In situoptical-reflectance study ofH2physisorption inC24K

An in situ study of the optical reflectance of a second-stage graphite-potassium compound undergoing adsorption of hydrogen gas at T=77 K is presented. From the small shift in the position of the Drude edge in the reflectance spectrum due to the hydrogen uptake we find the resulting electron backdonation ($f_B$) to be less than 0.020 electron per K atom, or 0.0065 electron per H atom. This low value of $f_B$ indicates that the T=77 K adsorption of hydrogen into the stage-2 graphite-K compound is consistent with the traditional view of physisorption of the molecular species (${\mathrm{H}}_2$) in which the adsorbed molecules are attracted to the host’s lattice via a van der Waals interaction.