Excited states from luminescence spectra

The divalent rare-earth ion Eu has the 4f electronic configuration at the ground states and the 4f 5d electronic configuration at the excited states. The broadband absorption and luminescence of Eu are due to 4f - 4 f 5d transitions. The emission of Eu is very strongly dependent on the host lattice. It can vary from the ultraviolet to the red region of the electromagnetic spectrum. Furthermore, the 4f-5d transition of Eu decays relatively fast, less than a few microseconds [33]. 

Excited-state lifetimes can be measured directly by monitoring the decay of luminescence, but impurities present affect both the lifetime and the luminescence spectrum. Also, because low temperatures are necessary for phosphorescence studies, the excited-state properties determined may differ from those at room temperature. 

The luminescence of Bi " is quite diverse and depends strongly on the host lattice (Boulon 1987 Blasse and Grabmaier 1994 Blasse et al 1994). For the heavy Bi " the transitions between the ground state and the Pi state becomes additionally allowed by spin-orbit mixing of the Pi and Pi states. After excitation at low temperature, the system relaxes to the lowest excited state. Consequently, the emission at low temperatures can be ascribed to the forbidden transition Pq- Sq and has a long decay time. Nevertheless, both Pi and Po are emitting levels and they are very close so that at higher temperatures the luminescence from the Pi level may appear with a similar spectrum, but shorter decay (Fig. 5.49). 

Figure 3.10 The electronic transitions [absorption in (a)] of small molecules show vibrational and rotational lines in addition to the purely electronic spectrum, (b) Luminescence emission is resonance fluorescence (f), and chemical reactions (R) can originate from several excited states...

From a practical standpoint, much of the interest in the role of excited states in ionic interactions stems from their importance in ionospheric chemistry.Ih In addition, it has been realized more recently that certain ion-neutral interactions offer a comparatively easy means of populating electronically excited reaction products, which can produce chemiluminescence in the visible or UV region of the spectrum. Such systems are potential candidates for practical laser devices. Several charge-transfer processes have already been utilized in such devices, notably He+(I,He)I + and He2+(N2,2He)N2+.3 Interest in this field has stimulated new emphasis on fundamental studies of luminescence from ion-neutral interactions. 

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