Cascade-free lifetime measurements

Fig. 6.93 Cascade-free lifetime measurements despite the simultaneous excitation of many levels (a) preexcitation by collisions in a gas cell with subsequent laser excitation (b) decay of level k) without cascading curve a), its feeding by cascades curve b), and resulting population Nk t) with cascading and decaying curve c). (c) The fluorescence I x,X) is measured alternately with and without selective laser excitation...

Fig. 6.94 Experimental arrangement for cascade-free lifetime measurements in fast beams of ions or neutrals with fluorescence collection by conically shaped optical-fiber bundles...

A method for measuring fluorescence quantum yields and cascade free lifetimes for open shell cations has been reported. The lifetimes are calculated from the coincidence between undispersed fluorescence photons and energy selected photo-electrons. A similar system has been used to evaluate the fluorescence lifetimes of and and fluorobenzene cations." ... 

The observation of transient effects is closely connected to the feature of Doppler switching by sudden changes of the ion velocity. Applications include cascade-free lifetime and quantum beat measurements by observation of the free decay after a short excitation region. Following the first experiments by Andra/ the alternative to produce a short excitation pulse by crossing the ion and laser beams has been applied extensively (cf. Part B, Chapter 20 by H. J. Andra). 

These two isoelectronic sequences clearly illustrate the usefulness of the lifetime measurements made by the beam-foil technique. Since the uncertainty in the beam velocity is often less than 2 per cent, the accuracy of the results for those levels which are free of cascade is usually limited by the photon counting statistics. However, for levels which do show cascade contributions uncertainties arise, especially if the decay rates of the different exponential components are of similar magnitude. It appears that these difficulties were not always realized in some of the early beam-foil experiments. 

Electrons are transferred singly to any species in solution and not in pairs. Organic electrochemical reactions therefore involve radical intermediates. Electron transfer between the electrode and a n-system, leads to the formation of a radical-ion. Arenes, for example are oxidised to a radical-cation and reduced to a radical-anion and in both of these intermediates the free electron is delocalised along the 7t system. Under some conditions, where the intermediate has sufficient lifetime, these electron transfer steps are reversible and a standard electrode potential for the process can be measured. The final products from an electrochemical reaction result from a cascade of chemical and electron transfer steps. 

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