Preferred Label : emission anisotropy;
IUPAC acronym : radiant intensit;
IUPAC definition : Used to characterize luminescence (fluorescence, phosphorescence) polarization resulting
from photoselection. Defined as: \[r \frac{I_{\parallel} - I_{\perp }}{I_{\parallel}
2I_{\perp }}\] where I and I 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
I 2I is proportional to the total fluorescence intensity I.;
Scope note : in time-resolved fluorescence with d-pulse excitation, the theoretical value at time
zero is identified with the fundamental emission anisotropy.; 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, r0, depends on the angle abetween the absorption
and emission transition moments in the following way: r0
3 a 2 1 5 where
denotes an average over the orientations of the photoselected molecules. r0can take
on values ranging from 15 for a 90 (perpendicular
transition moments) to 25 for a 0 (parallel
transition moments). in spite of the severe assumptions, the expression is frequently
used to determine relative transition-moment angles.; luminescence polarization spectroscopy, 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.; on continuous illumination, the measured emission anisotropy is called steady-state
emission anisotropy ( r ) and is related to the time-resolved anisotropy by:
r 0 t rt it 0 t it where rt
is the anisotropy and it is the radiant intensity of the emission, both at time
tfollowing a d-pulse excitation.; fluorescence polarization may also be characterized by the polarization ratio, also
called the degree of polarization p, p i¿ i¿
i¿ i¿ for parallel absorbing and emitting transition moments the (theoretical)
values are rp 2512 ; when the transition moments are perpendicular, the
values are rp 1513 . in many cases, it is preferable to use emission
anisotropy because it is additive; the overall contribution of n components ri,
each contributing to the total fluorescence intensity with a fraction fi
ii i , is given by: r i 1 n fi
ri with i 1 n fi 1;
Origin ID : ET07370;
See also
Used to characterize luminescence (fluorescence, phosphorescence) polarization resulting
from photoselection. Defined as: \[r \frac{I_{\parallel} - I_{\perp }}{I_{\parallel}
2I_{\perp }}\] where I and I 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
I 2I is proportional to the total fluorescence intensity I.
