Suppose that I have an atom in one corner of a room, and I fire a photon toward the opposite corner (and assume that it is absorbed there into the wall). There is essentially zero probability that that photon will interact with the atom (either be captured, or stimulate emission, or whatever). As the angle at which I fire the photon relative to the position of the atom becomes smaller, this probability increases until I'm aiming essentially at the nucleus of the atom, at which (I assume) the interaction probability will be largest. Several questions: 1. Am I framing this correctly? That is, it is a matter of interaction probabilities and the spatial relations determine these. 2. (Here's my real question) How does one frame this mathematically, esp. regarding the excitation state of the atom? 3. Suppose that instead of an atom and a photon I'm shooting another atom at the first one, say, I'm shooting one ground state H at another the same, v. H+ at H+, or an excited H at another? In these cases I'm hoping to find an equation that relates the excitation state of the atoms to the angles of interaction and the interaction probabilities. Thanks!