The values of the stationary energy levels of the hydrogen atom are -Ry, -Ry/4, -Ry/9, ... where Ry =13.6 eV is the Rydberg constant.

At the energy level -Ry/4 there are the excited stationary states 2s and 2p.

Transitions exist from higher energies, say from 3p, to the state 2s, thus the state 2s is certainly reachable.

Once the state 2s is reached the only stationary state with lower energy is the ground state 1s, its energy being -Ry.

There is a selection rule according to which transitions from say, state ms to state ks (m, k integers) cannot happen. In particular the transition 2s->1s is "forbidden".

The rule means that the excited state 2s is stable.

Is there experimental confirmation of this particular instance of QM selection rules?. Or has the selection rule been disproved by experiment? In other words: Are laboratory experiments for or against the selection rule?