Experimental and Theoretical Studies of Electron-Impact Excitation of Neon

The electron excitation functions of some fifty states from the $2p^5\mathrm{ns}$, $2p^5\mathrm{np}$, and $2p^5\mathrm{nd}$ configurations have been measured by the optical method. Corrections due to cascade from the upper levels are important only for the $\mathrm{np}$ states and typically amount to 50% of the total population. The scheme of characterizing the excitation behaviors of helium by the quantum numbers $L$ and $S$ of the states can be carried over to neon if one expresses the wave functions of the excited states of neon in terms of the $\mathrm{LS}$ eigenfunctions. For example, all the optically allowed states exhibit very broad excitation functions and large cross sections, and the purely triplet states have narrow excitation functions. For the other states, the wave functions are expressible as linear combinations of those of triplet states and dipole-forbidden singlet states; hence, the excitation functions have intermediate widths. From group-theoretical arguments, we have shown that within a configuration $2p^5\mathrm{nl}$, the states with odd values of $J+l$ have larger excitation cross sections than the ones with even values. This theoretical rule has been well confirmed by our experimental data. In addition, we have deduced theoretically that $Q(J=0\mathrm{})>Q(J=2\mathrm{})\mathrm{}$ for the $\mathrm{np}$ states, and $Q(J=1\mathrm{})>Q(J=3\mathrm{})\mathrm{}$ for $\mathrm{nd}$; and in a more restricted way, that $Q(J=1\mathrm{})>Q(J=3\mathrm{})\mathrm{}$ for $\mathrm{np}$ and $Q(J=2\mathrm{})>Q(J=0,4\mathrm{})\mathrm{}$ for $\mathrm{nd}$. These relations are in good agreement with experiment with only a few exceptions. Most of the qualitative and semiquantitative features of the experimental data can be understood from generalization of the results of helium and by simple theoretical considerations. The excitation cross sections of the states with odd values of $J+l$ have been calculated by the Born approximation using atomic wave functions constructed by the Hartree-Fock-Slater orbitals, together with a semiempirical treatment of the intermediate vector coupling. The agreement between the theoretical and experimental values is generally satisfactory. The even-($J+l$) states are strongly influenced by indirect coupling; thus, no attempt was made to compare the experimental data of these states with the Born-approximation calculations.