Is photon absorption an elastic or inelastic collision? neither?
Under resonant conditions you can look at the absorption process as an elastic scattering event and as an inelastic one while off resonance. On resonance it also dependends on what decay channels the excited electron has.
The term "elastic collision" usually refers to a two-body scattering where each body has the same final energy in the center of mass system as its initial energy. With this definition, absorption is an inelastic process.
If the photon is "absorbed" it means that its energy is totally or partially transferred to the absorbing system, so the process it's inelastic by definition. However, if another photon of the same energy is emitted immediately after the absorption, the overall process can be considered as elastic.
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