Re: Fluorescent Spectrum - Spikes vs Broad Slopes

On Thu, 2 Mar 2006 20:34:22 +0200, "Ioannis"
<morpheus@xxxxxxxxxxxx> wrote:

"Victor Roberts" <xxx@xxxxxxxxxxxxxxxxxxxxx> wrote in message
news:v7jc025acku2rhf07dikeqgt4oo64ra7dt@xxxxxxxxxx

On Wed, 01 Mar 2006 20:35:14 -0500, Victor Roberts
<xxx@xxxxxxxxxxxxxxxxxxxxx> wrote:

Let's see how may typos I made in this :-)

Well, what I left out is that while the density of 6^3P1
atoms in a high pressure mercury lamp is determined mostly
by the local temperature, the work of Karabourniotis shows
that the density of this state is not fully in equilibrium
with the local temperature. Therefore absorption of 254 nm
photons can increase the density of the state. However,
this departure from LTE is so slight that most of the
absorption of Hg visible lines you see in your measurements
is controlled by the temperature profile of the discharge
and not 254 nm photon absorption.

Thanks. If I am to repeat the gist of what you wrote, you are saying that in
a (non LTE) low pressure mercury discharge, any self-absorption that appears
is controlled solely by absorption of 254nm photons,

No, not exactly. For the lines I mentioned the answer is
yes. However, other lines, such as the 577 and 579 nm lines
terminate on the 6^1P1 level, which is also the origin for
the 185 nm line, so absorption of 185 nm photons will effect
the absorption of the 577 and 579 lines.


BTW - here is a quite taken directly from your own web site
that supports what I wrote yesterday:

"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."

while in a (near LTE)
high pressure mercury discharge, self-absorption has little to do with
absorption of 254nm photons.

Yes.

Therefore, if I was to measure the lines of a lpm discharge at high
resolution using a suitable device (such as a Fabry-Perot etalon), like on
the references for the 254nm line given above, any self-absorption I would
see would come from the absorption of 254nm photons, even on other lines,
while on my photos it indeed comes from the cooler gas around the tube
walls.

The self-absorption is a function of the density of the
lower state of the transition. All excited states are
populated by the a number of mechanisms, including electron
impact collisions, and adsorption of photons. Photon
absorption can modify the density of an excited state and
therefore the self-absorption of a line that terminates
there, but it is rarely the only mechanism that is
responsible for the density of that state. The only time
that self-absorption is even the dominant mechanism that
determines the density of an excited state is when you start
with a high excited level, that has a very low density due
to normal mechanisms, such as electron collisions, and use
absorption of in intense beam of light at a specific
wavelength (usually using a laser) to intentionally excite
the atoms from a lower state to the excited state of
interest, thereby increasing its density. This is done for
purposes of studying the physics of the atom.

As an aside, the explanation I got for the Fraunhoffer absorption on the
tube walls on the high pressure discharges on my web page, came from physics
professor Howard Goldberg of the University of Illinois, when I was a senior
there.

When were you at Illinois? I know a professor there and
some colleagues who went to school there.

--
Vic Roberts
http://www.RobertsResearchInc.com
To reply via e-mail:
replace xxx with vdr in the Reply to: address
or use e-mail address listed at the Web site.

This information is provided for educational purposes only.
It may not be used in any publication or posted on any Web
site without written permission.

.