Radiative and Nonradiative Processes

The excited ions of the pumped laser materials in a laser resonator can be de-excited by various radiative (either laser or luminescence) and nonradiative (electron-phonon interaction or energy transfer) processes. Also, the amount of ions participating in laser emission is dependent on the laser emission efficiency ( /i). The laser emission is produced by the excited ions inside the laser mode volume and pumped above the threshold. The excited ions inside the pumped volume but outside the laser mode volume and those that form the inversion of population at the laser threshold can be de-excited by luminescence and nonradiative processes. 

The relevance of the superposition integral of the laser mode and the pumped volume t]y is determined by the regime of laser emission. The material factors that influence the laser threshold and thus the laser emission efficiency rii, in the case of CW emission, have been evaluated. Several of these factors, such as the emission quantum efficiency and the effective lifetime, can be influenced by the conditions of the experiment (concentration of the doping ions, temperature), whereas the quantum defect ratio is influenced by the pump wavelength. 

At concentrations of Nd , self-quenching of the emission due to the downcon-version or upconversion within the system of the doping ions can reduce the emission quantum efficiency (rjqe), which influences the emission threshold and the generation of heat. The effects of Cn4 and the pump intensity on i/qe in Nd-doped laser materials have been well documented. The calculated concentration dependences of effective lifetime (rgg = Tf /qe) for Nd-doped YAG and GSGG have been calculated and compared [24]. 

The increased self-quenching of emission, as shown in Eig. 9.3, has been widely acknowledged as a disadvantage of Nd laser materials with increasing doping concentration [30]. Actually, the effect of Cn 3 should be evaluated by considering both the pump absorption efficiency rj and the emission quantum efficiency that have effects on the laser parameter, i.e., the incident pump power [29, 30, 32]. 

The calculated product CNd qe for Nd YAG is shown in Fig. 9.4, indicating that the overall effect of CNd in certain concentration ranges, e.g., the range of 3-4 at.%, on threshold could be positive [30]. Furthermore, the increased at high C d offers 

---

For a simplified case, one can obtain the rate of CL emission, =ft GI /e, where /is a function containing correction parameters of the CL detection system and that takes into account the fact that not all photons generated in the material are emitted due to optical absorption and internal reflection losses q is the radiative recombination efficiency (or internal quantum efficiency) /(, is the electron-beam current and is the electronic charge. This equation indicates that the rate of CL emission is proportional to q, and from the definition of the latter we conclude that in the observed CL intensity one cannot distii pish between radiative and nonradiative processes in a quantitative manner. One should also note that q depends on various factors, such as temperature, the presence of defects, and the... 

The effect of the substitution of a heavy-atom directly onto the nucleus of aromatic compounds (internal heavy-atom effect) on intercombinational radiative and nonradiative processes can be seen by examination of experimental data obtained for naphthalene and its derivatives. The data obtained by Ermolaev and Svitashev<104) and analyzed by Birks(24) to obtain individual rate constants for the various processes are collected in Table 5.9. 

The pK of tyrosine explains the absence of measurable excited-state proton transfer in water. The pK is the negative logarithm of the ratio of the deprotonation and the bimolecular reprotonation rates. Since reprotonation is diffusion-controlled, this rate will be the same for tyrosine and 2-naphthol. The difference of nearly two in their respective pK values means that the excited-state deprotonation rate of tyrosine is nearly two orders of magnitude slower than that of 2-naphthol.(26) This means that the rate of excited-state proton transfer by tyrosine to water is on the order of 105s 1. With a fluorescence lifetime near 3 ns for tyrosine, the combined rates for radiative and nonradiative processes approach 109s-1. Thus, the proton transfer reaction is too slow to compete effectively with the other deactivation pathways. 

To test the above ideas, Weitz etal.(i2) performed experiments on the fluorescence decay from a thin layer of europium(III) thenoyltrifluoracetonate (ETA) deposited on a glass slide covered with Ag particles approximately 200 A in diameter. The fluorescence decay rate was found to increase by three orders of magnitude in comparison with that of ETA in solid form. In addition to the large increase in decay rate, there was also evidence for an increase in overall fluorescence quantum efficiency. It is not possible from Eq. (8.11) to say anything about the manner in which is partitioned between radiative and nonradiative processes, y should be written in terms of a radiative part yr and a nonradiative part ynr y = yr + y r. Since the radiative rate for dipole emission is given by... 

P. M. Fauchet, Porous Silicon Photoluminescence and Electroluminescent Devices C. Delerue, G. Allan, and M. Lannoo, Theory of Radiative and Nonradiative Processes in Silicon Nanocrystallites L. Bros, Silicon Polymers and Nanocrystals... 

It would be elegant to finish the part on photophysics and photochemistry of liquid alkanes by giving a picture that unifies the temperature- and energy-dependence results obtained in fluorescence and photodecomposition studies. However, the spectroscopic information available for alkane molecules is not sufficient to identify the exact excited states involved in the radiative and nonradiative processes [55]. Because of the lack of information, there are different views on the positions and identities of excited states involved [52,55,83,121,122]. 

Scheme 2 Trapping dynamics of WS2 nanoparticles following photoexcitation. Radiative and nonradiative processes are indicated with double-line arrows and wavy arrows, respectively.

This photoperturbation technique has been applied to a number of different spin-equilibrium complexes. Its success is apparently due to the fact that the relaxation times of the spin equilibria are longer in each case than the radiative and nonradiative processes in the excited states. 

Figure 24. Schematic representation of the proposed radiative and nonradiative processes occurring in nanocrystalline Mn2+ CdS. The straight lines represent radiative processes and the curved lines represent nonradiative processes. (1) Absorption to generate excitonic excited state. (2) Energy transfer to defect. (3) Energy transfer to Mn2+ via defect. (4) Radiative decay of defect. (5) Radiative decay of Mn2+. (6) Direct energy transfer to Mn2+. [Adapted from (122).]...

Examples of studies on multiphoton absorption processes and nonlinear second-and third-harmonic generation processes will be discussed along with some possible radiative and nonradiative processes. The selection rules for multiphoton absorption will be mentioned in Section 7.3, and molecular examples will be shown along with their correlating photophysical properties in Section 7.4. The effect of some parameters relating to second-order activity along the lanthanide... 

Another important application of the first-order model is the examination of the ground- and excited-state kinetics of atoms and molecules.79 These systems are characterized by competing first-order transitions representing both radiative and nonradiative processes. The radiative processes normally... 

The lifetimes of molecular fluorescence emissions are determined by the competition between radiative and nonradiative processes. If the radiative channel is dominant, as in the anthracene molecule, the fluorescence quantum yield is about unity-and the lifetime lies in the nanosecond range. In molecular assemblies, however, due to the cooperative emission of interacting molecules, much shorter lifetimes—in the picosecond or even in the femtosecond range—can theoretically be expected an upper limit has been calculated for 2D excitons [see (3.15) and Fig. 3.7] and for /V-multilayer systems with 100 > N > 2.78 The nonradiative molecular process is local, so unless fluorescence is in resonance by fission (Section II.C.2), its contribution to the lifetime of the molecular-assembly emission remains constant it is usually overwhelmed by the radiative process.118121 The phenomenon of collective spontaneous emission is often related to Dicke s model of superradiance,144 with the difference that only a very small density of excitation is involved. Direct measurement of such short radiative lifetimes of collective emissions, in the picosecond range, have recently been reported for two very different 2D systems ... 

Figure 6.8. Schematic representation of the potential energy surfaces relevant for the photochemical conversion of 1,4-dewarnaphihalcnc to naphthalene. Radiative and nonradiative processes postulated are shown and probabilities with which each path is followed are given (by permission from Wallace and Michl, 1983).

A review dealing with the implications of the breakdown of the BO approximation for the description of radiative and nonradiative processes can be found in J. Jortner,... 

A. vanDijken, E. A. Meulenkamp, D. Van-maekelbergh etal.. The kinetics of the radiative and nonradiative processes in nanocrystalline ZnO particles upon photoexcitation, J. Phys. Chem. B 2000, 104(8), 1715-1723. 

Photochemistry is inherently related to photophysics, the study of those radiative and nonradiative processes that convert one electronic state into another electronic state without chemical change. Central to both photochemistry and photophysics is the classification of UV-vis radiation in terms of its energy. Because electromagnetic radiation is quantized, it has properties like those of a particle, and a mole of photons is called an einstein. Electromagnetic radiation also has the properties of a wave, and equation 12.1 gives the relationship between the energy of UV-vis radiation in kcal/mol and its wavelength in run. ... 

Grinberg M, Mandelis A, Fieldsted K, Othonos A (1993) Spectroscopy and analysis of radiative and nonradiative processes in Ti Al203 crystals. Phys Rev B 48 5922... 

---