Speaker
Description
Using angle-resolved photoemission spectroscopy, in particular photoemission orbital tomography (POT), we investigate the coverage-dependent electronic and geometric properties of the organic oligomer α-sexithiophene (6T) on Cu(110)-p(2×1)O [1]. The oxygen-induced reconstruction of the copper surface electronically decouples the molecules from the substrate [2]. By combining momentum-resolved experimental data with density functional theory calculations, we trace the transition of the film structure from a substrate-templated monolayer to bulk-like multilayers in a quantitative manner. Specifically, the tilt of the 6T molecules decreases from 37° (2 ML) to 31° (8 ML). The former is consistent with DFT and is governed by the oxygen row spacing, while the latter matches the structure of bulk 6T.
A previous study of this system [3] succeeded in disentangling effects of intra- and intermolecular delocalization and dispersion. To this end, band maps were measured either along or perpendicular to the long molecular axis. In our case, using photoemission momentum microscopy, we gained access to the 2D momentum space, enabling a significantly more comprehensive characterization. In particular, we identify an electronic band that exhibits clear intermolecular-dispersion character in one direction, while in the perpendicular direction electronic states remain confined at the molecular scale, revealing typical intramolecular dispersion. This suggests that, for certain states, electron delocalization within a molecule is comparable to that between molecules, marking the limit of a purely local orbital picture. This also implies that in optically excited states, excitons will likely extend over several adjacent molecules, which has recently been observed in a dedicated time-resolved POT study [4].
References:
[1] Stettner et al., arXiv: 2603.06204 (2026)
[2] Yang et al., Chem. Commun. 54 (2018)
[3] Berkebile et al., Appl. Phys. A 95 (2009)
[4] Theilen et al., arXiv:2511.23001 (2025)