Collision of hyperthermal Cs+ ion beams with a Si(111) surface partially covered with water gives rise to emission of CsX(+) cluster ions (X is a surface-atom or molecule) even when the monomer X(+) ions are not produced. The yield for atomic and cluster ion emission is examined as a function of Cs+ collision energy, based on which, we propose that CsX(+) species are formed by recombination of the scattered, low kinetic energy Cs+ ions and the gaseous neutral species emanating from a surface. It is also demonstrated that under this condition the secondary neutral flux contains a large fraction of molecular units.