Selection Rules and Why Some Transitions Never Appear
A spectrum is missing far more lines than it shows. Whether a transition appears depends on an integral over the two states and the light's interaction with the molecule, and that integral is often exactly zero.
What a learner can do afterwards
- States the gross selection rule for infrared and for microwave absorption in terms of a changing dipole
- Explains why homonuclear diatomics show no infrared absorption but do show Raman scattering
- Applies the specific selection rules on vibrational and rotational quantum numbers to predict which lines occur
- Distinguishes a forbidden transition from a weak one and gives an example of each
1 · Read
A spectrum shows far fewer lines than the levels allow. A photon carries one unit of angular momentum, so the jump must hand that over properly. Selection rules follow from conservation laws like this one. They decide which pairs of levels are connected; a blocked pair usually gives no line in emission or absorption.
The gross rule: the molecular dipole must change during the vibration, or no infrared absorption happens at all. Stretching nitrogen or oxygen never builds a dipole, so those gases absorb no infrared. Some carbon dioxide vibrations change its dipole, which is why it acts as a greenhouse gas while they do not.
Nitrogen shows no infrared band, yet it still scatters light in a Raman spectrum. Stretching the molecule deforms its electron cloud, so the polarisability changes even though no dipole appears. Raman needs a changing polarisability where infrared needs a changing dipole. That is why the two methods see different jumps.
Vibration normally steps by one quantum, rotation by one unit of J. Overtones, two-step jumps, are weak but visible, because real bonds are not perfect springs. Some forbidden transitions stay missing, like the delta J = 0 line in HCl's band. Others, like the spin blocked triplet step, leak through slowly: slow triplet glow is phosphorescence, fast singlet flash fluorescence. The P and R branches flank the gap where that missing line would sit.
A changing dipole opens infrared absorption, and the step rules decide which of those jumps actually appear.
The rewritten readings were written for this dataset.
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.