SOFT X-RAY IRRADIATION OF H2S ICE AND THE PRESENCE OF S2 IN COMETS

Little is known about the effects of X-rays in interstellar ices. To understand the sulfur depletion in dense clouds and the presence of S-2 in comets, we simulated experimentally the soft X-ray processing (0.3 keV) of H2S ice for the first time. Experiments were performed under ultrahigh vacuum conditions at 8 K using infrared and quadrupole mass spectrometry to monitor the solid and gas phases, respectively. A UV irradiation experiment using a similar dose was made for comparison. After X-ray irradiation, an infrared absorption appears near 4.0 mu m which is attributed to H2S2 formation in the ice. This identification is also supported by the desorption at 133 K of m/z 66, 65, 64, corresponding to the mass fragments of H2S2. The H2S2 species is expected to be present in interstellar and cometary ices that were processed by X-rays. Further irradiation leads to dissociation of this molecule forming S-2 and larger S-molecules up to S-8, which may explain the depletion of sulfur in dense clouds. CS2 was so far the parent molecule proposed for S-2 formation in comets. But the abundance of H2S2, formed by irradiation of pure H2S or H2S in an H2O-ice matrix, should be larger than that of CS2 in the ice, the latter requiring a carbon source for its formation. Based on our experimental results, we propose that S-2 in comets could be formed by dissociation of H2S2 in the ice.