Ellis and Randy are looking at a molecular absorption spectrum. The spectral lines appear to fall into two groups with a gap in the middle. Ellis says that this must be an absorption spectrum for transitions between the v = 0 and v = 1 vibrational states of a diatomic molecule. Randy says the gap in the middle must occur because a ΔJ = 0 transition is forbidden. Which one, if either, is correct, and why?
A) Ellis, because rotational levels are separated by greater energy differences than vibrational levels.
B) Randy, because rotational levels are separated by greater energy differences than vibrational levels.
C) Ellis, because the ΔJ = 0 transition is the most prominent spectral line in transitions between vibrational levels.
D) Neither, because rotational levels are separated by greater energies that vibrational levels and ΔJ = 0 transitions are not forbidden.
E) Both, because vibrational levels are separated by greater energy differences than rotational levels and ΔJ = 0 transitions are forbidden.

Question

Quizplus · Accepted Answer

Ellis is correct because vibrational transitions typically involve larger energy differences than rotational transitions, leading to distinct groups of spectral lines for different vibrational states. Randy is correct because in rotational transitions, a ΔJ = 0 transition (where J is the rotational quantum number) is indeed forbidden due to selection rules, explaining the gap in the spectrum.

Ellis and Randy Are Looking at a Molecular Absorption Spectrum