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.
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.
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.