Absorption and emission processes

In addition to absorption and stimulated emission, a third process, spontaneous emission, is required in the theory of radiation. In this process, an excited species may lose energy in the absence of a radiation field to reach a lower energy state. Spontaneous emission is a random process, and the rate of loss of excited species by spontaneous emission (from a statistically large number of excited species) is kinetically first-order. A first-order rate constant may therefore be used to describe the intensity of spontaneous emission this constant is the Einstein A factor, Ami, which corresponds for the spontaneous process to the second-order B constant of the induced processes. The rate of spontaneous emission is equal to Aminm, and intensities of spontaneous emission can be used to calculate nm if Am is known. Most of the emission phenomena with which we are concerned in photochemistry—fluorescence, phosphorescence, and chemiluminescence—are spontaneous, and the descriptive adjective will be dropped henceforth. Where emission is stimulated, the fact will be stated. 

We referred in the last paragraph to the calculation of concentrations of excited species from emission intensity measurements, ft may, however, not always be possible to determine Am directly, and some other method of evaluating A factors may be needed. The A coefficient may be calculated from the B coefficient for the same transition by using the relation 

In this section we have distinguished between spontaneous and induced transitions, and we have shown how the probabilities for these processes, the Einstein A and B coefficients, are related to each other. The next section deals with experimental measurements of absorption, and the relation between these measurements and the theoretical quantities is explored. 

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Figure 2.2 (a) Absorption and emission processes between states m and n. (b) Seeding... 

To aid our understanding of absorption and emission processes, Eq. (2.1) can be expanded in terms of electronic, vibronic (vibrational components of an electronic transition), and spin wave functions ... 

If the energy is transferred by trivial emission/reabsorption, it will lengthen the measured lifetime of the donor emission, not shorten it as happens in resonance energy transfer. This comes about because intervening absorption and emission processes take place prior to the final fluorescence emission (the reabsorption cannot take place until the photon has been emitted) the two processes do not compete dynamically, but follow in a serial fashion. In FRET, such an emission/reabsorption process does not occur, and the fluorescence lifetime of the donor decreases. This is an experimental check for reabsorption/reemission. 

It is useful to view optical absorption and emission processes in such a system in terms of transitions between distinct vibrational levels of the ground and excited electronic states of a metal atom-rare gas complex or quasi-molecule. Since the vibrational motions of the complex are coupled with the bulk lattice vibrations, a complicated pattern of closely spaced vibrational levels is involved and this results in the appearance of a smooth, structureless absorption profile (25). Thus the homogeneous width of the absorption band arises from a coupling between the electronic states of the metal atom and the host lattice vibrations, which is induced by the differences between the guest-host... 

Even in these cases, over 90% of such atoms are likely to remain in the ground state if cooler flames, e.g. air-propane, are used (Table 8.7). The situation should be contrasted with that encountered in flame photometry which depends on the emission of radiation by the comparatively few excited atoms present in the flame. However, because of fundamental differences between absorption and emission processes it does not follow that atomic absorption is necessarily a more sensitive technique than flame emission. 

The PL decay following a short excitation pulse provides valuable information about absorption and emission processes. Therefore the temporal behavior of the PL spectra has been studied at time-scales from picoseconds to tens of milliseconds for different temperatures. 

The shift in O — O transition due to solvent interaction in the two States of different polarity can be explained with the help of Franck-tondon principle (Figure 4.9) for absorption and emission processes. 

Figure 5.9 The Jablonski diagram describing absorption and emission processes.

Here A is a constant that depends on the efficiencies of the absorption and emission processes, and the summation is over all fluorescent groups in the system contributing to the... 

At this stage, before discussing the emission aspects, we would like to stress that one has to be cautious when characterizing the relaxation phenomena in the excited states on the basis of the Stokes shift. As a matter of fact, if the same excited state is involved in the absorption and emission processes, then the Stokes shift that is defined as the difference between the 0-0 lines in absorption and emission is always negligible, as found for the PPV oligomers and in the best PPV samples. However, such behaviour is not at all inconsistent with the presence of significant relaxation processes in the excited state, as has been shown above. Note that a Stokes shift is... 

The result of the absorption and emission process is to reduce the speed of the atom, provided that its initial speed is larger than the recoil velocity from scattering a single photon. If the absorption and emission are repeated many times, the mean velocity, and therefore the kinetic energy of the atom will be reduced, thus cooling the atoms. 

The following sections are purposely separated into specific structural classes of square planar Pt" complexes of the general formulae Pt(NAN)(C=CR)2, [Pt(NANAN) (OCR)]+, Pt(NANAC)(C=CR), rra s-Pt(PR3)2(OCR)2, and d.v-Pt(PAP)(( =CR)2, where NAN is a bidentate 2,2 -bipyridine, NANAN and NANAC are tridentate polypyridines, PR3 is a monodentate phosphine, and PAP is a bidentate phosphine ligand. The final section of this work is dedicated to recent electronic structure calculations on these molecules with an emphasis on the successful application of DFT (density functional theory) and TD-DFT (time-dependent density functional theory) methods towards understanding the absorption and emission processes of these chromophores. 

D) Ag° - Support Interactions Recall that the major absorption and emission processes of gaseous Ag atoms in the uv-range are rather straightforwardly interpreted in terms of electronic transitions between an isotropic 4d105s1, 2 Si 2... 

So far, this discussion of selection rules has considered only the electronic component of the transition. For molecular species, vibrational and rotational structure is possible in the spectrum, although for complex molecules, especially in condensed phases where collisional line broadening is important, the rotational lines, and sometimes the vibrational bands, may be too close to be resolved. Where the structure exists, however, certain transitions may be allowed or forbidden by vibrational or rotational selection rules. Such rules once again use the Born-Oppenheimer approximation, and assume that the wavefunctions for the individual modes may be separated. Quite apart from the symmetry-related selection rules, there is one further very important factor that determines the intensity of individual vibrational bands in electronic transitions, and that is the geometries of the two electronic states concerned. Relative intensities of different vibrational components of an electronic transition are of importance in connection with both absorption and emission processes. The populations of the vibrational levels obviously affect the relative intensities. In addition, electronic transitions between given vibrational levels in upper and lower states have a specific probability, determined in part... 

The photophysical properties of Ceo and C70 have been reported.Ceo does not display any detectable fluorescence or phosphorescence, but is an efficient producer of singlet oxygen ( O2) with a quantum yield of unity (532nm). The properties of C70 are similar, except that a weak fluorescence is now apparent and the quantum yield of O2 is slightly less than unity. These differences are explained by the lower symmetry of the C70 molecule relaxing the forbidden nature of some of the absorption and emission processes. In addition, the fluorescence, phosphorescence, and nonlinear optical... 

The small S-T splitting in Ceo and C70 ( 10kcal/mol in Cgo, [18,23] 7.0 kcal in C70 [33]) is probably a result of the large diameter of the molecules and the resulting small electron-electron repulsion energy. Overall, the behavior of C70 is similar to that of Ceo. but the lower symmetry relaxes the forbiddenness of some absorption and emission processes. 

If a sample is irradiated with polarized light, only those molecules with absorption axes parallel to the plane of polarization will absorb appreciable energy. The emission from the molecule is also polarized, and its plane of polarization will be fixed in relation to its absorption axis. If the molecule has not moved between the absorption and emission processes, all the emitted radiation will be in one plane of polarization. The spread in the plane of polarization of the emitted light is a function of the lifetime of the excited state and the rate of molecular movement. Polarization data give information on molecular size and shape and may be obtained by a combination of spectrum scanning with modulation of the emission signal by rotation of a polarizing film interposed between the sample and detector (K7). Most manufacturers supply a simple, manually operated attachment for polarization studies. 

This is what accounts for the discrete values of frequency v in emission spectra of atoms. Absorption spectra are correspondingly associated with the annihilation of a photon of the same energy and concomitant excitation of the atom from En to Em Fig. 1.9 is a schematic representation of the processes of absorption and emission of photons by atoms. Absorption and emission processes occur at the same set frequencies, as is shown by the two types of line spectra in Fig. 1.7. 

A Jablonski diagram, shown in Fig. 14.11, is a simplified representation of some possible absorption and emission processes in molecules. Assuming that the ground electronic state is a singlet, designated So, absorption of radiation can occiii to several vibrational levels of the lowest excited singlet state Si. Several things can then happen to the excited molecule. One possibility, which we described above for a diatomic molecule, is radiationless relaxation to the lowest vibrational level of Si followed by emission of a photon, usually within several nanoseconds of the absorption. This is fluorescence, which returns the molecule to one of the... 

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