[en] Time-of-flight spectrometry has been used for measurements of Cs⁺ ions emitted from cesium iodide targets by 600 keV hydrogen cluster ions Hn⁺, with n ranging from 5,7,9,... to 23. Evidence for an enhanced yield with respect to the sum of the individual atoms in a cluster is reported. The increase of the yield as a function of the projectile mass exhibits a fourth power dependence on the additive stopping power. Similar trends are found from preliminary results on the variation of the yield as a function of the cluster energy. Such non-linearity is discussed on the basis of existing models for monoatomic ions. (author)