Final answer:
The additional rule for allowed transitions in an absorption spectrum is that the change in the quantum number associated with the magnitude of orbital angular momentum, ΔI, must equal +1.
Step-by-step explanation:
The additional rule for allowed transitions in an absorption spectrum is that for a hydrogen-like atom, atomic transitions involving the emission and absorption of photons obey a selection rule. This selection rule states that the change in the quantum number associated with the magnitude of orbital angular momentum, ΔI, must equal +1. This means that the rotational quantum number, I, must change by one unit for the transition to be allowed. The additional rule for allowed transitions in an absorption spectrum is that the change in the quantum number associated with the magnitude of orbital angular momentum, ΔI, must equal +1.