Glossary of Terms Used in Photochemistry
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BACK ELECTRON TRANSFER
A term often used to indicate thermal reversal of excited state electron
transfer restoring the donor and acceptor in their original oxidation
level. In using this term one should also specify the resulting electronic
state of the donor and acceptor.
BANDGAP ENERGY (Eg)
The energy difference between the bottom of the conduction band and
the top of the valence band in a semiconductor or an insulator.
See conduction band,
Fermi level.
BANDPASS FILTER
An optical device which permits the transmission of radiation within
a specified wavelength range and does not permit transmission of radiation
at higher or lower wavelengths. It can be an interference filter.
See also cut-off filter.
BARTON REACTION
Photolysis of a nitrite to form a -nitroso
alcohol. The mechanism is believed to involve a homolytic RO-NO cleavage,
followed by -hydrogen
abstraction and radical coupling.
BATHOCHROMIC SHIFT
(Effect)
Shift of a spectral band to lower frequencies (longer wavelengths)
owing to the influence of substitution or a change in environment
(e.g., solvent). It is informally referred to as a red shift and is
opposite to hypsochromic shift.
BEER-LAMBERT LAW (or
Beer-Lambert-Bouguer Law)
The absorbance of a beam of collimated monochromatic radiation in
a homogeneous isotropic medium is proportional to the absorption pathlength,
l, and to the concentration, c, or - in the gas phase - to
the pressure of the absorbing species. This law holds only under the
limitations of the Lambert law and for absorbing species exhibiting
no concentration or pressure dependent aggregation. The law can be
expressed as
where the proportionality constant, ,
is called the molar (decadic) absorption coefficient. For l in cm
and c in mol dm-3 or M,
will result in dm3 mol-1 cm-1 or
M-1 cm-1 , which is a commonly used unit. The
SI unit of
is m2 mol1. Note that spectral radiant power
must be used because the Beer-Lambert law holds only if the spectral
bandwidth of the light is narrow compared to spectral linewidths in
the spectrum.
See absorbance, attenuance,
extinction coefficient,
Lambert law.
BIOLUMINESCENCE
Luminescence produced by living systems.
See luminescence.
BIPHOTONIC EXCITATION
Also called two-photon excitation. The simultaneous (coherent) absorption
of two photons (either same or different wavelength) the energy of
excitation being the sum of the energies of the two photons.
BIPHOTONIC PROCESS
A process resulting from biphotonic excitation.
See multiphoton
process
BIRADICAL (Synonymous with
diradical)
An even-electron molecular entity with two (possibly delocalized)
radical centres which act nearly independently of each other.
Species in which the two radical centres interact significantly are
often referred to as biradicaloids. If the two radical centres are
located on the same atom, they always interact strongly, and such
species are called carbenes, nitrenes, etc.
The lowest-energy triplet state of a biradical lies below or at most
only a little above its lowest singlet state (usually judged relative
to kT, the product of the Boltzmann constant k and the absolute temperature
T). The states of those biradicals whose radical centres interact
particularly weakly are most easily understood in terms of a pair
of local doublets.
Theoretical descriptions of low-energy states of a biradical display
the presence of two unsaturated valences (biradicals contain one fewer
bond than permitted by the rules of valence): the dominant valence
bond structures have two dots, the low energy molecular orbital configurations
have only two electrons in two approximately nonbonding molecular
orbitals, two of the natural orbitals have occupancies close to one,
etc.
BIRADICALOID
Biradical-like.
BLEACHING
In photochemistry this term refers to the loss of absorption or emission
intensity.
BLUE SHIFT
Informal expression for hypsochromic shift.