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Chemistry / Re: What happens when electrons move between energy levels in atoms?
« on: 29/05/2016 01:12:31 »
http://hyperphysics.phy-astr.gsu.edu/hbase/optmod/lascool.html <- Laser cooling
Of note:
You specifically need to use an absorption event rather than simple scattering (i.e. what happens when the photon energy doesn't coincide with an energy level difference) and you actually need to use light slightly under the energy of the edge in order to selectively excite only the atoms you want to excite (the ones coming towards you that would be slowed instead of the ones going away that would speed up). Using just simple scattering the momentum transfer is generally much much smaller than an absorption event and you lose any hope of selectively interacting with only the atoms you can cool.
Of note:
Quote from: The Link
A conceptual problem is that an absorption can also speed up an atom if it catches it from behind, so it is necessary to have more absorptions from head-on photons if your goal is to slow down the atoms. This is accomplished in practice by tuning the laser slightly below the resonance absorption of a stationary sodium atom. From the atom's perspective, the headon photon is seen as Doppler shifted upward toward its resonant frequency and it therefore more strongly absorbed than a photon traveling in the opposite direction which is Doppler shifted away from the resonance. In the case of our room temperature sodium atom above, the incoming photon would be Doppler shifted up 0.97 GHz, so to get the head on photon to match the resonant frequency would require that the laser be tuned below the resonant peak by that amount.
You specifically need to use an absorption event rather than simple scattering (i.e. what happens when the photon energy doesn't coincide with an energy level difference) and you actually need to use light slightly under the energy of the edge in order to selectively excite only the atoms you want to excite (the ones coming towards you that would be slowed instead of the ones going away that would speed up). Using just simple scattering the momentum transfer is generally much much smaller than an absorption event and you lose any hope of selectively interacting with only the atoms you can cool.