Observing the spatial and temporal evolution of exciton wave functions

TL;DR

This paper demonstrates a novel femtosecond time-resolved photoemission orbital tomography technique to directly image and analyze the quantum wave functions of excitons in organic semiconductors, revealing their spatial and phase dynamics.

Contribution

It introduces a new experimental method and a quantitative model to reconstruct exciton wave functions in real space, including phase information, and observes ultrafast exciton contraction.

Findings

Reconstructed exciton wave function shows delocalization over three molecular units.

Observed a 20% reduction in exciton radius within 400 fs.

Validated experimental results with ab initio many-body perturbation theory calculations.

Abstract

Excitons, the correlated electron-hole pairs governing optical and transport properties in organic semiconductors, have long resisted direct experimental access to their full quantum-mechanical wave functions. Here, we use femtosecond time-resolved photoemission orbital tomography (trPOT), combining high-harmonic probe pulses with time- and momentum-resolved photoelectron spectroscopy, to directly image the momentum-space distribution and ultrafast dynamics of excitons in $\alpha$-sexithiophene thin films. We introduce a quantitative model that enables reconstruction of the exciton wave function in real space, including both its spatial extent and its internal phase structure. The reconstructed wave function reveals coherent delocalization across approximately three molecular units and exhibits a characteristic phase modulation, consistent with ab initio calculations within the framework of many-body perturbation theory. Time-resolved measurements further show a $\sim 20$\% contraction of the exciton radius within 400 fs, providing direct evidence of self-trapping driven by exciton-phonon coupling. These results establish trPOT as a general and experimentally accessible approach for resolving exciton wave functions -- with spatial, phase, and temporal sensitivity -- in a broad class of molecular and low-dimensional materials.