Emission from “forbidden” transitions

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). 

This apparently is based on the assumption that the cross-section depends on exp (-AEjkT). However, Frish and Bochkova141 claim to have demonstrated that the 8P state is produced only in very low yield. They observed the emission from excited sodium atoms in an electric discharge in He, Hg and Na mixtures at very low total pressures. Their data appear to show that the cross-section for excitation of the 8P state, the process with the smallest energy discrepancy and for which optical transitions of both atoms are allowed, is smaller than the cross-section into the 9S state, which is optically forbidden for the sodium. 

The decay time of this emission is very long, viz. some 5 ms [57,58]. There are two reasons for this. First the transition involved is spin forbidden [48, 51] secondly, the spin-allowed transition from which the spin-forbidden transition steals its intensity is unusually weak [58, 59],... 

Experiments with filters have shown that CL emission from polymers is in the blue-violet region (-400-500 nm). The low intensity of emission indicates a forbidden transition from a triplet to a singlet state, i.e. phosphorescence. The emission spectra of CL often agree with that of carbonyl cro-mophores [6, 7, 8]. Deactivation of an excited carbonyl is generally believed to be the source of chemiluminescence from polymers (see Scheme 1). 

Our objective was to probe fluorescence over a time domain as large as possible. To this end we combined two different detection techniques, FU and TCSPC, allowing us to perform measurements from 100 fs to hundreds of nanoseconds. Notably, we use the same laser excitation source the third harmonic of a titane sapphire laser (267 nm, 100 fs). This is important because the excited state population is created under identical conditions in the two types of experiments. The time-resolution obtained after deconvolution of the recorded signals is 100 fs and 10 ps for FU and TCSPC, respectively. For reasons explained below, FU only detects emission corresponding to highly allowed transitions. TCSPC, on the other hand, is capable to monitor not only allowed but also very weak or forbidden transition. Therefore, particular care must be taken when merging data obtained by these two techniques as described in Ref. 10. 

Light absorbed by an atom or molecule excites it from the initial ground (or excited) state to a higher-energy excited state for low-intensity light, this occurs, provided that the various applicable quantum rules for the transition are satisfied (electric-dipole "allowed" transitions). If quantum rules "forbid" a transition, then the transition is either absent ("strongly forbidden transition") or very weak ("weakly allowed transition"). The "Jablonski"110 diagram (Fig. 3.16) depicts various forms of absorption and emission from... 

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