Basic Photophysical and Photochemical Concepts

Abstract

Although man was not present to observe the event, the first photochemical reaction probably occurred billions of years ago. High-energy solar radiation (λ < 2000 Å) undoubtedly was an important factor in the development of large molecules, polymers, and eventually polypeptides from the primitive earth’s reducing atmosphere (methane, ammonia, and hydrogen). However, from man’s point of view, the greatest photochemical “breakthrough” occurred when the first few quanta of light were absorbed by a rudimentary photosynthetic unit, resulting in the release of molecular oxygen to the atmosphere, thus paving the way for all higher life:

$$nH_2 O + nCO_2 + light\,\,\,absorder \xrightarrow{{hv}}\,\,\left( {CH_2 O} \right)n + nO_2$$

(1.1)