In rovibrational transitions we have following selection rules
$$ \Delta v = \pm 1 $$
$$ \Delta J = \pm 1 $$
where $v$ is the vibrational quantum number and $J$ is rotational quantum number.
- All $\Delta J = +1$ transitions give rise to R-branch lines.
- All $\Delta J = -1$ transitions give rise to P-branch lines.
Literature also notes that R-branch lines are higher in frequency (and thus energy) than P-branch lines.
This is how it's defined but notice how I emphasized transition above? What confuses me is this: it is not specified which transition is meant, is it absorption or emission (of a photon)? Because it seems to me that it makes a difference.
Let's take one example transition, say between levels (v=0, J=0) to the (v=1, J=1) then we have:
- in case of absorption J goes from 0 to 1 so: $\Delta J = +1$
- in case of emission J goes from 1 to 0 so: $\Delta J = -1$
So it's the same transition but different $\Delta J $ based on whether it's emission or absorption. So to which branch does this transition belongs then?