THEORETICAL-STUDIES OF THE ELECTRONIC AND OPTICAL-PROPERTIES OF PERMETHYL-OLIGOSILANES

A systematic study was made on the optical absorption spectra of perme thyl oligosilanes, CH3[CH3-Si-CH3]NCH3, as a function of their backbon e chain length from N = 2 to 16 by using first principles [density fun ctional with local density approximation (LDA)] electronic structure c alculations. The general features of the calculated spectra agree with those of experimental measurements. Analysis of the nature of optical transition revealed the existence of two qualitative crossovers at ch ain lengths of N = 2 and around N = 10 and 12. The first one is associ ated with the molecular orbitals giving rise to the first ultraviolet absorption peak, while the second one is associated with the crossover from ''molecular'' to ''polymeric'' as the oligomers become larger. T he broadening of the first absorption peaks is estimated by the vibrat ion modes and electronic relaxation energies in the oligomers. Compari son with calculated results by a model Hamiltonian based on the Sandor fy C model is made and the relative merit of semiempirical and LDA cal culations is discussed. (C) 1996 American Institute of Physics.