[PPS09-P05] 二酸化炭素ハイドレートの形成に伴う炭素同位体組成の変化
キーワード：アモルファス氷、クラスレートハイドレート、赤外分光法、同位体
In interstellar molecular clouds, various molecules such as H2O, CO, CO2, CH3OH, H2CO, and NH3 are condensed onto dust grains. Water exists as amorphous ice on the dust grains and is transformed into various structures depending on thermal conditions and compositions of coexisting molecules. From the results using transmission electron microscopy and Fourier transformed infrared spectroscopy, it was found that the vapor deposited amorphous ice including CH3OH and CO2 is transformed into clathrate hydrate at around 120 K [1]. This suggests that the cometary ice exists as clathrate hydrate. Clathrate hydrates are inclusion compounds consisting of water molecules and a variety of guest molecules. From infrared (IR) astronomical observation, it was found that the 12C/13C isotope ratio of comets is larger than that of interstellar matter [2]. To investigate the effect of clathrate hydrate formation on 12C/13C isotopic ratio, IR spectra of vapor deposited amorphous ice including 12CO2 and 13CO2 were measured. Using the spectra, the variation process of 12CO2/13CO2 ratio during warming was analyzed.
Amorphous ice including 12CO2 and 13CO2 was prepared with vapor deposition of mixture of 12CO2, 13CO2 gases and distilled and degassed water on a substrate of oxygen-free copper at 42 K. In the gas mixture, the 12C/13C isotopic ratio was 92, and (12CO2 +13CO2)/ H2O was 1.4. The total pressure in the vacuum chamber was kept at about 3.5 × 10–5 Pa during the deposition. After the deposition of amorphous ice, the substrate was warmed to 180 K at a rate of 1–4 K/min. The IR spectra were measured using a spectrometer (Shimadzu IRPrestage-21) at 2 K intervals during warming. To analyze the sublimation behaviors, the temperature programed desorption (TPD) curves were also measured using quadrupole mass spectrometer (Pfeiffer QME220) during warming.
From the analyses of the wave numbers of the O–H stretching modes of H2O, 12C=O asymmetric stretching mode of 12CO2, and 13C=O asymmetric stretching modes of 13CO2 during the warming, remarkable changes were found at around 100 K. From the TPD curve, furthermore, a remarkable gas release was observed at this temperature. Because of the closed wave number of the 12C=O asymmetric stretching mode at 100 K with that of CO2 clathrate hydrate [3], the formation of CO2 clathrate hydrate at 100 K was confirmed. It was found that the 12CO2/13CO2 ratio increases at around 100 K. These results suggest that the formation of clathrate hydrate from amorphous ice with warming can be a cause of 12C condensation.
References
[1] D. Blake, L. Allamandola, S. Sandford, D. Hudgins, F. Freund, Science 254, 548 (1991)
[2] V.Vanysek, J.Rahe, The Moon and the Planets 18, 441–446 (1978)
[3] F. Fleyfel, J. P. Devlin, J. Phys. Chem. 95, 3811 (1991)
Amorphous ice including 12CO2 and 13CO2 was prepared with vapor deposition of mixture of 12CO2, 13CO2 gases and distilled and degassed water on a substrate of oxygen-free copper at 42 K. In the gas mixture, the 12C/13C isotopic ratio was 92, and (12CO2 +13CO2)/ H2O was 1.4. The total pressure in the vacuum chamber was kept at about 3.5 × 10–5 Pa during the deposition. After the deposition of amorphous ice, the substrate was warmed to 180 K at a rate of 1–4 K/min. The IR spectra were measured using a spectrometer (Shimadzu IRPrestage-21) at 2 K intervals during warming. To analyze the sublimation behaviors, the temperature programed desorption (TPD) curves were also measured using quadrupole mass spectrometer (Pfeiffer QME220) during warming.
From the analyses of the wave numbers of the O–H stretching modes of H2O, 12C=O asymmetric stretching mode of 12CO2, and 13C=O asymmetric stretching modes of 13CO2 during the warming, remarkable changes were found at around 100 K. From the TPD curve, furthermore, a remarkable gas release was observed at this temperature. Because of the closed wave number of the 12C=O asymmetric stretching mode at 100 K with that of CO2 clathrate hydrate [3], the formation of CO2 clathrate hydrate at 100 K was confirmed. It was found that the 12CO2/13CO2 ratio increases at around 100 K. These results suggest that the formation of clathrate hydrate from amorphous ice with warming can be a cause of 12C condensation.
References
[1] D. Blake, L. Allamandola, S. Sandford, D. Hudgins, F. Freund, Science 254, 548 (1991)
[2] V.Vanysek, J.Rahe, The Moon and the Planets 18, 441–446 (1978)
[3] F. Fleyfel, J. P. Devlin, J. Phys. Chem. 95, 3811 (1991)