|dc.description.abstract||Astronomers observed the evidence of solid phase S-bearing molecules, OCS and SO2, in the Interstellar Matter (ISM). However, in the other astronomical object like comet and moons of the Jupiter they also observed the solid H2S molecule, but H2S was absent in the ISM. So far the incomplete inventory of S atoms in the interstellar ice is yet an unsolved puzzle. We choose the H2S+CO and H2S+CO2 as ice mixtures and compare their photolysis products and formation mechanism by VUV photon in order to explore the formation mechanism of S-bearing molecules. Also we will try to understand how were the Sn molecules played the role of missing inventory of sulfur atoms.
In this study, different source of the C atom from H2S+CO and H2S+CO2 ice mixtures were evaluated and their difference in producing complex molecules in the photolysis process were investigated. Also the relation between the ice mixture composition such as H2S: CO=1:7 & 1:20 and H2S:CO2=1:7 & 1:20 and its correlation to the product were studied. We used a Hydrogen Microwave Discharge Lamp(HMWDL) with MgF2 & CaF2 window and High Flux beamline of the Synchrotron Radiation Research Center (NSRRC) as VUV source to explore the photolysis processes and the variation of products under different UV energy. The Fourier Transform Infrared spectroscopy (FTIR) and Quadrupole Mass Spectrometer (QMS) were employed to analyze the formation mechanism and yield of the products. The results indicate that the OCS yield in the H2S+CO ice mixture is higher than that in the H2S+CO2 ice mixture. It is noted that the OCS yield is only dependent on the initial composition of the ice mixture but not the photon energy. On the other hand, the OCS formation rate only depends on the photon energy but not the initial composition of ices.
We observed the S-bearing molecules including H2S2、OCS、CS2 and SO2 in this work. We also confirmed that the SO2 only could be produced in rich O atom environment, i.e. in the H2S+CO2 ice mixture. While studying the formation of CS2, it was found that there were two different reaction paths of the CS2 in the different photon energy. In the NSRRC with 30.4 nm and 58.4 nm, the UV energy is greater than the HMWDL energy. Therefore, the CO molecule can be dissociated by the photon into C atom and O atom, then the O atom and the HS radical combined to form SO molecule, and finally react with CS to produce CS2. While using the HMWDL with MgF2 and CaF2 window, the OCS molecule and H2S combined to form CS2 and H2O. Due to lacking the O atom formation in the HMWDL experiment, the S atom combined each other to form the S2 molecule. But the S2 molecule has a symmetrical structure and consequently will not have absorption feature in the IR spectrum. The missing S atom in the interstellar ice could be proposed to be present in the Sn (n=2-8) structure. We report the S2 molecule thermal desorption evidenced in our mass spectrum during the sample warming up process. Unfortunately, the larger structure molecules have the higher thermal desorption temperature than 300K. It is beyond our experimental capability at this moment to directly observe the thermal desorption of larger Sn molecules should they present in our ice residue.
It is worth to mention that observed peak position of the OCS absorption feature will be affected by the H2S+CO ice mixture composition. In the higher CO concentration ice mixture, the OCS peak is shifted to higher energy. This blue shift of photolysis produced COS molecule feature is found absent in H2S+CO2 ice mixture.
In summary, we report the photolysis products and their possible formation mechanism in the H2S+CO and H2S+CO2 ice mixture. Comparison with the astronomical observations and the related experimental results using ion bombardment were discussed and found in good agreement. Wish the experimental results in this work can lay down a milestone for the S-bearing interstellar ice related research in the future.