Context. Chemical models predict the presence of S-bearing molecules such as hydrogen sulfide (H 2 S) in interstellar icy grain mantles in dense molecular clouds. Up to now only two S-bearing molecules, namely sulfur dioxide (SO 2 ) and carbonyl sulfide (OCS), have been detected in the solid phase towards young stellar objects (YSOs), while upper limits for solid H 2 S have been reported towards the same lines of sight. The estimated abundance of S-bearing molecules in icy grain mantles is not able to account for the cosmic S abundance. Aims. In this paper we studied the effects of ion irradiation on different icy targets formed by carbon monoxide (CO) and SO 2 or H 2 S as mixtures and, for the first time, as layers. Methods. We carried out several irradiation experiments on ices containing SO 2 or H 2 S mixed or layered with CO. The samples were irradiated with 200 keV protons in a high-vacuum chamber (P < 10 −7 mbar) at a temperature of 16-20 K. IR spectra of the samples were recorded after various steps of irradiation and after warm-up. Results. We have found that the column density of H 2 S and SO 2 , as well as CO, decreases after irradiation, and the formation of new molecular species is observed. In the case of CO:SO 2 samples, OCS, sulfur trioxide (SO 3 ), ozone (O 3 ), and carbon dioxide (CO 2 ) are the most abundant species formed. In the case of CO:H 2 S samples the most abundant species formed are OCS, SO 2 , carbon disulfide (CS 2 ), hydrogen persulfide (H 2 S 2 ), and CO 2 . The profile of the OCS band formed after irradiation of the CO:H 2 S mixture compares well with the profile of the OCS band detected towards the high mass YSO W33A. Conclusions. Our results show that on a time scale comparable to the molecular cloud lifetime, the column density of H 2 S is strongly reduced and we suggest that this could explain the failure of its detection in the solid phase in the lines of sight of YSOs. We suggest that the solid OCS and SO 2 detected in dense molecular clouds are formed after ion irradiation of icy grain mantles.

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Context. The surfaces of airless bodies in the Solar System are continuously altered by the bombardment of micrometeoroids and irradiation by solar wind, flares, and cosmic particles. Major effects of this process -space weathering -are darkening and "reddening" of the spectra of surface materials, as well as a "degrading" of absorption features. Aims. We studied the changes induced by energetic ion irradiation in the ultraviolet-visual-near-infrared (UV-Vis-NIR) (0.2-0.98 μm) reflectance spectra of targets selected to mimic the surfaces of airless bodies in the inner Solar System. Our chosen targets are olivine pellets, pure or covered by an organic polymer (polystyrene), which is transparent before irradiation. Polystyrene is used as a template for organic matter of low volatility that can be present on asteroidal surfaces. Moreover we measured the changes induced by ion irradiation in the absorption coefficient of the polymer. The purpose was to have a tool to better compare laboratory with observed spectra and distinguish between planetary objects with pure silicate surfaces and those whose surface is covered by organic matter exposed to cosmic ion bombardment. Methods. The samples were irradiated in vacuum, at room temperature, with 200 keV protons or 200-400 keV argon ions. Before, during, and after irradiation diffuse reflectance spectra were acquired. Polystyrene films were also deposited on quartz substrates and irradiated while transmittance spectra were recorded. Results. We measured the variations of the absorption coefficient of polystyrene as a function of ion fluence. We showed that after ion irradiation the diffuse reflectance spectra of the samples covered by organics exhibit a much more significant variation than those of pure silicates. The spectra of targets made of olivine plus polystyrene can be fitted by using the measured absorption coefficient of polystyrene. Conclusions. The results obtained for pure olivine extend to the UV the spectral range of previous experiments. The data concerning the absorption coefficient of polystyrene are available on our web site (http://web.ct.astro.it/weblab/dbindex.html) and can be used to compare laboratory with astronomical spectra. This will possibly allow us to obtain information about organic matter on the surface of a given object, as well as on the relevance of the exposure to cosmic ions (space weathering).

The resonance Raman spectra of cyano-and aquocobalamin have been obtained from aqueous solutions. In order to check the vibronic theory of Raman intensity in the rigorous resonance case, the excitation profile of the 1502 cm-' vibration was measured and analyzed using a model of two displaced harmonic oscillators for the ground and the resonant electronic state. A very good agreement is shown between the calculated and the experimental values of both the excitation profile and the absorption spectrum. Information on the corrin ring symmetry and evaluation of the vibronic interaction energies have been obtained. The appearance of several overtones and combinations is also reported. peak (p band, at 525 nm for cyano-and 500 nm for aquocobalamin) is assigned to the 0-1 vibronic components. At about 420 nm a weak band of controversial origin ap-Journal of Raman Spectroscopy 2 (1974) 351-362. All Rights Reserved Copyright 0 1974 by D. Reidel Publishing Cornparry, Dordrecht-Holland