Transient absorption spectra of ion radicals were observed for tetrasilanes with rigid syn or anti Si units by electron beam pulse radiolysis. Two transient absorption bands at 4.1 and 2.8 eV were clearly resolved for the anion radical of syn-tetrasilane.. while for anti-tetrasilane, the anion radical produced a broad absorption band at 4.4 eV. Cation radicals of the tetrasilanes gave similar optical transitions at 3.6 and 2.1 eV for the syn form, and 3.8 eV for the anti form. The spectra for the ion radicals of anti-tetrasilane are identical to those of ion radicals of permethylated pentasilanes or hexasilanes, suggesting that the ion radicals of oligosilane without configurational constraint take an anti conformation. The unique spectra of ion radicals of syn-tetrasilane reflect the large gap between the second and third HOMO (LUMO) of neutral molecules. which is also induced by the configurational constraint. (C) 2003 Elsevier B.V. All rights reserved.