In an effort to elucidate the role of molecule-metal interaction in determining the electronic structure and excited state dynamics at interfaces relevant to molecule-based electronic devices, two-photon photoemission (2PPE) is applied to three systems of molecular films on metal surfaces: C60 on Cu(111) and Au(111) and C6F6 on Cu(111). The technique of femtosecond time-resolved 2PPE permits the probing of occupied and transiently occupied electronic states in energy, time and momentum spaces, information not accessible by traditional photoemission techniques. The energetic alignment of occupied and unoccupied molecular orbitals with respect to the metal Fermi level and the electronic coupling strength between the molecule and metal, two critical pieces of information for current molecular electron transport theories, are directly probed. Comprehensive studies of C60/Cu(111) and C 60/Au(111) are undertaken, revealing image states, molecular excitons, interfacial charge transfer states and many-body correlation effects. Measurement of the distance dependence of molecular exciton lifetimes permits quantification of the molecule-metal spectral density. Results from very thin films reveal purely interfacial effects, including intrinsic image state localization on one monolayer (ML) C60/Cu(111). Upon deposition of an additional layer, nearly-free-electron-like behavior is restored, which is observed as a modulation of the image state band structure in the periodicity of the superstructure reciprocal lattice. Computational modeling of the C60/Cu(111) image state system confirms the central significance of the surface dipole lattice due to charge transfer from the substrate in the first layer. Image state localization on 1ML C60/Au(111), however, is not observed and is attributed to the absence of substrate charge transfer. A two-dimensional nearly-free-electron (2D-NFE) model is developed to compare image state localization in different systems and extract the Fourier components of the surface pseudopotential probed by the image state electron.