Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    DrClaude

    User Avatar

    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.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Collisional excitation: selection rules for rotations?
  1. Selection Rules (Replies: 0)

  2. Selection Rules (Replies: 2)

Loading...