RADIATION PRESSURE

the pressure on a surface resulting from electromagnetic radiation that impinges on it, which results from the momentum carried by that radiation; radiation pressure is doubled if the radiation is reflected rather than absorbed. Although the pressure of solar radiation is exceedingly small, a sufficiently large surface could produce a force that would be technologically useful. For example, it has been calculated that a solar sail could be designed large enough to propel a spacecraft. Secondary effects of radiation Purely physical effects With respect to radiation effects the terms primary and secondary are used in a relative sense; the usage depends on the situation under study. Thus, ionization and excitation may be considered as primary with respect to some physical and chemical effects. For other chemical effects, production of free radicals (molecular fragments) may be considered as primary even though that process requires a much longer time for its accomplishment. Still longer times are involved in biologic processes, in which the end product of an earlier chemical reaction may be considered as primary. Generally, an atomic solid (a material consisting of only one atomic species) exhibits little or no permanent chemical change upon irradiation. Important among the atomic solids are such materials as metals and graphite. Production of molecular carbon (C2) or bigger clusters upon irradiation of carbon and graphite may, in a certain marginal sense, be considered a chemical change. Ionization of a condensed atomic medium followed by recombination regenerates the same atom, but its locale may be affected. For a molecular medium the situation is quite different. Excited electronic states are often dissociative for a molecule and yield chemically reactive radicals. Positive ions, similarly produced, can experience a variety of reactions even before neutralization occurs. Such an ion may fragment all by itself, or it may react with a neutral molecule in what is called an ionmolecule reaction. In either case new chemical species are created. These transformed ions and radicals, as well as the electrons, parent ions, and excited states, are capable of reacting with themselves and with molecules of the medium, as well as with a solute (a dissolved substance) that may be present in homogeneous distribution. The end products of the reactions can be, on the one hand, new stable compounds or, on the other, regenerated molecules of the original species, as in the case of water irradiation. A variety of purely physical effects have been observed in different substances under irradiation. They may be broadly classified as: (1) structural change in the crystal, sometimes accompanied by change in the structural dimensions, (2) change in static mechanical properties, such as elasticity and hardness, (3) change in dynamic mechanical properties, such as internal friction and strain, and (4) changes in transport properties, such as heat conductivity and electrical resistivity. Such changes are considered below in Tertiary effects of radiation on materials. Molecular activation Figure 1: Energy states in molecular systems (see text). A molecule is considered activated when it absorbs energy by interaction with radiation. In this energy-rich state it may undergo a variety of unusual chemical reactions that are normally not available to it in thermal equilibrium. Of special importance is electronic activationi.e., production of an electronically excited state of the molecule (see Figure 1). This state can be reached (1) by direct excitation by photon absorption, (2) by impact of charged particles, either directly or indirectly through charge neutralization, or by excitation transfer from excited positive ions, and (3) by charge transfer in collision with (relatively) slow incident positive ions. Among the variety of ensuing processes is light emission, or luminescence.