Nonequilibrium thermodynamics of luminescent processes in solids

Abstract

The kinetic mechanism of luminescent processes in solids is discussed on the grounds of linear nonequilibrium thermodynamics. By applying the principle of minimum entropy production, the densities of electrons in the conduction band and holes in the valence band are found to remain steady during the thermoluminescent emission, while, during the thermoluminescence buildup and the accompanying photoluminescent emission, they increase with a rate equal to half the rate of pair production by ionizing radiation. Analogously, the densities in intermediate levels lying in the forbidden energy gap are found to behave always steadily. In this way, simple equations are derived to represent these processes both in the case of single-level luminescent centers and when they are constituted by two-level states for localized electron-hole pairs. It is shown that the linear approximation for nonequilibrium thermodynamics becomes more reliable the more the thermoluminescent process approaches its end. On the contrary, for thermoluminescent buildup and photoluminescent emission the best reliability is found at the beginning, while it gets worse as these processes go on, thus removing the system from the initial state of thermodynamic equilibrium.