Structures, Energetics, and Dynamics of Helium Adsorbed on Isolated Fullerene Ions

Helium adsorbed on $\mathrm{C}_{60}{}^{+}$ and $C_{70}{}^{+}$ exhibits phenomena akin to helium on graphite. Mass spectra suggest that commensurate layers form when all carbon hexagons and pentagons are occupied by one He each, but that the solvation shell does not close until 60 He atoms are adsorbed on $\mathrm{C}_{60}{}^{+}$, or 62 on $\mathrm{C}_{70}{}^{+}$. Molecular dynamics simulations of ${\mathrm{C}}_{60}\mathrm{He}_n{}^{+}$ at 4 K show that the commensurate phase is solid. Helium added to ${\mathrm{C}}_{60}\mathrm{He}_{32}{}^{+}$ will displace some atoms from pentagonal sites, leading to coexistence of a registered layer of immobile atoms interlaced with a nonregistered layer of mobile atoms.

Figure 1

Mass spectrum of helium droplets doped with ${\mathrm{C}}_{60}$.

Figure 2

Ion yields of ${}^{12}\mathrm{C}_{60}\mathrm{He}_n{}^{+}$ and ${}^{12}\mathrm{C}_{70}\mathrm{He}_n{}^{+}$ versus size $n$.

Figure 3

(a) Calculated dissociation energies $D_n$ for loss of He from ${\mathrm{C}}_{60}\mathrm{He}_n{}^{+}$. The inset shows the dissociation energies derived from ion yields. (b) Average site occupancy for ${\mathrm{C}}_{60}\mathrm{He}_n{}^{+}$.

Figure 4

(a) Number density of helium atoms versus distance $R$ from the fullerene center for $n=32,40,45,50,55,60$. The inset shows the density in the second shell for larger $n$. (b) Angular He distribution for ${\mathrm{C}}_{60}\mathrm{He}_{31}{}^{+}$. The white circle marks the unoccupied pentagonal site. (c) Angular distribution for ${\mathrm{C}}_{60}\mathrm{He}_{33}{}^{+}$.