UV/EUV photolysis of interstellar ice analogs

• Main Authors: Yu-Jung Chen, 陳俞融
• Other Authors: Tai-Son Yih
• Format: Others
• Language: zh-TW
• Published: 2007
• Online Access: http://ndltd.ncl.edu.tw/handle/64976531179229350308

博士 === 國立中央大學 === 物理研究所 === 95 === In the last decades, many results were obtained from laboratory experiments of ion and photon irradiation of ice mixtures at low temperature. Photon irradiation studies were almost all focused on the effects of 121.6 nm (Lyman α) photons. In this report, we used synchrotron light provided by the high-flux beamline of the National Synchrotron Radiation Research Center (NSRRC) as an ultraviolet (UV)/extreme ultraviolet (EUV) photon source. Since this high-flux beamline can provide photons with energies ranging from 4 to 45 eV, we can study the photolysis of interstellar ice analogs with different photon energies. The irradiation of H2O+CO2+NH3 ice mixtures could allow us to confirm that small organic compounds were formed after photolysis and warming up to room temperature thanks to infrared spectroscopy, mass spectrometry and isotope labeling of the starting mixtures. For instance, carbamic acid (NH2COOH) was observed to be formed in its neutral form after a 4–20 eV photon irradiation around 250 K, unlike what was reported previously after energetic proton bombardment. We also irradiated ice mixtures containing polycyclic aromatic hydrocarbons (PAHs). These compounds are common throughout the Universe, naphthalene being the smallest PAH since it contains only two aromatic cycles. Two broad-band energy ranges of UV/EUV photons (4–20 eV and 13–45 eV) were used for the irradiation of H2O+NH3+C10H8 ice mixtures at 15 K. We observed that polycyclic aromatic nitrogen heterocycles (PANHs) such as quinoline (C9H7N) and phenanthridine (C13H9N) might have formed, and could estimate the production yields of the photo-products CO, CO2 and OCN- and see that they are significantly higher in the 13–45 eV irradiation experiment than when 4–20 eV photons are used. We finally irradiated H2O+N2+CH4 ice mixtures in order to study the difference of photo-products formed after different irradiation photon energies, namely HI (121.6 nm), HeI (58.4 nm) and HeII (30.4 nm) molecular and ion lines. We observed CN bearing molecules were produced in 30.4 nm and 58.4 nm experiments. In all the experiments mentioned before, amino acids were formed after UV/EUV photons (4–20 eV) irradiation at 15 K and subsequent warming up to room temperature. This result, obtained in experiments simulating astrophysical environments, is important since such organic molecules are probably formed in extraterrestrial environments and are believed to have played a significant role for the origin of life, after having been delivered to the early Earth by comets, asteroids and their interplanetary dust particles.