Absolute photoabsorption oscillator strengths by electron energy loss methods: the valence and S 2p and 2s inner shells of sulphur dioxide in the discrete and continuum regions (3.5-260 eV)

Absolute photoabsorption oscillator strengths (cross-sections) for the vale nce shell discrete and continuum regions of sulphur dioxide from 3.5 to 51 eV have been measured using high resolution (similar to 0.05 eV FWHM) dipol e (e,e) spectroscopy. A wide-range spectrum, covering both the valence shel l and the S 2p and 2s inner shells, has also been obtained from 5 to 260 eV at low resolution (similar to 1 eV FWHM), and this has been used to determ ine the absolute oscillator strength scale using valence shell TRK (i.e., S (0)) sum-rule normalization. The present measurements have been undertaken in order to investigate the recently discovered significant quantitative er rors in our previously published low resolution dipole (e,e) work on sulphu r dioxide (Cooper et al., Chem. Phys. 150 (1991) 237; 150 (1991) 251). Thes e earlier measurements were also in poor agreement with other previously pu blished direct photoabsorption measurements. We now report new absolute pho toabsorption oscillator strengths using both high and low resolution dipole (e,e) spectroscopies. These new measurements cover a wider energy range an d are much more consistent with the previously published direct photoabsorp tion measurements, The accuracy of our new measurements is confirmed by an S(-2) dipole sum-rule analysis which gives a static dipole polarizability f or sulphur dioxide in excellent agreement (within 3.5%) with previously rep orted polarizability values. Other dipole sums S(u) (u = - 1, - 3 to - 6, - 8, - 10) and logarithmic dipole sums L(u) (u = - 1 to - 6) are also determ ined from the presently reported absolute oscillator strength distributions . (C) 1999 Elsevier Science B.V. All rights reserved.