Photochemistry

PHOTOCHEMISTRY is that division of physical chem istry which deals with the chemical action of radiant energy on matter, and with the direct production of radiation by chemical reactions. The field of photochemistry might be regarded as being equally as extensive as the range of electromagnetic vibra tions ; from the tiny ultra-X-rays to the huge waves of "wire less." Actually, limitations exist and photochemical reactions are considered primarily as the transformations of matter asso ciated with radiations between and including the trans-ultra violet and infra-red radiations. This range includes the visible radiations, and a band on either side of approximately the same width as that covered by the visible radiations. The wave-lengths included are those between io,u/./ and i,000pq . The micron, A, a measure of length, is cm. or mm., and the millimicron, ALA, is cm. or mm. The direct production of radiation by chemical reaction is termed chemiluminescence. It is differen tiated from the radiation produced in such reactions as combus tion, by the absence of thermal equilibrium between the matter and the radiation concerned (see RADIATION).

Principle of Photochemical Equivalence.

Previous to about 191o, photochemistry had but one accepted generaliza tion, the absorption law, according to which only the light energy absorbed by a body is active in promoting chemical change. This is a special case of the principle of conservation of energy, which states that energy may be neither created nor destroyed. Nearly a century after the first expression of this law, a principle of photochemical equivalence was put forward, in 1922, by A. Einstein. It is founded on Planck's quantum theory (see QUAN TUM THEORY) of radiation, according to which all exchange of energy as radiation between atoms and molecules takes place discontinuously by quanta of finite magnitude e , where v is the frequency of vibration, h a universal constant, equal to 6.55x ergs/sec. ; E varies continuously in magnitude with the multiplying frequency v, but Planck's h is an invariable quantum of action.

The photochemical equivalence principle of Einstein states that in a primary photochemical process, the decomposition of one molecule requires one quantum of energy he , or that one gram-molecule requires Nhy, where N is Avogadro's number of gas molecules per litre, 6.o6 X In his original derivation he postulates a reversible reaction of simple unimolecular decom position the bimolecular "dark" reverse reaction giving the chemilumines cent radiation by ; the reaction was supposed to occur in a gaseous or dilute system. In a later deduction, this primary photo chemical event was supposed to be limited to the excitation of an atom or molecule the atom or molecule C being raised to a higher energy level. It may be stated at once that this generalization is not so com plete or sufficient that all photochemical phenomena can be in terpreted by its sole aid. It has, none the less, completely changed

the outlook, beside leading to improvements in the technique.

Experiment and Technique.

Modern investigation of a photochemical reaction essentially involves determining its quan tum efficiency, in the sense of Einstein's principle. This requires certain elements of technique :—(a) a light source of sufficient intensity adequately monochromatized; (b) exact measurement of the energy absorbed during a given reaction period; (c) dis tinction of the primary photochemical reaction, if possible, from secondary dark reactions; (d) determination of the molecular "order" of the reaction involved, and analytical measurement of the product of the photochemical reaction proper.

As to the first, there are not many available reactions occurring in light of the visible spectrum range. As visibility coincides fairly closely with the spectral distribution of energy in sunlight —a matter of natural selection—it is obvious that on the whole only systems stable in such illumination will be readily found. Much more frequent are photochemical reactions in the ultra violet; and artificial light sources rich in these rays are most used in laboratory research, as well as for certain industrial applica tions. The ultra-violet radiation from electric glow lamps is rela tively feeble and does not extend far to the shorter waves. Re course has to be made to spark and arc discharges. The latter are generally more convenient. The three chief types used may be described as carbon arcs, magnetite and high-melting metal arcs, as tungsten arcs, and metal vapour arcs; of these, the mer cury vapour arc in fused quartz is the most important. The spectrum is discontinuous, and it is not difficult to secure ap proximately monochromatic light at several different wave lengths by the use of colour screens or ray filters. Unfortunately when the far ultra-violet rays are in question (X less than 200 /1/2 ) there are no very satisfactory filters available, the best requir ing the use of chlorine and bromine gases. When a high degree of monochromatic purity is required, the light must be dispersed and quartz monochromators employed, with great reduction of intensity. With regard to the other requirements, the measure ment of the energy absorbed in a reaction is determined with a radiometer (q.v., such as a thermopile, radiomicrometer or bolo meter) as the difference between the energy incident upon and transmitted through the reaction mixture; the latter is usually contained in a vessel with plane-parallel quartz walls, and main tained at constant temperature. The distinction of the primary photochemical reaction, the determination of the molecular order and of the quantum efficiency are then matters of interpreting the analytical results for different periods of the reaction.