Relation between spontaneous emission, absorption and re-emission

[fisico30]

Hello Forum,
when a wavefield is incident of an atom and the photon have the right energy (equal to the band gap energy), the photon is absorbed and disappear...
But isn't the photon re-emitted later on and allowed to propagate through the material or does all its energy get lost into heat?


If the photon frequency omega is not equal to the resonance frequency, will the photon still excited the atoms in the material but the photon will be able to move through the material it as if the material was transparent? So each atom would absorb the photon and re-radiate it elastically if omega is not a resonant frequency?

If omega was a resonant frequency the atoms should still absorb, and re-emit the photon via spontaneous emission...I tend to associate spontaneous emission no with the disappearance of the photon but with a re-emission of it...

thanks,
fisico30

 

[SergioPL]

Your question is quite complex for me, but you can found answers about it in Richard P. Feyman's book "Quantum electrodynamics". (Advanced book classics collection).

As far as I know (for this book), if the photon is far away from the resonance frequency the material will be transparent for it.

When the frequency is in the band gap energy, atoms can absorb the photon raising the energy level of its electrons but it can also happen a double transition where the atom changes from an initial state L to another higher energy state M and then to a lower state N. In this case the incoming photon has been scattered by the atom (it has been absorbed and re-emitted with another direction and energy). It's also possible to make a L --> M --> L transition in which the incoming and outgoing photons will have the same energy.

All these things and more (and much better) are explained on the book mentioned above.


Sergio