Why do aromatics fluoresce & scintillate?

[Keep_i_real]

I'm currently trying to figure out exactly why aromatics (e.g. polyvinyl toluene [PVT], polystyrene [PS], anthracene) fluoresce and scintillate...

I know that its got something to do with the pi system causing dense clouds of electrons to form in the ring(s), and that the addition of a methyl group to PS [i.e. PVT] increases the density of electrons in the ring which is why PVT is generally more widely used than PS in plastic scintillators (albeit with them both doped slightly with anthracene)...

Also, why are PS / PVT scintillators usually doped with anthracene? I'm thinking something along the lines of resonance energy transfer to improve quantum efficiency.

Beyond that, I can't quite fathom why this allows it to scintillate...

My understanding of fluorescence is that excited electrons/atoms/molecules relax thermally to S1, then a proportion of those relax with emission from S1 -> S0 - so why would several scintillation photons be produced by a single X-ray photoelectric interation?
Does the photoelectron then excite many more electrons, or does it just emit several photons as it relaxes (e.g. S3->S2 then S2->S1, S1->S0)?


Thanks in advance, and if you could advise me on any relevant books / papers, that would be great!

Apologies if there's any massive misuses of terminology above, I'm more of a physicist than a chemist (and I originally wrote this with the intent of posting on Yahoo Answers...).

-Mark

 

[jbsteele]

The fluorescence and scintillation properties of aromatics are related to the fact that they possess an extended pi electron system, which leads to efficient transition between electronic energy levels. In the case of PVT and PS, when exposed to X-rays, the electrons of the aromatic molecule can be excited to higher energy levels. As these electrons relax back to the ground state, they emit light in the form of fluorescence photons. Anthracene is added to PVT and PS to promote a more efficient transition from the excited state to the ground state, which is known as resonance energy transfer. This helps to increase the quantum efficiency of the scintillator material, meaning more scintillation photons per X-ray photon. In addition to the fluorescence process, the excited electrons in aromatics can also undergo a process known as intersystem crossing, in which they transition from a singlet state to a triplet state before finally returning to the ground state. This process is slower than the fluorescence process, but it can lead to the emission of multiple scintillation photons from a single X-ray photoelectric interaction. For further reading, I suggest looking into the literature on molecular fluorescence and scintillation. Some good books to start with are Fluorescence Spectroscopy by J.R. Lakowicz and Molecular Luminescence Spectroscopy by E. Mataga and T. Kubo.