AN EVALUATION OF ATOMIC AND MOLECULAR MIXTURE RULES AND GROUP ADDITIVITY CONCEPTS FOR THE ESTIMATION OF RADIATION ABSORPTION BY LONG-CHAINED, SATURATED-HYDROCARBONS AT VACUUM UV AND SOFT-X-RAY ENERGIES

The feasibility of using atomic and molecular mixture rules as well as group additivity concepts for predicting valence shell photoabsorption oscillator strengths (cross sections) for long-chained alkane molecules has been investigated over a wide energy range from 18 to 220 eV. The predictions are discussed with reference to recently reported experimental measurements (Chem. Phys. 173 (1993) 209) for normal alkanes, C(n)H2n + 2 (n = 1-8). Atomic mixture rules based on either theoretical or experimental atomic oscillator strength sums are found to be unsatisfactory, giving very large errors at most photon energies. A wide range of molecular mixture rules based on linear combinations of measured oscillator strength values for small `component' alkane molecules and molecular hydrogen have also been evaluated. Although good agreement with experiment is obtained with some linear combinations, many others result in substantial errors. Molecular mixture rules constructed using oscillator strengths for larger component alkanes generally give better estimates of the experimentally measured data; however, since no other a priori physical or chemical reasons can be advanced for any particular choice of molecular mixture rule, this procedure is unsatisfactory for general application. In contrast, a group additivity procedure based on oscillator strength estimates for the methylene (CH2) and methyl (CH3) alkane group fragments, derived entirely from the photoabsorption measurements for lower alkanes, provides excellent agreement with the measured oscillator strengths for C8H18 over the entire energy range studied (18-220 eV). The absolute photoabsorption group oscillator strengths derived for the CH2 and CH3 fragments should be applicable to assessing the contributions from saturated hydrocarbon groupings to vacuum UV and soft X-ray absorption in larger chemical and biochemical systems.