Editing Raman spectroscopy (section)

== Theory ==
{{Main|Raman scattering}}
{{more citations needed|date=February 2025}}

The magnitude of the Raman effect correlates with the polarizability of the [[electron]]s in a molecule. It is a form of inelastic [[light scattering]], where a [[photon]] excites the sample. This excitation puts the molecule into a [[Virtual state (physics)|virtual energy state]] for a short time before the photon is emitted. Inelastic scattering means that the energy of the emitted photon is of either lower or higher energy than the incident photon. After the scattering event, the sample is in a different [[Rigid rotor#Quantum mechanical rigid rotor|rotational]] or [[molecular vibration|vibrational state]].

For the total energy of the system to remain constant after the molecule moves to a new [[rovibronic coupling|rovibronic]] (rotational–vibrational–electronic) state, the scattered photon shifts to a different energy, and therefore a different frequency. This energy difference is equal to that between the initial and final rovibronic states of the molecule. If the final state is higher in energy than the initial state, the scattered photon will be shifted to a lower frequency (lower energy) so that the total energy remains the same. This shift in frequency is called a [[Stokes shift]], or downshift. If the final state is lower in energy, the scattered photon will be shifted to a higher frequency, which is called an anti-Stokes shift, or upshift.

For a molecule to exhibit a Raman effect, there must be a change in its electric dipole-electric dipole polarizability with respect to the vibrational coordinate corresponding to the rovibronic state. The intensity of the Raman scattering is proportional to this polarizability change. Therefore, the Raman spectrum (scattering intensity as a function of the frequency shifts) depends on the rovibronic states of the molecule.<ref>{{Cite book |last1=Roberts |first1=John D. |title=Basic principles of organic chemistry |last2=Caserio |first2=Marjorie C. |last3=Caserio |first3=Marjorie Constance Beckett |date=1977 |publisher=Benjamin |isbn=978-0-8053-8329-4 |edition=2. |location=Menlo Park, Calif.}}</ref>

The Raman effect is based on the interaction between the electron cloud of a sample and the external electric field of the monochromatic light, which can create an induced dipole moment within the molecule based on its polarizability. Because the laser light does not excite the molecule there can be no real transition between energy levels.<ref>{{Cite book|title=Spectroscopy for the biological sciences|last=Hammes|first=Gordon G.|date=2005|publisher=Wiley|isbn=9780471733546|oclc=850776164}}</ref> The Raman effect should not be confused with emission ([[fluorescence]] or [[phosphorescence]]), where a molecule in an excited electronic state emits a photon and returns to the ground electronic state, in many cases to a vibrationally excited state on the ground electronic state potential energy surface. Raman scattering also contrasts with infrared (IR) absorption, where the energy of the absorbed photon matches the difference in energy between the initial and final rovibronic states. The dependence of Raman on the electric dipole-electric dipole polarizability derivative also differs from IR spectroscopy, which depends on the electric dipole moment derivative, the atomic polar tensor (APT). This contrasting feature allows rovibronic transitions that might not be active in IR to be analyzed using Raman spectroscopy, as exemplified by the [[rule of mutual exclusion]] in [[centrosymmetry|centrosymmetric molecules]]. Transitions which have large Raman intensities often have weak IR intensities and vice versa. If a bond is strongly polarized, a small change in its length such as that which occurs during a vibration has only a small resultant effect on polarization. Vibrations involving polar bonds (e.g. C-O , N-O , O-H) are therefore, comparatively weak Raman scatterers. Such polarized bonds, however, carry their electrical charges during the vibrational motion, (unless neutralized by symmetry factors), and this results in a larger net dipole moment change during the vibration, producing a strong IR absorption band. Conversely, relatively neutral bonds (e.g. C-C , C-H , C=C) suffer large changes in polarizability during a vibration. However, the dipole moment is not similarly affected such that while vibrations involving predominantly this type of bond are strong Raman scatterers, they are weak in the IR. A third vibrational spectroscopy technique, inelastic incoherent neutron scattering (IINS), can be used to determine the frequencies of vibrations in highly symmetric molecules that may be both IR and Raman inactive. The IINS selection rules, or allowed transitions, differ from those of IR and Raman, so the three techniques are complementary. They all give the same frequency for a given vibrational transition, but the relative intensities provide different information due to the different types of interaction between the molecule and the incoming particles, photons for IR and Raman, and neutrons for IINS.