The Infrared Spectrum of Protonated Naphthalene and Its Relevance for the Unidentified Infrared Bands

Laboratory infrared spectroscopy of PAHs

“…Bandwidths of just a few cm −1 have been obtained for PAH species that are estimated to be expansion cooled to temperatures on the order of 20 K. These methods require the availability of IR laser sources that are continuously tunable in the 5 -15 µm spectral range and thus far free electron lasers and optical parametric oscillators have been used. Moreover, the PAHs need to be entrained in a seeded molecular beam expansion to form the weakly bound PAH-rare gas complexes, which has thus far only been possible for the smallest PAHs naphthalene (Piest et al 1999a;Ricks et al 2009) and phenanthrene (Piest et al 2001).…”

“…The group of Duncan recently recorded IR spectra of protonated benzene (Douberly et al 2008) and naphthalene (Ricks et al 2009) using an optical parametric oscillator (OPO) source that provides mJ pulse energies down to wavelengths of about 10 µm. Argon tagged protonated molecules are produced in the expansion from a pulsed discharge nozzle and mass-selected in the first arm of a reflectron TOF mass spectrometer.…”

Laboratory Spectroscopy of Protonated PAH Molecules Relevant For Interstellar Chemistry

“…Ion and cluster ion generation has been accomplished in an electron-impact or dischargedriven supersonic plasma expansion. This approach was used to produce protonated benzene and naphthalene, C 6 H + 7 and C 10 H + 9 , and their weakly-bound clusters with ligands L = Ar, N 2 , CH 4 , and H 2 O (Solca & Dopfer 2002;Solca & Dopfer 2003b;Douberly et al 2008;Ricks et al 2009). Alternatively, isolated H + PAH can be generated by chemical ionization of PAH in an ion cyclotron resonance (ICR) mass spectrometer using protonating agents, such as CH + 5 or C 2 H + 5 , as successfully demonstrated for C 6 H + 7 and C 10 H + 9 (Jones et al 2003;Dopfer et al 2005;Lorenz et al 2007).…”

Gas-phase infrared spectra of cationized nitrogen-substituted polycyclic aromatic hydrocarbons

“…These species are known as polycyclic aromatic nitrogen heterocycles (PANHs). They have been identified in carbonaceous chondrite meteorites (Sephton 2002) and are expected to form in the interstellar medium (ISM) if nitrogen-bearing molecules are present (Ricca et al 2001). The formation and photostability of small PANHs was previously discussed with the conclusion that only an astrophysical environment typical of a dense molecular cloud can host these species (Peeters et al 2005).…”

“…Indeed, because of the high proton affinity of these species, it is not unlikely that they occur in their protonated forms. Recent experiments involving ion spectroscopy of various protonated PAH species report absorption bands very close to the 6.2 μm interstellar band (Douberly et al 2008;Knorke et al 2009;Lorenz et al 2007;Ricks et al 2009;Vala et al 2009a,b), although a detailed comparison between all protonated PAH spectral features and the interstellar UIR bands has not been performed to date. These protonated PAHs also have a closed-shell electronic structure.…”