Investigation of the ultraviolet, visible, and near-infrared absorption spectra of hydrogenated polycyclic aromatic hydrocarbons and their cations

The formation and the presence of PAHs containing excess H atoms, or hydrogenated PAHs (H-n-PAHs) in interstellar clouds has recently been discussed. It has been suggested that H-n-PAHs contribute to the IR emission bands and that they might be among the molecular precursors of the carbon particles that cause the interstellar UV extinction curve. The spectroscopy of H-n-PAHs is investigated, and the implications for interstellar spectra are discussed. The UV, visible, and near-IR absorption spectra of a series of H-n-PAHs and their photoproducts formed by vacuum UV irradiation were measured for the first time in inert-gas (neon) matrices at 5 K. It is shown that the spectra of both the neutral H-n-PAH and cationic H-n-PAH(+) species exhibit vibronic band systems with similar spectral positions and relative spectral intensities to their nonhydrogenated PAH chromophore counterparts. The results are discussed in the context of the nature of the origin of the diffuse interstellar bands.