Adsorption of sodium and cesium on aggregates of C60

Abstract

Abstract.We explore the formation of C60 sodium and C60 cesium complexes in superfluid helium nanodroplets. Anomalies in mass spectra of these doped droplets reveal anomalies in the stability of ions. (C60)mCs + n ions (m 6) are particularly abundant if they contain n = 6m + 1 cesium atoms; (C60)mCs 2+ n dications (m 3 or 5) are abundant if n = 6m + 2. These findings are consistent with the notion that alkali metal atoms (A) transfer their valence electrons into the three-fold degenerate lowest unoccupied orbital of C60, resulting in particularly stable C60A6 building blocks. However, (C60)4Cs 2+ n dications display an entirely different pattern; instead of an expected anomaly at n = 6 × 4 + 2 = 26 we observe a strong odd-even alternation starting at n = 6. Also surprising is the effect of adding one H2O or CO2 molecule to (C60)mCsn mono-or dications; anomalies shift by two units as if the impurity were acting as an acceptor for two valence electrons from the alkali metal atoms.

Stable structures of exohedrally decorated C60-fullerenes

“…In these early works, they found for lithium certain "magic numbers" which can only arise from an homogeneous distribution of twelve Li atoms on the top of the pentagonal faces of fullerene. Some other elements were found to form clusters above a certain concentration, or to form other non-homogeneous distributions on the surface [18,19,20,21,22,23]. This behavior is detrimental for hydrogen adsorption, because the clustering of the metal atoms on the surface of C 60 reduces the active adsorption sites for H 2 [24].…”

Positively and Negatively Charged Cesium and (C60)mCsn Cluster Ions

“…We did not notice these features in our previous report on heavily coated C 60 cluster cations, but close inspection of Figure 1 in that report reveals a steep rise in the ion abundance of (C 60 ) m Cs n + from n = 4 to 5 followed by a plateau, for m = 2, 3, and 4. 8 The abundance of ions with n ≤ 4 was simply too small to discern any anomalies.…”

“…In a recent report we had measured abundance distributions of (C 60 ) m Cs n + cations containing up to six C 60 and several dozen Cs. 8 Local maxima in the ion abundance suggested that ions with n = 6 m + 1 are particularly stable. 22 The findings were consistent with earlier research by Martin and co-workers on potassium- and rubidium-doped fullerene aggregates; the authors suggested the presence of C 60 A 6 building blocks in which the alkali metal atoms transfer their valence electrons to the 3-fold degenerate lowest unoccupied orbital (t 1u ) of the fullerene, forming essentially ionic bonds.…”

“… 13 , 15 , 23 , 24 Further support for the model comes from a shift of the anomalies to n = 6 m + 2 for (C 60 ) m Cs n 2+ dications and to n = 6 m + 3 for X(C 60 ) m Cs n + where X = H 2 O or CO 2 . 8 …”

“…In a recent report we presented ion abundances of (C 60 ) m Cs n + cations prepared in the gas phase by electron ionization of helium nanodroplets doped with C 60 and Cs. 8 A key finding was that (C 60 ) m Cs n + ions ( m ≤ 6) are particularly abundant if they contain n = 6 m + 1 cesium atoms; (C 60 ) m Cs n 2+ dications were abundant (with the exception of (C 60 ) 4 Cs n 2+ ) if n = 6 m + 2. By and large these results were consistent with earlier photoionization experiments by Martin and co-workers involving potassium and rubidium which indicated that up to six alkali metal atoms per C 60 transfer their valence electrons into the t 1u orbital, resulting in particularly stable C 60 A 6 building blocks.…”