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As far as I know, the discrete energy levels of many atoms form quasicontinuum energy levels or bands. In a semiconductor, when a photon with energy bigger than the bandgap "hit" the electron, this electron moves from the valence band to the conduction band, and an exciton is formed. What I don´t understand is: If I have a single atom, let´s say Si, it has 2 electrons in 3s and 2 in 3p levels, which means four electrons in the valence shell, so, if a photon hits one of these electrons a) which of these electrons is likely to be hit? (the most energetic one?) and where does it move? I mean, where is the conduction "shell"? In Si there are 4 allowed states in the 3p level, are these allowed states, the allowed states in the conduction "shell" or the 3d allowed states? Where are these states? Thanks for any insight about this.

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    $\begingroup$ Crystal energy bands are not atom energy levels. $\endgroup$
    – Jon Custer
    Apr 11, 2020 at 15:14

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If we start with a single atom, the electronic energy levels are rather well defined -- the associated uncertainty is usually called line width. The data can be looked up using NIST database.

If light shines onto a single atom, the coupling between the electron and the photon is strongly influence by the wavelength of the light. If the wavelength is such, that the photon couples an electron to another electronic state via an (allowed) dipole transition, the coupling is usually "strong". However, if the energy of the photon is detuned from a transition, the coupling between the two electronic states becomes "small". So one way to picture this, is that the photon couples all possible combinations of electronic states, each with a different coupling strength. Hence, each transition has a different probability. Whether or not a transition takes place becomes a matter of probability.

Now, let's form a solid from "many" individual atoms. We start with many identical atomic levels, which are degenerated. However, by bringing the atoms closer together we introduce interactions between the atoms. The atoms bind and the already complicated atomic level structure becomes even more complicated: The degeneration of the atomic levels is partly lifted and energy bands are formed. Thus, the energy bands are due to the interaction between the atoms. They are not part of a the level structure of a single atom.

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