**4.4. Testcase #4**

We now illustrate how to use PopRatio to calculate collisionally excited emission line intensity ratios.

As an example we take the UV intercombination multiplet
2s2p^{2} ^{4}P -> 2s^{2}2p
^{2}P^{o} of C II at 2325 Å.
Intensities ratios of lines belonging to this multiplet may serve as a
useful indicator of
electronic densities in the range
10^{7}
*n*_{e}
10^{10} cm^{-3}
[23,
24].
These lines have been observed in the spectra of a variety of
astronomical objects: planetary nebulae
[25], giant stars
[26,
27],
symbiotic stars
[28]
and in the solar chromosphere and transition region
[29].

The emissivity of a line is given by [4]:

We calculate the following emissivity ratios as a function of electronic density:

We take into account collisions by electrons and protons and
fluorescence induced by a black
body radiation field of temperature *T*_{*} = 4000K
attenuated by a geometric dilution factor
*f* = 0.5 (which might be representative of a stellar chromosphere).

In the high density limit function LTEDEV is called to show that the level populations differ from LTE conditions by less than 0.4%.

To run this testcase the user must include the black body radiation field in the body of function URAD in module POPRATIO:

urad = Planck(4.0E3_WP,c/lambda*1.0E8_WP)