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Spectroscopy question

  1. Jan 14, 2008 #1
    1. The problem statement, all variables and given/known data

    For a many electron system with L=2,S=1 and J=2 calculate the angle between the L and S vectors both according to the old quantum theory and quantum mechanics.

    2. Relevant equations

    3. The attempt at a solution

    I suppose this problem is nothing but a simple application of cosine rule.The external angle is the answer.
    But I am thinking twice as they mention: both according to the old quantum theory and quantum mechanics.

    Am I missing something?
  2. jcsd
  3. Jan 14, 2008 #2

    Tom Mattson

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    In the Bohr model (I think that qualifies as "old quantum theory"), we have [itex]L=n\hbar[/itex], where [itex]n=1,2,3,...[/itex]. Contrast that with modern quantum mechanics.

    Does that help?
  4. Jan 14, 2008 #3


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    In QM, use LS cos(theta)=L.S=[J(J+1)-L(L+1)-S(S+1)]/2.
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