DIBs in captivity (?)

Thomas J. Wdowiak*, Luther W. Beegle, Michael S. Robinson, Wei Lee

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

1 Scopus citations


Experiments involving the electrical discharge of trace amounts (0.5%) of methane in argon gas followed by freezing out of products of the discharge in an argon matrix at 10 K have produced absorption bands of known interstellar molecular species, including CH, C2 (anion form), C3 and HCO ("a frozen interstellar cloud"). These experiments also repeatedly produce absorption bands that correlate in approximate wavelength with those of some of the stronger diffuse interstellar bands (DIBs). Among the interesting absorption bands produced in our experiments is a strong, broad band at 4500 Å. It is well known that because there is a difference between interaction of lower and higher energy states of a molecule or ion with the matrix, the absorption for a matrix-isolated species occurs at a different wavelength(s) than for the gas phase. Progressing from polar matrices to argon. and on to neon, the absorption wavelength converges toward what occurs in or is expected for the gas phase. Neon matrices affect an isolated species the least, with the result that wavelengths of absorptions are very close (∼ 0.1%) to that of the gas phase. The two 9700 Å region bands of the matrix-isolated C60 cation shift blueward when isolated in neon, by 94 and 86 Å from what they are in argon (Gasyna. Z. et al., J. phys. Chem., 96, 1525-1527. 1992; Joblin. C. M., Ph.D. thesis, University of Paris. 1992). Isolated in neon, the pyrene cation exhibits an absorption at 4395 Å. which is blueward by 40 Å from the wavelength measured in argon (Salami, F. and Allamandola, L. J., Astrophys. J., 395, 301-306, 1992; Nature, 358, 42-43, 1992). These results strongly suggested that the 4500 Å band due to a reactive and as yet unidentified species isolated in argon in our experiments could shift blueward in the gas phase to coincide with the strong DIB at 4428 Å (Wdowiak. T. J. et al., The Diffuse Interstellar Bands, 1994; Molecules and Grains in Space, 1994). Recently, it has been demonstrated that the absorption seen in an argon matrix at 4500 Å for the photolysis product of methylpyrene shifts 60 Å to 4440 Å (Leger, A. and d'Hendecourt, L., The Diffuse Interstellar Bands, 1995) or 4442 Å (Salarna, F. and Allamandola, L. J., The Diffuse Interstellar Bands, 1995). All of this strongly argues for intensive investigation of the band found in our experiments at 4500 Å as a significant laboratory analog for the 4428 Å DIB. Efforts are in progress to better characterize the nature of the absorption, optimize its production in a more controlled manner and identify the 'culprit'. This report discusses the relative strength of the 4500 Å band as compared with that of those bands due to identified species such as C2 (anion) and C3, and the results of laser-induced fluorescence experiments. Future experiments will include the more difficult neon matrix isolation technique and laser desorption mass spectroscopy.

Original languageEnglish
Pages (from-to)1429-1435
Number of pages7
JournalPlanetary and Space Science
Issue number10-11
StatePublished - 1 Jan 1995

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