The kinetic diameters of helium adsorbate, hydrogen adsorbate and nitrogen adsorbate correspond to 0.2551, 0.2827–0.289 and 0.364–0.380 nm respectively. The interlayer spacing of graphite is typically 0.334 nm. I understand that nitrogen can therefore not enter a space which does not exceed its kinetic diameter, but do not understand why helium and hydrogen are not able to enter the spacings between graphitic layers?