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SHAOUL EZEKIEL: In
this demonstration,

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we're going to illustrate in a
simple way how molecules absorb

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light and emit fluorescence
as a result of the absorption.

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We're going to use the dye
laser, which is over here.

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And we're going to
go, for molecules,

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we're going to use iodide.

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That's molecular
iodine in a vapor.

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And what we'll do, will tune
the dye laser in wavelength

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as it passes through
the iodine vapor cell

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and observe the fluorescence as
a function of dye laser tuning.

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Again, in order to observe
the colors correctly,

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you will have to adjust your
monitor so that the colors come

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out as close to what is
supposed to be as possible.

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So now, we will
get the dye laser

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fired up so that you can
observe the iodine fluorescence.

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Now we're ready to
look at fluorescence

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in molecular iodine induced
by this tunable dye laser.

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So here we see-- again,
we see our dye laser.

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And the beam from the dye laser,
here it is on my little card.

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The beam from the
dye laser, then,

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is passed through
this vapor cell

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of iodine at room temperature--
molecular iodine, that is.

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And I don't know if you can
see with the room lights on.

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Maybe you can see the streak in
the middle of the tube, which

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is the fluorescence
in iodine induced

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by the dye laser at this
particular wavelength.

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Now, the fluorescence is
made up of many wavelengths.

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But sometimes you
get a big contrast

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in the colors in
the fluorescence as

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compared with the
exciting laser light.

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So in order to show you this,
we've done the following.

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We've taken the output
of the dye laser

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as it passes through the cell.

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And then we've scattered
it by a scatterer over here

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onto this screen so that
when we dim the room lights,

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we'll be able to see the
fluorescence over here,

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the iodine fluorescence
inside the tube.

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And the background will be the
color of the exciting dye laser

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light.

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So in this way--

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and again, if you've set
your monitors or your TVs,

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if you adjusted the
settings so that you

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can see the actual
colors, we'll be

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able to show you on the tape.

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So now we're going to
dim the room lights

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and then tune the dye lasers
and see the fluorescence that

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can be excited in
molecular iodine

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as a function of
dye laser tuning.

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Here we are.

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Now we see that the
fluorescence is orange.

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And the dye laser
is yellowish-green.

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Now I'm going to
tune the dye laser.

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Now you can see that the
fluorescence gets fainter.

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Now here we are.

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That's one color fluorescence.

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Now it becomes orange.

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Now it's very strong.

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The fluorescence
now is very strong.

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Now I tuned some more.

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Sometimes you see it flickering.

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It's because the laser
wavelength is not stabilized.

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And you're moving away from
the absorption lines in iodine.

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Now here we are.

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We have very bright fluorescent,
very intense fluorescence.

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And it looks orange-ish.

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But really, it's
not a pure color.

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It's made up of many
wavelengths in the fluorescence.

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And this depends, again, which
states are being excited.

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And here we are.

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This is weak absorption.

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Again, now, a very
faint absorption here.

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Now the light from the
dye laser is yellow.

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You can see there's
absolutely no--

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very little
absorption over here.

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And now the fluorescence
is almost the same,

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looks the same color
as the dye laser light.

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Now we have intense
fluorescence.

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And let me go through
and see what we can see.

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Oh, this one now--

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now its fluorescence
is yellowish,

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while over here the fluorescence
was more orange-ish.

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I said before, these are a
mixture of-- fluorescence

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is made up of mixture
of wavelengths.

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They're not pure wavelengths.

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Here we are.

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That's nice.

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Now you see a nice contrast.

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Now here's a nice contrast
where fluorescence is orange

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and the laser light is green.

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And here we are.

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That's at the end
of the spectrum.