Radiative Return to the Ground State Luminescence

The energy of an electronically excited state may be lost in a variety of ways. A radiative decay is a process in which a molecule discards its excitation energy as a photon. A more common fate is non-radiative decay, in which the excess energy is transferred into the vibration, rotation, and translation of the surrounding molecules. This thermal degradation converts the excitation energy into thermal motion of the environment (i.e., to heat). Two radiative processes are possible spontaneous emission, just like radioactivity, which is a completely random process where the excited state decays  

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In Chapter 2, several ways were considered in which the luminescent system can absorb the excitation energy. In the following chapters the several possibilities of returning to the ground state are considered. In this chapter we will deal with radiative return to the ground state in the case when the absorption and emission processes occur in the same luminescent center. This situation occurs when photoluminescence is studied on a luminescent center in low concentration in a non-absorbing host lattice (Fig. I.l). 

Fig. 34.9. The Dexter-Klick-Russell model for explaining a low luminescence efficiency or the absence of luminescence. The intersection point S of the two curves lies below the vibrational level reached after excitation. The non-radiative return to the ground state requires no activation energy (from Blasse and Bril, 1970).

The fluorescence and phosphorescence of luminescent materials are modulated by the characteristics of the environment to which these materials are exposed. Consequently, luminescent materials can be used as sensors (referred also as transducers or probes) to measure and monitor parameters of importance in medicine, industry and the environment. Temperature, oxygen, carbon dioxide, pH, voltage, and ions are examples of parameters that affect the luminescence of many materials. These transducers need to be excited by light. The manner in which the excited sensor returns to the ground state establishes the transducing characteristics of the luminescent material. It is determined by the concentration or value of the external parameter. A practical and unified approach to characterize the luminescence of all sensors is presented in this chapter. This approach introduces two general mechanisms referred as the radiative and the nonradiative paths. The radiative path, in the general approach, is determined by the molecular nature of the sensor. The nonradiative path is determined by the sensor environment, e.g., value or concentration of the external parameter. The nonradiative decay rate, associated with the nonradiative path, increases... 

The luminc.sccncc proces.ses in such a system arc as follows. The exciting radiation is absorbed by the activator, raising it to an excited state (Fig. 1.2). The excited state returns to the ground state by emission of radiation. This suggests that every ion anil every material shows luminescence. This is not the case. The reason for this is that the radiative emission process has a competitor, viz. the nonradiative return to the ground state. In that process the energy of the excited state is used to excite the vibrations of the host lattice, i.e. to heat the host lattice. In order to create efficient luminescent materials it is necessary to suppress this nonradiative process. 

The obvious characteristics to be measured on this system are the spectral energy distribution of the emission (the emission spectrum), and of the excitation (the excitation spectrum which in this simple case is often equal to the absorption spectrum), and the ratio of the radiative and the nonradiative rates of return to the ground state. The latter determines the conversion efficiency of our luminescent material. 

In Chapter 2, the luminescent center was brought into the excited state, whereas in Chapters 3 and 4 the return to the ground state was considered, radiatively and nonradialively, respectively. In this chapter another possibility to return to the ground state is considered, viz. by transfer of the excitation energy from the excited centre (S ) to tuioiher centre (A) ... 

Not every ntineral shows luminescence. The reason is that the radiative emission process has a competitor, namely, the nonradiative return to the ground state. In that process the energy of the excited state is used to excite the vibrations of the host lattice, i.e. to heat the host lattice. 

Fig. 6. Configuration coordinate diagram of a luminescent centre. Non-radiative return from the excited state to the ground state is possible via the crossover S. This requires an activation energy AE which can be supplied at higher temperatures. Exc excitation, em emission...

Up to this point it was assumed that the return from the excited state to the ground state is radiative. In other words, the quantum efficiency (q), which gives the ratio of the numbers of emitted and absorbed quanta, was assumed to be 100%. This is usually not the case. Actually there are many centers which do not luminescence at all. We will try to describe here the present situation of our knowledge of nonradiative transitions that is satisfactory only for the weak-coupling case. For detailed reviews the reader is referred to ref. 11. 

Radiative return from the excited state to the ground state (Chapter 3) is not the only possibility of completing the cycle. The alternative is nonradiative return, i.c. a return without emission of radiation. Nonradiative processes will always compete with radiative processes. Since one of the most important requirements for a luminescent material is a high light output, it is imperative that in such a material the radiative processes have a much higher probability than the nonradiative ones. 

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