Studies of collisions between C6o and K(np) Rydberg atoms with 35 ~ n ~< 125 indicate that C6o can attach very-low-energy electrons to form long-lived C~) ions. The rate constant for negative ion formation is observed to be small, ---10 -8 cm 3 s-i and to be independent of n pointing to s-wave capture. Possible mechanisms for this are discussed, including the formation of image-charge-bound C~o ions.
Experiments undertaken using both crossed beam and flowing afterglow/Langmuir probe (FALP) methods have shown that C6o readily attaches free electrons with energies up to --~ 12 eV [1-3]. The free electron studies, however, suggest that the electron capture rate drops at very small electron energies. This is indicative of a rotational barrier and points to p-wave capture, which is consistent with theoretical arguments that suggest that s-wave capture to C6o is forbidden due to the absence of low-lying L = 0 bound states of C~ [4]. However, C6o formation has recently been observed in collisions between C60 and Cs(np) Rydberg atoms having principal quantum numbers n in the range 14 to 42 [5]. In Rydberg atoms, the excited electron is so far from the nucleus and inner electrons, which together comprise a compact ionic core of unit net positive charge, that it can be considered as an independent particle ~ (see, for example, Ref. [7]). CSo formation can then be viewed as arising from a For a discussion of the independent particle model see, for example. Ref. [6].