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Cryopumping is one of effective methods to evacuate H$_2$ and has been used in various vacuum systems because of its cleanliness and high pumping speed. For development and utilization of cryopumps, it is indispensable to elucidate the physisorption characteristics of a cryosurface against H$_2$ in extremely high vacuum (XHV), where H$_2$ is the dominant residual gas. Several authors have reported unexpected outcomes in either the static or dynamic characteristics of H$_2$ cryopumping, such as the abnormal temperature dependence of the equilibrium pressure [1,2,3] and the deviation from the Dubinin-Radushkevich-Kaganer (DRK) equation [4,5], but their origins have not been clarified yet.
In this study, we aimed to obtain the adsorption isotherms and the mean residence time of H$_2$ and D$_2$ on a copper surface using the electron stimulated desorption (ESD) technique [6]. Since H$^+$ (D$^+$) ions are mainly generated by dissociative ionization of H$_2$ (D$_2$), the ESD yield of H$^+$ (D$^+$) is assumed to be proportional to the density of H$_2$ (D$_2$) physisorbed on the surface in the submonolayer range. We measured the adsorption isotherms at the equilibrium pressures between 10$^{-10}$ and 10$^{-6}$ Pa in the temperature range of 4.0 - 7.0 K and clearly observed the two-dimensional condensation. Whereas the adsorption isotherms of H$_2$ at pressures below 10$^{-8}$ Pa were affected by H$^+$ originating from surface impurities of hydrocarbons, those of D$_2$ were able to be obtained without their influences in the whole pressure range. We also measured the mean residence time by monitoring the ESD yields of H$_2$ and D$_2$ in transitional states.
[1] C. Benvenuti et al., J. Vac. Sci. Technol. 13, 1172 (1976).
[2] I. Arakawa et al., J. Vac. Sci. Technol. A 4, 293 (1986).
[3] I. Arakawa, J. Vac. Sci. Technol. A 4, 1459 (1986).
[4] E. Wallen, J. Vac. Sci. Technol. A 14, 2916 (1996).
[5] E. Wallen, J. Vac. Sci. Technol. A 15, 265 (1997).
[6] I. Arakawa et al., J. Vac. Sci. Technol. A 33, 021602 (2015).
