Nonradiative mode

W. H. Weber and G. W. Ford, Enhanced Raman scattering by adsorbates including the nonlocal response of the metal and excitation of nonradiative modes, Phys. Rev. Lett. 44, 1774-1777 (1980). 

We have seen that SPs are nonradiative modes and that they caimot be excited by incident light at a flat interface between two media. This follows from... 

The requited characteristics of dyes used as passive mode-locking agents and as active laser media differ in essential ways. For passive mode-locking dyes, short excited-state relaxation times ate needed dyes of this kind ate characterized by low fluorescence quantum efficiencies caused by the highly probable nonradiant processes. On the other hand, the polymethines to be appHed as active laser media ate supposed to have much higher quantum efficiencies, approximating a value of one (91). 

A typical ligand capable of generating a dendritic structure is 1,4,5,8,9,12-hexaazatriphenylene (HAT). Photophysical studies of trinuclear species based on HAT have been reported [14 a, 49]. Representative example of complexes of this type are 26, 27, and 28. For some of these complexes, the luminescence, originating from MLCT levels involving the central HAT ligand, was found to decay with multiexponential kinetics. Furthermore, the vibrational modes responsible for the nonradiative decay of the luminescent MLCT states are different in the polynuclear dendritic edifices with respect to the mononuclear [M(L)2(HAT)]2+ compounds [14a]. 

As a preliminary test of equations 1-3, we calculated the values of required to explain the observed nonradiative deactivation rates for 0s(phen)3 " and 0s(bpy)3 at 4.2 K.1 The resulting values, respectively, 0.29 and 0.33 are in good agreement with the value of 0.29 calculated by Byrne et al. for skeletal stretching modes in large aromatic molecules. 

Since the confined modes (whether they are waveguide modes or surface plasmons) are nonradiative (i.e., their wavevector parallel to the interface, is greater than the wavevector of the... 

Quenching of narrow-line emissions (as observed for many Ln3+ ions) has been explained by phonon emission to the lattice modes. Moos and co-workers (60) and others (67) have given many examples. Usually the nonradiative rate is described by Kiel s formula (62) for a single-frequency p-phonon process,... 

The excess free carriers (and excitons) do not represent stable excited states of the solids. A fraction of them recombine directly after thermahzation either radiatively or by multiphonon emission. In most materials, nonradiative transitions to defect states in the gap are the dominant mode of decay. The lifetime of free carriers T = 1/avS is determined by the density a of recombination centers, their thermal velocity v, and the capture cross section S, and may span 10-10 s. Electrons, captured by states above the demarcation level, and holes, captured by states below the hole demarcation level, may get trapped. The condition for trapping is given when the occupied electron trap has a very small cross section for recombining with a free hole. The trapping process has, until recently, not been well understood. 

When the symmetry is reduced by replacement of a bpy ligand by another bidentate, the symmetry restrictions are relaxed and the initially formed excited state is expected to be localized (600, 601). The rates of nonradiative decay of a large range of Os(II)-polypyridine have been calculated successfully in terms of a modified energy gap law in which low-frequency modes are explicitly considered (148). 

Since the writing of this review, Englman and Jortner have presented a new formulation of the theory of radiationless transitions.227 Their treatment rests on the assumptions that the molecular vibrations are harmonic and that the normal modes and their frequencies are the same in the initial and final states, except for displacements in the origins. They consider the nonradiative transition rate in the conventional form... 

The sound mode of nonradial oscillation, with the spherical harmonics Yjiin(0, 0) and the frequency u>, can exist in the propagation zone, where the bottom boundary locates at the position of = L [=Vfi(fl+l)cs/r], and the upper boundary does near the photosphere. Here, L is called as the Lamb frequency, and cs is the sound velocity. 

When the quencher contains heavy atoms nonradiative relaxation of the exciplej occurs via the triplet state (heavy atom perturbation). A second mode of exciplcx dissociation is through electron transfer between the excited molecule and the quencher. Ionization potential of the donor, electron affinity of the acceptor and solvent dielectric constant are important parameters in such cases. 

In Sec. IV we discuss another TPM dye, malachite green (MG), which was used as a molecular probe for glass transition of alcohols and polymers [11,12], Analysis of the temperature dependence of nonradiative relaxation in MG shed light on the understanding of the mechanism of glass transition. Novel experimental observations are divided into two classes. (1) The critical temperature (Tc) predicted by the mode-coupling theory (MCT) was undoubtedly... 

As a result of ion-phonon interaction, the population of the excited state decreases via nonradiative transition from the excited state to a lower electronic state. The energy difference between the two electronic states is converted into phonon energy. This process of population relaxation is characterized by a relaxation time, xj, which depends on the energy gap between the two electronic states, the frequencies of vibration modes, and temperature (Miyakawa and Dexter, 1970 Riseberg and Moos, 1968). At room temperature, the excited state lifetime is dominated by the nonradiative relaxation except in a few cases such as the 5Do level of Eu3+ and 6P7/2 level of Gd3+ for which the energy gap is much larger than the highest phonon frequency of the lattice vibrations. 

Although no quantum confinement should occur in the electronic energy level structure of lanthanides in nanoparticles because of the localized 4f electronic states, the optical spectrum and luminescence dynamics of an impurity ion in dielectric nanoparticles can be significantly modified through electron-phonon interaction. Confinement effects on electron-phonon interaction are primarily due to the effect that the phonon density of states (PDOS) in a nanocrystal is discrete and therefore the low-energy acoustic phonon modes are cut off. As a consequence of the PDOS modification, luminescence dynamics of optical centers in nanoparticles, particularly, the nonradiative relaxation of ions from the electronically excited states, are expected to behave differently from that in bulk materials. 

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