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 (I- 0) and the stopping anisotropy (A) as well as the first moment (I-1) o f the dipole oscillator strength distribution for 20 small molecules a nd molecular ions containing 6 to 30 electrons. We find that for excit ations polarized orthogonal (perpendicular) and parallel to the molecu lar high symmetry axis I-0(O) > I-0(P) for all linear molecules, while I-1(O) < l(0)(P) for all molecules other than HF. We note that the Bl och rule holds only for small, compact molecules, and that the Bragg r ule should not be used to determine the mean excitation energies of th e molecules that we consider here. The stopping anisotropy is positive for all but the C-3 nu molecules, and is largest for the linear, rod- shaped molecules.