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?"?