Instituto de Investigación en Comunicación Óptica,

(Recibido: 26 de julio de 2005; Aceptado: 8 de diciembre de 2005) The availability of isotopically pure semiconductors in the last fifteen years has triggered their scientific and technological interest. The effects of the electron-phonon interaction on the band structure can be experimentally investigated by measuring the temperature or the isotopic composition dependence of energy gaps. In this article, we discuss the effects of isotopic composition on the dielectric function of silicon by using spectroscopic ellipsometry in the energy range from 3.1 to 3.7 eV. The silicon crystals investigated are the isotopically pure 28Si and 30Si, and the natural Si (natSi, Mnat= 28.14 amu). At low temperatures, the energies of the interband transitions become mass-dependent through the dependence of the electron-phonon interaction and the lattice parameter on the average isotopic mass. We determine the mass dependence of critical point energies and other optical parameters as accurately as possible by analyzing the ellipsometric data in reciprocal (Fourier-inverse) rather than direct (frequency) space