Time-dependent chemistry in dense molecular clouds. II - Ultraviolet photoprocessing and infrared spectroscopy of grain mantles

Details Time-dependent chemistry in dense molecular clouds. II - Ultraviolet photoprocessing and infrared spectroscopy of grain mantles Time-dependent chemistry in dense molecular clouds. II - Ultraviolet photoprocessing and infrared spectroscopy of grain mantles

Apr 1986

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1986a%26a...158..119d&link_type=abstract

Astronomy and Astrophysics (ISSN 0004-6361), vol. 158, no. 1-2, April 1986, p. 119-134.

Astronomy and Astrophysics

Astronomy

136

Cosmic Dust, Infrared Astronomy, Interstellar Chemistry, Molecular Clouds, Photolysis, Ultraviolet Radiation, Absorption Spectra, Ice, Infrared Spectra, Photodissociation, Solid Phases, Time Dependence, Vapor Phases

Scientific paper

We have performed a series of experiments in which mixtures of gases condensed on a substrate at 10 K are irradiated with ultraviolet light in order to probe the photochemical processing of interstellar ices. The evolution of these simulated grain mantles is followed by infrared spectroscopy (2.5-20 μm). Upon photolysis, the appearance of new bands in the spectra is observed and their possible identification is discussed. Rough quantitative estimates of the photo-production rate of new molecules are given and some infrared cross sections of a few of these bands are discussed. The main results show that: i) CO2 and H2CO should be formed abundantly on grains via this process and their high production rate should influence the gas phase as well. ii) the similarities between the infrared spectra of processed ices and the spectrum of W 33 A show that the energetic processes modifying the composition of the icy mantles in most regions of dense clouds are very efficient. iii) infrared cross sections of a few bands observed in our spectra and in the spectrum of W 33 A are given, using conservative estimates of the number of molecules in the laboratory samples. These cross sections suggest that the species giving rise to the "6.8 μm" band are limited to a class of molecules in which the -CH2 and -CH3 groups are adjacent to unsaturated groups as found in ketones, esters and alcohols. Finally, the importance of laboratory work coupled with infrared astronomy in furthering our understanding of the composition of interstellar ices, is stressed.

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