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Collisional excitation: selection rules for rotations?

  1. Aug 25, 2015 #1
    To be specific, I am referring to CO molecules undergoing collisions with H2, resulting in CO transitioning to an excited vibrational state. I can't seem to find any rotational selection rules for collisions, meaning ΔJ could be essentially anything, as long as energy and angular/linear momentum are conserved. The rigid rotor approximation (i.e., rotational transitions for CO) selection rule is ΔJ = ±1, but apparently this only applies to electronic transitions. Can anyone shed any light on the reason behind this discrepancy?
  2. jcsd
  3. Aug 26, 2015 #2


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    Staff: Mentor

    The dipole selection rule is directly related to conservation of angular momentum. The photon having spin 1, its spin angular momentum has to be transferred to/from the molecule, hence the ##\Delta J = \pm 1## rule. Note that ##\Delta J = 0## (Q-branch) is also possible for some electronic transitions, where the angular momentum of the electronic state changes.

    So there is no discrepancy between absorption/emission and collisions. Conservation of angular momentum is the only thing in play.
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