Large polycyclic aromatic hydrocarbons (PAHs) are the most abundant complex molecules in the interstellar medium; however, their possible formation pathways from small molecular species are still elusive. In the present work, we follow and characterize the formation of PAHs in an electrical discharge, specifically the PAH naphthalene in a molecular beam of argon. The fragments, products and reaction intermediates are unambiguously structurally identified by mass-selective IR-UV spectroscopy combined with quantum chemical calculations. This experiment provides evidence of the formation of larger PAHs containing up to four cyclic rings in the gas phase originating from a non-radical PAH molecule as a precursor. In addition to PAH formation, key resonance stabilized radical intermediates and intermediates containing di-acetylenic side groups are unambiguously identified in our experiment. We thereby not only reveal competing formation pathways to larger PAHs, but also identify intermediate species to PAH formation that are candidates for detection in radio-astronomy.
Aims. In this work we determine the effects of anharmonicity on the mid-infrared spectra of the linear polycyclic aromatic hydrocarbons (PAHs) naphthalene, anthracene, tetracene and pentacene recorded using the free electron laser FELIX. Methods. Comparison of experimental spectra obtained under supersonic jet conditions with theoretically predicted spectra was used to show if anharmonicity explicitly needs to be taken into account. Results. Anharmonic spectra obtained using second-order vibrational perturbation theory agree on average within 0.5% of the experimental frequencies. Importantly, they confirm the presence of combination bands with appreciable intensity in the 5-6 µm region. These combination bands contain a significant fraction of the IR absorption, which scales linearly with the size of the PAH. Detection and assignment of the combination bands are a preliminary indication of the accuracy of far-IR modes in our anharmonic theoretical spectra. Detailed analysis of the periphery-sensitive CH out-of-plane band of naphthalene reveals that there is still room for improvement of the VPT2 approach. In addition, the implications of our findings for the analysis of the aromatic infrared bands are discussed.
Gas-phase
IR–UV double-resonance laser spectroscopy is an
IR absorption technique that bridges the gap between experimental
IR spectroscopy and theory. The IR experiments are used to directly
evaluate predicted frequencies and potential energy surfaces as well
as to probe the structure of isolated molecules. However, a detailed
understanding of the underlying mechanisms is, especially in the far-IR
regime, still far from complete, even though this is crucial for properly
interpreting the recorded IR absorption spectra. Here, events occurring
upon excitation to vibrational levels of polycyclic aromatic hydrocarbons
by far-IR radiation from the FELIX free electron laser are followed
using resonance-enhanced multiphoton ionization spectroscopy. These
studies provide detailed insight into how ladder climbing and anharmonicity
influence IR–UV spectroscopy and therefore the resulting IR
signatures in the far-IR region. Moreover, the potential energy surfaces
of these low-frequency delocalized modes are investigated and shown
to have a strong harmonic character.
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