| dc.description.abstract | The astronomical observations have shown that the main composition of interstellar ice is water. Astronomers observed H2O in gas phase in cold clouds (<10 K), it means that the ultraviolet photons and cosmic rays can induce H2O ice desorption in cold clouds, because the thermal desorption is negligible in those cold regions. In our study, we tried to understand the vacuum ultraviolet (VUV) photo-desorption process and VUV photolysis of pure H2O ice at different temperature conditions.
In this study, we used microwave-discharge hydrogen-flow lamp (MDHL) to mimic the interstellar UV field. A quadrupole mass spectrometer (QMS) was employed to detect the desorbed species during irradiation and warm-up periods. The Fourier transform infrared spectroscopy (FTIR) was used to monitor the variation of absorption intensity of H2O ice and products during VUV irradiation period. The laser interference system was used to monitor the ice thickness and optical properties. Experimental results show that the photodesorption yield of H2O ice increases while the temperature of deposited H2O ice increases, but the trend of production yield of products as a function of temperature is contrary to photodesorption yield of H2O ice.
When the thickness of H2O ice is thicker than 30 ML, the photo-desorption yield is independent with thickness, and the photo-desorption yield of H2O ice is about 0.052 molecules/photon.
The ice photo-desorption of H2O ice was studied at astronomy relevant temperatures (14 - 110 K). The most abundant desorbed species is H2. The ion signal of desorbed OH increases while temperature increases as well as desorbed H2O. O atom is active while temperature increases, the ion signals of O and O2 are obvious stronger than H2O while temperature increases, and ion signals of O & O2 increase rapidly at temperature above 50 K. Because the O atom is more active at higher temperature and easy to react with H2O2 to form O2.
The absorption intensity of OH dangling bond becomes weaker at higher deposition temperature and after VUV irradiation. We observed the productions of H2, H2O2, HO2, O2 decrease while deposition and irradiation temperature increases. From chemical reaction pathway and the experimental results, we found that O atom as a competitor to reduce the production of H2O2. The results also imply that HO2 will be produced by photon irradiation of H2O ice containing O2 in high abundance.
H2O ice can increase its average kinetic energy, and the structure of H2O ice transfers from amorphous phase to cubic phase at higher temperature. The comparison of the results of laser interference system, QMS and FTIR, the structure or optical property of H2O ice is changing at 147 – 155 K range. | en_US |