H_n-PAHs are PAHs which bear excess H atoms. We think that, along with normal PAHs, H_n-PAHs are produced in the outflows of aging carbon stars. Our experiments on the photo-chemistry of PAHs in water ice indicate that H_n-PAHs may also be formed from PAHs in dense molecular clouds. Like normal PAHs these molecules contribute to the IR emssion features (UIRs) seen coming from regions of space where there is intense UV radiation.

The infrared emission in the C-H stretching region from the Orion Bar (an HII) comapred with our lab spectrum of hexahydropyrene (H₆-pyrene) in an argon matrix.

The 3040 cm^-1 component is thought to be due to the normal aromatic C-H stretch of PAHs, but the precise identification of the weak features just longward of the 3040 cm^-1 emission feature has remained somewhat enigmatic. (for a brief 'tutorial' on molecular vibrations and infrared spectroscopy, click here) In most objects these features, falling near 2940, 2890, 2850, and 2810 cm^-1 (3.40, 3.46, 3.51, and 3.56 µm), are weak compared to the 3040 cm^-1 (3.29 µm) band and show a tendency to decrease in strength with decreasing frequency. However, in the spectra of a small number of early type objects spanning the evolutionary bridge between carbon-rich giants and planetary nebulae, the 2940 and 2850 cm^-1 bands (3.40 and 3.5 µm) are actually stronger than the 3040 cm^-1 feature. Several identifications for these features have been suggested, including (i) overtones and combinations of lower frequency PAH vibrational modes, (ii) "hot" bands associated with relaxation from higher vibrational levels which are anharmonic, (iii) the C-H stretching vibrations of aliphatic sidegroups on PAHs, and (iv) PAHs containing excess H atoms (H_n-PAHs). All the suggested possibilities probably contribute to the emission, the extent of each depending on conditions in the local environment, but this figure and the associated paper on this subject add support to the notion that, at least in H II regions, H_n-PAHs are the best explanation

The C-H bonds on the fully aromatic rings produce the normal aromatic C-H stretching feature near 3040 cm^-1, while the aliphatic -CH₂- structures in the rings with extra H atoms produce the lower frequency bands. The fact that the 3040 cm^-1 feature is stronger than the lower frequency C-H stretching features indicates that the aromatic structures dominate over the aliphatic structures in the interstellar medium. This is to be expected since fully aromatic PAHs should be more stable in the harsh radiation environments of emission nebulae.

Further support for this suggestion is provided by the observation that there are a few objects where the emission feature near 2950 cm^-1 is actually stronger than the feature near 3040 cm^-1. These rare cases are only seen in protoplanetary nebulae, nebulae that contain both very fresh material and relatively benign radiation environments. Thus, the features potentially associated with H_n-PAHs are only seen to dominate this spectral regions in locations where this less stable form of aromatic material might be expected to survive for any appreciable time in space.

For more detailed information and reviews on our laboratory work on Hn-PAHs and their possible relationships to the intrared interstellar emision bands and interstellar and cometary ice analogs, see:

Bernstein, M. P., Sandford, S. A., & Allamandola, L. J. (1996). Hydrogenated Polycyclic Aromatic Hydrocarbons (H_n-PAHs) and the 2940 and 2850 Wavenumber (3.40 and 3.51 Micron) Infrared Emission Features. Astrophys. J. 472, L127-L130.

Sloan, G. C., Bregman, J. D., Geballe, T. R., Allamandola, L. J., & Woodward, C. E. (1997). Variations in the 3 micron spectrum across the Orion Bar: Polycyclic aromatic hydrocarbons and related molecules. Astrophys. J. 474, 735-740.

Bernstein, M. P., Sandford, S. A., Allamandola, L. J., Gillette, J. S., Clemett, S. J., & Zare, R. N. (1999). UV Irradiation of Polycyclic Aromatic Hydrocarbons in Ices: Production of Alcohols, Quinones, and Ethers. Science 283, 1135-1138.

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