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Akira Kouchi1

1, Hokkaido University, Sapporo, , Japan

An ultrahigh vacuum TEM has been developed for in situ observation of ice (JEOL JEM-2100VL). A column of the microscope is evacuated by five ion pumps, two Ti-sublimation pumps, and two turbomolecular pumps. The pressure of the specimen chamber after baking is 1x10-7 Pa measured by an ionization gauge. Furthermore, since the specimen is surrounded by liquid nitrogen shroud, the pressure around the specimen is supposed to be lower than 1x10-7 Pa. We use a single tilt liquid He cooling holder (Gatan ULTST) for specimen cooling. Three ports are directed to the specimen surface with an incident angle of 55° for in situ studies. These are used for gas-inlet, UV irradiation (30 W deuterium lamp), and a quadrupole mass spectrometer. We used a 5-nm-thick amorphous Si film (SiMPore Inc.) as a substrate for ice deposition. To avoid the electron-beam damage to the ice sample, low magnification observation was performed with very low electron beam density. We will introduce two examples of observation on amorphous ices.
We have developed a new method for the formation of high-density amorphous ice, matrix sublimation method [1]. CO:H2O = 50:1 ice was deposited at 10 K, which was then allowed to warm. After the sublimation of CO at around 35 K, high-density amorphous ice was formed. This is similar to that formed under high-pressure condition.
We found that UV-irradiated amorphous H2O ice at 10 K shows liquid-like behavior at 50-140 K [2]. First, islands of crystalline ice Ic was made at 145 K, then cooled to 10 K and irradiated UV-rays. Crystalline ice Ic was easily amorphized within one minute. After 30 min. UV-irradiation, the sample was heated. We observed at T > 50 K that the height of the islands decreased and their area increased and finally overlapped like squashed liquid droplets. The viscosity estimated from the spreading velocity of islands is an order of 107 Pa s, 5 orders of magnitude smaller than that at the glass transition temperature (1012 Pa s), indicating that the UV-irradiated amorphous water ice behaves like a viscous liquid.

References
[1] Kouchi A. et al. (2016) Chem. Phys. Lett., 658, 287.
[2] Tachibana S. et al. (2017) Sci. Adv., 3, eaao2538.

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