Re: What energy and field is needed for an atom magnetic dipole transition?

Hello Bryan,

Magnetic dipole transitions can come in quite a few different systems and
at quite a few different energies.  However, perhaps the most famous of
magnetic dipole transitions is one of the simplest and also happens to be
forbidden.  "Forbidden" in this case doesn't mean "never occurs" but rather
"very, very seldom occurs."  The transition I'm referring to is that found
in a hydrogen atom.  The transition from the electron & proton spins
parallel to antiparallel is not allowed by simple quantum mechanics rules,
but it does in fact happen if you include a few additions(or have lots of
patience!).  The energy for this transition is quite small and corresponds
to a 21cm wavelength of radiation.  That comes out to around 59millionths
of a single electron-volt of energy.  

The reason it's so famous is that, other than just being a simple system
(ie, Hydrogen!), the universe at large is also populated by lots of
hydrogen.  Much of radio astronomy uses the 21cm wavelength to study the
universe.  Regions of the sky which may appear dark in visible light, will
be very bright with radio waves such as the 21cm ones produced by hydrogen
undergoing the magnetic dipole transition.

After the photon is emitted, the hydrogen atom will be in a lower energy
state than it was before.  Without getting energy from somewhere else it
will remain in the lower energy state.  You can get an idea for it from an
analogy with bar magnets.  Hold two bar magnets parallel and try to bring
them together from the side.  Now hold them anti-parallel and do the same
thing.  

Thank you very much for the questions.  Magnetic dipole transitions are a
bit outside my field of study.  It was fun to read upon them again as I
have not gotten to think about this subject much since graduate school.

best wishes,

Michael




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Michael S. Pierce
Materials Science Division
Argonne National Laboratory