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In atoms, there are electronic energy levels, when there is a match between those and a light photon energy, an electronic transition ocurs.

Example and question

Suppose an atom has a levels $0$, $1$ and $2$ with $E_2>E_1$ (relative to $E_0$). Then if a photon comes with energy $E_2>E_{\mathrm{ph}}>E_1$ why can't it be partially absorbed? So the electron moves to $ E_1$ and the extra energy of the photon is released. Is it possible?

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  • $\begingroup$ If the photon energy is below the threshold for exciting the atom, the photon still can scatter from atom elastically, but the absorbed energy-momentum by atom as a whole is normally too small to be noticed in the final photon. $\endgroup$ – Vladimir Kalitvianski May 30 '18 at 6:40
  • $\begingroup$ In case of non resonant photon an inelastic interaction is also possible; it is just much less likely than the elastic interaction. It is called Raman scattering. $\endgroup$ – Vladimir Kalitvianski May 30 '18 at 6:47
  • $\begingroup$ @VladimirKalitvianski So we have three phenomena. Elastic scattering non elastic scattering and absortion. But why we see only absortion lines on atomic spectra? Can't a photon with higher energy than the first transition be partially absorbed? $\endgroup$ – santimirandarp May 30 '18 at 6:54
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    $\begingroup$ Yes, it can. Read a Wikipedia entry for Raman scattering, please. $\endgroup$ – Vladimir Kalitvianski May 30 '18 at 6:55

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