# Single photon emissions

1. Mar 23, 2012

### Demon117

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?

2. Mar 23, 2012

### Cthugha

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.

Last edited: Mar 23, 2012
3. Mar 23, 2012

### Demon117

Ahh, that makes much more sense. Thank you!