Vacuum-ultraviolet photoionization of a grandPAH molecule. The interplay of alkyl groups and large polycyclic aromatic hydrocarbons upon photoionizaton

So-called grandPAH molecules are proposed to be omnipresent in various interstellar regions as they are large enough to be photostable against strong interstellar radiation fields. They are also notoriously difficult to synthesize and sublimate. Measuring their gas phase spectra and molecular properties is thus very difficult. We report on the experimental photoionization of a defined grandPAH cation (C_60H_18(C_12H_25)_4 1415 amu). The molecule consists of a large PAH containing 60 sp^2 carbon atoms and four long alkyl groups (-C_12H_25) attached to peripheral positions. The alkyl chains wrap around the aromatic core and stabilize the dication leading to a lowering of the ionization energy. m/z The ion yield spectrum of the grandPAH was recorded from 8.1 to 25 eV using an ion trap coupled to vacuum-ultraviolet (VUV) synchrotron radiation. To accurately extrapolate the experimental results toward the unsubstituted grandPAH itself, we perform a series of calculations to quantify this lowering of the ionization energy. The adiabatic ionization energy of the grandPAH cation was determined as 8.50 ± 0.35 eV. This dual experimental and theoretical methodology can be used to perform other spectroscopic measurements of large, alkylated PAHs to extract properties of the unsubstituted PAH. The implications of these results are discussed in the context of the behavior of large PAHs in interstellar and/or interplanetary dust particles as well as PAH units embedded in insoluble organic material which are found in meteorites.