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Re: starship-design: Casimir-Foreward balloon
> From firstname.lastname@example.org Thu Jan 13 20:05:32 2000
> From: "N. Lindberg" <email@example.com>
> I have a question about the 'phosphor' inside the balloon, and
> about phosphorescent materiels in general.
As far as I know:
> Apparently, a phosphor absorbs
> a high-energy photon, and re-emits low energy photons.
Sometimes other way around, but rarely.
Also, there is phosphorescence and luminescence.
The difference is in the delay between absorbing
(some energy, e.g., a photon) and emitting a photon.
Phosphorescent materials have long delay (even days),
so they shine long after the excitation ended
(e.g., materials glowing in the dark) while luminescent
materials stop to radiate almost immediately after
the excitation stopped.
Also, the excitation may be of different nature -
electromagnetic radiation (what we are talking about here),
corpuscular radiation (e.g. electrons - as in cathode-ray tube
screen and conventional TV displays), heat (thermoluminescent
materials, or just thermal radiation), etc.
> Could someone say something about how this works?
Somewhat like in a laser, but without the coherent emission
In general, the excitation pumps (exterior) electrons in molecules
of the material to higher energy levels, from which they
(after some delay, spontaneously or under some external influence)
fall down back to their previous levels (usually in several steps),
emitting photons along the way.
Higher energy of outcoming photons occurs when the incoming
photons shift upward an electron in several (e.g., two) steps
to such a level, that it falls down by a distance larger than
one step of its upward move. So, for several (say, two)
input photons we get one output photon, though of higher energy
> Are the photons emitted one at a time, or all together?
In a laser (or maser), the emitted photon excites other "pumped"
molecules to emit immediately and in phase with it, hence
the "all together" lasering effects and phase coherence of output.
In conventional materials every emission act works more or less
> What is the typical energy difference/proportion between
> absorbed & emitted light.
Ehem, what means "typical" here?
There is the whole range o proportions, byt of course,
always there is less energy emitted than absorbed
(usually much less - the efficiency of the process is rather low).
> Finally, is any energy left over?
Yes, usually a significant percentage (as stated above).
The sources of loss:
- not all exciting photons are absorbed (some go through,
some are reflected);
- still some incoming photons are absorbed in other ways
(eg., in inner electron layers) and are converted to heat;
- the emitted photons are of lower energy (longer wavelength),
or - where it does not apply - are of lower energy than the sum
of energies of those photons which were used in the process
of generating them;
- falling down of electrons not always occurs along the standard
route, so that they may emit other photons than desired - often
those that are then easily (re)absorbed and converted to heat;
- the "desired" photons may get absorbed and converted to other ones
or to heat before they manage to leave the material.
-- Zenon Kulpa