Chemical bonding effects in the oxygen Kα x-ray emission bands of silica

Abstract

Self-consistent-field (SCF) atomic and molecular orbitals and energy levels for O, SiO, and collinear Si2O are used to interpret the oxygen Kα soft-x-ray emission spectrum of silica. The O and Si2O orbitals are calculated in both the frozen-orbital (FO) and relaxed-orbital (RO) approximations. The results are used to calculate the emission-line energies and oscillator strengths for the oxygen Kα spectrum of each species. The calculated separation between the π and σ emission lines of collinear Si2O in both the RO approximation (6.9 eV) and the FO approximation (6 eV) are somewhat larger than the observed separation between the principal emission peak and low-energy satellite in silica (5 eV), but it is shown that the bending of the Si-O-Si bond in silica will reduce the separation. The calculated position of the principal line for Si2O (525.7 eV in the RO approximation) is in good agreement with the principal peak observed in silica (526 eV). The 3p and 3s atomic orbitals of silicon are found to be strongly modified from their free-atom shape in both SiO and Si2O; enough to make an order-of-magnitude difference between oscillator strengths calculated in the SCF and linear-combination-of-atomic-orbitals approximations. The modifications of the oxygen valence orbitals are relatively small. The oxygen s p hybridization is quite small. The atomic population of the 5σ(SiO) level is 88% 2s(O), 2% 2p(O), 4% 3p(Si) and 5% 3s(Si). The d hybridization is also small: a maximum of 5% 3d(Si) in the 2πu(Si2O) level and 1% 3d(O) in the 2πg(Si2O) level. Crossover transitions in a literal sense are negligible. The 2s(O) line in the silicon Kβ spectrum of SiO, for example, [the 5σ(SiO)→1σ(SiO) transition], is almost entirely a vertical transition which comes from the small 3p(Si) admixture. An atomic population analysis gives ionicities of Si+0.78O-0.78 and Si2+0.37 O-0.74. © 1973 The American Physical Society.