Formation of nitrogen-substituted carbon cluster anions by gas-feed Cs-sputtering from different forms of carbon

We have investigated the mechanism of formation of C-n(-) and CnN- cluster anions during Cs+-ion sputtering from surfaces of different crystallographic forms of carbon (polycrystalline graphite, amorphous C and C-60), in the presence of high purity N-2 or NO2 gas. The normalised yields of sputter-ejected CnN- (n less than or equal to 10) cluster anions display an oscillatory dependence on n: the odd n clusters being more abundant than the even n ones. Significantly, CnN- clusters produced from the three different forms of C, show similar oscillatory variations with n. Moreover, the CnN- yields follow a single power law decline as a function of cluster size for all three surfaces sputtered, and over a wide range of operating conditions. This strongly indicates that the intrinsic structure of the ion-induced surface does not play a significant role in the formation of sputtered cluster anions. The observed abundance patterns of C-n(-) and CnN- clusters also suggest that mono-nitrogen substitution of the carbon clusters does not allow the chain to ring transformation to occur up to n = 10.