Glossary of terms used in photochemistry1 INTERNATIONAL UNION OF PURE AND APPLIED CHEMISTRY ORGANIC CHEMISTRY DIVISION COMMISSION ON PHOTOCHEMISTRY GLOSSARY OF TERMS USED IN PHOTOCHEMISTRY 2nd Edition, March 1996 Prepared for publication by: S.E. Braslavsky , K.N. Houk (first edition) and J.W. Verhoeven* (2nd edition) INTRODUCTION TO THE 1996 REVISION A provisional version of this Glossary was published in the period 1983-1984 in the Newsletters of the European Photochemical Association, the Inter-American Photochemical Society, and the Japanese Photochemical Association.The first edition of the “Glossary of Terms Used in Photochemistry“ was prepared for publication in the Commission by S.E. Braslavsky and K.N. Houk. It was published in 1988 (Pure Quantities, Symbols and Units of Radiation Physics“, DIN (Deutsches Institut fr Normung) 5031 (1982), F.R.G. “Radiometric and Photometric Properties of Materials; Definitions Characteristics“, DIN 5036, Part 1 (1979), F.R.G. “Radiometric and Photometric Characteristics of Materials and their Measurement“, International Commission on Illumination (CIE) (1977) 38. Jan W. Verhoeven Amsterdam, March 1996 4COMMISSION ON PHOTOCHEMISTRY ALPHABETIC LIST OF TERMS AND DEFINITIONS ABSORBANCE (A) The logarithm to the base 10 of the ratio of the spectral radiant power of incident , essentially monochromatic, radiation (I =I d) to the radiant power of transmitted radiation (P ): A = log(P0/ P ) = logT In practice, absorbance is the logarithm to the base 10 of the ratio of the spectral radiant power of light transmitted through the reference sample to that of the light transmitted through the solution, both observed in identical cells. T is the (internal) transmittance. This definition supposes that all the incident light is either transmitted or absorbed, reflection or scattering being negligible. Traditionally (spectral )radiant intensity, I, was used instead of spectral radiant power, P , which is now the accepted form. (The terms: absorbancy, extinction, and optical density should no longer be used.) See absorption coefficient, absorptance, attenuance, Beer-Lambert law, depth of penetration, internal transmittance, Lambert law, molar absorption coefficient. ABSORPTANCE The fraction of light absorbed, equal to one minus the transmittance (T). See absorbance. ABSORPTION (of electromagnetic radiation ) The transfer of energy from an electromagnetic field to a molecular entity. ABSORPTION COEFFICIENT (decadic-a or Napierian- ) Absorbance divided by the optical pathlength, l : a = A / l = (1/ l)log(P0/ P ) Physicists usually use natural logarithms. In this case: = aln10 = (1/ l)log(P0/ P ) where is the Napierian absorption coefficient. Since absorbance is a dimensionless quantity, the coherent SI unit for a and is m-1. Also cm-1 is often used. See also absorptivity, molar absorption coefficient. ABSORPTION CROSS SECTION ( ) Operationally, it can be calculated as the absorption coefficient divided by the number of Glossary of terms used in photochemistry5 molecular entities contained in a unit volume of the absorbing medium along the light path: =/ N = 1/ (Nl)ln(P0/ P ) where N is the number of molecular entities per unit volume, l is the optical pathlength, and is the Napierian absorption coefficient. The relation between the absorption cross section and the molar (decadic) absorption co- efficient, , (units M-1cm-1) is = ( /NA) 0.1 ln 10 = 0.2303 ( /NA) = 3.825 x 10-24 where in in m2 and NA is the Avogadro constant. See attenuance, Beer-Lambert law. ABSORPTIVITY Absorptance divided by the optical pathlength. For very low attenuance it approxi- mates the absorption coefficient (within the approximation (1 - e-A) A). The use of this term is not recommended. ACTINOMETER A chemical system or physical device which determines the number of photons in a beam integrally or per unit time. This name is commonly applied to devices used in the ultraviolet and visible wavelength ranges. For example, solutions of iron(III) oxalate can be used as a chemical actinometer, while bolometers, thermopiles, and photodiodes are physical devices giving a reading that can be correlated to the number of photons detected. ACTION SPECTRUM A plot of a relative biological or chemical photoresponse (= y) per number of incident photons, against wavelength or energy of radiation under the same radiant power of light. This form of presentation is frequently used in the studies of biological or solid state systems, where the nature of the absorbing species is unknown. The action spectrum is sometimes called spectral responsivity or sensitivity spectrum. The precise action spectrum is a plot of the spectral (photon or quantum) effectiveness. By contrast, a plot of the biological or chemical change or response per absorbed photon (quantum efficiency) versus wavelength is the efficiency spectrum. See also excitation spectrum., efficiency spectrum. ADIABATIC ELECTRON TRANSFER Electron transfer process in whi