Mean excitation energies for the stopping power of atoms and molecules evaluated from oscillator-strength spectra

The mean excitation energy for the stopping power of matter, usually expressed by symbol I, is the only nontrivial material property in Bethe's [Ann. Phys. 5, 325 (1930)] asymptotic stopping-power formula. It is therefore a crucial input for the evaluation of stopping power for swift charged particles. To calculate the I value of a material from its definition, it is necessary to know the oscillator-strength spectrum of the material in question over the entire range of the excitation energy. We evaluate the mean excitation energies of 32 atoms and molecules from the oscillator-strength spectra that were published by Berkowitz in 2002 [Atomic and Molecular Photoabsorption: Absolute Total Cross Sections (Academic, San Diego, 2002)]. We find that most of the present I values are consistent with those given in the literature. The I values of NO2, O-3, and C-60 in particular are evaluated in the present work. For buckminsterfullerene C-60, an estimation of the I value is made also using the local-plasma approximation, in an attempt at understanding differences in the I values of carbon in a free atom, a C-60 molecule, and graphite. Systematic trends of I values obtained in the present calculation are discussed, in the context of the Thomas-Fermi model. (c) 2006 American Institute of Physics.