Infrared Spectra Of Protonated Aromatic Hydrocarbons And Their Neutral Counterparts In Solid Para-hydrogen

Protonated polycyclic aromatic hydrocarbons (H$^{+}$PAH) have been reported to have infrared (IR) bands at wavenumbers near those of unidentified infrared (UIR) emission bands from interstellar objects. However, recording IR spectra of H$^{+}$PAH in laboratories is challenging. Two spectral methods have been employed previously to yield IR spectra of H$^{+}$PAH. One employs IR multiphoton dissociation (IRMPD) of H$^{+}$PAH, but the bands are broad and red-shifted. \footnote {O.~Dopfer, \textit{ PAHs and the Universe,} \underline{\textbf{46}}, 103 (2011).} Another measures the single-photon IR photodissociation action spectrum of cold H$^{+}$PAH tagged with a weakly bound ligand, such as Ar, but application of this method to large PAH is difficult.\footnote {A.~M.~Ricks, G.~E.~Douberly, M.~A.~Duncan, \textit{Astrophys. J.} \underline{\textbf{702}}, 301 (2009).} A new method for investigating IR spectra of H$^{+}$PAH and their neutral counterparts was developed using electron bombardment during ${p}$-H$_{2}$ matrix deposition. With this technique, we have recorded IR absorption spectra of protonated forms of benzene (C$_{6}$H$_{7}$$^{+}$), naphthalene (1- and 2-C$_{10}$H$_{9}$$^{+}$), pyrene (1-C$_{16}$H$_{11}$$^{+}$), coronene (1-C$_{24}$H$_{13}$$^{+}$), and their neutrals.\footnote {M.~Bahou, Y.-J.~ Wu, Y.-P.~Lee, \textit{J. Chem. Phys.} \underline{\textbf{136}}, 154304 (2012); M.~Bahou, Y.-J.~Wu, Y.-P.~Lee, \textit{Phys. Chem. Chem. Phys.} \underline{\textbf{15}}, 1907 (2013); M.~Bahou, Y.-J.~ Wu, Y.-P.~Lee, \textit{J. Phys. Chem. Lett.} \underline{\textbf{4}}, 1989 (2013); M.~Bahou, Y.-J.~ Wu, Y.-P.~Lee, \textit{Angew. Chem. Int. Ed.} \underline{\textbf{53}}, 1021 (2014).} The significant superiority of the spectra thus recorded to those with the Ar-tagging and IRMPD methods is demonstrated. The narrow widths of the lines enabled us to distinguish clearly between isomers 1-C$_{10}$H$_{9}$$^{+}$ and 2-C$_{10}$H$_{9}$$^{+}$; 2-C$_{10}$H$_{9}$$^{+}$ was unstable and converted to 1-C$_{10}$H$_{9}$$^{+}$ in less than 30 min. A survey of these experimental results shows that three major lines in the 7-9 $\mu$m region are red-shifted from 7.19, 7.45, and 8.13 $\mu$m of 1-C$_{16}$H$_{11}$$^{+}$ to 7.37, 7.53, and 8.21 $\mu$m of 1-C$_{24}$H$_{13}$$^{+}$, showing the direction towards the UIR bands near 7.6, 7.8, and 8.6 $\mu$m. In contrast, the line at 11.5 $\mu$m for 1-C$_{16}$H$_{11}$$^{+}$ is blue-shifted to 11.4 $\mu$m for 1-C$_{24}$H$_{13}$$^{+}$, showing the direction toward the UIR band near 11.2 $\mu$m. Other examples will be presented if time permits