Phosphorescence, monitoring triplet state population

Pd(2-thpy)2 is dissolved in an n-octane matrix and is excited at low temperature (T < 2 K) by a c.w. source (non-pulsed, e.g. at A = 330 nm [61]). Additionally, microwave irradiation is applied and scanned in frequency. The microwave radiation can cause transitions between the triplet sublevels in the case of resonance, and thus, the previously different and non-thermalized steady state populations of the substates are usually altered. Under suitable conditions (see below) a change of the phosphorescence intensity will result due to microwave perturbation. Usually, this effect is very weak. Therefore, microwave pulse trains are applied, for example, with a repetition rate of 150 Hz. Thus, one can monitor the microwave-induced intensity changes by a phase-sensitive lock-in technique [90]. 

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