86
E. Pelizzetti . P. Calza
It is also possible to use light fields in the laboratory. Systems such as solar simulators can approximate surface sunlight at most wavelengths. Since the underwater light
spectrum varies markedly with depth, broadband studies can accurately measure only
the near-surface effects for optically thin samples or vertically integrated effects and
absorption coefficients as a function of wavelength.
3.2.3
Factors Influencing Photoreactions
Photoreaction rates are determined by two factors:
1. The rate of absorption of light by the chromophores;
2. Quantum yield with which chemical change results after excitation of the
chromophore.
These factors may all vary over an enormous range, and accordingly the transformations involved vary enormously in their capacity to act as sources or sinks for different compounds. Compounds with a sufficiently high absorption coefficient and a
very efficient photodecomposition rate (high quantum yield) for instance will generally have little chance of being detected at the water surface or in the euphotic zone,
because their steady state concentration will be kept very low as a result of the extensive photochemical sink.
Basically, any solution photoprocess involves two, or often three relatively distinct
steps:
1. Absorption of a single photon by a single chromophore C with the generation of an
electronically excited state, C*;
2. Parallel and consecutive processes that degrade the electronic energy to heat, emitted radiation and primary photochemical products;
3. Secondary reactions of these primary photoproducts into the water.
It is frequently desired to measure or calculate the excitation rate of a chromophore
under various conditions such as in situ or in an experimental apparatus simulating
environmental light intensities and spectral distributions.
Several cases are possible. An example can be that of an optically thin layer near
the water surface in which the excitation rate is a function only of the spectrum and
intensity of the incident solar light and of the concentration and absorption spectrum
of the chromophores. Consideration of this system can be simplified by neglecting
reflection, backscattering, and internal scattering, so that in-air insolation measurements are assumed to represent the underwater light intensity.
An optically thick water layer that has a homogeneous chromophore and scatterer
distribution may represent another case. In addition to the parameters involved in the
optically thin case, the excitation rate now depends also on the optical properties of
the water. Since all photons are absorbed, the excitation of a given chromophore depends on its ability to compete with all other chromophores for photons. The depth
of such an optically thick layer may extend a few centimetres into turbid and/or absorbing waters, or nearly 100 m into relatively transparent oceanic waters.
E. Pelizzetti . P. Calza
It is also possible to use light fields in the laboratory. Systems such as solar simulators can approximate surface sunlight at most wavelengths. Since the underwater light
spectrum varies markedly with depth, broadband studies can accurately measure only
the near-surface effects for optically thin samples or vertically integrated effects and
absorption coefficients as a function of wavelength.
3.2.3
Factors Influencing Photoreactions
Photoreaction rates are determined by two factors:
1. The rate of absorption of light by the chromophores;
2. Quantum yield with which chemical change results after excitation of the
chromophore.
These factors may all vary over an enormous range, and accordingly the transformations involved vary enormously in their capacity to act as sources or sinks for different compounds. Compounds with a sufficiently high absorption coefficient and a
very efficient photodecomposition rate (high quantum yield) for instance will generally have little chance of being detected at the water surface or in the euphotic zone,
because their steady state concentration will be kept very low as a result of the extensive photochemical sink.
Basically, any solution photoprocess involves two, or often three relatively distinct
steps:
1. Absorption of a single photon by a single chromophore C with the generation of an
electronically excited state, C*;
2. Parallel and consecutive processes that degrade the electronic energy to heat, emitted radiation and primary photochemical products;
3. Secondary reactions of these primary photoproducts into the water.
It is frequently desired to measure or calculate the excitation rate of a chromophore
under various conditions such as in situ or in an experimental apparatus simulating
environmental light intensities and spectral distributions.
Several cases are possible. An example can be that of an optically thin layer near
the water surface in which the excitation rate is a function only of the spectrum and
intensity of the incident solar light and of the concentration and absorption spectrum
of the chromophores. Consideration of this system can be simplified by neglecting
reflection, backscattering, and internal scattering, so that in-air insolation measurements are assumed to represent the underwater light intensity.
An optically thick water layer that has a homogeneous chromophore and scatterer
distribution may represent another case. In addition to the parameters involved in the
optically thin case, the excitation rate now depends also on the optical properties of
the water. Since all photons are absorbed, the excitation of a given chromophore depends on its ability to compete with all other chromophores for photons. The depth
of such an optically thick layer may extend a few centimetres into turbid and/or absorbing waters, or nearly 100 m into relatively transparent oceanic waters.