| Pacific Rim Symposium on Surfaces, Coatings and Interfaces (PacSurf 2016) | |
| Thin Films | Tuesday Sessions |
| Session TF-TuP |
| Session: | Thin Films Poster Session |
| Presenter: | Yuki Kato, Gakushuin University, Japan |
| Authors: | Y. Kato, Gakushuin University, Japan K. Kawahara, Gakushuin University H. Yamazaki, Gakushuin University K. Yamakawa, Gakushuin University I. Arakawa, Gakushuin University |
| Correspondent: | Click to Email |
In order to achieve ultimate pressure in extreme high vacuum (XHV) region, it is necessary to exhaust hydrogen which is the dominant residual gas in XHV. Cryopumping has been regarded as an effective mean to exhaust hydrogen and widely used in a variety of vacuum systems. Several studies have reported unexpected outcomes in either the static or the dynamic characteristics of H2 cryopumping, which is likely due to incident thermal radiation, the diffusion of H2 in porous adsorbents, or the lack of equilibrium of the H2 distribution on a surface [1,2,3], but its origin has not been clarified yet. The adsorption isotherms measured in the pressure range below 10-8 Pa were far from being sufficient for quantitative analysis. This is mainly due to the difficulty in determining the density of H2 physisorbed on a cold surface. In the present study, we utilized electron stimulated desorption (ESD) and time-of-flight (TOF) techniques to determine the density of H2 physisorbed on a copper surface [4]. The ejection of H+ is mainly caused by the dissociative ionization of the H2 molecule physisorbed. We assume that, in submonolayer range, the ESD yield of H+ ions is proportional to the surface density of physisorbed H2. Using the above methods, we measured the adsorption isotherms of submonolayer H2 at equilibrium pressures between 10-10 and 10-6 Pa in the temperature range of 3.8 - 6.5 K. By monitoring the time development of the H+ ESD yield in the transition state approaching adsorption equilibrium, we also determined the mean residence time of physisorbed H2 at various temperatures. One example of the transition measurements, where the initial H2 coverage is less than 0.001 (the equilibrium pressure as low as 6×10-11 Pa) and the final coverage is 0.3 which is in equilibrium with the pressure of 3×10-8 Pa, gives the mean residence time of 4400 s.
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[4] I. Arakawa, H. Shimizu, T. Kawarabuki, K. Yamakawa and T. Miura, J. Vac. Sci. Technol. A33, 021602 (2015).