Dissociation mechanisms of cluster ions resolved using ab-initio molecular orbital calculations

We have examined the cluster-size effect on fragments in the collision-induced dissociation (CID) of small boron clusters ( B n + , n ≦ 8) by means of ab-initio molecular orbital (MO) calculation, for conditions representative of low-Z cationic clusters in plasma. Single cations B+ (leaving B n - 1 0 ) are predominantly fragmented from small clusters B n + (n < 5) by low-energy collisions (<10 eV) with Xe atoms. With increasing the size n, beyond n = 5, B n - 1 + (leaving B0) clusters are observed preferentially. This alternating trend in experiment has not been quantitatively explained by previous ab-initio MO calculations. We used a density functional method called B3LYP and basis functions 6-31+G∗∗ including diffuse functions, and have succeeded in reproducing the alternation of the competing two dissociation processes as cluster size increases. In addition, the accuracy of ionization potentials and binding energies were also improved.