| Abstract : | The universe is molecular in nature. More than 270 molecules have been detected in the interstellar environment. These molecular species are cations, anions, radical, and neutral. The molecular clouds in interstellar medium (ISM) consisting of water-rich icy grains, have been identified as a major source of complex organic molecules (COMs). Under this harsh circumstance, the unique chemistry of interstellar ice is expected to occur. Clathrate hydrates (CHs) are nonstoichiometric crystalline host–guest complexes in which guest molecules get entrapped in cages made of water molecules. CHs require high pressures and moderate temperatures for their formation on earth. Our focus has been to form CHs at extremely low pressure and temperature as in the ISM, since water and guest molecules (methane, carbon dioxide, carbon monoxide, etc.) exist in space. Our research group has developed a state-of-the-art ultrahigh vacuum (UHV) instrument capable of creating ISM conditions (~10-10 mbar and 10 K). The instrument has reflection-absorption infrared spectroscopy (RAIRS), temperature programmed desorption-mass spectrometry (TPD-MS), and other analytical techniques. The thermal processing of interstellar organic molecules in amorphous solid water showed the enclathration of molecules in the water cages. This gives experimental evidence of the existence of CH in interstellar medium. CHs with guest molecules such as carbon dioxide, methane, ethane, acetone, tetrahydrofuran, and formaldehyde have been reported under UHV and cryogenic temperatures. We showed dissociating acetone and formaldehyde hydrates can crystallize amorphous solid water in cubic and hexagonal crystal structures at 130–135 K, respectively. These results suggest that interstellar ices possess dynamicity that facilitates the mobility of molecules, potentially accounting for the chemical evolutions observed in the ISM. |
