M2L11e.txt

#
# File:   content-mit-8-421-2x-subtitles/M2L11e.txt
#
# Captions for 8.421x module
#
# This file has 91 caption lines.
#
# Do not add or delete any lines.
#
#----------------------------------------

Let me now briefly mention important applications
of field ionization.

One is close to 100% detection efficiency for atoms.
If you want to detect single atoms-- for instance,
you have a krypton sample, and you
want to find a rare isotope of krypton
for dating the material.
You need an extremely high sensitivity
and you may just have a few single atoms in the sample.
One way to do it would be that you excite the atom, maybe
through an intermediate state to Rydberg state.

And then by just applying a few volts per centimeter
you get an ion.
And ions can be counted by particle detectors.
You can accelerate the ion, smash it into a surface,
and count the particles with close to 100% efficiency.
And this is one of the most sensitive detection schemes.
I remember in the aftermath of Chernobyl,
there was an interest in detection schemes
for radioactive strontium.
And on the wiki, I give you a reference
where some people developed this resonance ionization
spectroscopy for some atomic isotopes which, unfortunately,
appeared more frequently after the Chernobyl disaster.
And they developed an effort based
on excitation to Rydberg states which was more sensitive
than other methods.
You may ask, why don't you ionize it with a laser?
Well, the fact is, you can photoionize it with a laser.
That's another alternative.
But it takes much more laser power,
because if you excite into the continuum,
the matrix element is much smaller.
And often, if you want to have 100% ionization probability
to go into the continuum, you need such high laser power
that you may get some background form off resonant ionization
of other elements and such.
So Rydberg atoms is really the smart way to go.
You go to an almost unbound electron,
and then it's just the electric field which causes
the final act of ionization.
I also want to briefly mention the famous experiments
on Rydberg atoms by Herbert Walther and Serge Haroche
and collaborators would not have been possible
without field ionization.
I'll give you this one reference,
but I'm sure you'll find more in the actually very,
very nicely written Nobel lecture of Serge Haroche.
I just read it a few weeks ago, and it's
a delight to read how he exposes the field.
So they did QED experiments by having
microwave transitions between atoms
in two highly excited states.
Let's say, with principal quantum number 50 and 51.
So there is now conveniently in the microwave regime.
Then those atoms are passed through a cavity,
and in this cavity, single atoms interact with single photons.
And they have done beautiful quantum non-demolition
experiments of single photons and that's
really wonderful state of the art experiments.
But back to Rydberg atoms and field ionization.
Eventually the read out of those experiments
was, you prepare atoms homes in the state 50 or 51,
and afterwards if they have absorbed or emitted a photon,
they should be in a different state.
So we're interested in a very high detection
efficiency, which could distinguish between 50 and 51.
And of course there is a way to distinguish that.
And this is because of n to the 4.
You first apply an electric field
which can only field ionize 51, and then you
allow the atoms to propagate into a slightly
higher electric field, and then the 50s are ionized.
So the standard experiment is that you
pass those atoms between two field plates
and by putting an angle between them that indicates
the voltage is increasing.
And then you have two little holes.
You have some Channeltron particle detectors,
and the first detector will detect the 51's,
and the second detector will detect the lower lying states.
So you can detect every atom with high probability,
but also in a state selective way.

So this way of doing state selective field ionization,
based on the discussion we had earlier.
This is the method of choice for experiments
involving Rydberg atoms.