Non-Radiative Relaxation of Solids: Different Pathways to the Ground State
Non-radiative processes are a very general item, and they may occur equally well in gases, liquids, or solids. Here we are specifically interested in electronic radiationless processes in solids. The most important example is radiationless deactivation of the excited state, but phonon absorption for instance can also lead to an upper electronic state. In the simplest case, radiative and non-radiative transitions compete and occur in the same ion or more generally within the same luminescent center (“one-center processes”). As pointed out by Struck and Fonger, the case of a low phonon coupling typically leads to the so-called “exponential energy gap” law for multiphonon transitions, while the case of a strong phonon coupling often leads to a Mott-Seitz type law in the temperature range of interest i.e. an activation energy followed by a cascade phonon emission process. However, these authors realized that even when this law fits correctly the experimental results, the activation energy involved in the Mott-Seitz formula is not necessarily equal to the height of the crossing point of the configurational curves. The reason for this discrepancy is that the conventional Mott-Seitz process actually occurs only in the limiting case of very high temperatures, while at lower temperatures tunneling from low vibrational levels in the excited state to the ground state may occur.
In addition, even if radiationless transitions can be predicted to occur between the emitting excited state and the ground state, often the quenching processes involve excitation via one or several other excited levels (for instance this is the case in thermal quenching of ruby).
Both the number of emitted (or absorbed) phonons, and the strength of the coupling constant, are to be considered as well. Kun Huang has shown that the pathway to the ground state involves, in some cases the highest energy phonons, in other cases the largest coupling constant or the mean value of the phonon distribution. As a matter of fact, any specific case must be studied separately.
More intricate pathways take place, in which radiationless transitions occur via some kind of energy transfer from the excited center to a “Killer center”. The mechanism of impurity poisoning in conventional II – VI or III – V phosphors is not yet clear, but important progresses have been made in the recent years. Transfers have been shown to occur towards a center which emits I.R. photons for absorbed U.V. or visible exciting radiation.
Some attention is also given to trapping processes and Auger effects in semi-conducting luminescent materials. Characteristic examples of these items are discussed shortly.
KeywordsThermal Quenching Thermal Activation Energy Auger Effect Radiationless Transition Nonradiative Process
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