![]() For Cs n +, similar distributions were deduced from droplets co-doped with C 60 but the yield of Cs n – was very weak in the presence of C 60. Data were extracted from mass spectra recorded without C 60 doping. The temperatures of the ovens were varied in order to obtain the optimal experimental conditions for formation of complexes containing several C 60 molecules and up to ∼10 cesium atoms.ĭistributions of neat cesium cluster ions Cs n ± are presented in Figure 3a. C 60 (MER, purity 99.9%) was vaporized in a resistively heated oven in the first chamber metallic cesium (Sigma-Aldrich, purity 99.95%) was vaporized in the second chamber. (18) The resulting supersonic beam was skimmed by a 0.8 mm conical skimmer and traversed a 20 cm long pickup region consisting of two differentially pumped pickup chambers. At these temperatures helium nanodroplets contain an average number of 4 × 10 5 and 1.2 × 10 6 helium atoms, respectively. For the measurements of neat cesium anions the cryostat temperature was lowered to 8.8 K. We also report the dependence of the abundance of (C 60) mCs 3 – anions on the electron energy which shows an intriguing alternating pattern as m increases.įor the C 60Cs experiment neutral helium nanodroplets were produced by expanding helium (Messer, purity 99.9999%) at a stagnation pressure of 20 bar through a 5 μm nozzle, cooled by a closed-cycle refrigerator to 9.3 K, into a vacuum chamber (base pressure about 2 × 10 –6 Pa). (16, 17) However, in the literature we find no support for the proposed presence of stable alkali trimer and pentamer units in neutral or charged (C 60) mA n. We observe, indeed, corresponding anomalies at n = 3 and 5 in abundance distributions of neat Cs n + and Cs n –, consistent with theoretical predictions that these ions are particularly stable. The fact that these anomalies are independent of m and the charge state might suggest that they are due to particularly stable cesium cluster ions that are favored irrespective of the number of fullerenes. Another, weaker anomaly appears at (C 60) mCs 5 ±. A surprising finding is a pronounced maximum in the abundance of (C 60) mCs 3 ± cations and anions for all values of m except for C 60Cs n – whose abundance declines very rapidly with increasing n. Furthermore, we investigate cations as well as anions. In the present work we have doped helium nanodroplets with much smaller amounts of cesium than in our previous experiments, resulting in ions containing as many as 10 C 60 and between 1 and about 12 Cs atoms. (4) Indeed, the highest transition temperature of any alkali-doped fulleride has been reported for Cs 3C 60 which is an insulator at ambient pressure but becomes superconducting below T c = 38 K at elevated pressure without changing its body-centered cubic structure. ![]() The superconducting transition temperature in binary A 1B 2C 60 fulleride salts (A, B = alkali metal) was found to correlate with the lattice parameter. ![]() (3, 4) Superconductivity, however, is restricted to the A 3C 60 fulleride salts in which the alkalis transfer their valence electrons to the lowest unoccupied triply degenerate molecular orbital of t 1u symmetry, resulting in a half-filled conduction band of C 60. (2) Various crystal structures form depending on the dopant concentration which may be as large as 12:1 in Li 12C 60. Early experiments on fullerides doped with alkali (A) metals revealed metal–insulator transitions (1) and the appearance of superconductivity in potassium-doped fullerides below T c = 18 K. Since the successful development of methods to synthesize fullerenes in macroscopic quantities, the properties of metal-doped fullerene solids and their potential applications have been studied. ![]()
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