# Quick question about hydrogen atom perturbation

## Homework Statement

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

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

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

vela
Staff Emeritus
Homework Helper