Introductory Remark
Before listing short descriptions
of the various forms of optical activity, a general comment on the
terminology is in place. Two aspects need be distinguished: the underlying
optical property inherent to matter and the manifestations of
this property in specific optical experiments. For example, a dielectric
solid with the property called Linear Dichroism rotates the polarization
plane when acting on linearly polarized light but changes the ellipticity
when acting on circularly polarized light. An optical property of a material
is often referred to by one of its manifestations, which has lead to the
use of different synonyms for the same physical property. Some of the synonyms
have been indicated in parentheses after the preferred term. We mention
in this context Circular Birefringence and Optical Rotation,
the latter phenomenon (i.e., the rotation of plane polarized light) being
recognized to be a consequence of the former (i.e., the difference in the
Refractive Indices for left and right circularly polarized light) by Fresnel.
An optical medium can be characterized by so-called eigen vectors. These vectors represent polarized light (here, circular or linear) of which the polarization is not altered by interaction with the medium. The eigen vectors can depend on the direction of light propagation. A medium with eigen vectors which differ in absorption gives rise to Dichroism; a difference in the Refraction to Birefringence. The interactions of the eigen vectors with the medium define the eigen values of the medium. The term Linear in expressions like Linear Dichroism and Linear Birefringence refers to media which have eigen vectors corresponding to linearly polarized light. Accordingly, Circular refers media with eigen vectors corresponding to circularly polarized light. In general, the effect of a medium on polarized light can be predicted by: (i) Expansion of the polarized light in the eigen vector basis for the medium. (ii) Calculation of the effect of the medium on the eigen vector components. (iii) Composition of the resulting vectors (including the eigen value factors obtained under (ii)). The construction is illustrated by the Figure. The refractive index (n) has emerged as a basic physical quantity for the description of the interaction between matter and light and from mid 19th century on theories have been advanced to relate this quantity with the electronic structure of matter. Therefore, where possible, we have used terms like Circular Birefringence (preferred term) instead of Optical Rotation (derived term). Descriptions of the derived terms have been included under the preferred term.
List of Descriptions
Birefringence
Circular Birefringence
Circular Dichroism
Cotton-Mouton Effect
Dichroism
Dispersion
Dispersion Relations
Double Absorption
Double Refraction
Faraday Effect
Linear Birefringence
Linear Dichroism
Magnetic Circular Birefringence
Magnetic Circular Dichroism
Magnetic Linear Birefringence
Magnetic Linear Dichroism
Magnetic Optical Rotation
Magnetic Optical Rotatory Dispersion
Natural Circular Birefringence
Natural Circular Dichroism
Optical Activity
Refractive Index
Zeeman Effect
Birefringence (B)
(Synonym: Double Refraction) Birefringence is the dependence
of light refraction on light polarization. The refractive index
is related to the speed of light (See Refractive
Index). A beam of unpolarized light that is incident on a piece of
birefringent material is split at the surface into polarized beams propagating
along different trajectories with different speeds (called the normal and
extraordinary rays). This phenomenon is the origin of the terms Birefringence
and Double Refraction.
Circular Birefringence (CB)
(Synonyms: Circular Double Refraction; Optical Rotation;
Optical
Rotatory Strength; Optical Rotatory Dispersion) Circular
Birefringence is the difference in refraction (and the associated
speed of light) of left and right circularly polarized light. The effect
can been observed in liquids and solutions even if the chromophores are
randomly oriented in space. In the latter case, CB reflects the dissymmetry
(that is, the lack of mirror symmetry) of the liquid molecules or dissolved
chromophores and is only observed for molecules with a three-dimensional
structure. CB studies have played an important role in the development
of stereochemistry. The polarization plane of linearly polarized light
traversing a Circularly Birefringent medium is rotated. The rotation is
the result of an increment in the relative phase of the left and right
circularly polarized components in which the plane polarized light can
be decomposed. The increment (which can be either positive or negative)
is caused by differences in the interactions of left and right circularly
polarized light with a medium consisting of chiral molecules. The rotation
angle is given by
Circular Dichroism (CD)
Circular Dichroism is the difference in the absorption (or emission)
of left and right circularly polarized light. Like CB, the effect can already
been observed in liquids and solutions in which the chromophores are randomly
oriented in space. CD (also called natural CD) reflects the dissymmetry
(that is, the lack of mirror symmetry) of the liquid molecules or dissolved
chromophores and can only be observed for molecules with a three-dimensional
structure (See also: Circular Birefringence)
Cotton-Mouton Effect
The Cotton-Mouton Effect is the combination of Magnetic
Linear Birefringence and Magnetic Linear Dichroism
of molecular solutions (or colloidal suspensions) caused by the alignment
of the molecules (or colloidal particles) induced upon application of a
magnetic field (See Linear Birefringence and
Linear
Dichroism). The Cotton-Mouton effect should not be confused with the
intrinsic
MLB and MLD effects, which are independent of molecular alignment (SeeMagnetic
Linear Birefringence and Magnetic Linear Dichroism).
Dichroism (D)
(Synonym: Double Absorption) In this homepage we are
essentially concerned with electric dipole transitions in molecules.
With each electronic transition there is associated an electric transition
dipole moment, which is a 3-dimensional vector with, in general, complex
components. The vector has a definite spatial relationship ("direction")
with respect to the nuclear frame of the molecule. The absorption (or emission)
of polarized light by the molecule depends on the degree of alignment of
the electric radiation field with respect to the electric transition dipole
moment. This dependence confers upon the molecule an anisotropy
as to its interaction with polarized light. The anisotropy gives rise to
Dichroism (also called Natural Dichroism or Double Absorption), which is
defined as the effect of there being a difference in the absorption (or
emission) of monochromatic light depending on the polarization of the light.
Atoms, which are spherically symmetric, do not exhibit any Natural Dichroism
without the intervention of symmetry breaking by an applied field. Dichroism
of molecular assemblies is related to the Dichroism of the molecular constituents
(See Linear Dichroism and Circular
Dichroism). In the current definition, Dichroism is an effect that
can be observed for monochromatic light. This seems to conflict with the
literal meaning of the word "Dichroism = Two-coloredness". The explanation
for the term’s origin is the following. If one observes a crystal, such
as tourmaline, through a linear polarizer, one observes color changes by
rotating the polarizer. In other words, the rotation alters the polarized
spectrum (i.e., the functional dependence of the transmitted light intensity
on frequency). The spectral change is caused by the fact that the absorption
difference of the two linearly polarized light components is a function
of light frequency. The latter effect is called Dichroic Dispersion
and is usually what is meant if the term Dichroism is used.
Dispersion
Dispersion is the dependence of a physical quantity on the frequency
of light. The Dispersion of the wavelength of light (l(w))
defines the velocity of light (c’ = lw/2p)
as a function of color.
Dispersion Relations
A Dispersion Relation expresses the Dispersion
of the real part (n’) of the Refractive Index into the imaginary part (n")
of the Refractive Index, or vice versa. In the framework of linear response
theory, general expressions for the Dispersion Relations (n’(n") and n"(n’))
have been derived by Kramers and Kronig. These relations give a mathematical
formulation for the causality principle, viz. that cause precedes effect.
It follows that the Dispersion Curves for refraction and absorption are
interconvertable quantities and have essentially the same information content.
Similar relationships exist for the dispersions in Birefringenceand
Dichroism, eliminating the need to do both types
of measurement. Dichroic measurements have become the preferred technique
for practical reasons.
Double Absorption
(Synonym: Dichroism)
Double Refraction
(Synonym: Birefringence)
Faraday Effect
(Synonym: Magnetic Circular Birefringence)
Linear Birefringence (LB)
(Synonym: Linear Double Refraction) Linear Birefringence
is the difference in refraction (and the associated speed of light)
of linearly polarized light with orthogonal planes of polarization. The
conditions for its occurrence are similar to those described for Linear
Dichroism. The polarization plane of linearly polarized light does
not change when the polarization plane is parallel to one of anisotropy
axes of a Linearly Birefringent medium; only the relative phase of the
two orthogonal linearly polarized components is affected (See Introductory
Remark).
Linear Dichroism (LD)
Linear Dichroism is the difference in the absorption (or emission)
of linearly polarized light beams with orthogonal planes of polarization.
Atoms are strictly
achroic as a result of their spherical symmetry.
A certain degree of anisotropy is required to generate the effect. In general,
the electronic transitions in single molecules are Dichroic, with the possible
exception of transitions induced by polarized light incident along certain
molecular symmetry planes. The effect completely cancels in ensembles of
randomly oriented molecules (e.g., chromophores in solution) because with
respect to disordered systems all linear polarization planes are equivalent.
Linear Dichroism can only be observed in molecular ensembles in which the
molecules are to some extent aligned (e.g., in samples with "texture").
The effect is also referred to as Natural Linear Dichroism to emphasize
that no applied field is required for the effect to exist. If we consider
a thin slab of Dichroic material, the direction along which the chromophores
are aligned in the plane of the slab together with its normal define the
optical
anisotropy axes. Linearly polarized light incident on the slab with
a polarization plane parallel to one of the anisotropy axes is diminished
in intensity but not altered in polarization (for simplicity, we have assumed
here that there is no Birefringence). The two types
of polarized light can be considered as the eigen vectors
of the material (vectors that are not changed, apart from a constant, by
the action of the material) of which the corresponding eigen values are
related to the light absorption by the slab. Given the direction of the
electric transition dipole with respect to the molecular frame, LD can
be used to determine molecular alignment, or vice versa. LD can also be
observed from ordered arrays of molecular chromophores in crystals and
samples containing molecules that are aligned by an externally applied
field (See Cotton-Mouton Effect).
Magnetic Circular Birefringence (MCB)
(Synonyms: Faraday Effect; Magnetic Optical Rotation;
Magnetic
Optical Rotatory Dispersion (MORD)) Magnetic Linear Birefringence
is a kind of Optical Activity that arises upon placing
matter in a magnetic field. MCB is defined as the field-induced difference
in refraction (and the associated speed of light) of the left and
right circularly polarized components of light that is incident parallel
to the magnetic field. MCB is a universal effect and does not require any
of the prerequisites for natural CB, such as the
presence of chiral molecules. MCB leads to the rotation of the polarization
plane of linearly polarized light and is therefore also called Magnetic
Optical Rotation. The dependence of the rotatory effect on color is referred
to as Magnetic Rotatory Dispersion (MORD). The rotatory effect was discovered
by Faraday in 1849 and is also called the Faraday Effect. The discovery
played an important role in establishing light as an electro-magnetic phenomenon.
Magnetic Circular Dichroism (MCD)
Magnetic Linear Dichroism is a kind of Optical Activity
that arises upon placing matter in a magnetic field. MCD is defined as
the field-induced difference in the absorption (or emission)
of the left and right circularly polarized components of light that is
incident parallel to the magnetic field. MCD is a universal effect and
does not require any of the prerequisites for natural
CD to occur. The strongest MCD effects have been measured for paramagnetic
chromophores. These effects are far more intense than the accompanying
natural CD, a property that can be used for selectively probing metal centers
in proteins.
Magnetic Linear Birefringence (MLB)
Magnetic Linear Birefringence is a kind of Optical Activity that arises
upon placing matter in a magnetic field. The MLB effect is defined as the
difference in the refraction (or, equivalently, in the speed) of
linearly polarized light beams that are incident normal to the magnetic
field and of which the polarization planes are parallel and perpendicular
to the applied magnetic field. As for MLD, two effects can be distinguished:
intrinsic
MLB and orientational MLB (See Magnetic
Linear Dichroism).
Magnetic Linear Dichroism (MLD)
Magnetic Linear Dichroism is a kind of optical activity that is induced
by placing matter in a magnetic field. The MLD effect is defined as the
difference in the absorption (or emission) of linearly polarized
light beams that are incident normal to the magnetic field and of which
the polarization planes are parallel and perpendicular to the applied magnetic
field. Two kinds of MLD can be distinguished. (1) The intrinsic
MLD effect, which is a consequence of the Zeeman Effect (the relation between
the two effects has been analyzed in some detail in theoretical studies
by Voigt in 1902). (2) The orientational MLD effect, which results
from orientation of molecules by the magnetic field (also called the Cotton-Mouton
Effect). The latter effect resembles Natural Linear
Dichroism in that it originates from the preference of molecules to
orient themselves along a specified direction in space, but it differs
from Natural Linear Dichroism in that the molecules are oriented by an
external field and not "spontaneously" by intermolecular interactions.
Field-induced orientation originates from interactions of the magnetic
field with molecules having either a permanent magnetic moment or an induced
magnetic moment. In the latter case, the magnetic interactions result in
a torque if the magnetic polarization of the molecule is anisotropic. The
average molecular orientation is temperature dependent, an effect described
by Langevin. Orientational MLD is found to be stronger than intrinsic MLD
in studies of liquid samples (either solutions or pure liquids). As a result,
the intrinsic MLD escaped detection, despite great many attempts made by
Kerr, Majorana, Meslin, Cotton and Mouton to detect the effect in the period
1900-20. Only if the chromophores are immobilized by freezing the liquid,
the orientational effect can be eliminated and the intrinsic MLD measured.
Recently, cryogenic conditions in conjunction with instrumental improvements
afforded detection of an intrinsic MLD effect of molecular chromophores
in frozen solution. The information content of intrinsic MLD differs
from that of orientational MLD. While the former effect exclusively
depends on chromophore's electronic response to the magnetic field,
the latter effect rather reflects its molecular response and is
therefore of little use for identification and characterization purposes.
Moreover, intrinsic MLD is a universal effect, whereas orientational MLD
is not. Hereafter, we shall use the term MLD to indicate the intrinsic
effect.
Magnetic Optical Rotation
(Synonym: Magnetic Circular Birefringence)
Magnetic Optical Rotatory Dispersion
(MORD)
(Synonym: Magnetic Circular Birefringence)
Natural Circular Birefringence
(Synonym: Circular Birefringence)
Natural Circular Dichroism
(Synonym: Circular Dichroism) The adjective Natural
is often used to indicate that the Dichroism (or
Birefringence)
in question is not caused by an externally applied field.
Optical Activity
Optical Activity is the effect of light polarization on the interaction
of light and matter. This definition includes the various forms of (magnetic)
Birefringence and (magnetic) Dichoism listed here.
Refractive Index
The Refractive Index is a material specific complex number, n = n’+in".
The real part (n’) relates the speed of light in the material (c’) to the
speed of light in vacuum: n’ = c/c’; the imaginary part (n") is a measure
for the absorption of light. In general, the Refractive Index depends on
the light frequency (See Dispersion), the
direction of light propagation with respect to the anisotropy axes of the
medium, and polarization (See Introductory Remark).
Zeeman Effect
The Zeeman Effect is the splitting of a single unpolarized spectral
line into a multiplet of polarized spectral lines occurring upon
applying a magnetic field. The effect was discovered in atomic emission
spectra of sodium by Zeeman in 1897. The splitting is associated with the
alignment of electronic orbits (and spins) by the interaction of the magnetic
momenta generated by these periodic electronic motions and a magnetic field.
Depending on the direction of observation, the Zeeman spectra show either
Magnetic
Circular Dichroism or Magnetic Linear Dichroism.
The MCD and MLD effects in atomic absorption spectra were detected
in a series of studies that started mid 1960th.