Difference between fluorescence and phorforescence

[DaTario]

I all,

I am afraid of abusing the dictionary now, but by fluorescence I mean excitation and immediate emission. By phosforescence I means excitation and after a randomic time that can be of the order of minutes, the atom emits.
I woud like to confirm if the names are correct and if there is an etimologic reason for these terms.

Thanks


Sincerely

DaTario

 

[alxm]

Fluroescence is 'immediate' re-emission compared to phosphorescence, but the difference in time scales itself isn't the proper distinction between the two. (they both have time to undergo some vibrational relaxation, obviously things that visibly fluoresce in UV light are re-emitting a lower frequency) The difference is that phosphorescent re-emission requires a forbidden transition (typically a singlet-triplet transition). By which 'forbidden' really means 'very unlikely', which in turn means a long lifetime for the excited state.

Fluorescence is named for the mineral fluorite, which fluoresces in UV. Phosphorescence is named for glowing phosphorus - which is not actually a case of phosphorescence but chemiluminescence.

 

[DaTario]

Thank you alxm, but could you elaborate a little more on this?

It is clear that
1) fluorescence implies fast emiting response under stymulus of excitation.
2) phosphorescence implies slow response under the same stymulus
3) phosphorescence implies generaly forbiden transitions which involve low probability and then large times (is this explanation the same as saying that we have here largely stable metastable excited states?)

But chemiluminescence is not so clear.
Fluorescence implies reemiting in lower frequencies? (accompained by some sort of vibrational relaxation?)

thank you

Best wishes

DaTario

 

[alxm]

Your points are pretty much correct. To elaborate a bit: 'Fast' and 'slow' are relative terms. In both fluorescence and phosphorescence, the system often has time to undergo vibrational relaxation, see for instance: http://en.wikipedia.org/wiki/File:Franck-Condon-diagram.png" (excitation in blue, re-emission in green) A consequence of this (different states) is that you have different transition probabilities for the excitation and re-emission; so depending on what your excited state is and what your molecule is, the thing will linger in the excited state for longer or shorter periods of time. The state is metastable, but it's a real state.

(The reason I underline that is because http://en.wikipedia.org/wiki/Raman_scattering" can be viewed as excitation and re-emission from a virtual state, and is much faster than fluorescence or phosphorescence.)

To elaborate on forbidden transitions, it's typically a process that simply can't occur as a single-photon process, but rather requires the involvement of a virtual photon. (the transition probabilities are a lot trickier to calculate). In both cases the transition probabilities (and transitions themselves) can be altered drastically through the presence of other substances (http://en.wikipedia.org/wiki/Quenching_(fluorescence)" )

Chemiluminescence is just the fancy word for 'chemical reactions that give off light'.

 

[DaTario]

Thank you,

Sincerely

DaTario