The hypothesis that interstellar infrared emission originates from vibrationally excited polycyclic aromatic hydrocarbons (PAHs) requires that emission can arise from all vibrational levels that are energetically accessible. Due to anharmonicity, the emission from the upper vibrational levels is shifted to longer wavelengths from that of the υ = 1-0 transition. The authors show that structure in the 3 μm region is readily and quantitatively explained by emission from upper vibrational levels of excited PAHs that contain a maximum of 20 - 30 carbon atoms. The asymmetrical broadening of the 11.3 μm emission band may also be due to anharmonicity.
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Barker et al. (1987) studied this question.