Cooling Excited state deactivation

It has long been known that many lanthanide ions fluoresce under ultraviolet light, the fluorescence coming from f -> f transitions some ions which do not normally fluoresce at room temperature do so when they are cooled. This fluorescence property has led to lanthanide ions being incorporated in the phosphor of domestic fluorescent tubes and in the screens of colour televisions. When an ion is in an electronically excited state there is a competition between deactivation by radiative and non-radiative processes. For an ion to be a good emitter, any non-radiative process must be a poor second in the competition. If studies are carried out using aqueous solutions, it is found that the lanthanide ions at the centre of the lanthanide series are... 

Modern experimental measurements and the new computational techniques just discussed are now providing results that can rationalize issues such as the efficiency of 1C at a surface crossing, the competition with fluorescence when an excited state barrier is present, and the relationship between the molecular structure at the intersection and the structure of the photoproducts. Experiments on isolated molecules in cold-matrices or expanding-jets have revealed the presence of thermally activated fast radiationless decay channels. For example, Christensen et al. have proposed that (under isolated conditions in a cool-jet) trans — cis motion in all-tra 5-octa-1.3,5,7-tetraene (all-trow -OT) induces the opening of an efficient nonadiabatic radiationless deactivation channel on Si (2Ag). We now discuss this experiment and complementary theoretical results that illustrate the way in which theory and experiment can be used in concert. 

---