Excitation of the Mg andMg+resonance lines by electron impact on Mg atoms

We have measured the optical excitation functions and polarizations of the Mg resonance line (2852 Å) and of the ${\mathrm{Mg}}^{+}$ resonance lines (2796 Å, 2803 Å, unresolved) arising from ionizing excitation of magnesium atoms, for electron-impact energies from the excitation thresholds to 1400 eV. In our crossed-beam apparatus, the electron-beam energy resolution was ≈ 0.25 eV for energies below 10 eV, and the atom beam was optically thin. The excitation function of the ionic lines was normalized to that of the atomic line by relative intensity measurements. The 2852-Å excitation function, when normalized to Born theory in the high-energy limit, has a maximum cross section of $17.3\pi a_0^2\pm 3\%$ at 18.5 eV. This excitation function is graphically compared with the resonance-line excitation functions of Na, Ca, and Li previously measured in this laboratory. The 2852-Å polarization function is consistent with the theoretical threshold limit of + 100%, and has a sharp feature near 5 eV which cannot be due to cascading.