# Size effect in the ionization energy of PAH clusters

## TL;DR

This study measures and explains how the ionization energy of PAH clusters decreases with size, using experimental photoelectron spectroscopy and theoretical simulations, revealing a size-dependent trend and a notable discontinuity for pyrene hexamers.

## Contribution

First experimental measurement of near-threshold photo-ionization spectra of PAH clusters with theoretical comparison, highlighting size effects and temperature influence.

## Key findings

- Ionization energy decreases with cluster size.
- Temperature effects lower ionization energy, improving theory-experiment agreement.
- Discontinuity observed in pyrene hexamer ionization energy trend.

## Abstract

We report the first experimental measurement of the near-threshold photo-ionization spectra of polycyclic aromatic hydrocarbon clusters made of pyrene C16H10 and coronene C24H12, obtained using imaging photoelectron photoion coincidence spectrometry with a VUV synchrotron beamline. The experimental results of the ionization energy are confronted to calculated ones obtained from simulations using dedicated electronic structure treatment for large ionized molecular clusters. Experiment and theory consistently find a decrease of the ionization energy with cluster size. The inclusion of temperature effects in the simulations leads to a lowering of this energy and to a quantitative agreement with the experiment. In the case of pyrene, both theory and experiment show a discontinuity in the IE trend for the hexamer.