The selection rules for the appearance, of P branch in the rotational - vibrational absorption spectra of a diatomic molecule within rigid rotor-harmonic oscillator model are
(a)$\Delta \mathrm{v}= \pm$ and $\Delta \mathrm{J}= \pm 1$
(b)$\Delta \mathrm{v}=+1$ and $\Delta \mathrm{J}=+1$
(c)$\Delta \mathrm{v}=+1$ and $\Delta \mathrm{J}=-1$
(d)$\Delta \mathrm{v}=-1$ and $\Delta \mathrm{J}=-1$
Answer
Answer: C ✓ checked by 4AB · confidence high
Explanation
In the fundamental absorption band (Δv = +1), the P branch comes from ΔJ = −1 (c).