Excited State Electronic Crossovers

In the preceding sections, we have seen that the direction and magnitude of the variation of electronic energies with pressure depends on the nature of a given electronic state and its dependence on underlying crystal field and covalency effects. The differential response of electronic states to pressure provides an opportunity to use pressure to alter the ordering of electronic states and to stabilize new luminescence processes as a result. We refer to a re-ordering of electronic states as an electronic crossover and in this section we present examples of 

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The first pressure-induced excited state electronic crossover in a luminescent transition metal system was reported by Dolan et al. [175] in a study of Cr + K2NaGaFg. Cr + is an excellent candidate for observing an electronic crossover with pressure because, depending on its crystal field strength, it can exhibit either spin allowed T2 A2 emission (low field Cr ) or spin forbidden... 

Figure 2. Thermoluttiinescence depicted from the configurational coordinate model (top) and from the band model (bottom). In the coordinate model the trapped electron occnpies an excited state (111) slightly lower in energy than the main excited state (II). Heat can indnce vibrations, which move the electron into the (II) state by direct crossover at C. A transition can then occm to the gronnd state (I), resniting in emission. In the band model heat moves the trapped electron to the condnction band where it can travel thronghont the crystal rmtil it recombines with a hole at the bottom of the band (forbidden energy) gap. [Used by permission of the CRC Press, from Shionoya and Yen (1999) Figs. 49 and 50, p. 90.]...

In order to observe an electronic crossover of two excited states, a blue shift of the lower participating excited state normally must occur simultaneously with a red shift of the higher participating excited state. It is in principle possible to observe crossovers between states that shift in the same direction with appreciably different magnitudes, but no examples of this type have been reported. Electronic crossovers are likely in transition metal systems because of the wide variation in pressure shifts observed for electronic transitions in transition metal systems (see Sect. 3.2.1). 

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