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View Full Version : What Forbids the Forbidden Line?



Jeff Root
2014-Oct-17, 12:02 PM
For example, a great many astronomical objects have a
strong emission line at (rest frame) ~500.7 nm. This is
said to be caused by a particular electronic transition in
a particular oxygen ion, yet no such line has ever been
observed in any laboratory spectrum of oxygen (as far
as I know).
This greatly surprises me. Why is this line so difficult to
produce in a lab? Does the oxygen react with the walls
of the vacuum chamber?

-- Jeff, in Minneapolis

antoniseb
2014-Oct-17, 12:20 PM
I moved this to the Q&A Section since it was quite tangential to the "Researcher Shows Black Holes Don't Exist" thread in the Astronomy section. Plus I didn't want the answer to get lost in a long thread.

Ken G
2014-Oct-17, 12:31 PM
Forbidden lines have an extremely low probability of being emitted, because they don't obey the "selection rules" that make transitions much more likely to happen (often because, classically, they would require the electron to do something different from oscillating back and forth like a dipole). That means any one atom would take an incredibly long time to emit light in that transition, and at the kind of high densities we have in the lab, there will always be something else that happens to the atom first. However, in astronomy, we find extremely low densities at times (say, in the interstellar medium), and an atom can find itself in a "metastable state", which is a state that does not have any allowed downward transitions. If the density is low enough, nothing else will happen to that atom, it will just sit there waiting for the "forbidden" transition to happen. In that situation, the rate a huge collection of atoms will emit light in that forbidden transition is just the rate that atoms get put into the metastable upper level of that transition, and that can be fast enough that mostly the light you see is in that forbidden line.

ngc3314
2014-Oct-17, 03:31 PM
In more detail, a "forbidden" line may be described by the critical density where an ion is equally likely to lose that excitation energy by emitting the photon as by undergoing a collision (usually with an electron). In fact, some ions such as S+ have lines with similar upper energy levels but different critical density, giving an important tool for density measurements. The strong [O III] line at 5007 A has critical density 6x105 per cm3; [O II] at 3727 is a doublet with critical densities 630 and 3200, making this pair useful for density measurements. (There is a table of such values compiled here (http://astronomy.ua.edu/keel/galaxies/emission.html)). All these values are on par with listed values for the hardest laboratory vacuum chambers (I don't know of any detections of these lines in the laboratory, although the obvious set of search terms is not terribly helpful with Google).

Jeff Root
2014-Oct-17, 08:00 PM
Is this particular line a significant/major/primary component of
the green glow of atomic oxygen in Earth's upper atmosphere?

Does the temperature of the oxygen play a major role in
determining whether it can emit in this forbidden line?

Does UV or visible light prime the oxygen ions in Earth's upper
atmosphere for emission in this line?

Is the mean time between collisions (between an oxygen ion
and anything else) the immediate determiner?

A graph I have shows that atomic oxygen is the most abundant
component of Earth's atmosphere between 200 km and 600 km.
A table in the same data set shows that the density doesn't get
down to 6x105 until between 950 and 1000 km. At that
altitude the mean time between collisions is about 20 minutes!

-- Jeff, in Minneapolis

ngc3314
2014-Oct-18, 12:12 AM
Is this particular line a significant/major/primary component of the green glow of atomic oxygen in Earth's upper atmosphere?

That would be the [O I] 5577 line, the strongest airglow line from most sites most of the time (accompanied by [O I] 6300 and 6363, which also occur in gaseous nebulae and active galaxies).


Does the temperature of the oxygen play a major role in determining whether it can emit in this forbidden line?

To the extent that the temperature influences the ionization state of the oxygen, and which upper levels are populated by particle collisions. At low densities the second may be more important. To be picky it's the electron temperature that does this (at low densities various species can have different kinetic temperatures).


Does UV or visible light prime the oxygen ions in Earth's upper atmosphere for emission in this line?

Electron collisions can do this as well (which dominates when the same spectral lines occur in aurorae). For the airglow this site (http://www.atoptics.co.uk/highsky/airglow2.htm) says it's solar radiation by an indirect mechanism of chemical formation and exchange reactions.