Abstract
Helium tagging of charged C-60(q+) ions (q = 1-3) has been used for measuring electronic and infrared gas phase spectra, allowing astronomers to improve the identification of fullerenes in space. Here, we present a detailed study of the attachment of He to cold mass selected fullerene ions.
Experiments were performed in the temperature variable radio frequency (rf) ion trap ISORI at high He densities and a few K. For all three charge states, the ternary rate coefficients for forming He-C-60(q+) are below 10(-31) cm(6)S(-1) and all three show temperature dependences, proportional to exp(-T/T-0) with T-0.< 1.5 K.
While most of our knowledge on the interaction of C-60(+) with He atoms comes from ab initio calculations, from gas phase ion mobility experiments, and from fullerene ions ejected from He-droplets, the sequential addition of He to C-60(q+) in ter- or maybe bimolecular collisions provides complementary information. Our experimental results are tentatively explained with difficulties to form and stabilize the short-lived He-fullerene complexes while relaxation of hot fullerenes in collisions with cold He seems to be rather efficient.
Severe restrictions are due to the fact that thermalized C-60(q+) ions behave like fast rotating rigid spheres. Despite the low efficiency in attaching the first He, measurements reveal a fast subsequent growth as soon as a few He atoms stick to the fullerene.
In the case of triply charged ions, almost all primaries can be quickly converted into the particular structure H-32-C-60(3+). IR excitation of this thermalized complex with one 1329 cm(-1) photon provides interesting insight into the decay kinetics.