# The expectation of 'z' and 'x+iy'

## Main Question or Discussion Point

can anyone give me any ideas on how to evaluate this:

<z>=<$$\Phi$$1|z|$$\Phi$$2>

(for say hydrogen wavefunctions). Similarly

<x+iy>=<$$\Phi$$1|x+iy|$$\Phi$$2>

FYI, I'm trying to understand how radiation is polarised (an external B field polarises radiation, so we must consider the dipole transition matrix thus:

<r>=<$$\Phi$$1|r|$$\Phi$$2>

so I am simply resolving 'r' into two components (in the xy plane and z axis).

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The only hard part in the calculation is knowing $$\left\langle r \right\rangle$$,
from which
$$x = r \sin\theta \cos\varphi$$
$$y = r \sin\theta \sin\varphi$$.
Can you think of a spherical harmonic equal to $$\sin\theta e^{i\varphi}$$? Then you need to know how to compute $$\left\langle Y^\ell_m\right\rangle$$, which is easy enough for simple cases, but if you want you can consult a table of 3-j symbols.

That should be all that you need.

Good luck.

aah, that's clever. thanks Ibrits.

for hydrogen like wavefunctions, x+iy=$$\sin\theta e^{i\varphi}$$=Y(l=1,m=1) so yeah its just the expectation of that.

what about the expectation of z?

I assume you know the representation of $$z$$ in spherical coordinates. I also assume you have a table of spherical harmonics handy. It shouldn't be hard to figure the rest out =)

oh i see, i was being really stupid (as per usual)... z=rcos(theta), so its just <r><Y1,0>. thanks:)