Laboratory Photo-chemistry of PAHs: Ionization versus Fragmentation

TL;DR

This study experimentally investigates how interstellar PAHs undergo ionization or fragmentation upon UV radiation, revealing that larger PAHs tend to ionize rather than fragment, indicating greater photo-stability.

Contribution

It provides new experimental data on size-dependent ionization and fragmentation behaviors of PAH cations under UV radiation.

Findings

Fragmentation dominates for small PAHs like coronene.

Ionization becomes dominant as PAH size increases.

Large PAHs are more photo-stable than previously thought.

Abstract

Interstellar polycyclic aromatic hydrocarbons (PAHs) are expected to be strongly processed by vacuum ultraviolet photons. Here, we report experimental studies on the ionization and fragmentation of coronene (C24H12), ovalene (C32H14) and hexa-peri-hexabenzocoronene (HBC; C42H18) cations by exposure to synchrotron radiation in the range of 8--40 eV. The results show that for small PAH cations such as coronene, fragmentation (H-loss) is more important than ionization. However, as the size increases, ionization becomes more and more important and for the HBC cation, ionization dominates. These results are discussed and it is concluded that, for large PAHs, fragmentation only becomes important when the photon energy has reached the highest ionization potential accessible. This implies that PAHs are even more photo-stable than previously thought. The implications of this experimental study for the photo-chemical evolution of PAHs in the interstellar medium are briefly discussed.