Rotational-Vibrational Spectroscopy: P,Q,R Branches

[photon79]

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.

 

[Claude Bile]

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.

Claude.

 

[photon79]

Hi claude...thanks for the replies, I got some thing out of them,.

 

[Claude Bile]

Always good to know .

Claude.

 

[beautiful1]

Though its not the case you mentioned, it is also possible that electronic angular momentum may yield a $$\Delta J = 0$$ selection rule. This may occur when a rotational transition accompanies an electronic transition between, say, a $$\Sigma$$ singlet state to a$$\Pi$$ 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.