Used to characterize
luminescence (
fluorescence,
phosporescence)
polarization resulting from
photoselection. Defined as:
where
and
are the intensities measured with the
linear polarizer for
emission parallel and perpendicular, respectively, to the electric vector of linearly polarized
incident electromagnetic radiation (which is often vertical). The quantity
is proportional to the total
fluorescence intensity
.
Notes:
- Fluorescencepolarization may also be characterized by the polarization ratio, also called the degree of polarization
,
For parallel absorbing and emitting transition moments the (theoretical) values are
;
when the transition moments are perpendicular, the values are
.
In many cases, it is preferable to use emission anisotropy because it is additive; the overall contribution of
components
,
each contributing to the total fluorescence intensity with a fraction
,
is given by:
with
- On continuous illumination, the measured emission anisotropy is called steady-state emission anisotropy
(
)
and is related to the time-resolved anisotropy by:
where
is the anisotropy and
is the radiant intensity of the emission, both at time
following a δ-pulse excitation.
- Luminescencepolarizationspectroscopy, with linear polarizers placed in both beams, is usually performed on isotropic samples, but it may also be performed on oriented anisotropic samples. In the case
of an anisotropic, uniaxial sample, five linearly independent luminescence spectra, instead of the two available for an isotropic sample, may be recorded by varying the two polarizer settings relative to each other
and to the sample axis.
- The term fundamental emission anisotropy describes a situation in which no depolarizing events occur subsequent to the initial
formation of the emitting state, such as those caused by rotational diffusion or energy transfer. It also assumes that there is no overlap between differently polarized transitions.
The (theoretical) value of the fundamental emission anisotropy,
,
depends on the angle
between the absorption and emission transition moments in the following way:
where
denotes an average over the orientations of the photoselected molecules.
can take on values ranging from
for
(perpendicular transition moments) to
for
(parallel transition moments). In spite of the severe assumptions, the expression
is frequently used to determine relative transition-moment angles.
- In time-resolved fluorescence with δ-pulse excitation, the theoretical value at time zero is identified with the fundamental
emission anisotropy.
Source:
PAC, 2007, 79, 293
(Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006))
on page 332
Cite as:
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by
A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997).
XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic,
J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8.
https://doi.org/10.1351/goldbook.
