Excess Adsorption of Helium in Extremely Narrow Slit Pores

Wave functions of quantum helium in narrow slit pores are strongly restricted; as such, quantum helium condensed in narrow slit pores displays different behaviors from that in bulk. Herein, we report the densities of helium adsorbed on carbon surfaces and in carbon slit pores with average pore widths of 0.7, 0.9, and 1.1 nm at 2-5 K. The density of adsorbed quantum helium in the 0.7-nm slit pores was significantly higher than those in the larger slit pores and bulk. The average layer density of helium in the 0.7-nm pores was also significantly higher than those in the larger slit pores, suggesting solid-like structure formation even under helium vapor condition. The highly dense state of helium in narrow slit pores is due to strong attractive potential effects in such slit pores.