2006ApJ...643..923M

We present the results of UV irradiation with Lyα photons of carbon grains with a water ice cap at 11 K. Formation of CO and CO₂molecules takes place during irradiation. An estimation of the formation cross section of these molecules by Lyα photons has been obtained from the intensity increase of their infrared stretching bands as a function of the photon fluence. The fraction of carbon in the grains converted to CO and CO₂by UV photons is 0.06 and 0.05, respectively. The spectral profile of the CO stretching feature and that of the CO₂bending mode indicate a polar environment for these molecules. On the basis of the present laboratory results and those obtained in previous work on ion irradiation of similar samples, it has been possible to estimate the contribution of polar CO and CO₂produced on carbon grains by energetic processing to the observed column densities of these molecules for dense clouds whose visual extinction is known. A significant amount of polar CO and CO₂is produced through the mechanism we have studied. Furthermore, we have found that the laboratory profile of the bending band of CO₂produced on carbon grains is compatible with that observed toward the field star Elias 16.