Confused about fluorescence at resonance

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  • Thread starter Malamala
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Hello! I am a bit confused about a resonance signal that is obtained by measuring the fluorescence signal from overlapping a laser beam with some atoms. Based on the signal shape, the maximum number of counts corresponds to the resonant frequency of the transition (ignoring for this questions effects that might skew the shape of the signal). But I am not totally sure I understand why. If we use the laser at exactly the resonant frequency (we can ignore the linewidth of the laser), the electrons will have the highest probability of doing a transition to the upper level. However, a laser is basically an oscillating electric field (consider only electric dipole transitions). So the electron that we want to move to the upper state will have a high probability of transitioning to the upper state, but it will also have an equally big probability of going from the upper to the lower state (by stimulated emission) so the electron will oscillate between the 2 states. At resonance this oscillation will be higher (as the probability is higher), but why would one expect that when the laser is off and we measure the fluorescence signal, the electrons will end up in the upper level? Isn't it equally likely to be in the lower level too?
 

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DrClaude
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I'm not sure I understand what kind of experiment your are considering, so my answer may be off.

Yes, a laser resonant with a transition will lead to Rabi cycling between the ground and the excited state, but you have to compare the Rabi frequency with the duration of the experiment. If the laser is not intense, you will probably never reach a the point where the population of the excited state is being lowered by the laser.
 

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