I'm doing some personal research on how matter interacts with radiation. Specifically, I am looking through the treatment of Bransden and Joachain. I've taken two semesters of quantum in the past (a while ago), but now I'm coming across something that I've either never seen or never stopped to question. In discussing Einstein Coefficients, I keep seeing this |rba|2 term, where b and a both refer to energy states, with Eb > Ea. Now, I'm having some trouble visualizing any significance to this term. It is defined in the usual way, as the sum of |xi|2, but I just have no idea what this means, as energy levels don't ever really have definite locations, as far as I understand. Could anyone help shed some intuition on what this "distance" means? Also, sorry in advance if this belongs in the homework/coursework forum, I wasn't quite sure.