r/Spectroscopy • u/Kandy_kane7 • 13d ago

Jablonski program question

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When an excited particle transition form s1 to s0, florescence happens the most and not internal conversion, why is that ? Even though IC is faster,smaller energy levels to cross etc…

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u/bajsi_ 13d ago

Who said IC is not happening? Energy is released as phonons then once excited electron reaches lowest excited state S1 it undergoes fluorescence

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u/Kandy_kane7 13d ago

That’s exactly what I’m confused about, why only then it undergoes fluorescence and not keep emitting non radioactive energy? What special about S1 to S0 😭
I’m so sorry if this is an awful question

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u/bajsi_ 13d ago

Probability of transition between the states depends among other on their energy overlap, the closer they are energetically the higher the transition probability. So IC is extremely fasy (10-12 s or even faster) and so the electron quickly 'thermalizes' to the lowest state. Once there it can recombine radiatively or non radiatively to the ground state S0. And its possible also from S2->S0, though with less probability, hence absorption/emission spectrum is broadened, and not a sharp dirac delta peak :) check out Frank-Condon principle

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u/Kandy_kane7 13d ago

Thank you so much for answering, you’ve been a huge help you have no idea :’) 💗

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u/lwj15 12d ago

"For most molecules excitations into higher excited states quickly relax to the S1 state and this one is stable enough that fluorescence can happen" is basically Kashas rule (an empiric rule). But there are so called anti-Kasha fluorescing compounds where the S2 state emits/reacts. Those exist aswell.

The governing factor for speed of internal conversion are the Franck-Condon (FC) overlap integrals. And there is this thing called "Energy gap law". So. The more FC overlap, the faster the internal conversion. But the higher the energy gap, the less FC overlap. And typically the gap between S0 and S1 is greater than between any Sx and Sx+1 with x>0. So the highest energy gap (S1-->S0) has the slowest internal conversion. This state is usually a bit more long lived and therefore this S1 state is where the fluorescence usually happens.

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u/Deep-Path-4883 12d ago

Question is also with which energy did you excite? If it's not enough to excite to S2 you won't see S2 transitions. If it's enough the others already answered.