Context : As a part of interstellar dust , polycyclic aromatic hydrocarbons ( PAHs ) are processed by the interaction with vacuum ultraviolet ( VUV ) photons that are emitted by hot young stars .
This interaction leads to the emission of the well-known aromatic infrared bands but also of electrons , which can significantly contribute to the heating of the interstellar gas .

Aims : Our aim is to investigate the impact of molecular size on the photoionization properties of cationic PAHs .

Methods : Trapped PAH cations of sizes between 30 and 48 carbon atoms were submitted to VUV photons in the range of 9 to 20 eV from the DESIRS beamline at the synchrotron SOLEIL .
All resulting photoproducts including dications and fragment cations were mass-analyzed and recorded as a function of photon energy .

Results : Photoionization is found to be predominant over dissociation at all energies , which differs from an earlier study on smaller PAHs .
The photoionization branching ratio reaches 0.98 at 20 eV for the largest studied PAH .
The photoionization threshold is observed to be between 9.1 and 10.2 eV , in agreement with the evolution of the ionization potential with size .
Ionization cross sections were indirectly obtained and photoionization yields extracted from their ratio with theoretical photoabsorption cross sections , which were calculated using time-dependent density functional theory .
An analytical function was derived to calculate this yield for a given molecular size .

Conclusions : Large PAH cations could be efficiently ionized in H I regions and provide a contribution to the heating of the gas by photoelectric effect .
Our work provides recipes to be used in astronomical models to quantify these points .