High resolution infrared spectra of protonated benzene isolated in solid parahydrogen

Identification of infrared (IR) spectra of protonated polyaromatic hydrocarbons (PAH) is important in understanding the unidentified IR bands of interstellar media. We demonstrate a new method that is superior to the Ar-tagging IR photodissociation or the IR-multiphoton-dissociation (IRMPD) methods currently used. The protonated benzene (C$_{6}$H$_{7}^{+})$ was produced on electron bombardment of a mixture of benzene (C$_{6}$H$_{6})$ and \textit{para}-hydrogen ($p$-H$_{2})$ during deposition. IR features of C$_{6}$H$_{7}^{+}$ and C$_{6}$H$_{7}$ were identified by observing the change in intensity upon photolysis and comparison with theoretical calculations. Lines of C$_{6}$H$_{7}^{+}$ decreased in intensity when the matrix was irradiated with light at 365 nm, those of C$_{6}$H$_{7}$ increased in intensity. Similar experiments were performed for a sample of C$_{6}$D$_{6}$/$p$-H$_{2}$ and the production of C$_{6}$D$_{6}$H$^{+}$ was confirmed. Observed wavenumbers, relative IR intensities and deuterium isotopic shifts agree with those predicted for C$_{6}$H$_{7}^{+}$ and C$_{6}$H$_{7}$. Compared with previous methods, this method provides a wider spectral coverage with much narrower lines and more accurate relative IR intensities, and may be readily applied to larger protonated and neutral PAH.