8.2  Fe XIV

If the population of metastable level (m) is comparable with the ground level (g), then other excited levels (k) can be populated from this metastable level as well as from the ground level and the dependence of the intensity on electron density becomes:

Ik,m @ Neb        2 < b < 3

(25)

Fe XIV is formed at coronal temperatures (1.8 10⁶ K), it is a good example of the latter diagnostic case. The metastable level is actually the upper level in the ground configuration (3s²3p ²P_1.5). The transition probability to the ground level is only 60 sec^-1. For low electron densities (10⁸ cm^-3), almost all the population for Fe XIV is in the ground level, but as electron density is increased (10¹⁰ cm^-3), the upper level begins to have a significant population aswell.

Figure 18: Above: [from Peter Young] Diagram of some of the main levels and lines in Fe XIV. Below: level populations from CHIANTI. Bottom: line ratio.

The spectral line at 334.2 Å (²P_1/2 - ²D_3/2) is excited from the ground level(3s²3p ²P_1/2), whereas the line at 353.8 Å (²P_3/2 - ²D_5/2) is excited from the upper level in the ground configuration (3s²3p ²P_3/2).

The intensity ratio of these two lines (Figure ) varies with electron density, reflecting the level population changes in the ground configuration.