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Pure Appl. Chem., Vol. 68, No.12, pp. 2223-2286, 1996

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 delta-nitroso alcohol. The mechanism is believed to involve a homolytic RO-NO cleavage, followed by delta-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, epsilon, is called the molar (decadic) absorption coefficient. For l in cm and c in mol dm-3 or M, epsilon will result in dm3 mol-1 cm-1 or M-1 cm-1 , which is a commonly used unit. The SI unit of epsilon 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.




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Online version compiled by European Photochemistry Association (EPA)

 

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