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 are conservation laws in disguise. They decide which pairs of levels are connected, and the rest stay missing from both emission and 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.
Learn the specific steps too. Vibration normally steps by one quantum and rotation by one unit of J. Overtones that jump two vibrational steps are weak but visible. A forbidden transition breaks a rule and stays missing, like the delta J = 0 jump in a band or a spin blocked triplet step. The P and R branches flank the gap where that missing line would sit. Slow triplet glow is phosphorescence; fast singlet flash is fluorescence.
A changing dipole opens infrared absorption, and the step rules decide which of those jumps actually appear.
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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.