Abstract
A number of vibronic progressions based on low-energy vibrational modes of a large molecule have been found in the diffuse interstellar band (DIB) spectrum of HD 183143. Four active vibrational modes have been identified with energies at 5.18 cm–1, 21.41 cm–1, 31.55 cm–1, and 34.02 cm–1. The mode at 34.02 cm–1 was previously recognized by Herbig. Four bands are associated with this molecule, with origins at 6862.61 A, 6843.64 A, 6203.14 A, and 5545.11 A (14589.1 cm–1, 14608.08 cm–1, 16116.41 cm–1, and 18028.9 cm–1, respectively). The progressions are harmonic and combination bands are observed involving all modes. The appearance of harmonic, rather than anharmonic, terms in these vibronic progressions is consistent with torsional motion of pendant rings, suggesting that the carrier is a floppy molecule. Some constraints on the type and size of the molecule producing these bands are discussed.
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