Primer on Collisions
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II) Electron-Impact Excitation out of the ground state of Atoms

C. Excitation of Neon, Argon

We next consider the heavy rare gases such as neon and argon. The ground state of each of these atoms is the 1S0 state that arises from a np6 configuration. Excited states are formed by moving one of the np6 electrons into a higher orbital, forming a np5n' l configuration.

Click here for a neon energy level diagram    (GIF file: 950×700).
Click here for an argon energy level diagram (GIF file: 950×700).

Neon 2p5ns levels


As 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.

The 2p5ns configuration has a total of four LS-terms:


1P1,    3P0,    3P1,    3P2

L = 1   and  S = 0, 1

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 1P 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 1s2, 1s3, 1s4, and 1s5. We now expect that both J=1 levels have some 1P character, and thus both should have a broad peak.
J=1    Broad excitation functions
J=1    Broad excitation functions
J=0    Narrow excitation functions
J=0    Narrow excitation functions

And the experimental data:

Neon 2p5np levels


We now use the same approach for the 2p5np levels. With LS coupling we can have L=0,1,2 and S=0,1. This gives rise to ten LS-terms.
1S0, 3P0 1P1, 3S1, 3P1, 3D1 1D2, 3P2, 3D2 3D3
Mixing --Mixing-- Mixing J=3
Thus, one level is purely triplet (3D3) and should have a narrow excitation function. All the others have a singlet component and thus should have broader excitation functions.

And the experimental data:

Argon 3p54p levels


So 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 cm2.
LevelsJQ(100 eV)          LevelsJQ(100 eV)
2p1 059    2p2 113
2p5 025    2p4 114
2p3 228    2p7 116
2p6 223    2p10 118
2p8 239    2p9 313
Note the trend that the cross sections for excitation into levels with even J are larger than cross sections into odd J levels. Let us see how this trend can be explained using the same multi-pole field expansion as with helium.


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,
Ar( 3p6, J=0) --> Ar( 3p54p, J'=0 or 2)    allowed
Ar( 3p6, J=0) --> Ar( 3p54p, J'=1 or 3)    forbidden in 1st approximation


Thus, our standard multipole expansion arguement agrees with the experimental data:

Q( 3p54p, even J ) » Q( 3p54p, odd J)

Click here to see this exact case repeated for the neon 2p53p levels.

Neon 2p54d levels


This 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,

with     k=1:       J=0-->J'=1    both favorable
    k=3:       J=0-->J'=3


Thus, the multipole field analysis would support

Q( 2p54d, odd J ) » Q( 2p54d, even J)


This analysis is once again supported by the experimental data for the twelve levels of the 2p54d configuration of neon in units of 10-20 cm2.
LevelsJQ(100 eV)          LevelsJQ(100 eV)
4s1' 121    4s1" 21.6
4d2 161    4s1"" 21.3
4d5 113    4d1" 21.6
4s1"' 33.1    4d3 21.2
4d1" 32.1    4d6 00.3
4d4 33.4    4d4' 41.9
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