Absolute scale determination for photoabsorption spectra and the calculation of molecular properties using dipole sum rules

An assessment of the absolute scales of photoabsorption differential oscillator strength (df/dE) spectra which were originally normalized using valence shell TRK (i.e. S(0)) sum-rule normalization is presented for a series of dipole (e,e) measurements for 5 noble gases and 52 small molecules. This comprehensive data set involves previously published absolute high resolution dipole (e,e) spectra of the valence shell discrete region combined with very wide range low resolution measurements in the ionization continuum for each atom or molecule. The absolute scales established for the dipole (e,e) spectra using the S(0) sum-rule in the originally published works (data available on the world wide web at ftp://chem.ubc.ca/pub/cooper or by anonymous ftp - see end of present paper for more details) are assessed by deriving static dipole polarizabilities for each atom and molecule using the S(-2) sum-rule. These values are found to be highly consistent with experimental and theoretical literature values of the static dipole polarizability and in almost all cases well within the estimated +/-5% accuracy of the originally published TRK sum-rule normalized absolute photoabsorption differential oscillator strength spectra. Significant errors of 8% and 19% in the previously published absolute oscillator strength scales for CCl4 (G.R. Burton, W.F. Chan, G. Cooper, C.E. Brion, Chem. Phys. 181 (1994) 147) and SiF4 (X. Guo, G. Cooper, W.F. Chan, G.R. Burton, C.E. Brion, Chem. Phys. 161 (1992) 453, 471) respectively are revealed by the dipole polarizability analysis and appropriate corrections are recommended. Alternative methods of absolute scale determination for photoabsorption spectra using static or dynamic dipole polarizabilities from refractive index or dielectric constant measurements are also considered. These are found to be particularly useful especially where data are restricted to lower photon energies (< 60 eV) or where phenomena such as low lying inner shells: continuum shape resonances or Cooper minima preclude the use of the valence shell TRK sum-rule. As a result of the assessment of the absolute scales for the published dipole (e,e) spectra and since polarizabilities and refractive index data can typically be obtained with even higher precision( +/- 1%), these data have been used to further refine the measured differential oscillator strength scales. On this basis, dipole oscillator strength sums S(u) (u = -1, -2, -3, -4, -5, -6, -8, -10) and L(u) (u = -1, -2, -3, -4, -5, -6) are obtained from the df/dE spectra. For many of the systems considered, the presently reported dipole sums and derived molecular properties are more accurate than previously reported values. In fact, for 28 of the 52 small molecules these sum-rule values represent the only available values. The dipole sums can be used to calculate normal Verdet constants which are involved in the Faraday effect. It is also shown that accurate values of the rotationally averaged C-6(A,B) dispersion coefficients for the long-range interaction of all possible pairs of atomic and/or molecular species can be obtained from the measured differential oscillator strength spectra. Alternatively, an approximation to C-6(A,B), requiring only the S(-2) and L(-2) sums, is shown to provide very reliable estimates of C-6(A,B). (C) 1997 Elsevier Science B.V.