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Re: Quantum question?



I have heard astronomers refer to this 'collisional' broadening as
seen in high pressure sodium discharge lamps.

Collisional broadening also occurs in stellar atmospheres, I
suppose, but it is overpowered by doppler broadening. It is not
usually astronomical phenomena that astronomers are talking
about when collisional broadening is mentioned. We hate the
high pressure lamps because light pollution from them is broad
spectrum. It is easier to filter out the narrow band lines
produced by low pressure sodium lighting.

Collisional broadening is just what *doesn't* happen in the
interstellar medium, and that is responsible for some of the
narrowest lines known to man. The famous 21 cm line of hydrogen
is incredibly narrow, its natural linewidth being due to a
lifetime of about a million years in the upper state. This line
can't be reproduced in the laboratory because collisional
broadening with the walls of a vessel usually disturbs the
higher level atoms before they can decay radiatively. Lots of
hydrogen atoms suffer no collisions for a million years in
space, and so they get a chance to decay radiatively. Since
space is very, very big even this dilute population can produce
enough radiation to be a useful tool in sufficiently dense (or
sufficiently thick) nebulae.

Is this mechanism also describable as 'Pauli Exclusion' broadening,
something that solid state folks use to explain the creation of
conduction 'bands' as isolated atoms come close to each other.

No. The bands in a solid are a consequence of high spatial density
of electrons. (Well, the bands aren't, but the degenerate nature of
the electron distribution in the bands and the resulting unbound
high energy conduction electrons are consequences of Pauli exclusion.)

There is a simple 'Kroenig-Penny' model that can be programmed
eassily to illustrate the bands arising as the atoms (with their
initial discrete levels) get closer together.

That is correct. The periodic nature of the crystalline potential
is responsible for the bands (that is, for the gaps between the
bands) in what would otherwise be a simple parabolic population
distribution in kinetic energy (N(E) ~ E^1/2) of electrons.

Leigh