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Hund's cases for diatomic molecules

  1. Feb 12, 2016 #1
    Dear All,

    May anyone please advise me to the following questions in case of diatomic molecules:

    1. How do we choose which Hund's case ((a), (b), or (c)...) that best describes a particular diatomic molecule?

    2. How can we deduce from Hund's cases molecular electronic states (2s+1)ΛΩ (e,g. Σ+/-, Π, Δ, Φ, Γ...)?

    3. When Λ-type doubling for non-sigma states should be taken into consideration?

    4. How do we obtain the spin-orbit interaction terms A and B (where Y=A/B)?

    Any help would be greatly appreciated...
    Best wishes
  2. jcsd
  3. Feb 12, 2016 #2


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    Staff: Mentor

    It depends on the strength of the spin-orbit coupling in comparison to other couplings, such as the residual electron-electron interaction. As far as I know, it can be difficult to predict in advance which coupling case is the right one, and it is the actual spectrum that will give the answer (like LS vs jj coupling in atoms).

    The term symbol is obtained from the electronic configuration. The Hund cases will affect how the term symbol can be written. For instance, in Hund's case C, Λ is not defined, so one uses Ω instead.

    I don't know. Someone more knowledgeable may chime in.

    I don't know what this means. Can you explain the notation?
  4. Feb 12, 2016 #3
    Is there a way we can predict whether L and S are good quantum numbers to choose case (a), for example?

    The electronic energy of a multiplet term is given to a first approximation by: Te = To + AΛΣ.
    where To is the term value when the spin is neglected (spin-free) and A is a constant for a given multiplet term (for spin-orbit). The coupling constant A determines the magnitude of the multiplet splitting. If A>0 , the spin-orbit terms are considered as regular states (2Π1/2, 2Π3/2). For A<0, we have an inverted terms (2Π3/2, 2Π1/2).
    How do we evaluate or find A and B terms? I am searching for a relation for A and B, but I am not finding any!
  5. Feb 12, 2016 #4


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    Science Advisor

    Landau, Lifshitz, vol 3, Quantum Mechanics, contains a nice discussion of the Hund's coupling cases.
  6. Feb 12, 2016 #5
    Thank you so much DrDu!
  7. Feb 14, 2016 #6
    Thank you DrClaude for your help and continuous support...
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