title: Double ionization of quaterrylene (C40H20) in water-ice at 20 K with Ly alpha (121.6 nm) radiation.
abstract: Polycyclic aromatic hydrocarbon (PAH) molecules undergo facile ionization in cryogenic water-ices resulting in near quantitative conversions of neutral molecules to the corresponding singly charged radical cations. Here we report, for the first time, the production and stabilization of a doubly ionized, closed shell PAH in water-ice. The large PAH quaterrylene (QTR, C40H20) is readily photoionized and stabilized as QTR 2+ in a water-ice matrix at 20 K. The kinetic analysis of photolysis shows that the QTR 2+ is formed at the expense of QTR +, not directly from QTR. The long-axis polarized S1-S0 (1(1)B(3u) <-- 1(1)Ag) transition for QTR 2+ falls at 1.59 eV (782 nm). TD-DFT calculations at the B3LYP level predict that this transition falls at 1.85 eV (670 nm) for free gas-phase QTR 2+, within the 0.3 eV uncertainty associated with these calculations. This red shift of 0.26 eV is quite similar to the 0.24 eV red shift between the TD-DFT computational prediction for the lowest energy transition for QTR + (1.68 eV) and its value in a water matrix (1.44 eV). These results suggest that multiple photoionization of such large PAHs in water-ice can be an efficient process in general.
authors.affiliation: NASA Ames Research Center, Mail Stop 245-6, Moffett Field, California 94035, USA. gudipati@glue.umd.edu
authors.forename: Murthy S
authors.initials: MS
authors.lastname: Gudipati
authors.forename: Louis J
authors.initials: LJ
authors.lastname: Allamandola
mesh_terms: D002412:Cations / Q000737:chemistry; D003198:Computer Simulation; D004583:Electrons; D007053:Ice; D007700:Kinetics; D008956:Models, Chemical; D008958:Models, Molecular; D044367:Phase Transition; D017785:Photons; D011084:Polycyclic Aromatic Hydrocarbons / Q000737:chemistry* / Q000528:radiation effects; D011839:Radiation, Ionizing; D014867:Water / Q000737:chemistry
chemical_list: D002412:Cations; D007053:Ice; D011084:Polycyclic Aromatic Hydrocarbons; D014867:Water