Observation of optical coherence in a disordered metal-molecule interface by coherent optical two-dimensional photoelectron spectroscopy

Coherent multidimensional optical spectroscopy methods overcome some of the limitations found in their one-dimensional counterparts and allow us, for example, to resolve overlapped spectral features, to separate homogeneous and inhomogeneous broadening, and to track the energy transfer kinetics and coherent dynamics in complex quantum systems. In their most common configurations, signal detection is achieved by optical means, making these techniques widespread in studies of bulk systems but less common in the surface and interface sciences. In this paper, we demonstrate an inherently surface-sensitive two-dimensional coherent spectroscopy scheme, based on photoelectron detection, by studying the interface formed between tris(8-hydroxyquinolinato)aluminium (Alq(3)) and a ferromagnetic Co surface. Despite the inhomogeneous linewidth broadening (approximate to 800 meV) in the ensemble of disordered molecules, we resolve two narrow resonances (approximate to 10 meV linewidth) with an energy spacing of approximate to 80 meV. By combining experimental data and simulations, using the Lindblad master equation, we identify these resonances as lowest unoccupied molecular orbital (LUMO) to LUMO + 1 transitions in chemically decoupled second-layer Alq(3) molecules and deduce related optical coherence lifetimes of at least 120 and 240 fs. These observations establish that Alq(3) molecules in a disordered adsorbate layer exhibit well-defined and rather homogeneous internal electronic transitions, although the absolute energetic positions of the involved states with respect to the substrate reference are significantly affected by the disorder. The results indicate that inhomogeneous line broadening in a disordered Alq(3) layer and pure dephasing have only a minor impact on optical transitions in individual molecules. This opens interesting opportunities for coherent control schemes and emphasizes the importance of optical coherences for all electron dynamics, even in disordered hybrid metal-molecule interfaces.