The charge survival yield for a cluster anion, I - 2 (CO 2 ) n (n = 0-30) during its collision onto a silicon surface covered with silicon oxide layers of 2 nm in thickness was measured as a function of the number of the CO 2 molecules, n, and the collision energy (per I - 2 ) in the range of 1-80 eV. A monotonic increase in the charge survival yield with n was observed. This means that the efficiency of the charge transfer from the core ion, I - 2 , to the surface is reduced with the increase of n. It was concluded that the CO 2 molecules suppress the charge transfer by behaving as an electrostatic stabilizer of the core ion and as a spacer between the core ion and the surface.