Natural Science Forum / Physics / Optics / November 2005

Well Tony may not have looked at it for a while, but thats an excellent
summary!

Some extra points of detail:
Re point one, an important aspect is that the cross section for exciting N2
by electron impact is high at electron energies which can be obtained in a
nice stable discharge, & that at those energies very little else gets
excited. (Look at a multi-kW CO2 laser discharge - it is a pretty faint
glow; very little goes into useless electronic states.) CO2 & N2 molecules
that do end up in higher vibrational states (and lots do; these modes have
temperatures of a few thousand K) tend to cascade down very efficiently &
with minimal energy loss into the upper laser level. The collision cross
sections are such that vibration of the N2/CO2 coupled modes can be at
several thousand K whilst rotation & translation can stay not far above
300K; the electron energy is funneled to where you want it.

And re point 3, that the lower laser level, E2 in the above can be rapidly
depopulated, partly because you have two levels in Fermi resonance close
together + a third nearby, partly because two of these three are 'overtones'
of the bending vibration, & that (plus helium collisions) is a fast 'route
down'.

Another useful fact is that CO2 lasers have reasonably high gain, so that
efficiency is not too sensitive to small losses, and that the system is
essentially free of parasitic losses such as excited state absorption (ESA.)
In fact the gain is somewhat anomolous, because this is a 'difference
transition' in which *two* states change their quantum numbers, & generally
such transitions are quite weak, this one being stronger than the usual
rules of thumb would suggest.

It also happens that CO2 is 'almost' mono-isotopic ~99% 12C16O2 (13C, 17,
18O are quite rare, < ~1%) which also helps. And because 16O has no nuclear
spin, half the rotational levels are absent, which ~ doubles the gain
compared to if 17O happened to be the common isotope.

Overall, no one single factor; a comination of helpful circumstances come
together in one molecule. The spectroscopically closely related N2O laser is
far less efficient for example. (Although N2O is NNO, not NON, which reduces
the symetry.)

I think you will find the *original* CO2 laser (CKN Patel?) had no N2 in it
& was pretty pathetic.
Sorry, cant check, the papers are in my destroyed office :-(

Harvey