Re: Absorption spectra

[brian whatcott <inet@INTELLISYS.NET>]

I'll attempt to supplement George's response with a pragmatic,
perhaps idiosyncratic view not untypical of engineers.

There's no such thing as an indefinitely thin spectrum line.

BECAUSE this requires an indefinitely long preexistence of this
emission - of which there are absolutely (?) no examples in nature.
(You recall the reciprocal nature of extension in time and
in frequency implied by the Fourier transform...)

A measure used in technical applications to assign a width to
spectral lines is the Quality Factor "Q".
This is sometimes defined as the ratio of the width of a resonant
response at its half power width to its center frequency.

  The coils in your radio may be connected for Q in the 50 to
200 range - this selectively amplifies a central band of
 frequencies coding the audio signal but attenuating frequency
 components further removed from the center -these are unwanted
signals from adjacent or other channels.

But what was once a matter of strict national secrecy was that
an electromagnetic wave resonating in a microwave cavity, an
arrangement rather more like the probability wave existing in
 an atom which we call an electron, can display an exceptionally
high Q  of hundreds of thousands or higher -
but shsh...don't tell anyone!

 Referring to the celebrated experiments on breathing modes of
the Sun (yet again...) whereby one can assign a value to the
mean velocity of patches of Solar visible surface, a value as
low as (say) 10 meters per second
- it's easy to see that a sodium line of the order of Q=10E7
is needed for the Doppler frequency shifted measurement.

This easily leads to the position that George explained: though
a particular photon has some limited extension in space and time
 - there is some atomic mechanism to closely align the frequency
of such sequential emissions.

This is like the phase synchronicity we have been reading about in
this list,  in connection with  the stimulated emissions
in lasers.

   You will see that in finally arriving agreeably at George's
position, I started from an opposing supposition - that oscillator
 outputs have finite even low Q, but can be strikingly narrow in
atomic mechanisms, compared to the macroscopic oscillators
existing in man-made circuits.

Brian


At 13:51 5/13/99 -0400, you wrote:
> Absorption (or emission lines) are broadened by a variety of mechanisms.
> Most of what is observed is Doppler broadening - atoms are in random thermal
> motion which affects the actual frequencies absorbed and emitted.  The
> intrinsic width of the spectral line, essentially due to the time-energy
> version of the Heisenberg Indeterminacy Principle, is too small to observe.
> The energy levels can also be broadened by electric and magnetic fields,
> especially when low dispersion spectroscopes are used.
>
> ************************************************************************
> "Before I came here I was confused about this subject.
> Having Listened to your lecture, I am still confused, but
> on a higher level."
>                                                      -  Enrico Fermi
> ***********************************************************************
> George Spagna
> Department of Physics
> Randolph-Macon College
> P.O. Box 5005
> Ashland, VA 23005-5505
> phone: (804) 752-7344     FAX (804) 752-4724
> e-mail: gspagna@rmc.edu
> http://www.rmc.edu/~gspagna/gspagna.html
>
>
>
> -----Original Message-----
> From: GARY HEMMINGER [mailto:Hemmig@D-E.PVT.K12.NJ.US]
> Sent: Thursday, May 13, 1999 1:41 PM
> To: PHYS-L@LISTS.NAU.EDU
> Subject: Absorbtion spectra
>
>
> A student has asked the following question which seems like a good
> one to me:
> If the black lines in an absorbtion spectra correspond to
> individual wavelengths,  then shouldn't they be too thin to be seen?
> Any thoughts?  Do they have a "width"?
>
> *****************************************
>  Gary Hemminger
>  Dwight-Engelwood School
>  315 E. Palisade Ave.
>  Englewood, New Jersey
>             07631
>  e-mail: hemmig@d-e.org
> *******************************************
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