ИСТИНА

Polycyclic aromatic hydrocarbons (PAHs) are important molecules in astrochemistry, since they are estimated to contain up to 20% of carbon in our galaxy. [1] In the interstellar medium or protoplanetary discs these molecules are subjected to harsh XUV radiation. This leads to different photochemical processes, namely ionization and/or fragmentation. [2,3] Therefore studying in details these processes on the femtosecond time scale is useful for understanding the chemical processes occurring in the extraterrestrial conditions.[4] We have performed simulations for pump-probe behavior of three PAHs: fluorene, phenanthrene, and pyrene. In particular, we investigated the decay of each of the dicationic states. To aid this, we have adopted a recently proposed method for blackbox simulations of electronic impact mass spectra (MS) based on Born-Oppenheimer molecular dynamics simulations (BOMD).[5,6] It replaces the explicit treatment of excited states by internal excess energy (IEE), representing the averaged excitation energy, which is gradually drained into vibrational degrees of freedom with a rate of internal conversion. In order to adopt this methodology to XUV/IR excitation, we have proposed an explicit model for the value of the IEE based on the maximum entropy principle and a new model for the internal conversion rate. Following this methodology, we were able to qualitatively reproduce the experimentally observed ionic yield pumpprobe dependences for ions appearing from PAH2+ (see figure). The BO surface in the simulations was produced by the GFN2-xTB method [7] implemented in the XTB [8] software. Literature: [1] C.Joblin, A.Tielens, EAS Publications Series 2011, 46, 3–10. [2] D.Hagan, J.Eland, Rapid Commun. Mass Spectrom. 1991, 5, 512–517. [3] A.Marciniak et al., Nat. Commun. 2015, 6, 7909. [4] E.Van Dishoeck, Proc. IAU 2018, 13, S332, 3–22. [5] S.Grimme, Angew. Chem. Int. Ed. 2013, 52, 6306–6312. [6] Ásgeirsson V. et al., Chem. Sci. 2017, 8, 4879–4895. [7] C.Bannwarth et al., JCTC 2019, 15, 3, 1652–1671. [8] https://www.chemie.uni-bonn.de/pctc/mulliken-center/software/xtb/xtb