Vibronic Lineshapes of PTCDA Oligomers in Helium Nanodroplets

TL;DR

This study investigates the vibronic lineshapes of PTCDA oligomers in helium nanodroplets, revealing broad spectral features explained by a Frenkel exciton model coupled to vibrational modes, advancing understanding of organic semiconductor spectroscopy.

Contribution

The paper introduces a simple theoretical model that reproduces the broad vibronic spectra of PTCDA oligomers using only one vibrational mode, providing insights into exciton-vibrational interactions.

Findings

Oligomer spectra have broader peaks than monomers.

A single vibrational mode suffices to reproduce main spectral features.

Including multiple vibrational modes results in quasi-continuous spectra.

Abstract

Oligomers of the organic semiconductor PTCDA are studied by means of helium nanodroplet isolation (HENDI) spectroscopy. In contrast to the monomer absorption spectrum, which exhibits clearly separated, very sharp absorption lines, it is found that the oligomer spectrum consists of three main peaks having an apparent width orders of magnitude larger than the width of the monomer lines. Using a simple theoretical model for the oligomer, in which a Frenkel exciton couples to internal vibrational modes of the monomers, these experimental findings are nicely reproduced. The three peaks present in the oligomer spectrum can already be obtained taking only one effective vibrational mode of the PTCDA molecule into account. The inclusion of more vibrational modes leads to quasi continuous spectra, resembling the broad oligomer spectra.