Preferred Label : molecular orientation;
IUPAC definition : Absorption probability (referred to electric dipolar absorption) for a molecular transition
with its electric transition (dipole) moment at an angle θ with the electric vector
of the light is proportional to cos 2θ. For the whole sample it is proportional to
the orientation factor Kθ cos 2 θ , averaged over all sample molecules. This
average is 1 for a sample with all transition moments perfectly aligned along the
electric vector of the light, 1/3 for an isotropic sample and 0 for a sample where
all transition moments are perpendicular to the electric vector.;
Scope note : order parameters (related to wigner matrices) are an alternative to the directional
cosine-based description of molecular alignment. order-parameter methods also work
well for non-uniaxial samples and provide a seemingly more complex, but in other ways
convenient, description of molecular orientation distributions. wigner matrices are
used as a basis set for an expansion of the orientation–distribution function.; a related, commonly used description is based on diagonalized saupe matrices:s¿ (3k¿-1)/2the
principal (molecular) coordinate system (x,y,z) forming angles¿ a,ß,¿with the light
electric vector should be chosen such that the matrixkand the tensor s¿ are diagonal.to
describe processes involving two or more photons, such as luminescence of a uniaxial,
aligned sample, an expansion of the directional cosines to the fourth power is required.; the directional cosines provide, especially for uniaxial samples, a simple description
of exactly those orientation properties of the sample that are relevant for light
absorption. with the principal coordinate system (x,y,z), forming angles ¿ a,ß,¿ with
the light electric vector in the zdirection, all orientation effects induced by light
absorption are contained ink¿¿ k¿. since the sum of k¿for three perpendicular molecular
axes is equal to1, only two independent parameters are required to describe the orientation
effects on light absorption.;
Origin ID : MT07422;
See also
Absorption probability (referred to electric dipolar absorption) for a molecular transition
with its electric transition (dipole) moment at an angle θ with the electric vector
of the light is proportional to cos 2θ. For the whole sample it is proportional to
the orientation factor Kθ cos 2 θ , averaged over all sample molecules. This
average is 1 for a sample with all transition moments perfectly aligned along the
electric vector of the light, 1/3 for an isotropic sample and 0 for a sample where
all transition moments are perpendicular to the electric vector.
