Abstract. The first observation of capture of He atoms by negatively charged Buckminsterfullerene (Coo) is reported. The collision energy dependence of the production of the endohedral complex is presented in the region of the energetic threshold and compared directly with results obtained using positively charged Coo. The threshold for capture by negative ions is shifted by ~ 1.5 eV to higher energies. This shift can be accounted for by considering the different internal energies of the positive and negative ions. 36.40. +d; 34.90. +q One of the many interesting properties of Coo is its ability to form endohedral compounds in collisions with atoms or ions. This phenomenon was first reported by Weiske et al. [1] and has since been looked at by a number of groups, e.g. [2][3][4][5]. The threshold energy for the capture process in collisions between positively charged Coo and neutral He and Ne atoms has been reported by Campbell et al. [4]. Molecular dynamics calculations using empirical forces [2,6] give good agreement with the experimentally determined threshold energies as do estimates obtained by using ab initio molecular orbital calculations for the barrier for penetration of a benzene ring by the rare gas atom [7]. Interestingly, very different threshold energies are obtained for the collision systems C + ÷ Ne (9 __ 1 eV [4]) and Ne + ÷ C6o ( > 20 eV [5]). This difference may be partly due to differences in the internal energy of the Coo used in the two experiments. The positively charged Buckminsterfullerene was produced by UV laser desorption [8] and could have internal energies > 40 eV as estimated by considering the production of metastable fragments from the hot C + on passage through the time-of-flight mass spectrometer or the observation of delayed ionisation [9]. The neutral Coo, on the other hand, was produced by heating to 350 °C [5] and thus had an internal energy of about 3.5 eV estimated from the known vibrational frequencies [ 10]. Recent molecular dynamics simulations [6] have shown, however, that the temperature effect can only account for a difference of 1.5 to 2 eV in the threshold energy so that the observations are probably mainly due to the increased polarisation effects expected for the Ne + + Coo case. PACS:In this rapid note we report, for the first time, capture of He atoms by negatively charged C60 and observe a shift in the threshold energy of approximately 1.5 eV to higher energies compared with positively Charged C6o in excellent agreement with the estimated temperature effects mentioned above.We have investigated the collision energy dependence of the formation of He@C6~ and He@C~0 using a new tandem reflectron time-of-flight mass spectrometer with improved mass resolution compared with previous measurements from our group [5]. The ion source is a Coo oven heated to 500 °C with a 2 mm diameter nozzle. Positive ions are produced by electron bombardment from a heated tungsten filament at electron energies variable between 20 and 350 eV. For the results presented here an accelera...