Re: what is the difference between fluorescence & chemiluminescence?

To explain this, I will need to give a brief sketch of the electronic
state of molecules.

When a molecule is formed through the combination of atoms, it achieves an
intertwining of all of the atomic energy levels to form new arrangements
which are called "molecular orbitals". These are similar to the orbitals
for atoms except that they are orbitals for the molecule. However, they 
follow the sorts of rules for filling as atomic orbitals - Hund's Rule of 
Maximum Spin Multiplicity, Pauli's Exclusion Principle, the Aufbau 
Principle, etc.

So, when a molecule is put together, all of the molecular orbitals fill 
with all of the available electrons. The result is the "electronic ground 
state". That is, the "resting" or lowest energy state of the molecule. 
Above this state are higher energy configurations where, for example, spin 
pairing might occur that is not present in the ground state configuration.

Fluorescence and Chemiluminescence have a common origin. They both result
from the decay of a molecule in a higher energy or "excited" state back 
down to the ground state. This decay results in the release of energy and 
the emission of a photon.

But they are caused by different phenomenon. Fluorescence results from
electronic excitation - a consequence of the molecule absorbing a photon
initially. That is, a photon of light is absorbed and very quickly 
re-emitted. However, the same colour is not necessarily re-emitted as the
molecule can lose energy, while in the excited state, through vibrational
deactivation. This is why fluorescence essentially changes the colour of
the light - for example, blue in, green out. And why fluorescence takes
UV light and converts it to visible.

Chemiluminescence is caused by a molecular reaction of two (or more) 
ground state molecules producing a final molecule in an excited state.
That is, the energy that is in the reactants is translated to the products
and, while forming the products, it also excites them. All molecular
interactions form products in "vibrationally" excited states - a bit like
ringing a bell, it takes time for the vibrations to die away. This 
birational excitation leads to "infrared chemiluminescence. But reactions
can also result in the formation of molecules in "electronic" excited 
states and, in this case, that results in the emission of a photon of 
light.

Hope this answers your question.