The intent of this thread (for now) is to introduce the basic theoretic principles of rotational/vibrational spectroscopy, follow this up with a brief sketch of the expermiental techniques and finally zoom in on the experimental and theoretical aspects of a specific technique of recent interest - for which I've chosen Matrix Isolation Spectroscopy. As of now, this is what I have in mind (and I expect to devote no more than a couple of posts to each sub-topic, barring questions or other responses) : PART 1 : Theory of rotational/vibrational spectroscopy : - - - classical derivation of Raman/IR/Rayleigh scattering - - - useful intuitions for determining Raman and IR active modes of simple inorganic molecules - - - sketch of quantum picture and comparison with derived classical results - - - (maybe) a brief recap of the rigid rotor model for vibrational levels of simple molecules - - - selection rules and some explanation of their origin - - - application to identification of organic molecules/functional groups - - - (maybe) qualitative explanation of a real spectrum based on theory developed in thread [At this point - and, in fact at any point in the "discussion" - others are invited to correct me, add insights or ask questions about things I've posted so far.] PART 2 : Experimental Techniques : - - - Sample preparation : brief overview - - - Raman Spectroscopy; Surface Enhanced Raman - - - IR Spectroscopy; FT-IR - - - (maybe) FT-IR, Raman microscopy PART 3 : Matrix Isolation Spectroscopy - - - Introduction - - - list of references for some recent papers in MIS - - - "free for all" discussion Parts 2 and 3 are less well-planned in my head right now, and will likely evolve significantly from the above outline, as the thread progresses. As a disclaimer, I should add that most of what I know and will talk about is stuff that I've read from various papers, texts or websites or have thought about over lunch. Very little of what I know comes from formal, classroom instruction, so others that are more strongly trained in the areas of discussion are urged to help, correct or supplement me. <photon, I'll get to your question in a little bit> It's hard to find any readable introductions to Spectroscopy, so I'll give you a simple outline. If anyone has links to any simple introductions to spectroscopy, please post them. The idea of using spectroscopy as an analytical (qualitative) tool is based on the fact that different molecules have different electronic, vibrational and rotational energy levels which can serve as identifiers. The process simply involves irradiating the molecules under study with light and studying the spectrum of absorbed or emitted radiation. The incident photons are absorbed by the molecules/electrons causing them to be promoted to excited states. The electron/molecule then falls from this excited state to a lower state, and from there to yet another lower state and so on. Each of these lowerings is accompanied by the emission of a photon (to ensure conservation of energy). Thus by looking at the energies of the emitted photons, one gets an idea about the energy differences between different quantum states of the molecule and from this data, can deduce what the molecule is.