The mean excitation energy, usually expressed by symbol I, is the only nontrivial property of matter in the Bethe asymptotic stopping-power formula. It is therefore an important input for the evaluation of stopping power for swift charged particles, together with various corrections [1]. The I value can be obtained by three methods: 1. Fitting of measured stopping-power data to the Bethe formula with correction terms. 2. Calculations from oscillator-strength spectra, which are empirically determined. 3. Ab initio theoretical calculations, which are feasible only for exceptional cases. We report on a substantial step forward in applications of the second method.
　　Berkowitz [2] surveyed extensive data on oscillator-strength spectra of gaseous atoms and molecules, critically examined them with the use of sum rules and other general constraints, and determined a recommended set of data for each of 32 species.
　　We calculated I values for all the 32 species treated by Berkowitz [2]. Our results are generally consistent with experimental and other theoretical values in the literature. Contributions of each shell or subshell to the I value indicate useful insights into the quantity. Our results also reveal useful systematics of the I values for different species. The mean excitation energy per electron I/Z, where Z is the total number of electrons, reflects subtleties of electronic structure. An example concerns trends in an allotropic series such as (H, H₂), (N, N₂), and (O, O₂, O₃). The I value per carbon atom is 72 eV for the C₆₀ molecule in gas according to the present work, while it is about 80 eV for graphite and 62 eV for the free atom according to the literature.

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