where q is the charge and V is the voltage. This implies that the kinetic energy depends only on the particle's charge, not its mass. Consequently, primary particles with a given charge have a specific kinetic energy. However, the distribution of this energy among the atoms in the primary particle varies with the number of atoms. For instance, a Bi1 + ion and a Bi5 + ion both have the same total kinetic energy, but the energy per atom in Bi5 + is one-fifth that of Bi1 + . This variation of the energy per atom (depending on the nature of primary ion) influences the ion penetration in the solid and the sputtering process, including the secondary ion yields and fragmentation patterns observed in ToF-SIMS (40) .