Sodium doped silicon clusters (SinNam; 1 less than or equal to n less than or equal to 14, 1 less than or equal to m less than or equal to 5) produced by two types of laser vaporization were studied. The adsorption of Na atoms on the Si-n clusters leads the substantial lowering of the ionization energy, E-i, of SinNam clusters. Their reactivity toward NO molecules was measured with a fast flow reactor, sind the anticorrelation between the E(i)s and the reactivity was clearly observed: species having low E-i exhibit high reactivity and vice versa. Moreover, the clear parallelism between the E(i)s of SinNa and the EAs of Si-n is found. This is consistent with the fact that the structure of SinNa clusters keeps the frame of the corresponding Si-n cluster unchanged and that the electronic structure of SinNa is similar to that of the corresponding negative ion Si-n(-). In addition to the experimental studies, the geometries, adsorption energies, and vertical ionization energies of SinNa (n=1-7) were investigated with ab initio MO calculations including electron correlation; The Moller-Plesset perturbation theory was used and the configuration interaction (CI) calculation was carried out, particularly for a diatomic molecule, SiNa. (C) 1997 American Institute of Physics.