Each orbital of a molecule in its ground or unexcited states is usually occupied by two electrons, if the molecule has an even number of electrons. The Pauli Exclusion Principle demands that electrons occupying the same orbital have their spins aligned in opposite directions. If one electron in the molecule is excited to a higher level, its spin can either be orientated parallel or antiparallel to the spin of the electron remaining in the orbital from which the excited electron was removed. If the spins are antiparallel the molecule is said to be in a singlet state if the spins are parallel the molecule is said to be in a triplet state. Alternatively the molecule can be said to have a “multiplicity” of one or three respectively. The term “phosphorescence”, usually refers to the light emitted by an excited molecule which many accompany the transition from the lowest triplet to the ground singlet state of the molecule. Theoretically, if angular momentum us to be conserved, transitions can only occur between states of the same multiplicity. In practice, because of a perpetuation to the electron’s spin quantum number due to coupling between the rotation of the electron on its axis and its orbital motion around the nucleus, there is a finite probability of a radiative transition between triplet and singlet states. This probability is however, very much smaller than between stated of the same multiplicity. This lower radiative transition probability manifests itself as a longer lived luminesce than fluorescence, which is the emission accompanying a radiative degradation between electronic states of the same multiplicity. For example, fluorescence typically has a life time of the order of nanoseconds whereas phosphorescence has a lifetime from 10-⁴ to 10 seconds. Because of its long lifetime, phosphorescence is very difficult to observe in the gaseous or liquid phases due to faster competing non-radiative quenching processes. A quenching reaction is a process in which the excitation energy possessed by the triplet state us dissipated by some chemical species present in the system. Phosphorescence may often be readily observed from irradiated molecules in the solid state where molecular movement has largely been “frozen out”. Under these conditions quenching reactions become very much slower than in the liquid or gaseous phases and consequently phosphorescence may compete with the quenching reaction far more successfully. Phosphorescence has been observed in a number of solid state environments. In 1896, Schmidt was the first to observe phosphorescence from molecules trapped in a “glassy” matrix. This technique was employed by Vavilov and later extensively used by Lewis and Terenin in the 1940’s. Today, glasses commonly used, are those formed on cooling 3-methylpentane, Methylcyclohexane – isopentane mixtures, and a variety of other organic solvent mixtures. Oster and Melbuish have employed plastics as rigid media for phosphorescence studies. Among the plastics that have been found useful are polycarbonates, acrylics, polystyrene, and cellulose polymers. A good deal of work has also been done on the phosphorescence of molecules in the crystalline state.