Calculated molecular mean excitation energies for some small molecules

Using the random phase approximation, we have calculated the total and directional components of the mean excitation energy for stopping (I₀) and the stopping anisotropy ( ) as well as the first moment (I₁) of the dipole oscillator strength distribution for 20 small molecules and molecular ions containing 6 to 30 electrons. We find that for excitations polarized orthogonal (perpendicular) and parallel to the molecular high symmetry axis I₀^O > I₀^P for all linear molecules, while I₁^O < I₁^P for all molecules other than HF. We note that the Bloch rule holds only for small, compact molecules, and that the Bragg rule should not be used to determine the mean excitation energies of the molecules that we consider here. The stopping anisotropy is positive for all but the C_3v molecules, and is largest for the linear, rod-shaped molecules.