The physics of discharges is severely complicated. Without going into
too much detail, the spectra emitted fall under three categories:
Linear spectra (from excited element gases or vapors)
Banded spectra (from excited molecules)
Continuous spectra (from highly heated or ionized matter)
Unofficially, there is a fourth category, the mixed (or
degenerate) kind, which are spectra which may contain linearity and
continuity simultaneously (Na). In reality,
whenever continuum is to be found around or close to certain lines in
the emission spectrum, the phenomenon of self-reversal takes
place. This phenomenon, (otherwise known as spontaneous reversal or the corona phenomenon:
Note, that there is another phenomenon called "corona discharge", which
refers to electric discharges of a certain type. This is not the one I
am discussing here) occurs, whenever more energy than the energy
required to ionize a certain electron is being provided for a certain
transition in the atom involved. Look at the figure below, depicting a
simplistic schematic for a Na discharge tube:
Now look at the phenomenon as seen through the Phasmatron,
displaying precisely this behavior optically:
You can see the continuum emitted by the Arc around the absorption
band, and then the dark absorption band with the two Sodium lines in
the middle. So the phenomenon actually occurs because Kirchoff
absorption occurs inside the tube, along with continuous emission.
A more detailed spectral distribution shows clearly what's happening. The absorption that occurs on the D1/D2 doublet is clearly visible.
Self-absorption usually happens with resonance lines, i.e. with lines whose transitions terminate at the ground state. For example, the Na D1/D2 lines above are resonance lines. However, when the density of the discharge plasma is high, enough absorption on the resonance lines causes self-absorption on other lines as well. Here is a similar absorption which happens on the lines of Mercury under high pressure, although less pronounced. This is the blue Mercury line, followed by the corresponding spectral distribution.
In general, most conventional discharge sources used for lighting
exhibit this phenomenon, even on non-resonance lines, whenever the plasma pressure is high. Most of the times though, the spectroscopes
are not powerful enough to show self-reversal because the width of the
absorption within the line is of an order of a couple of Angstroms,
sometimes even less than an Angstrom. The second photo before the end
in the Hg (same as the one above), which shows the blue Mercury line,
needs at least an R=50,000 to show the effect. The effect is less
obvious as the ionization levels for a particular element go up in eV and as one moves to non-resonance lines.
Sodium displays this behavior because it has a relatively low
ionization energy level and because the D doublet is a resonance line. Mercury on the other hand needs much higher
energies to ionize. In any case, Alkali are prone to showing the effect
on relatively low power spectroscopes, as are some easily ionizable
metals. The effect can be seen on Li, Ca
and Na sometimes using an R as low as 700-800. For color photographs which show this effect, click here.
The actual width of the lines in arc generated spectra is never
infinitely thin. It is always of finite width δλ, which
depends on the energy given to the arc. The light distribution around
the emission wavelength is almost Gaussian (usually it is Lorentzian and or Voigt). The only known δλ~0 sources were constructed around 1926 by Bogros and Paul using atomic
beams and electron guns, but had the disadvantage that the lines were
weak.
The best spectroscopes to view self-absorption are
spectroscopes with Fabry-Perot etalons which can reach resolving powers
of the order of 1,000,000.
