High-resolution IR absorption spectroscopy of polycyclic aromatic
  hydrocarbons in the 3 micron region: role of hydrogenation and alkylation
  Aims. We aim to elucidate the spectral changes in the 3 micron region that
result from chemical changes in the molecular periphery of polycyclic aromatic
hydrocarbons (PAHs) with extra hydrogens (H-PAHs) and methyl groups (Me-PAHs).
Methods. Advanced laser spectroscopic techniques combined with mass
spectrometry were applied on supersonically cooled
1,2,3,4-tetrahydronaphthalene, 9,10-dihydroanthracene, 9,10-dihydrophenathrene,
1,2,3,6,7,8-hexahydropyrene, 9-methylanthracene, and 9,10-dimethylanthracene,
allowing us to record mass-selective and conformationally selective absorption
spectra of the aromatic, aliphatic, and alkyl CH-stretches in the 3.175-3.636
micron region with laser-limited resolution. We compared the experimental
absorption spectra with standard harmonic calculations and with second-order
vibrational perturbation theory anharmonic calculations that use the SPECTRO
program for treating resonances. Results. We show that anharmonicity plays an
important if not dominant role, affecting not only aromatic, but also aliphatic
and alkyl CH-stretch vibrations. The experimental high-resolution data lead to
the conclusion that the variation in Me- and H-PAHs composition might well
account for the observed variations in the 3 micron emission spectra of
carbon-rich and star-forming regions. Our laboratory studies also suggest that
heavily hydrogenated PAHs form a significant fraction of the carriers of IR
emission in regions in which an anomalously strong 3 micron plateau is
observed.
