Molecular-Orbital Studies of the Electronic States of Amorphous Selenium

The electronic energy structure of Se chains and rings are studied by a molecular-orbital approach, i.e., using the hybrid orbitals as bases and using the semiempirical Hamiltonian matrix elements. The choices of the bases and the semiempirical parameters are discussed. The calculated density of states is in excellent agreement with that obtained for amorphous Se by photoemission experiments (Nielsen). The simulated joint density of states resembles the observed ${\epsilon }_2$ spectra (Leiga, Stuke). A new interpretation for the double peaks in the valence-band density of states and in the ${\epsilon }_2$ spectra is given.