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M2L11c.txt
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M2L11c.txt
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#
# File: content-mit-8-421-2x-subtitles/M2L11c.txt
#
# Captions for 8.421x module
#
# This file has 97 caption lines.
#
# Do not add or delete any lines.
#
#----------------------------------------
So what we have is the following.
We have, of course, the structure
of atoms in excited field.
Here is the electric field, and let me just pick three n
values-- 18, 19, 20.
And now the structure, of course,
is that the criterion I actually gave
you was the criterion for the application of perturbation
theory.
If the energy levels are smaller than the matrix element,
you have to re-diagonalize between those levels,
and that gives you then, not a quadratic but a linear effect.
So therefore the structure is here,
that you have a region where you have strong l mixing,
so you have to use general perturbation
theory for the different l states,
but the many faults in n, the principle quantum number,
are still well separated, but then
eventually, when you go further, you have a region which
is called n mixing.
So now the electric field is really
completely re-diagonalizing your states with different quantum
numbers n.
So the result of this discussion is
that highly excited states of atoms
behave very differently from ground state atoms.
An end to the five scaling, a sensitivity
to volt per centimeter, level mixing all over the place,
and that's why for those highly excited states,
people have coined the word Rydberg atoms or Rydberg meta.
That means atoms with higher principle quantum numbers.
And the study of Rydberg atoms was pioneered-- well,
the early pioneering work-- by our own Dan Kleppner
and then Herbert Walther in Munich,
who happened to be my PhD advisor,
and finally Serge Haroche, who was recognized
with the last Nobel Prize, together with Dave Wineland.
So this is not just theory.
What I am showing to you here is spectroscopy
done at MIT by Dan Kleppner.
So what is done here, is from the ground state,
they excite to an excited state, and whenever you hit an excited
state, you see a signal that's focused on the upper part,
so if at a given electric field, you scan the laser,
you get one of the those traces.
You find peaks, peaks, peaks, and those peaks
correspond to the different n manifolds with strong Stark
mixing.
And eventually when you go to somewhat higher field,
you have states all over the place,
and this is the regime where you have done n mixing.
So in the '70s and '80s, there was really--
those experiments obtained a clear understanding
and description of atoms in-- well,
I would say high electric fields,
but the fields were not so high.
It was just the atoms were very sensitive that already
had low electric fields, they reached or disregarded
the high field limit.
Now the question is, when we recorded signal suddenly
the traces stop, and that means the electric field is now
so high that the atom no longer has a stable state.
The electric field is so high that it literally rips
the electron away from atom, and if you go to higher states,
the electric field where this happens is lower.
This is the process of field ionization,
and that's what we want to discuss next.
Question?
For what atoms?
Those studies were actually done for lithium.
It's actually peculiar.
Dan Kleppner really liked hydrogen. Dan Kleppner
is the person who tried to do almost all experiments
with hydrogen-- the famous PZ experiment.
He also had the Rydberg experiment,
which was just in Building 26, where Vladan Vuletic teaches
now his labs.
This is where spectroscopy of hydrogen
were done, with a goal of a precision
measurement of the Rydberg constant,
so he excited hydrogen to some of those high levels
but, as probably the experts know,
the hydrogen atom is the hardest atom to work with,
because you need Lyman-alpha.
You have this huge gap to the first excited state,
and that's why if you can get away,
you try to work with other atoms.
In those experiments, those people
worked with the lithium atom.
So the lithium atom has a [? quantum ?] effect,
in contrast to hydrogen where the [? quantum ?] effect is 0,
and this will actually be very, very
important for field ionization, as I want to discuss now.
Other questions?