Raman Spectroscopic Investigation of $H_2$, HD, and $D_2$ Physisorption on Ropes of Single-Walled, Carbon Nanotubes

We have observed the $S$- and $Q$-branch Raman spectra of ${\mathrm{H}}_2$, HD, and ${\mathrm{D}}_2$ adsorbed at $85\mathrm{K}$ and pressures up to 8 atm on single-walled, carbon nanotubes (SWNT). Comparative data for ${\mathrm{H}}_2$ on graphite and ${\mathrm{C}}_{60}$ were also collected. Frequency-downshifted and upshifted features were observed in the $Q$-branch spectra of ${\mathrm{H}}_2$ on ${\mathrm{C}}_{60}$ and SWNT. These shifts are small and are therefore inconsistent with charge transfer. An ${\mathrm{H}}_2$-surface potential with van der Waals and electrostatic terms was developed and used to estimate the shifts in the frequency of the $Q\left(0\right)$ transition of ${\mathrm{H}}_2$ adsorbed in two types of sites. These calculations corroborate the experimental findings and indicate physisorption in multiple sites of the SWNT ropes.