EUV-VUV Photolysis of CO and CO+H2O+NH3 Icy Systems at 10 K

We wish to report our experimental results on the spectral identification of IR absorption features produced through EUV-UV photon-induced chemical reactions in cosmic ice analogs. The ice systems studied in the present work are the CO pure ices and CO+H2O+NH3 (1:1:1) mixed ices at 10 K. A tunable intense synchrotron radiation light source available at the National Synchrotron Radiation Research Center, Hsinchu, Taiwan, was employed to provide the required EUV-VUV photons. The photon wavelengths used to irradiate the icy samples include those centered at the prominent solar lines, namely, the 30.4 nm, 58.4 nm, and 121.6 nm. In the EUV-VUV photolysis of pure CO ices the new molecules produced in the ice are found to be primarily C3O2, C2O, CO2, and CO3. The production yields show a clear increase around 25 eV, which appears to correlate with the multiple-electron transitions of the CO molecule in the gas phase. The results support the assertion that photon-induced chemical reactions in pure CO ices depend on photon energy, as expected. The typical yields are of the order of 10-3 per photon, which is comparable to typical yields found in the photon-induced processes in gaseous phase. The results of the CO+CH4+NH3 (1:1:1) ice system at 10 K produced by photolysis at 30.4 nm, 58.4 nm, and 121.6 nm are mainly the C2H6, C3H8, HCO, and H2CO. The sharp HCN feature at 2090 cm-1 and CH2N2 feature at 2099 cm-1 are clearly discernible in both the 30.4 nm and the 58.4 nm photolysis. Tentative assignments are given to two other features at 2070 cm-1 for CN- and 2026 cm-1 for HCCO radical [e.g., see Moore and Hudson, Icarus, 161 (2003) 486]. The spectrum of the Difference of Absorbances obtained from the 121.6 nm photolysis does not show any clear absorption features for HCN, CN-, and the HCCO radical. The results obtained from the present study are important to our further understanding of chemical synthesis in the cometary-type ices, interstellar grains, and icy satellites of planetary systems. The detailed results of this work will be presented. This research is based on work supported by the NASA Planetary Atmospheres Program under Grant NAG5-11960.