# Quick question about hydrogen atom perturbation

1. Mar 31, 2017

### davon806

1. The problem statement, all variables and given/known data

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 ?

2. Relevant equations

3. 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

2. Mar 31, 2017

### Staff: Mentor

What is the quantum number related to the projection of angular momentum?

3. Mar 31, 2017

### davon806

Oh,do you mean that the problem asks you to compute the probability from 1s -> |210> because m = 0 in the z-direction? So basically the remaining two n = 2 states have nothing to do with this question?

4. Mar 31, 2017

### vela

Staff Emeritus
Check out http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/vecmod.html to see why your guess isn't correct.

The problem asked you to calculate the probability for a given final state. It could have specified a different final state and given you a different expression for the probability.