نبذة مختصرة : As a part of interstellar dust, polycyclic aromatic hydrocarbons (PAHs) in so-called photodissociation regions (PDRs) are processed by the interaction with ultraviolet (UV) photons that are emitted by hot stars. After absorption of a UV photon, an isolated PAH can undergo different relaxation processes: ionization, dissociation, and radiative cooling, including infrared (IR) fluorescence which results in the aromatic infrared bands (AIBs) observed in many astronomical objects. This interaction influences their charge state and photodissociation dynamics, ultimately determining their photostability in space. In return, it impacts the energy balance of the interstellar gas by photoelectric heating. PAHs are also proposed to act as catalysts for the most abundant molecule in space, H2, and play thus an ubiquitous role in the physics and chemistry of PDRs. This PhD thesis concentrates on the experimental investigation of cationic PAHs, addressing independently and for different molecular families two major aspects which concern the evolution of PAHs under UV irradiation and the possibility to form several isomers. The experimental measurements are supported by complementary theoretical calculations. Two main experimental campaigns have been carried out throughout this PhD work. At the SOLEIL synchrotron, large PAH cations (30 − 48 carbon atoms) were submitted to tunable vacuum UV (VUV) radiation in the range of 9.5 to 20.0 eV and their photofragments and dications were mass-analyzed as a function of photon energy. We could derive branching ratios, action spectra, and photoionization and photodissociation cross sections, demonstrating that ionization is the dominant channel in the photoprocessing of these large PAHs by VUV irradiation. Using theoretical photoabsorption cross sections from time-dependent density functional theory (TD-DFT), we were also able to determine photoionization yields and to give recipes which can directly be implemented in astrochemical modeling studies. At the FELIX facility, IR ...
Processing Request
No Comments.