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

Abstract

Helium adsorbed on C 60 þ and C 70 þ 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 C 60 þ , or 62 on C 70 þ . Molecular dynamics simulations of C 60 He n þ at 4 K show that the commensurate phase is solid. Helium added to C 60 He 32 þ 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.

Adsorption of helium on isolated C60and C70anions

“…These 'magic numbers' have been previously observed by our group in the studies of positively charged fullerene-helium complexes [11]; those data are included in Figure 2 as open dots. For n < 60, the size dependences of anions and cations are quite similar although there is more scatter in the anion data, probably due to poorer statistics.…”

“…In a series of experiments we have recently studied the adsorption of helium [11] and several molecular gases [12] on isolated C + 60 and C + 70 cations. Mass spectra indicate a strong energetic preference for formation of the 1 × 1 phase in which the 12 pentagons plus 20 hexagons of C 60 (25 hexagons for C 70 ) are decorated with one particle each.…”

“…Two other studies were devoted to cationic He n C + 60 . Leidlmair et al reported shell closure at n = 74 based on classical molecular dynamics; the value decreased to about 58 if quantum effects were included [11]. Calvo applied path-integral simulations and concluded that the first layer accommodates about 72 atoms [14].…”

“…On the other hand, the commensurate 1 × 1 layer of helium (or hydrogen) on C 60 is underdense. Additional helium atoms can be accommodated although they displace atoms from hollow sites resulting in a layer that is either partially [11,13] or completely [14] liquid. Upon completion, the first layer becomes solid but it remains incommensurate [13,14].…”

The adsorption of helium atoms on coronene cations

“…Studies of the decoration of both C60 + and C60 -by helium have been reported, as well as the equivalent ions for C70. 9,10 In these experiments the fullerene molecules were first isolated in liquid helium nanodroplets and then subjected to electron bombardment, either at low energy (for anions) or higher energy (for cations). The mass spectra show enhanced abundances for ions with extra stabilities which are often referred to as magic number ions.…”