g). In the time scale associated with these thermal transitions any selection rules formally associated with them are relaxed. Since there can be energy differences of the order of 1 eV or more between the highest occupied [Refs. on p. 43] Thermal Versus Optical Transitions 35 levels of g and u parity, this factor can make a significant contribution to the difference under consideration. Finally, one must recognize that the diagram in Fig.
The case shown has both AS and AH positive. The point B corresponds to the solution for T = 00 (or for AH = 0), and demonstrates that the term in AS 'biases' the infinite temperature site fraction. It can be shown that this simple case cannot account for many of the experimental observations. 2 Interacting centers (r #- 0) From our earlier remarks, we can generate the graphical solution by drawing the straight line with slope -2rjkB T through the point 1 AH AS) Th 1. h . ( 2' kB T - k B ' e resu tmg constructIOn IS sown m FIg.