Editing Circular dichroism (section)

=== Circularly polarized luminescence (CPL) ===
Circularly polarized luminescence (CPL) is the emission analog of CD. While CD examines the geometry of molecules in their ground state, CPL, typically observed perpendicular to the incident light beam, provides insight into the geometry of molecules in their excited or luminescent state. In a CPL experiment, the emitted light is analyzed for its contents of left and right circularly polarized light.

The difference in intensity between left and right circularly polarized emitted light, '''Δ''I''''', is given by:

'''Δ''I'' = ''I''<sub>L</sub> – ''I''<sub>R</sub>'''

where '''''I''<sub>L</sub>''' and '''''I''<sub>R</sub>''' are the intensity of left and right circularly polarized emitted light, respectively. The average luminescence intensity is given by:

'''''I'' = (''I''<sub>L</sub> + ''I''<sub>R</sub>) / 2'''

Both '''Δ''I''''' and '''''I''''' are in relative units because in luminescence experiments not all the luminescent light is detected, only that in a restricted solid angle. Moreover, an absolute value for the concentration of luminescent molecules may be hard to obtain. For these reasons, a dissymmetry factor in the luminescence is defined as:

'''''g''<sub>lum</sub> = Δ''I'' / ''I'''''

with '''''g''<sub>lum</sub>''' being an absolute number that is unique to each CPL-active molecule under specific conditions.

Many commercially available CD spectrophotometers can be adapted with an accessory to measure the circularly polarized luminescence of a sample.