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P,Q and R branches!

  1. Jan 30, 2006 #1
    In rotational-vibrational spectroscopy P-branch corresponds to low level rotational transitions "delta J = -1" and R-branch to high rotations "delta J = +1". And I am confused about Q branch which corresponds to "delta J = 0" ; which means that no rotational change is there and it is allowed in some cases. I want to know when it is allowed and how?. Because when vibrational or electronic transitions are there rotational quantum number should change.
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  3. Feb 5, 2006 #2

    Claude Bile

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    The condition that delta J = 0 is derived from the rigid rotor model. Molecules that are not well approximated by the rigid rotor model can therefore potentially exhibit Q branch transitions.

  4. Feb 6, 2006 #3
    Hi claude...thanks for the replies, I got some thing out of them,.
  5. Feb 6, 2006 #4

    Claude Bile

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    Always good to know :wink: .

  6. Feb 21, 2006 #5
    Though its not the case you mentioned, it is also possible that electronic angular momentum may yield a [tex]\Delta J = 0 [/tex] selection rule. This may occur when a rotational transition accompanies an electronic transition between, say, a [tex]\Sigma[/tex] singlet state to a[tex]\Pi[/tex] singlet state. In this case, the photon angular momentum is transferred to the electronic degrees of freedom as opposed to the rotational motion of the nuclei.
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