What's the difference between an excitation spectrum and an absorption spectrum for the same molecule? Are the words interchangeable, or are they completely different processes? In Sharma, A. and Schulman, S. G. (1999). Introduction to Fluorescence Spectroscopy. Wiley interscience, it says "generally", so when is the excitation spectrum not proportional to the absorption? I don't have that textbook, I found the quote from http://en.wikipedia.org/wiki/Fluorescence_spectroscopy#_note-1 Thanks.