A research team led by a Dutch group showed, by using the DESIRS beamline of SOLEIL, that the size of polycyclic aromatic hydrocarbons influences the photon-induced processes they undergo in the interstellar medium (ISM): ionization versus fragmentation. These results, published in the Astrophysical Journal Letters, bring new information on the PAHs stability within the ISM, where they are abundant, and on the key mechanisms related to far-ultraviolet radiation that governs their survival.
What if large polycyclic aromatic hydrocarbons (PAHs) were even more stable than previously thought? That is what tends to indicate the results obtained by scientists from the Leiden University (Netherlands), CNRS, Université de Toulouse and SOLEIL synchrotron facility. They studied the dominant phenomena of PAHs transformation within the ISM and evidenced that for small hydrocarbons, fragmentation (hydrogen atoms loss) is more important than ionization. When the size of the species increases, ionization becomes more and more important, and eventually starts to be predominant. Therefore, for large PAHs, it appears that fragmentation only becomes important when the photon energy has reached the highest ionization potential accessible.
In this study, three PAHs mono-cations produced through vaporization and ionization by electron impact, and covering an astrophysical relevant range of sizes, have been stocked into an ion-trap: coronene (C24H12), ovalene (C32H14) and hexa-peri-hexabenzocoronene (HBC, C42H18). Scientists followed, by mass spectrometry, the evolution of the cations irradiated by a photon flux in the 1014 1015 photons s-1 energy range in a 7% bandwidth. The experiment has been carried out on the DESIRS beamline, working in the vacuum ultraviolet (VUV) range (5-40 eV). Spectra recorded at different energies indicate that for a given photon energy, the ionization yield increases with the PAH size, while fragmentation is more important for small PAHs. Therefore, after a 20eV irradiation, coronene and ovalene cations show clear evidence for the loss of two and four hydrogen atoms. At the exact same energy, only 3% of the HBC is fragmented, versus 30% (+/-10%) for ovalene and 75% (+3%) for coronene. With increasing photon energy, both fragmentation and ionization yields increase but the same pattern remains.
Normalized intensity of PAH cation before and after irradiation. Top row: irridiation for 200 ms at 20.0 eV (62 nm). Bottom row: irradiation for 500 ms at 13.2 eV (93.3 nm). From left to right in each row, HBC, ovolene and coronene cations.
Two explanations have been proposed for these PAHs size-dependent dominant effects. First, a larger number of electrons lead to a higher density of electronic states. Therefore, within large PAHs, the excited state relaxation is slower since the system explores a larger phase space. This longer electronic lifetime then leads to a higher ionization yield by coupling with the electronic continuum. Secondly, larger species, because of their higher number of vibrational modes into which the energy can be distributed, need a higher internal energy than small PAHs to reach a same excitation level that could lead to fragmentation. Hence, fragmentation is not fostered for larger hydrocarbons, and only happens when the photon energy has reached the highest ionization potential accessible. In consequence, large PAHs are extremely chemically stable in the interstellar medium.
Polycyclic aromatic hydrocarbons are abundant in the ISM, containing about 10% of the interstellar carbon. They could be responsible for the diffuse interstellar bands, which are irregular absorption features seen in the spectra of astronomical objects and which play a paramount role in the ISM radiation balance. Therefore, studying them is crucial to better estimate the space stability. The results obtained here evidence that it is necessary to consider both fragmentation kinetic parameters and ionization properties of PAHs. Further quantum chemistry studies are now mandatory to complete the present results.