Raman spectroscopy Raman spectroscopy () (named after Indian physicist C. V. Raman) is a spectroscopic technique typically used to determine vibrational modes of molecules, although rotational and other low-frequency modes of systems may also be observed. Raman s ...

, the depolarization ratio is the

intensity Intensity may refer to: In colloquial use * Strength (disambiguation) *Amplitude *Level (disambiguation) *Magnitude (disambiguation) In physical sciences Physics *Intensity (physics), power per unit area (W/m2) * Field strength of electric, ma ...

ratio between the perpendicular component and the parallel component of Raman scattered light. Early work in this field was carried out by George Placzek, who developed the theoretical treatment of bond polarizability. The Raman scattered light is emitted by the stimulation of the electric field of the incident light. Therefore, the direction of the vibration of the electric field, or

polarization Polarization or polarisation may refer to: Mathematics *Polarization of an Abelian variety, in the mathematics of complex manifolds *Polarization of an algebraic form, a technique for expressing a homogeneous polynomial in a simpler fashion by ...

direction, of the scattered light might be expected to be the same as that of the incident light. In reality, however, some fraction of the Raman scattered light has a polarization direction that is perpendicular to that of the incident light. This component is called the perpendicular component. Naturally, the component of the Raman scattered light whose polarization direction is parallel to that of the incident light is called the parallel component, and the Raman scattered light consists of the parallel component and the perpendicular component. The ratio of the peak intensity of the parallel and perpendicular component is known as the depolarization ratio (ρ), defined in equation 1. For example, a spectral band with a peak of intensity 10 units when the polarizers are parallel, and intensity 1 unit when the polarizers are perpendicular, would have a depolarization ratio of 1/10 = 0.1, which corresponds to a highly polarized band. The value of the depolarization ratio of a Raman band depends on the symmetry of the molecule and the normal vibrational mode, in other words, the

point group In geometry, a point group is a mathematical group of symmetry operations ( isometries in a Euclidean space) that have a fixed point in common. The coordinate origin of the Euclidean space is conventionally taken to be a fixed point, and every ...

of the molecule and its irreducible representation to which the

normal mode A normal mode of a dynamical system is a pattern of motion in which all parts of the system move sinusoidally with the same frequency and with a fixed phase relation. The free motion described by the normal modes takes place at fixed frequencies. ...

belongs. Under Placzek’s polarizability approximation, it is known that the depolarization ratio of a totally symmetric vibrational mode is less than 0.75, and that of the other modes equals 0.75. A Raman band whose depolarization ratio is less than 0.75 is called a polarized band, and a band with a depolarization ratio equal to or greater than 0.75 is called a depolarized band. For a spherical top molecule in which all three axes are equivalent, symmetric vibrations have Raman spectral bands which are completely polarized (ρ = 0). An example is the symmetric stretching or "breathing" mode of

methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Ear ...

(CH₄) in which all 4 C–H bonds vibrate in phase. However for the asymmetric mode in which one C–H bond stretches while the other three contract, the Raman scattered radiation is depolarized. For molecules of lower symmetry ( symmetric tops or asymmetric tops), a vibration with the full symmetry of the molecule leads to a polarized or partially polarized Raman band (ρ < 0.75), while a less symmetric vibration yields a depolarized band (ρ ≥ 0.75).

References

Raman spectroscopy Ratios {{CMP-stub