ECAMP 15

In the interstellar medium (ISM) and circumstellar environments, photoionization or the photoelectric effect emerges as a prevalent phenomenon. In regions exposed to UV photons, either stellar or secondary photons induced by cosmic rays, polycyclic aromatic hydrocarbons (PAHs) liberate electrons through the photoelectric effect, efficiently contributing to the heating budget of the gas. In contrast to shielded areas, neutral and cationic PAHs can scavenge electrons and retain a significant portion of the cloud’s electronic charge. This dual behavior of PAHs not only influences the gas thermal and dynamical behavior but also plays a pivotal role in shaping the chemistry of the environments. We have measured the photoelectron kinetic energy distribution of PAHs of varying sizes, symmetries, and C:H ratios to describe the photoelectron kinetic energy distribution with a law to be implemented in astrophysical photoelectric models that describe gas heating. We used a double-imaging photoelectron photoion coincidence spectrometer coupled with the DESIRS VUV beamline at the SOLEIL synchrotron to record the gas phase spectra of a series of sublimated PAHs with different sizes and structures in the 13 to 20 C atom range. Our data were used in astrophysical dust photoelectric models to describe the PAH charge and gas photoelectric heating in the ISM. We show that although subtle differences between the molecules in our data set arise from individual electronic structures, the photoelectron KED of PAHs of different sizes and symmetry display remarkable similarities. A general law can thus be implemented in sophisticated ISM astrochemical models to describe their photoelectron KED behavior. Our results predict a maximum photoelectric efficiency that is significantly lower than the previous models, implying a lower interstellar gas temperature and emission.