What determines light-matter interaction processes?

[carllacan]

Hi.

Sorry for the awkwardly phrased title, I couldn't think of anything better that summarizes my question.

I've been learning about different types light-matter interaction, specially photon- electron processes, and there's something I don't get: what determines whether this process will be elastic or inelastic?

Thank you for your time.

EDIT: I just saw that my title got cropped, and on top of that I made a typo. Would any admin be so kind as to change it to "What determines light-matter interaction processes?"?

 

[A. Neumaier]

QED determines all light-matter interaction, in particular photon- electron processes. A process is inelastic whenever one or both of the particles involved changes its binding energy. The photon cannot, being always massless, but an electron can, if it is in an external potential where it can exist in a bound state. Thus excitation, deexcitation, capture, or emission of an electron in/by a molecule are inelastic processes.

 

[carllacan]

Thanks for the answer. Just to get it clear:

an electron can, if it is in an external potential where it can exist in a bound state.

Do you mean an electron can undergo inelastic scattering in those conditions or that it will? If it doesn't happen every time those conditions are met, is the fraction of times it does happen determined by QED, or are there other factors?

 

[A. Neumaier]

Thanks for the answer. Just to get it clear:Do you mean an electron can undergo inelastic scattering in those conditions or that it will? If it doesn't happen every time those conditions are met, is the fraction of times it does happen determined by QED, or are there other factors?

It does if and only if it actually changes its energy eigenstate. What actually happens is determined by the S-matrix (which is determined by QED together with the external potential specified). The squares of the absolute value of the appropriate S-matrix elements give the probabilities of the various possibilities for elastic and inelastic scattering.