CALCULATION OF ELECTRONIC-STRUCTURE AND PHOTOABSORPTION SPECTRA OF MONOSILANE MOLECULES SIH4, SIF4, AND SICL4

The electronic structure, ionization potentials, and photoabsorption spectra of monosilane molecules SiH4, SiF4, and SiCl4 were calculated using the discrete variational (DV) X-alpha method. Valence molecular orbitals (MOs) of SiH4 consist (from the lowest) of two occupied bonding MOs between Si and H, a1 and t2. Inner valence MOs of SiF4 and SiCl4 consist of the bonding MOs between Si and halogen, a1 and t2, and outer valence MOs consist of bonding MOs a1 and t2, and the MOs e, t2, and t1 localized on halogen. The lowest unoccupied MOs of SiH4 include two antibonding states t2 and a1, and two localized states, e and t2. The lowest unoccupied MOs of SiF4 and SiCl4 are antibonding states a1 and t2 between Si and halogen. Calculated ionization potentials agree well with measured photoelectron spectra. Calculation of the photoabsorption spectrum for Si 2p core excitation for SiH4, SiF4, and SiCl4 shows that peak positions and intensities agree well with measured photoabsorption spectra in both gas and solid phases. The absorption bands of SiH4, measured near the edge and at about 125 eV, consist of transitions from core to antibonding states consistent with experiments. The four main absorption bands of SiF4 and SiCl4 measured between 105 and 140 eV are assigned to transitions from the core Si 2p level to antibonding MOs a1, t2, e, and t2. Calculated photoabsorption spectrum for valence excitation of SiH4 agrees well with measurements. The first and second absorption bands measured at about 138 and 128 nm correspond to the transition from bonding to antibonding states between Si and H. This is consistent with the facts that monosilane is photolyzed in Xe 147 nm ultraviolet light in a vacuum and that fluorescence has not been observed from 130 to 150 nm, because these are interpreted in terms of photodissociation by transition from bonding to antibonding states. Photoabsorption spectra for valence excitation of SiF4 and SiCl4 are also calculated. We found that the first absorption band consists of two transitions from localized states on halogen to antibonding states between Si and halogen (t2 --> a1 and t1 --> t2).