Nonradiative excitation processes

Figure 24-4 Emission or chemiluminescence processes. In (a), the sample is excited by the application of thermal, electrical, or chemical energy. These processes do not involve radiant energy and are hence called nonradia-tive processes. In the energy-level diagram (b), the dashed lines with upward-pointing arrows symbolize these nonradiative excitation processes, while the solid lines with downward pointing arrows indicate that the analyte loses its energy by emission of a photon. In (c), the resulting spectrum is shown as a measurement of the radiant power emitted Pg as a function of wavelength, A.

The chemical structure of the CL precursor, not only the central portion containing the electronically excited group, but also the side chain The nature and concentration of other substrates affecting the CL pathway and favoring other nonradiative competition processes The selected catalyst... 

Fig. 10 Energy level diagram showing the excited states involved in the main photophysical processes (excitation solid lines radiative deactivation dashed lines, nonradiative deactivation processes wavy lines) of the 2 Nd3+ [Ru(bpy)2(CN)2] three-component system. For the sake of clarity, naphthyl excimer energy level has been omitted...

There are several conformational arrangements with different interactions of the fluorophores with surrounding groups of atoms. Such interactions may affect differently nonradiative deexcitation processes in the excited state, and the decay times for these conformational states will differ. If each of these states is characterized by singleexponential decay kinetics, then the number of constants f, will correspond to the number of aromatic groups in the protein that are in conformationally different states. 

Raman spectra are usually represented by the intensity of Stokes lines versus the shifted frequencies 12,. Figure 1.15 shows, as an example, the Raman spectrum of a lithium niobate (LiNbOs) crystal. The energies (given in wavenumber units, cm ) of the different phonons involved are indicated above the corresponding peaks. Particular emphasis will be given to those of higher energy, called effective phonons (883 cm for lithium niobate), as they actively participate in the nonradiative de-excitation processes of trivalent rare earth ions in crystals (see Section 6.3). 

Figure 5.16 Configurational coordinate diagrams to explain (a) radiative and (b) nonradiative (multiphonon emission) de-excitation process. The sinusoidal arrows indicate the nonradiative pathways.

Emission from the upper electronic excited states of polyatomic molecules, in violation of Kasha s rule which allows emission only from the lowest excited states Q), have been observed in a reasonable variety of molecules (2 11). With notable exceptions of azulene and thioketone, however, such emission is usually very weak, because the rates of nonradiative decay processes greatly exceed the rates of radiative processes when excited states other than the lowest excited states are involved. 

It is very important, in the theory of quantum relaxation processes, to understand how an atomic or molecular excited state is prepared, and to know under what circumstances it is meaningful to consider the time development of such a compound state. It is obvious, but nevertheless important to say, that an atomic or molecular system in a stationary state cannot be induced to make transitions to other states by small terms in the molecular Hamiltonian. A stationary state will undergo transition to other stationary states only by coupling with the radiation field, so that all time-dependent transitions between stationary states are radiative in nature. However, if the system is prepared in a nonstationary state of the total Hamiltonian, nonradiative transitions will occur. Thus, for example, in the theory of molecular predissociation4 it is not justified to prepare the physical system in a pure Born-Oppenheimer bound state and to force transitions to the manifold of continuum dissociative states. If, on the other hand, the excitation process produces the system in a mixed state consisting of a superposition of eigenstates of the total Hamiltonian, a relaxation process will take place. Provided that the absorption line shape is Lorentzian, the relaxation process will follow an exponential decay. 

The actual lifetime T of an excited molecule is usually less than xr because of the competing nonradi-ative processes. The sum of their rate constants can be designated /cnr. The fluorescence efficiency (or quantum yield) ( )F is given by Eq. 23-15. 

Fig. 16. Schematic configurational coordinate diagram of the ground and 3LC and 3MLCT excited states of the Ir3 + complexes. The full and broken lines refer to the state energies and relaxation pathways of the complex in a crystal or in solution, respectively. Straight arrows correspond to radiative and curved arrows to nonradiative relaxation processes. The shaded area indicates the range, in which the 3MLCT state can be found, depending on the environment...

The photophysical properties of compounds 31—34 are presented in Table 7. Compared to the Pd-based materials, the Pt-based ones exhibit longer emission lifetimes. This is rationalized by the more stable Pt-Pt and Pt-L bonds compared to the more photolabile Pd-Pd and Pd-L ones. Therefore, the energy-wasting photo-induced cleavage (here bond cleavage process) does not occur or at least in a much less efficiency for the Pt-materials, thus reducing efficient nonradiative excited state deactivation. 

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 ... 

The first optical laser, the ruby laser, was built in 1960 by Theodore Maiman. Since that time lasers have had a profound impact on many areas of science and indeed on our everyday lives. The monochromaticity, coherence, high-intensity, and widely variable pulse-duration properties of lasers have led to dramatic improvements in optical measurements of all kinds and have proven especially valuable in spectroscopic studies in chemistry and physics. Because of their robustness and high power outputs, solid-state lasers are the workhorse devices in most of these applications, either as primary sources or, via nonlinear crystals or dye media, as frequency-shifted sources. In this experiment the 1064-mn near-infrared output from a solid-state Nd YAG laser will be frequency doubled to 532 nm to serve as a fast optical pump of a raby crystal. Ruby consists of a dilute solution of chromium 3 ions in a sapphire (AI2O3) lattice and is representative of many metal ion-doped solids that are useful as solid-state lasers, phosphors, and other luminescing materials. The radiative and nonradiative relaxation processes in such systems are important in determining their emission efficiencies, and these decay paths for the electronically excited Cr ion will be examined in this experiment. 

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