--- title: "Selection Rules and Why Some Transitions Never Appear" description: "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 ex" canonical: https://lightmysky.com/learn/science/selection-rules-and-why-some-transitions-never-appear-mt_oJDr81W4fW source: https://lightmysky.com/learn/science/selection-rules-and-why-some-transitions-never-appear-mt_oJDr81W4fW.md retrieved: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # 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. Subject: Science · Area: Chemistry · Ages 19 to 20 Page: https://lightmysky.com/learn/science/selection-rules-and-why-some-transitions-never-appear-mt_oJDr81W4fW ## Ready when they can - 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 ## Lesson: Why some spectral lines never appear 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. **Example.** 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. **Recap.** A changing dipole opens infrared absorption, and the step rules decide which of those jumps actually appear. ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Molecular Motion Quantised: The Oscillator and the Rotor](https://lightmysky.com/learn/science/molecular-motion-quantised-the-oscillator-and-the-rotor-mt_dHgi-i9pxc) ## Opens up - [Reading Bond Lengths and Force Constants from Spectra](https://lightmysky.com/learn/science/reading-bond-lengths-and-force-constants-from-spectra-mt_5m7J1mFw68) - [Reducible Representations and Predicting Infrared and Raman Activity](https://lightmysky.com/learn/science/reducible-representations-and-predicting-infrared-and-raman-activity-mt_DP4JfkS6c0) - [Electronic spectra of complexes and Tanabe-Sugano diagrams](https://lightmysky.com/learn/science/electronic-spectra-of-complexes-and-tanabe-sugano-diagrams-mt_GQFMqha0es) - [Character Tables and What Symmetry Predicts](https://lightmysky.com/learn/science/character-tables-and-what-symmetry-predicts-mt_L8cTQjeKwf)