Photochemistry, the study of chemical changes that are initiated by light. Photochemists are concerned with the interaction of light (in the form of photons) with molecules and the physical and chemical changes that result.
The first law of photochemistry, known as the Grotthuss-Draper law, states that light must be absorbed by a chemical substance in order for a photochemical reaction to take place. In other words, molecules that do not absorb light of a particular frequency will not undergo a photochemical reaction when irradiated at that frequency. The second law of photochemistry, the Stark-Einstein law, states that for each photon of light absorbed by a chemical system, only one molecule is activated for photochemical reaction. This latter law is also known as the photoequivalence law and was derived by Albert Einstein at the time when the quantum (photon) theory of light was being developed by the German physicist Max Planck, the French physicist Louis de Broglie, and others. The law means that each photon of light can cause a photochemical reaction of only one light-absorbing molecule. A related law states that the amount of photoreaction that takes place is directly proportional to the product of the light intensity and the time of illumination. In other words, more light produces more photoproduct.
Photoreactions take place easily (provided absorption of light can occur) because the absorption of light promotes the molecule to an excited state that contains more energy than the stable ground state. Because the excited molecule contains more energy, it is more reactive. The advantage of photochemistry, where it works, is that it provides a short, direct route for chemical reaction.
Another significant advantage of photochemical over thermal reactions, which require heat for activation, is selectivity. Different frequencies of light can be used to promote entirely different and unique reactions of the same chemical substance. The frequency (u) of light that will be absorbed by a molecule, multiplied by h, Planck's constant, must exactly match the separation in energy between the ground state of the molecule (E1) and the excited state (E2) : E2 - E1 = hu. By changing the frequency of the radiation, it is possible to excite selectively the molecule to different excited states. This may result in a completely different photochemical reaction, depending on the frequency and on the number and types of excited states available in the molecule.
The majority of chemical substances that absorb light will not react photochemically because the molecule may "deactivate" rapidly, losing its energy before reaction can occur. The average lifetime of the excited state must be sufficiently long to allow reaction. See Luminescence.
Important examples of photochemical processes include the following. In the photosynthesis of green plants, molecules of the pigment chlorophyll absorb photons of sunlight, making energy available for the manufacture of carbohydrates. Sunlight also promotes the photodissociation of ozone in the atmosphere. The resulting absorption of light by ozone helps to screen out the more harmful ultraviolet radiation from the sun. Photography is a photochemical process in which silver bromide (AgBr) is converted to metallic silver by the action of light. The process of vision itself involves the photochemical isomerization of the protein rhodopsin in the retina of the eye.