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 S2 in comets, we simulated experimentally the soft X-ray processing (0.3 keV) of H2 Si ce 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 μm 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 .T he H 2S2 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 S2 and larger S-molecules up to S8, which may explain the depletion of sulfur in dense clouds. CS2 was so far the parent molecule proposed for S2 formation in comets. But the abundance of H2S2, formed by irradiation of pure H2 So r H 2 Si n an H 2O–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 S2 in comets could be formed by dissociation of H2S2 in the ice