Editing Selection rule (section)

=== Coupled transitions ===
{{Coupling in molecules}}
[[File:HCl rotiational spectrum.jpg|thumb|left|The infrared spectrum of [[HCl]] gas]]
There are many types of coupled transition such as are observed in [[rotational–vibrational coupling|vibration–rotation]] spectra. The excited-state wave function is the product of two wave functions such as vibrational and rotational. The general principle is that the symmetry of the excited state is obtained as the direct product of the symmetries of the component wave functions.<ref>Harris & Berolucci, p. 339.</ref> In [[rovibronic coupling|rovibronic]] transitions, the excited states involve three wave functions.

The infrared spectrum of [[hydrogen chloride]] gas shows rotational fine structure superimposed on the vibrational spectrum. This is typical of the infrared spectra of heteronuclear diatomic molecules. It shows the so-called ''P'' and ''R'' branches. The ''Q'' branch, located at the vibration frequency, is absent. [[Rotational spectroscopy|Symmetric top]] molecules display the ''Q'' branch. This follows from the application of selection rules.<ref>Harris & Berolucci, p. 123.</ref>

[[Resonance Raman spectroscopy]] involves a kind of vibronic coupling. It results in much-increased intensity of fundamental and overtone transitions as the vibrations "steal" intensity from an allowed electronic transition.<ref>{{cite book |last=Long |first=D. A. |title=The Raman Effect: A Unified Treatment of the Theory of Raman Scattering by Molecules |publisher=Wiley |date=2001 |isbn=0-471-49028-8 |chapter=Chapter 7: Vibrational Resonance Raman Scattering}}</ref> In spite of appearances, the selection rules are the same as in Raman spectroscopy.<ref>Harris & Berolucci, p. 198.</ref>