Auger and photoelectron line energy relationships in aluminum–oxygen and silicon–oxygen compounds

Abstract

Silicon–oxygen and aluminum–oxygen compounds exhibit significant XPS Auger and photoelectron chemical shifts that are accurately measurable. Chemical state plots of KLL Auger kinetic energy versus 2p photoelectron energy permit identification of chemical species from the locations of their points on the plots. The KLL Auger electrons of Al and Si were generated by the bremsstrahlung component of the radiation, with conventional instrumentation. The location of points on the plots can be understood on the basis of polarizability of the environment (on the Auger parameter grid of lines, slope +1) and on the basis of the factors contributing to the energy of the final state ion in the Auger transition (a grid of line, slope −1). Tetrahedral aluminum has a significantly smaller Auger parameter than octahedral aluminum, and this difference is repeated, but with reduced magnitude on the similar plots for silicon and oxygen lines for the same compounds. Otherwise, the Auger parameters for this class of compounds are remarkably uniform. The Auger parameter values for oxygen and sodium in these compounds, using the 1s and KLL lines, are relatively small compared to those of other compounds of oxygen and sodium. For compounds of similar Auger parameter, differences in Auger final state ion energy are interpretable on the basis of electron density on aluminum and silicon atoms in the initial state, due to extent of bonding to oxygen, or to amount of negative formal charge on the silicate structure. Inclusion of tetrahedral aluminum enhances the negative charge and decreases the final state ion energy in high alumina zeolites. The difference between the energies of the O1s and Si2p lines in the inorganic silicon compounds is almost invariant, 429.0 to 429.6 eV. The three silicon polymers examined have a significantly larger line difference, 429.8 to 430.1 eV, making possible a differentiation between silicones and silicates. The oxygen KVV lines, with Auger transition final vacancies in valence levels, have shapes characteristic of chemical structure. The uncharged Si–O–Si structure exhibits a well-defined shoulder; in Al–O–Si the shoulder is so close in energy it merely gives rise to asymmetry in the peak; Al–O–Al and charged Si–O–Si give oxygen KVV lines as single sharp peaks.