Soft X-Ray Irradiation of H2s Ice and the Presence of S2in Comets

This is the authors' abstract. We don't add key points for this paper.

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 H 2 S 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 μm which is attributed to H 2 S 2 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 H 2 S 2 . The H 2 S 2 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. CS 2 was so far the parent molecule proposed for S 2 formation in comets. But the abundance of H 2 S 2 , formed by irradiation of pure H 2 S or H 2 S in an H 2 O–ice matrix, should be larger than that of CS 2 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 H 2 S 2 in the ice.