Re: bleaching

[John Denker <jsd@MONMOUTH.COM>]

At 05:56 AM 10/22/99 -0400, Ed Schweber wrote:
>    I am starting by accepting the model that a substance's lack of
> transparency is due to the absorbtion of photons.

Good!

>   What if the substance were exposed  to such an intense light that a
> majority of its atoms were in an excited state? Would the substance then
> become transparent to any additional light at this frequency?

Absolutely.  It's called bleaching.

>   Has (or could) something like this be done experimentally?

It is done every day.  See e.g.
  http://materialscience.uoregon.edu/pslas.htm
which says in part:

> Laser pump/probe techniques have become the preferred means for
> performing time-resolved measurements in a broad range of fields,
> including chemistry, physics, materials science, and biophysics.  No
> longer limited to traditional transient absorption/bleach measurements,
> pump/probe measurements now encompass such diverse techniques as ...

Given a material with an assortment of absorbers, you can selectively
bleach one frequency and not others, leaving a remarkable absorption
spectrum with a notch in it.

Depending on details, the material might not *stay* bleached for very long.
 The timescale might be set by the physics of re-radiation or
thermalization, or it might be set by chemistry (since the excited state
might be reactive).

See the recently-announced prize
  http://www.nobel.se/announcement-99/chemistry99.html
if you're interested in the chemistry tangent.

______________________________________________________________
copyright (C) 1999 John S. Denker             jsd@monmouth.com