Collisional excitation: selection rules for rotations?

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SUMMARY

This discussion focuses on the collisional excitation of CO molecules interacting with H2, specifically addressing the absence of rotational selection rules during these collisions. While the rigid rotor approximation indicates that ΔJ = ±1 applies to electronic transitions, this rule does not hold for collisions, where ΔJ can vary freely as long as energy and angular momentum are conserved. The dipole selection rule, which is tied to the conservation of angular momentum, clarifies that the spin angular momentum of the photon must be transferred to or from the molecule, allowing for transitions including ΔJ = 0 in certain electronic states.

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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?
 
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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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