As a part of interstellar dust, polycyclic aromatic hydrocarbons (PAHs) are processed by the interaction with VUV photons that are emitted by young stars. After absorption of a VUV photon, an isolated PAH can undergo different relaxation processes: ionization, dissociation and radiative cooling, including IR fluorescence which results in the aromatic infrared bands (AIBs) observed in many astronomical objects. This PhD work combines experimental techniques and theoretical calculations to get better insights into the contribution of the different relaxation channels as a function of energy and molecular properties which govern the stability of PAHs in astrophysical environments.

Similar to an earlier study [1], we investigate the two relaxation processes of ionization and dissociation of large PAH cations ranging from 30 to 48 carbon atoms. The ions are trapped in the LTQ linear ion trap of the DESIRS beamline at the synchrotron SOLEIL and energized by VUV photons in the range of 8 – 20 eV. All resulting photoproducts are mass-analyzed and recorded as a function of photon energy. Ionization is found to be the dominating relaxation channel, with the photoionization yield increasing with number of carbon atoms. Action spectra are obtained from the photoproducts’ relative intensities and compared to the photoabsorption cross sections. The latter have been computed using the real time, real space implementation of TD-DFT from the OCTOPUS code [2].

Models of the chemical evolution of PAHs in HI photodissociation regions have shown that the dissociation of large PAHs may require multiple photon absorption [3]. To further investigate this process, we use the PIRENEA setup which combines cryogenic trapping and laser interaction. We show how this sequence of experiments can give insights into the photoprocessing of PAHs in different astrophysical environments.

References
[1] Zhen, J., Rodriguez Castillo, S., Joblin, C., et al. 2016, ApJ, 822, 2
[2] Malloci, G., Mulas, G., & Joblin, C. 2004, A&A, 426, 105
[3] Montillaud, J., Joblin, C., & Toublanc, D. 2013, A&A, 552, A15