Subject: fluorescence and phosporescence

From: "Thoms, Travis" <[email-masked]>
Date: Mon, 2 Apr 2001 12:20:46 -0700

Greetings:
	I hope this question is not too far off-topic; it does actually
relate to my feeble attempts at modeling fluorescent and phosporescent
materials.	
	I have reviewed a couple of physical chemistry books concerning
these two topics, and to be quite honest, either they have not explained it
well, or I am missing the big picture.  If you don't mind, I would like to
explain to you what I think is going on, and then you could correct me, or
refer me to some good sources to set me straight.

Fluorescence:
A photon comes in and excites a molecule in a ground state (are most ground
states "singlet states"?) to an excited state (I am assuming that one cannot
excite to a triplet state). There is a percentage or probability that this
excited state will relax back down to the ground state by emissive means,
emitting a photon of light that is smaller than the homo-lumo band gap of
the ground state molecule.  The molecule can also lose its energy through
non-emissive means such as heat.  The energy cannot move from the excited
singlet state to a lower energy triplet state due to the need to conserve
momentum.

Phosphorescence:  Exitation occurs as above, except energy in this case is
allowed to move from the excited singlet state to a lower energy triplet
state through intersystem crossing (which was forbidden in the previous
scenario).  The triplet state in turn can relax to the (singlet?) ground
state via emission because of spin orbital coupleing with the heavy
nucleus).

Does this sound right?  Have I left something out which is important?

Thanks.

Travis Thoms
Research Associate
Canon R&D Center Americas. inc.
Telephone: 1-408-468-2864
Facsimile:1-408-468-2810
[email-masked]