Abstract
The interaction between an organic and an inorganic material is important to understand the mechanism of forming interfacial electronic states. Although the impact of the weak interaction on the electronic state has hardly been investigated, non-negligible features have been observed in occupied states of an organic molecule and inorganic substrate. For example, angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) of the pentacene (PEN) monolayer (ML) on graphite shows that the spectral fine features of the highest occupied molecular orbital (HOMO) state are modified from those of an isolated gaseous molecule [1-3]. A simulated photoemission spectrum including interfacial interactions is in good agreement with the experiment [4]. These results indicate that deep insight into the fine features of ARUPS is necessary to fully understand the electronic structure of weakly interacting molecular systems beyond the simple approximation. The unoccupied states are expected to be significantly affected in the spectral features due to a larger spread and overlap of the electron cloud against these tiny changes in the occupied valence states. The unoccupied state is accessible in some specific cases by so-called angle-resolved secondary-photoelectron spectroscopy [5] although photoelectron spectroscopy is known to detect the band structure of the occupied state in general.
In the LEED measurement, it is observed that the PEN (ML) structure is arranged from the liquid-like disorder (above the transition temperature, 130 K) to the incommensurate ML crystal (below transition temperature). Figs. 1 (a–c) show three characteristic ARUPS of PEN/HOPG taken at the crystalline phase. The dispersive convex (hole-like) band appears in the kinetic energy range of 1.4 to 2.4 eV as a constant final state (CFS) overlapping with the discrete non-dispersive HOMO (00) where the vibronic coupling fine features (01) and (02) are resolved. The convex band is absent in the valence band of pure graphite in both the experiment and theory [6] in the observed energy and momentum region. The CFS dispersive band consists of positive and negative intensities depending on the excitation energy, indicating Fano resonance [7] involving a discrete molecular state that couples a continuum state in different excitation paths (Figs. 1 (d, e)). In both ML phases, a characteristic asymmetric Fano profile with respect to the excitation energy is observed. In a photoelectron emission, several examples are has been reported for core excitation with the Auger decay process [8] and valence excitation of a rare gas superstructure on the surface [9], however no results has yet been reported for the complicated molecular orbital-related systems so far. The continuum state at the PEN/graphite interface could originate from a newly formed conduction band at the weakly bound interface, indicating a strong impact of the weak electronic coupling on the wave function connection via a larger spread of the unoccupied states demonstrating measuring a fingerprint of the weak interaction at the vdW interface. We discuss a concept to describe the impact of weak interaction on the electronic states by Fano profile analysis.
References
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