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II) Electron-Impact Excitation out of the ground state of AtomsC. Excitation of Neon, ArgonWe next consider the heavy rare gases such as neon and argon. The ground state of each of these atoms is the ^{1}S_{0} state that arises from a np^{6} configuration. Excited states are formed by moving one of the np^{6} electrons into a higher orbital, forming a np^{5}n' l configuration.Click here for a neon energy level diagram (GIF file: 950×700). Neon 2p^{5}ns levelsAs a starting point, let us approach the problem of neon similar to the analysis of helium. First, we describe each configuration by its LS-terms (i.e. singlet, triplets). Next we use optical selection rules to determine the shape of the excitation functions.
If we apply the rules developed for helium, we thus expect to see the three sharply peaked excitation functions for the spin-forbidden triplets, and one broad excitation function for the dipole-allowed ^{1}P term. However, neon and the heavier rare gases do not generally conform to the LS-coupling scheme. LS-terms with the same total angular momentum J within each configuration are scrambled together. Hence, the two J=1 LS-terms are mixed, while the sole J=0 and sole J=2 LS-terms are left alone. The resulting four levels are labeled using Paschen notation as 1s_{2}, 1s_{3}, 1s_{4}, and 1s_{5}. We now expect that both J=1 levels have some ^{1}P character, and thus both should have a broad peak.
And the experimental data: Neon 2p^{5}np levelsWe now use the same approach for the 2p^{5}np levels. With LS coupling we can have L=0,1,2 and S=0,1. This gives rise to ten LS-terms.
And the experimental data: Argon 3p^{5}4p levelsSo far, we have only considered the shape of the excitation functions. Let us now examine the magnitudes of the cross sections. For variety, we also switch from neon to argon. The following table lists the cross section at 100 eV for the ten 2p (Pachen notation) of argon in units of 10^{-20} cm^{2}.
This is the same as a one electron excitation 3p (l=1)-->4p (l'=1). From the mulipole field analysis (k=0,1,2,3,...) we find that for l=1-->l'=1, only terms with k=0 and k=2 are compatible and will contribute signifcantly to the cross section. Thus,
Thus, our standard multipole expansion arguement agrees with the experimental data: Click here to see this exact case repeated for the neon 2p^{5}3p levels. Neon 2p^{5}4d levelsThis is the same as a one electron excitation 2p (l=1)-->4d (l'=2). From the mulipole field analysis (k=0,1,2,3,...) we find that for l=1-->l'=2, only terms with k=1 and k=3 are compatible. Thus,
Thus, the multipole field analysis would support This analysis is once again supported by the experimental data for the twelve levels of the 2p^{5}4d configuration of neon in units of 10^{-20} cm^{2}.
references: Experimental and Theoretical Studies of Electron-Impact Excitation of Neon Francis A. Sharpton, Robert M St. John, Chun C. Lin, and Fredric E. Fajen, Physical Review A 2 (1970) 1305-1322. Electron-Impact Excitation of the Argon Atom James K. Ballou, Chun C. Lin, and Fredric E. Fajen, Physical Review A 8 (1973) 1797-1807. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
last updated: May-15-1997 |