LABORATORY PHOTO-CHEMISTRY OF PAHs: IONIZATION VERSUS FRAGMENTATION

Abstract

Interstellar Polycyclic Aromatic Hydrocarbons (PAH) are expected to be strongly processed by Vacuum Ultra-Violet (VUV) photons. Here, we report experimental studies on the ionization and fragmentation of coronene (CH), ovalene (CH) and hexa-peri-hexabenzocoronene (HBC; CH) 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 (ISM) are briefly discussed.

Dissociative Photoionization of Polycyclic Aromatic Hydrocarbon Molecules Carrying an Ethynyl Group

“…The complete chemistry of PAH species in the ISM, however, comprises other mechanisms, e.g., ion-neutral reactions, electron attachment, double ionization, and multiphoton processes, which have been evaluated with various models (Le Page et al 2001Page et al , 2003Montillaud et al 2013). In support to double ionization, a recent experimental study by Zhen et al (2015) concluded that, in the ISM, photoionization prevailed over fragmentation in PAH species containing about 50 C atoms and more, leading to the formation of doubly and even triply charged cations. As a consequence, these large species were found to be photostable in interstellar conditions, in agreement with previous theoretical studies (Allain et al 1996a;Montillaud et al 2013).…”

Restructuring and destruction of hydrocarbon dust in the interstellar medium

“…Consequently, there should be no small, hydrogenated, carbon dust grains in the "ordinary" ISM, since they are very rapidly transformed into aromatic grains. The restructuring of larger dust grains takes longer: when [72], according to which ionization dominates for photon energies from 8 to 40 eV only for large dust grains, while a more important channel for small dust grains is their dissociation (loss of a hydrogen atom).…”

Infrared Spectra of Hexa-peri-hexabenzocoronene Cations: HBC+ and HBC2+

“…This fragmentation follows the available pathways (i.e., dehydrogenation or loss of C 2 or C 2 H 2 units), producing PAH derivative species along a top-down chemical scenario (Zhen et al 2014a;Bouwman et al 2016;Petrignani et al 2016). Photoionization and photofragmentation compete, and their relative importance depends on both the geometry of PAH molecules and the excitation wavelengths used (Zhen et al 2015(Zhen et al , 2016). …”