Mechanism of wedge effect in splitting of chemical bond by impact of X-2(-)(CO2)(n) onto silicon surface (X = Br, I)

Dissociation of Br-2(-) (into Br and Br-) in a cluster anion, Br-2(-)(CO2)( n), by impact of Br-2(-)(CO2)(n) on a silicon surface was investigated as a function of the number of CO2 molecules, n, at a collision energy per Br-2 (-) of 30-50 eV. The branching fraction of the Br-2(-) dissociation used as a propensity of the Br-2(-) dissociation rate showed a marked n-dependence similar to that observed in the collisional dissociation of I-2(-)(CO2)(n) on a silicon surface. The result is explained in terms of wedge effect in which a CO2 molecule at a midpoint of Br-2(-) splits the Br-2(-) bond as a wedge vs. cage effect in which the bond splitting of Br-2(-) in the CO2 sol vent cage is suppressed. The agreement of the n-dependence between the Br-2 (-)(CO2)(n) and I-2(-)(CO2)(n) collisions lends a further support for the v alidity of the bond splitting by the wedge action. A molecular dynamics sim ulation reproduces the wedge effect in the Br-2(-)(CO2)(n) collision, as we ll. The cage effect appearing in the vicinity of n=12 corresponds to the co mpletion of the first CO2 solvent shell as is the case of the I-2(-)(CO2)(n ) collision. (C) 1999 American Institute of Physics. [S0021-9606(99)01115-0 ].