| dc.description.abstract | Numerous solid molecules have been observed in various interstellar regions, and the abundant gaseous molecules are also detected in some environments with low temperatures beyond expectation, like the interstellar medium (ISM) and cold dense clouds. The radiation of the energetic cosmic rays, VUV photons, electrons, and X-rays impinge on the dust grains, resulting in the desorption of the ice mantles. The non-thermal photodesorption process has been seen as an indispensable origin of the overabundance of molecules in the gas phase in the cold regions.
Hence, energetic radiation experiments are of significance for studying the details of photodesorption and photochemical processes. The VUV and X-ray photo-processing on the pure and ice mixtures are focused to figure out the governing parameters on the photodesorption yield, as well as the photochemistry happening simultaneously during irradiation. The fundamental physical properties of the ice are involved in the discussion.
The experiments are conducted through the ultra-high vacuum (UHV) chamber, equipped with a closed-cycle helium cryostat to mimic the low pressure and low temperature near the cold dense clouds and the interstellar medium. The evolution of the solid ice is observed by a transmission Fourier transform infrared spectrometer (FTIR), while a quadrupole mass spectrometer (QMS) is used to detect the gaseous desorbing signals. Different kinds of ice samples are selected to be the sample, such as pure CO2, CO, CH4, the mixtures with N2, and the H2O:CO:NH3. The samples are irradiated by the VUV photons, generated by the Microwave-Discharge Hydrogen-flow Lamp (MDHL) produced in the laboratory with the wavelength 110--180 nm. The other energetic source performed in this thesis is the X-ray, provided by the National Synchrotron Radiation Research Center (NSRRC) with the energy 250 to 1250 eV. Various parameters are designed to investigate the photon-induced desorption and chemical reactions, including the different deposition temperatures, deposition angles, ice mixing ratios, and photon fluxes.
The parameters affecting photodesorption are revealed in detail. In the case of deposition temperature issue on the pure CO2 and CO ice, only the latter one shows the temperature dependency, and the desorption factors are developed for determining the photodesorption yields. Another important role is the absorption cross section of the solid ice, which should be considered when we discuss photodesorption. Moreover, we found that the angle between gas inlet and the substrate falls at 70$degree$ can create a maximum photodesorption yield of CO ice, along with a columnar structure of ice. The energy transfer between CO2/CO ice with N2 is discussed, confirming that the indirect desorption induced by electronic transition (DIET) dominates the photodesorption processes. During irradiation, the solid products are observed on the IR spectra according to the ice samples and the radiation sources. There are Complex Organic Molecules (COMs) identified by both the IR spectra and the mass spectra, and the chemical formation routes of these molecules are represented. Some larger molecules are detected during the temperature-programmed desorption (TPD) processes, supporting the molecular diversity in the interstellar regions. The infrared spectra of the ice at different temperature environments are required not only to identify the peak position of the molecules but also to provide information to track the temperature history of the ice mantles on the dust grains. With the experiments performed in this thesis, we hope to contribute to a more complete knowledge of astrophysics and astrochemistry. | en_US |