Rayleigh scattering by atomic ions of low nuclear charge

Rayleigh scattering amplitudes and elastic photon-atom scattering cross sections have been calculated for ${\mathrm{C}}^0$, ${\mathrm{C}}^{1+}$, ${\mathrm{C}}^{2+}$, ${\mathrm{C}}^{4+}$, ${\mathrm{Ne}}^0$, ${\mathrm{Ne}}^{1+}$, ${\mathrm{Ne}}^{2+}$, ${\mathrm{Ne}}^{4+}$, ${\mathrm{Ne}}^{6+}$, ${\mathrm{Ne}}^{8+}$, and ${\mathrm{Ne}}^{9+}$ atoms and ions in their ground state. Some data were also obtained for oxygen. X-ray energies considered are in the regions above and below the threshold energy for K-shell photoionization. Except in the immediate neighborhood of this threshold energy, the primary effect of ionization on the atomic cross section is to reduce the number of electrons from which scattering occurs. The resonance region near threshold widens with increasing ionization, as with vacancies in deeper-lying states transitions to intermediate states further below threshold are allowed. Above the threshold energy, K-shell amplitudes in an isonuclear sequence, except for hydrogenlike-ion cases, share a common curve differing only in the position of the threshold. Further, when the photon energy is expressed in units of the K-shell threshold energy, both carbon and neon imaginary amplitudes share a common curve. Well above the threshold, the real part of amplitudes approach form-factor predictions.