I'm in undergrad doing research with a professor. He's leaving for vacation for a few weeks, and has tasked me with a project. I've been analyzing some spectra for some Carbon reactions, and he posed a question to me on why we don't see other higher-energy states, even if the experiment is ran at higher energies. Obviously, he doesn't expect me to come up with an exact answer, and is just having me feel the waters - he's told me before that I'm gonna tell another professor in our department why his theory is wrong. However, I would like to have some background in this area so I can come up with a theory, even if it's not the best. I'm not 100% sure about what I should look out for in a book to read. If someone could give me some recommendations, that would be great. Thanks in advanced. Edit: I'm going into my senior year in physics in the fall, the only courses that I haven't really taken thus far are E&M I & II.