Size Effect in the Ionization Energy of PAH Clusters

Area of Science:

• Physical Chemistry
• Astrochemistry
• Quantum Mechanics

Background:

• Polycyclic Aromatic Hydrocarbons (PAHs) are abundant in space.
• Understanding PAH ionization is crucial for astrochemistry.
• Previous studies lacked experimental ionization data for PAH clusters.

Purpose of the Study:

• To experimentally measure near-threshold photoionization spectra of pyrene and coronene clusters.
• To compare experimental ionization energies with theoretical calculations.
• To investigate the influence of cluster size and temperature on ionization energy.

Main Methods:

• Utilized imaging photoelectron-photoion coincidence spectrometry.
• Employed a VUV synchrotron beamline for measurements.
• Performed electronic structure calculations for ionized molecular clusters.

Main Results:

• Observed a consistent decrease in ionization energy with increasing cluster size for both pyrene and coronene.
• Achieved quantitative agreement between experimental and theoretical ionization energies when including temperature effects.
• Identified a discontinuity in the ionization energy trend for the pyrene hexamer.

Conclusions:

• The study validates theoretical models for describing the electronic structure of PAH clusters.
• PAH clusters are likely ionized in astronomical environments.
• Experimental data provides benchmarks for theoretical advancements in PAH cluster research.