Primary Photochemical Processes

Abstract

Electromagnetic radiant energy interacts with atomic and molecular systems such that electronic and nuclear motions are affected. In all inelastic photon-molecule interactions radiant energy is converted into kinetic energy of electrons or nuclei. With such interactions the photon can be annihilated, a replica photon can be created, or it can emerge from the collision with a little less, or a little more, energy than it had initially. The interaction results in momentum transfer; and since electrons are much lower mass than carbon, oxygen, hydrogen, etc., nuclei, then interactions that transfer momentum to electrons are more easily accomplished than those that transfer momentum to nuclei. Hence cross-sections (molar absorptivities) for electronic transitions are much higher than those for nuclear vibrational transitions. The electromagnetic radiation spectrum is a continuous distribution of frequencies. Photons of any energy can exist; no frequencies are forbidden or specially favored. On the other hand, the kinetic and potential energies of electrons, nuclei, and subatomic particles are restricted to discrete, quantized states. It is this quantum requirement that governs whether or not a given molecule can extract momentum from a given photon. Thus most of the molecule-photon momentum transfer rules are set by the molecule. The photon only restricts itself to giving up either all its energy on to receiving or losing a small fraction (up to ca. 10%) of its energy content. Photons are not known to lose half of their energy, say, in an inelastic scattering erent.