I have already solved the problem, but I don't really understand why the orbital angular momentum in the z-direction has to be taken to 0 ?
The Attempt at a Solution
Suppose the component of orbital angular momentum in the z-direction is non-zero, that means there are extra angular momentum apart from l = 1 in the 2p state ? So when excitation occurs,it is no longer a "2p" state? Just a guess