VUV photo-processing of PAH cations: quantitative study on the ionization versus fragmentation processes

Area of Science:

• Astrochemistry
• Interstellar Medium Physics
• Photochemistry

Background:

• Polycyclic aromatic hydrocarbons (PAHs) are key molecules in the interstellar medium (ISM).
• Their interaction with vacuum ultraviolet (VUV) photons influences their stability, charge state, and role in ISM chemistry and energy balance.
• The branching ratio of ionization versus fragmentation for PAH cations under VUV irradiation is poorly understood.

Purpose of the Study:

• To investigate the photo-ionization and photo-fragmentation branching ratios of PAH cations exposed to VUV photons.
• To determine how molecular size and structure affect these processes.
• To provide crucial data for modeling PAH behavior in diverse ISM environments.

Main Methods:

• Action spectroscopy was employed using synchrotron radiation (SOLEIL beamline, DESIRS) in the 7–20 eV energy range.
• Relative intensities of photo-fragmentation and photo-ionization were measured for eight PAH cations (14–24 carbon atoms).
• Second and third ionization potentials (IPs) were experimentally determined.

Main Results:

• Fragmentation dominates for smaller PAH cations below ~13.6 eV, while ionization becomes competitive for larger species.
• Ionization yield increases with photon energy, reaching 0.8–0.9 at ~18 eV for all species.
• Molecular structure primarily influences fragmentation cross-sections, not ionization cross-sections.
• Measured IPs confirm that PAH cations can undergo further ionization in the diffuse ISM.

Conclusions:

• PAH cations exhibit distinct photo-response depending on photon energy, size, and structure.
• The ionization and fragmentation data are essential for accurate modeling of PAH populations in the ISM.
• Further modeling is needed to determine the abundance of PAH dications in interstellar environments.