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

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  • #1
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Main Question or Discussion Point

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

<z>=<[tex]\Phi[/tex]1|z|[tex]\Phi[/tex]2>

(for say hydrogen wavefunctions). Similarly

<x+iy>=<[tex]\Phi[/tex]1|x+iy|[tex]\Phi[/tex]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>=<[tex]\Phi[/tex]1|r|[tex]\Phi[/tex]2>

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

Answers and Replies

  • #2
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The only hard part in the calculation is knowing [tex]\left\langle r \right\rangle[/tex],
from which
[tex]x = r \sin\theta \cos\varphi[/tex]
[tex]y = r \sin\theta \sin\varphi[/tex].
Can you think of a spherical harmonic equal to [tex]\sin\theta e^{i\varphi}[/tex]? Then you need to know how to compute [tex]\left\langle Y^\ell_m\right\rangle[/tex], 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.
 
  • #3
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aah, that's clever. thanks Ibrits.

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

what about the expectation of z?
 
  • #4
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I assume you know the representation of [tex]z[/tex] in spherical coordinates. I also assume you have a table of spherical harmonics handy. It shouldn't be hard to figure the rest out =)
 
  • #5
62
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oh i see, i was being really stupid (as per usual)... z=rcos(theta), so its just <r><Y1,0>. thanks:)
 

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