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There is some kind of reciprocity, e.g. when an atom absorbs light and it passes into a higher energy level, it will spontaneously emit light going back to the initial energy level, with about the same probability with which it has absorbed the light. However this reciprocity is frequently circumvented, because atoms and ions have a lot of energy levels. Instead of re-emitting the light, the atom may pass more quickly to another energy level, and from there it may emit light with a very different probability (and of a different frequency, i.e. this is fluorescence). While in fluorescence light with a lower frequency is emitted, there is also the opposite case. In very intense light, e.g. from lasers, multi-photon absorption may happen. In that case there is also no reciprocity, because the atom has jumped an energy difference higher than that of the incoming photons. So it may re-emit light with a higher frequency. With rubidium atoms, multi-photon absorption is very frequently used, for Doppler-effect-free spectroscopy (by absorbing photons that come from opposite directions, so that the effects of the movement of the atom will cancel). In comparison with other atoms, rubidium vapor cells are easy to procure, for spectroscopy experiments, or for use in frequency or wavelength standards, but they still are rather expensive, especially when enriched in only one of the two rubidium isotopes (e.g. if you want just rubidium 87, instead of natural rubidium, a Rb vapor cell may cost close to $1200). |