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M2L7d.txt
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M2L7d.txt
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#
# File: content-mit-8-421-2x-subtitles/M2L7d.txt
#
# Captions for 8.421x module
#
# This file has 100 caption lines.
#
# Do not add or delete any lines.
#
#----------------------------------------
How do you measure an 8,000 second lifetime?
Well, there is a recent paper a few years
ago-- a physical review letter, Determination of the Lifetime
of Metastable Helium.
Traditionally people have measured the lifetime
of metastable helium in the following way-- no,
they haven't observed helium for 8,000 seconds
because if you have helium in the metastable state
maybe after a few seconds there are some collisions-- collision
or deactivation.
8,000 seconds you would just have
many, many systematic effects.
What people did is they put some atoms
into the metastable state, measured the number of excited
atoms, and then maybe during 100 milliseconds 1 out
of 80,000 atoms should decay.
And by determining what small fraction of the atoms
has decayed or how many photons you observe
in 100 milliseconds divided by the total number of atoms,
that's how you can measure the lifetime.
Of course the problem is, yes, you
can measure the number of photons
but if you want to divide by the number of atoms
you have to know very accurately how many atoms you have.
And this Australian group used a very clever trick
to eliminate this uncertainty and this is as follows.
We have this extremely long lived
8,000 seconds lifetime here and this is the transition.
But you can now excite the atom to the triplet P_O state, which
is also-- it's a triplet state but-- well, I
assume because it has a p state it has spin orbit coupling.
The lifetime is, well, for an atom is very long.
Typically atoms decay nanoseconds but this lifetime
is milliseconds.
So what they did is they just excited all the atoms
with a strongly saturating laser to the triplet P_0 transition
and then within a few milliseconds all the atoms
emitted.
And so all they had to do is measure accurately
the ratio of the VUV photons emitted
on by the triplet ground state and by the triplet P_O state.
And then the number of atoms cancelled out,
all they had to do is measure the ratio of two VUV
intensities.
So based on that research now we have higher accuracy
on the lifetime of metastable helium.
The question is, when I said we have
to violate the above assumptions to get
any decay between singlet and triplet.
Metastable helium, pretty much everything
you can think about it is forbidden.
So, for a long time people believed
it was a two photon transition, that one photon transition was
forbidden but a two photon transition was allowed.
We know now that the decay path is
one photon M1 transition-- magnetic dipole transition.
If you don't know what M1 is, we discuss it later in the course.
But you may find it interesting that until rather recently
it was not clear what that the decay path is.
Well, recently means probably a few decades ago,
but for a long time it wasn't even clear
and the calculations were not accurate enough--
what was the mechanism by which the metastable triplet ground state
of helium could decay.
When I prepared the class I wanted to give you
a simple picture and say hey, it is this mechanism
which causes the transition from the triplet count
state to the singlet count state but I couldn't
find a simple operator.
It seems it's a little bit more complicated.
The best I can tell you is that the decay path requires
higher order terms using the Dirac equation with coupling
to the electromagnetic field.
Any questions?
Yes, Nancy.
In the measurement of the lifetime of that state,
what is the limiting factor?
What limits this measurement
to be that more precise?
Do you know?
Here is the reference.
I mean this was a highly accurate measurement
for that it improved all previous measurements.
What the current limit of accuracy is, I don't remember.
But for instance, they had to put all atoms
into the excited state or by a strong saturation.
Of course saturation is never complete,
maybe there is a limitation to that.
Finally, you have to measure the ratio
of photons emitted by that state and emitted by that state.
Rather they are the vacuum UV, but you
may have a systematic effect if your photo detector has
a slightly different detection efficiency for 58
nanometer and 62 nanometer.
I mean, nothing is perfect, but I really
have to refer you to the reference for details.