No abstract

The electronic structures of different phosphates were studied by using the DV-Xa cluster molecular orbital method. Experimental data on crystal structures from x-ray difiaction measurements were used to set up realistic model clusters. Theoretical XPS valence band spectra obtained from the DV-Xa calculations were compared to our high-resolution XPS measurements on polycrystalline powder Li,PO., , Na,PO, , Na,P,O, and (NaPO,), samples. These new measurements made possible a more rigorous test of the theoretical approach and more reliable shape analysis than the previous set of data obtained by non-monochromatized measurements combined with the resolution enhancement technique. Systematic study of different clusters showed that the cations around the PO4 cluster and the distortion of the PO., tetrahedra due to the anisotropic crystal forces play only a minor role and their influence on the valence band spectra was found to he negligible. In the case of polyphosphates, considerable changes were found in the valence band spectra as a consequence of small changes in the bond angle of the P-0-P bridges. Our results, based on cluster parameters specific to the anhydrous crystals, are in good agreement with the experiments.

The local electronic structure around the core-ionized phosphorus and oxygen atoms in solid phase Na, PO,, (NaPO,), and Li,PO, compounds has been studied by XPS and x-ray excited Auger spectroscopy. Core level and valence band photoelectron spectra, as well as the 0 KLL Auger spectra have been measured using monochromated A1 K a excitation, while the high energy P KLV spectra have been excited using Mo-x-rays.From the similarity of the structures of the P KLV spectra it can be seen that the effect of the common tetrahedral symmetry is dominant while the effect due to the change of the cation is much smaller. This observation is supported by comparing the respective valence band photoelectron spectra to each other, and to the results obtained from cluster MO calculations. From the evaluation of the 0 KVV and 0 1s spectra the oxygen Auger parameters have been derived and the 2p2p and 2s2p hole repulsion energies have been determined for each compound. The experimental values of the different Auger parameter shifts (Aa', A F ) place the three oxyanions into the class of oxides, hydroxides and metallic anions instead of the class of nonmetallic anions and indicates the low electronic polarizability (similar to that of SO,) of these phosphorus oxyanions.

The electron energy distribution of the backscattered electrons from manganese and manganese films deposited on a silicon substrate was studied. The Monte Carlo technique was used to simulate the backscattered electron energy distributions and these were compared with the measured reflected electron spectra. A good agreement was found in general between our calculations and the experimental results. In addition, the applicability of Tougaard’s method for the determination of the energy loss function from reflected electron energy loss spectroscopy and simulated backscattered electron spectra of manganese films deposited on a silicon substrate were investigated.

For exciting K-Auger transitions of elements with atomic number between 13 and 26, Mo x-rays (Mo bremsstrahlung + Mo Lm, P, from a high voltage (max. 30 kV) source were used. High-resolution K-Auger spectra were measured by the help of a high luminosity hemispherical electron spectrometer. Spectral intensities obtained using the Mo x-ray source are compared to the respective intensities measured using an Al anode and source parameters usual for electron spectrometers. The attainable gain in Auger yields is between 2 and 15 for most elements investigated and reaches a factor of 20.5 in the case of P KLL. Optimum conditions of K-Auger excitation (regarding the atomic number region mentioned) are also discussed.As an example for applications, the phosphorus K-Auger parameter has been determined for Gap, taking an advantage of the presence of the P Is (Mo La) photoelectron line in the spectra excited by Mo x-rays. The higher analytical sensitivity, attainable by measuring Mo x-ray-induced P KLL Auger electrons compared to the sensitivity of the conventional XPS (A1 Ka excited P 2p), is demonstrated in the case of a phosphoric compound (corrosion inhibitor) adsorbed on a polycrystalline Fe surface.