Can anyone please settle an ongoing dispute..... Is fluorescence a form of scattering like Raman or not ?

It probably depends on the definition but I think scattering is immediate but fluorescence is slightly delayed- it's characterised by a half life so some photons are delayed by a considerable time.

That is the argument if scattering is defined as an intereaction between a photon and electron causing a change of direction and a possible change of wavelength, flourescence is a form of scattering, regardless of the time difference. I would also note there is a time delay with Raman especialy if a phonon is involved.

Fluorescence involves the absorption of a photon, and the emission of a second photon having a lower energy (longer wavelength). You're not getting the same photons out that you put in.

why ??? if you are correct what happens to the incident photon and where does the new photon come from ?

An electron gets excited by a photon and jumps to an increased energy level. There are a number of ways in which the electron can return to it's origonal level.

1) it can just make a single jump back to ground - thus the re-emmitance and scatter.

2) it can take several smaller jumps back to ground through a number of intermediate levels - in this case the electron takes longer to get to get there - so you get the delay before the all the energy is released - Thus you see the fluorescence.

3) the electrons can get trapped in 'meta stable' states on the way down - these take even longer to release all their energy and can take several minutes before the after glow can fade - this is phosphourescene.

 

 

Hope this helps.

Fluorescence involves the absorption of a photon, and the emission of a second photon having a lower energy (longer wavelength). You're not getting the same photons out that you put in.

 

In atomic physics the term is used for transitions involving the same states, as long as there is a half-life involved, i.e. there is an actual absorption, with a time delay for emission that decreases exponentially.