Activation Energy of CH Bond Formation in Carbon Grains Irradiated with Hydrogen Atoms

Details Activation Energy of CH Bond Formation in Carbon Grains Irradiated with Hydrogen Atoms Activation Energy of CH Bond Formation in Carbon Grains Irradiated with Hydrogen Atoms

Aug 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006apj...647l..49m&link_type=abstract

The Astrophysical Journal, Volume 647, Issue 1, pp. L49-L52.

Astronomy and Astrophysics

Astronomy

14

Astrochemistry, Ism: Dust, Extinction, Infrared: Ism, Ism: Lines And Bands, Methods: Laboratory

Scientific paper

We present the results of an experimental study of the interaction of nano-sized hydrogen-free carbon grains with H atoms of different temperatures. The effects of H irradiation have been analyzed in situ with infrared spectroscopy. Exposure of carbon grains to atomic hydrogen results in the formation of aliphatic CH bonds. The formation cross section has been derived from the increase of the intensity of the CH stretching band with H atom fluence. The cross section decreases with decreasing H beam temperature. We have estimated an activation energy of aliphatic CH bonds of 6 meV (~70 K), a value considerably lower than the barrier for chemisorption of H atoms on graphite. This result suggests that in dense clouds hydrogenation of carbon grains by atomic hydrogen cannot counteract destruction of CH bonds by cosmic rays and internal UV field, even if ice mantles are absent. The small activation energy, obtained on realistic analogs of interstellar carbon dust, supports theoretical models that predict an important role of chemisorption on carbon grains in the formation of H2.

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