Abstract
We report the discovery of a charge transfer (CT) related low binding energy feature at a molecule–metal interface by the application of resonant photoelectron spectroscopy (RPES). This interface feature is neither present for molecular bulk samples nor for the clean substrate. A detailed analysis of the spectroscopic signature of the low binding energy feature shows characteristics of electronic interaction not found in other electron spectroscopic techniques. Within a cluster model description this feature is assigned to a particular eigenstate of the photoionized system that is invisible in direct photoelectron spectroscopy but revealed in RPES through a relative resonant enhancement. Interpretations based on considering only the predominant character of the eigenstates explain the low binding energy feature by an occupied lowest unoccupied molecular orbital, which is either realized through CT in the ground or in the intermediate state. This reveals that molecule–metal CT is responsible for this feature. Consequently, our study demonstrates the sensitivity of RPES to electronic interactions and constitutes a new way to investigate CT at molecule–metal interfaces.
Highlights
One of the crucial questions for the performance of organic electronics is dynamical charge transfer (CT) across the metal–organic interface
First we present the observation of a low EB feature in a photoelectron spectroscopy (PES) map recorded in the energetic region of the largest resonant enhancement of the highest occupied molecular orbital (HOMO)
The quantities extracted from the valence PES and resonant photoelectron spectroscopy (RPES) experiments which can be calculated within the cluster model are the energetic separation and the intensity ratio of the main line and its satellite
Summary
We report the discovery of a charge transfer (CT) related low binding energy feature at a molecule–. Metal interface by the application of resonant photoelectron spectroscopy (RPES). Within a cluster model description and DOI This feature is assigned to a particular eigenstate of the photoionized system that is invisible in direct photoelectron spectroscopy but revealed in RPES through a relative resonant enhancement. Interpretations based on considering only the predominant character of the eigenstates explain the low binding energy feature by an occupied lowest unoccupied molecular orbital, which is either realized through CT in the ground or in the intermediate state. This reveals that molecule–metal CT is responsible for this feature. Our study demonstrates the sensitivity of RPES to electronic interactions and constitutes a new way to investigate CT at molecule–metal interfaces
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