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.