Polycyclic aromatic hydrocarbon-isomer fragmentation pathways: case study for pyrene and fluoranthene molecules and...

Area of Science:

• Atomic and Molecular Physics
• Physical Chemistry
• Chemical Physics

Background:

• Polycyclic Aromatic Hydrocarbons (PAHs) are crucial in astrochemistry and combustion.
• Understanding PAH ionization and fragmentation is key to interpreting astronomical observations and combustion processes.
• Isomeric differences in PAHs can influence their reactivity and spectral properties.

Purpose of the Study:

• To investigate the ionization and fragmentation of pyrene and fluoranthene isomers under Xe(20+) ion impact.
• To compare the behavior of single PAH molecules versus PAH clusters.
• To identify and understand the influence of isomeric differences on fragmentation patterns.

Main Methods:

• Experimental measurements of ion-molecule collisions using Xe(20+) projectiles.
• Analysis of fragment spectra resulting from PAH ionization and dissociation.
• Comparison of results for pyrene and fluoranthene monomers and clusters.
• Theoretical calculations using Density Functional Theory (DFT).
• Application of a classical over-the-barrier model for ionization cross-section estimation.

Main Results:

• Fragment spectra for both pyrene and fluoranthene were surprisingly similar for both monomer and cluster targets.
• Weak but significant isomer effects were observed, linked to small differences in binding energies (<2.5 eV) of the monomers.
• DFT calculations provided ionization and dissociation energies, supporting the observed effects.
• A classical over-the-barrier model was used to estimate ionization cross sections.

Conclusions:

• Isomeric differences in PAHs, though small, can lead to observable effects in ion-molecule collisions.
• Binding energy differences are the root cause of these isomer effects.
• The study provides insights into PAH-ion interactions relevant to astrophysical and combustion environments.