Re-exciting organic scintillator

[rphys]

I've attached an image from the book "nuclear physics" by John Lilley.

It says "in only one case is the restulting photon energy sufficient to cause re-excitation". My question is what is this one case? I recall my lecturer saying only decay to S₀₁ is sufficient to cause re-excitation, but I can't find any evidence for this.

Thanks for any help

 

[mfb]

Most electrons are in S₀₀, they need an energy of at least S₁₀-S₀₀ to be excited. There is only one way an electron in S₁₀ can release that much energy: If it goes to S₀₀. That is usually filled so it is a rare process. You can also have S₁₀->S₀₁ but then you need an electron in S₀₁ to get excited. Transitions to down higher energy levels need even higher energy levels of electrons for re-excitations but they are getting really rare.

 

[rphys]

Most electrons are in S₀₀, they need an energy of at least S₁₀-S₀₀ to be excited. There is only one way an electron in S₁₀ can release that much energy: If it goes to S₀₀. That is usually filled so it is a rare process. You can also have S₁₀->S₀₁ but then you need an electron in S₀₁ to get excited. Transitions to down higher energy levels need even higher energy levels of electrons for re-excitations but they are getting really rare.

So would you say the book is referring to fluorescence photons emitted during decay to S₀₀ as being the ones with sufficient energy to cause re-excitation? I am tempted to repeat what the book has said (only one case) and keep it vague because I am not sure about this.

 

[mfb]

So would you say the book is referring to fluorescence photons emitted during decay to S₀₀ as being the ones with sufficient energy to cause re-excitation?

That's how I would interpret the statement of the book. It depends on the temperature as well but you'll have empty S_0x states accessible at every reasonable temperature.