Femtosecond studies of electron dynamics and structure at metal-molecular interfaces

Femtosecond angle resolved two photon photoemission spectroscopy is used to study the electronic structure and electron dynamics at interfaces. At interfaces of thiolates chemisorbed on Ag(111), the adsorbate molecular electronic orbitals are observed to be nondispersive at low coverages and become dispersive at higher coverages. This is attributed to a phase transition of the layer. The molecules initially adsorb with their chains parallel to the surface. As the coverage is increased, the molecules order into a layer with the chains standing up from the surface. This closer packing results in a larger overlap between neighboring molecular orbitals and a dispersive electronic state. The lack of a change in the n = 1 image potential state electron lifetimes as a function of chain length indicate that the electrons reside in the layer. The n = 2 and 3 image potential state electron lifetimes decrease as the chain length is increased. This is attributed to the repulsive potential of the alkyl chains pushing the electron density into the sulfur portion of the layer. At a layer of acetonitrile molecules adsorbed on Ag(111), the image potential state electrons interact strongly with the adsorbate molecular dipoles. The dipoles rotate to solvate the electron, resulting in a decrease of the observed photoemitted electron kinetic energy as a function of time delay between population and photoemission. This is attributed to a change in the local workfunction resulting from the reorganization of the adsorbate layer molecules. For two layers of acetonitrile adsorbed on the Ag(111) substrate, dynamic electron localization is also observed.