Fluorescence and phosphorescence
In general, a small, simple molecule luminesces in the ultraviolet, and a more complex one emits near the blue-violet end of the visible spectrum. Dye molecules, on the other hand, may emit throughout the visible region, including the red end. The ground electronic state of most molecules is a singlet state. Usually, therefore, the optically allowed emission, or fluorescence, is from the lowest excited singlet state to the ground state. The lowest triplet state of the molecule lies somewhat below the excited singlet. Light emission from this triplet state is forbidden by the quantum-mechanical selection rules, but it does occur by default when other processes are even less probable. Such emission is called phosphorescence. It is relatively weak, slow, and shifted toward longer wavelength. Triplet states may be produced from higher singlets by processes called internal conversion and intersystem crossing. The states may also be produced in excitation from the ground state by impact of relatively slow charged particles, such as electrons.
Much of the effect of optical radiation in a condensed system is not on the molecule in which the energy is initially absorbed but on a more remote molecule to which the energy is transferred in a variety of possible processes. They include excitation transfer either directly between adjacent molecules, by a direct quantum-mechanical interaction of an excited molecule with a remote one at a distance of 40 angstroms (4 × 10-7 centimetre) or less, or by the so-called trivial process of fluorescence emission from one molecule and reabsorption by one at any distance. These processes are studied mostly in regard to fluorescence and phosphorescence phenomena.
With high-energy radiation (such as that of electrons, X rays, and gamma rays), an additional mechanism involving ions is also available. In the case of a solute M in a solvent S, for example, a simplified description of some possible effects of radiation is represented by the following expressions, in which the symbol ☢ is read, “is acted upon by high-energy radiation to give” and e represents an ejected electron:
Any actual process is considerably more complicated and involves a larger number of species.
Photographic process
One of the most important effects of radiation on matter is seen in photographic action. Apart from its various uses in art, commerce, and industry, photography is an invaluable scientific tool. It is used extensively in spectroscopy, in photometry, and in X-ray examinations. Also, photographic emulsion techniques have been widely used in the detection and characterization of high-energy charged particles. It is important to note that all speculation regarding the primary phenomena involves the notion that, in an energy absorption process, either direct or sensitized, a chloride (or other halide) ion in a silver halide lattice loses an electron. That electron is thereafter captured by a silver ion located at such a point in the lattice that under suitable conditions of exposure and development a silver grain grows to a size representative of the duration and intensity of the light exposure.
Ionization and chemical change
Earlier in this section, the ionization phenomenon was briefly discussed as a special case of molecular activation. The ionization process, however, does have certain characteristic features. Most notably, the probabilities (or cross sections) for ionization by light (photoionization) and for ionization by charged-particle impact are different in magnitude and in lowest—radiation—energy of occurrence (i.e., threshold behaviour) for the same atom or molecule. The photoionization cross section shows abrupt onset (i.e., a step behaviour) to a high value at threshold, falling thereafter only gradually with increase of photon energy. Electron-impact ionization in simple atoms (such as hydrogen and helium) begins at the ionization potential, increases in direct proportion to the energy near the threshold, and shows a peak at an incident energy of about 100–200 eV. With molecules the behaviour is similar except that the peak is broad and much less pronounced. When the incident energy is high and the ejected electron has kinetic energy (energy of motion) largely in excess of its binding energy, the cross section for the process approaches a limit called the classical Rutherford value, after the British physicist Ernest Rutherford.
In general, the initial processes resulting from the action of high-energy radiation on matter involve the intermediate production and participation of positive ions (both stable and unstable), electrons, negative ions, excited species, and free radicals and atoms, which in turn may enter into the processes of classical reaction kinetics.
Ordinary low-energy (or optical) processes usually involve only excited species and free radicals and atoms—all formed by processes that do not involve outright transfer of electric charge (i.e., electrons) between different atoms and molecules.
The important feature that characterizes the chemistry both of optical processes (photochemistry) and of high-energy radiation (radiation chemistry) is that they are conveniently employed and their kinetics studied at room temperature and lower.
Photochemistry
There are two “laws” of photochemistry. The first, the Grotthuss–Draper law (named for the chemists Christian J.D.T. von Grotthuss and John W. Draper), is simply: for light to produce an effect upon matter it must be absorbed. The second, or Stark–Einstein law (for the physicists Johannes Stark and Albert Einstein), in its most modern form is: one resultant primary physical or chemical act occurs per photon absorbed. The quantum yield of a particular species of product is the number of moles of that product divided by the number of einsteins of light (units of 6.02 × 1023 photons)—or the number of molecules of product per photon—absorbed. In the ideal case the quantum yield, frequently denoted by the Greek letters gamma, γ, or phi, Φ, is unity. In real cases, Φ may approach zero on the one hand—particularly if a back reaction is involved—or it may be of the order of 1,000,000, in which case the primary product may start a chain reaction, as in a clean, dry mixture of hydrogen (H) and chlorine (Cl). In the following chemical equations each symbol for an element stands for one atom, and the number of atoms bonded into a molecule is given as a subscript following the symbol, while the number of molecules precedes the formula; the arrow indicates the course of the reaction:
in which reactions 2 and 3 reoccur repeatedly in a chain reaction. The symbol →hν may be read “when a photon of light frequency, symbolized by the Greek letter nu, ν (which is always stipulated), is absorbed, gives.” Because h is Planck’s constant of action (approximately 6.6 × 10-27 erg second) and ν is expressed in reciprocal seconds (i.e., second-1), the product hν indicates the energy absorbed per photon. Some reactions may give two primary products; e.g.,
In that case, there are different quantum yields for each of the primary reactions, and the ratio of those yields varies with the frequency, ν, of the light absorbed.