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dumb laser question

dumb laser question - Chemistry Forum

dumb laser question - Chemistry Forum. Discuss chemical reactions, chemistry.


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  #1  
Old 02-17-2004, 03:33 PM
Allan Adler
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In the book of Butcher, Pike and Mayo, "Microscale organic laboratory",
there is one experiment in which one takes trans-1,2-dibenzoylethylene,
exposes it to a UV lamp for 12 hours, and obtains cis-1,2-dibenzoylethylene.

One of the questions at the end of the experiment asks: "If the trans
isomer of the compound used in this experiment is the most stable, how
is it possible to make this reaction provide exclusively the cis isomer?"

I'm not a chemist or a chemistry student. I'm just reading the book for the
fun of it. So I don't really know the answer. But I'm inclined to make
the following guess: Although the trans isomer is more stable, the
probability at any moment that a cis molecule will degenerate to a
trans molecule is so low that it will actually take a long time
for all or a significant portion of the cis product to decay into
the trans form. However, if one leaves the product alone for a long
time, it will all eventually decay into the trans isomer.

That's my guess. Assuming it is correct, the following idea occurred
to me (here, I should probably also admit that I'm not a physicist or
a physics student): We can look at the cis isomer as an excited state
of the trans isomer and this experiment achieves a population inversion
between the ground state (the trans isomer) and the excited state (the
cis isomer). If that is correct, can one then hope to design a laser
based on the transitions between the trans and cis conformations of
1,2-dibenzoylethylene? I don't care if it is a very crummy laser,
I'm just interested in knowing whether the idea is correct is principle.

Assuming it is correct, how would one design it? I have the CRC "Handbook
of lasers with selected data on optical technology" (1971) but I don't
know yet how to use it, nor whether it is the only reference one would
need to rely on. (I say "one", instead of "I", since I usually need
more references than "one" would).

For a while, a couple of decades ago, I was hanging around a laboratory
where people were doing atomic physics and working with atomic beams and
with dye lasers. They had glass tubing on one of the shelves for use in
setting up CO2 lasers, but I could never find anyone willing to set it up
for me to see how it was done. I don't know if one has the right to expect
a "trans-cis laser", if there is such a thing, to be anything like a CO2
laser. The CRC reference mentioned above does have the term "chemical lasers"
in the index, and that term does include CO2 lasers. Maybe that is a reason
for encouragement, and maybe not.

Ignorantly,
Allan Adler
[Only registered users see links. ]

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  #2  
Old 02-17-2004, 04:32 PM
Mark Folsom
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"Allan Adler" <[Only registered users see links. ].mit.edu> wrote in message
news:[Only registered users see links. ].mit.edu...
cis-1,2-dibenzoylethylene.
the

If the transition doesn't produce radiation, no. If there are only two
states, no.

Mark Folsom



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  #3  
Old 02-17-2004, 09:57 PM
Franz Heymann
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"Allan Adler" <[Only registered users see links. ].mit.edu> wrote in message
news:[Only registered users see links. ].mit.edu...
cis-1,2-dibenzoylethylene.
the
lasers"
reason

It sounds interesting. What is the energy difference between these isomers?
And remember that it is much easier to maintain the population inversion in
three-stste lasers than in two-state ones.

Franz


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  #4  
Old 02-18-2004, 12:20 AM
Mark Folsom
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Default dumb laser question

"Richard Schultz" <[Only registered users see links. ].ack.il> wrote in message
news:c0tmpk$n4h$[Only registered users see links. ].ac.il...
principle.
"stimulated"
molecules
energy
isomerization,
have

Okay, but if it didn't have the problem you describe, it would still have to
have a transition that produces radiation.


IIRC, the OP was talking about shining UV radiation on it to change its
state. That would make it an optically pumped laser.


Mark Folsom




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  #5  
Old 02-18-2004, 06:09 AM
Bruce
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In sci.physics
[Only registered users see links. ] (Richard Schultz) wrote:


Warning!
Do not look into laser with remaining eye.
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  #6  
Old 02-18-2004, 05:22 PM
Mark Tarka
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[Only registered users see links. ] (Bruce) wrote in message news:<[Only registered users see links. ]>. ..

BZZZZZT.

Sorry. The correct answer is "Level Three".


Mark (Here's the next toss-up question. Why...?)
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  #7  
Old 02-19-2004, 12:10 AM
Repeating Rifle
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in article c0uu6h$gpc$[Only registered users see links. ].ac.il, Richard Schultz at
[Only registered users see links. ] wrote on 2/17/04 9:45 PM:


Item 1.
Three-level lasers are inherently of lower effeciency than four-level lasers
if all other things are equal. You must invert half of the laser population
before reaching threshold. Compare ruby lasers to neodymium lasers. Ruby
does have the advantage of a longer fluorescent decay time thereby allowing
longer pump pulses.

Item 2.
You really need only two states for a laser to work, initial states and
final states. The problem is to get an inversion so that there are more
initial states than final states. If the final state is also the ground
state, then it is necessary to get more initial states than the residual
ground states in order to reach thresshold. That is why a thermally
depopulated final state in neodymium lasers ordinarily enables much lower
thresholds than can be obtained with ruby where the final state is the
ground state. By the way, neodymium lasers can be operated with the final
state being the ground state.

With cooling, the threshold for neodymium lasers drops like a rock because
the final state is depopulated even more. That cannot be done with ruby.

I don't know enough about excimer lasers to answer your question
definitively.

Bill

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  #8  
Old 02-19-2004, 05:37 AM
Mark Folsom
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"Richard Schultz" <[Only registered users see links. ].ack.il> wrote in message
news:c11h19$b13$[Only registered users see links. ].ac.il...
lasers.

If the ground state empties quickly by decomposition, then it's a three
state laser--upper state, unstable lower state, and atomic ground state. If
it were a true two-state laser, the lower state would be filling up as the
upper state emptied, and gain would stop when the excited population fell
below 50%.

halogen

And what happens after the radiating transition?

Mark Folsom



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  #9  
Old 02-19-2004, 01:21 PM
Andrew Resnick
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In <c11h19$b13$[Only registered users see links. ].ac.il> Richard Schultz wrote:
<snip>

Your argument seems a little polemical- semiconductor lasers could be
considered two-level by your reasoning as well: the states are the
valence band and the conduction band. But that obscures a lot of
important details, like the existence of a threshold current.

From what I understand of excimer lasers, you start with two
dissasociated atoms, excite one of them, they combine to an exciplex,
decay at the laser transition, and then dissasociate. So there is an
initial state, a metastable excited state, the exciplex, and the final
state which equals the initial state.

Another oddball example is a free-electron laser, which, AFAIK, is also
a (superficially) two-level system. But again, that obscures a lot of
the important physics, like the role of bunching in amplification.

--
Andrew Resnick, Ph. D.
National Center for Microgravity Research
NASA Glenn Research Center
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  #10  
Old 02-19-2004, 05:23 PM
Repeating Rifle
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in article c11h19$b13$[Only registered users see links. ].ac.il, Richard Schultz at
[Only registered users see links. ] wrote on 2/18/04 9:19 PM:


A key to understanding this is the meaning of stability. In the excimer
laser, the ground state is not unstable over time periods equal to the
excimer lifetime. It is just that the unpopulated ground state forms because
there never was such a state until the excimer was formed. Once the excimer
decomposes, the excimer ground state is no longer there.

Bill

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