Why Can't H-Atoms Emit a Single Photon in the 2s to 1s Transition?

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Why is it that for the H-atom there is no single photon emission in the transition from 2s to 1s? I realize that two-photon emission is valid in this transition but why not single-photon emission?
 
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You need to obey conservation laws. For single photon optical transitions you need \Delta l=\pm 1 which corresponds to conservation of angular momentum. As a photon always carries angular momentum (it is a spin 1 particle), the angular momentum of the atom inquestion must change accordingly and only those transitions which fulfill this condition can be dipole-allowed.
 
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Ahh, that makes much more sense. Thank you!
 

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