Multiphoton Photoemission Electron Microscopy of Porphyrin Films on Nanostructured Ag: Molecular Resonances and Plasmonic Field Enhancement

Dye molecules like porphyrins alter their optical properties upon condensation to a solid film. Besides intermolecular interactions, specific substrates can influence their optical characteristics, especially if the molecular film is in contact to plasmonic nanostructures. We apply multiphoton photoemission electron microscopy (nP-PEEM) with tunable laser excitation to the laterally resolved spectroscopy of magnesium-tetraphenylporphyrin (MgTPP) films deposited on nanostructured silver substrates. The high molecular specificity of nP-PEEM is demonstrated by the observation of a strong resonance at about 425 nm caused by optical excitation of the S0 → S2 transition (Soret band) of the MgTPP molecules. This molecular excitation gives rise to remarkably strong three-photon photoemission. The spectral position of the Soret resonance in our nP-PEEM laser spectra points to reduced excitonic coupling between the surface molecules of the MgTPP film. By comparison of the photoemission intensities from MgTPP on nanostructured and unstructured silver regions, we conclude noticeable plasmon-mediated enhancement of photoemission at resonant Soret excitation even under off-resonance conditions for the localized surface plasmons (LSPs). Combining molecular specificity with high sensitivity to field enhancements, we demonstrate that multiphoton PEEM is an excellent tool for the investigation of solid dye films and their interaction with plasmonic substrates.