Nonradiative phonon relaxation

The first term is due to spontaneous radiative relaxation and nonradiative phonon relaxation as described in eq. (13), where / , is the probability of ion i in the excited state. The second term is due to energy transfer induced by ion-ion interaction, where W es and W A are rates of resonant and phonon-assistant energy transfer, which depend on distance between donor and acceptor RtJ. For resonant energy transfer... 

A class of futuristic solar cells, often called hot carrier solar cells, seeks to harvest the full energy of solar photons. Such cells would utilize the additional energy content of a blue photon relative to ared one.126 In present-day solar cells, equilibrated carriers are collected and hence all absorbed photons with energy greater than the bandgap contribute equally to the measured efficiency. The realization of such hot carrier solar cells therefore requires electron transfer processes that are competitive with nonradiative decay of molecules or phonon relaxation in solids.126 Literature data indicate that such relaxation occurs on a femtosecond timescale. The ultrafast... 

Single-ion nonradiative decay for Ln3+ diluted into transparent host elpa-solite crystals, where the energy gap is greater than the Debye cutoff, is primarily due to multiphonon relaxation (with rate kmp). In some cases, first order selection rules restrict phonon relaxation between states, such as between Tig and T4g, or between T2g and T5g, CF states for MX63- systems. The dependence of the multiphonon relaxation rate, kmp, upon the energy gap to the next-lowest state (AE) has been investigated for other systems and is given by a relation such as [353, 354]... 

Figure 8 schematically illustrates a four-level laser system such as that for a Nd YAG laser. Population inversion and lasing is established between levels E2 and Ej. The optical pump populates level E3 that may be a broad range of closely spaced levels in practice. Decay from E3 down to the metastable upper laser level E2 may occur via radiative or nonradi-ative relaxation processes. Random radiative emission, or spontaneous emission, occurs without a stimulating electric field, in contrast to stimulated emission. Nonradiative relaxation implies energy transfer via lattice vibration also called phonon modes. 

The above dynamical description of the polymerisation strongly parallels that of nonradiative transitions and this is not accidental althouth the monomer crystal from which the polymeric one is issued, do fluoresce, the polymeric one does not, despite its strong absorption at 2 eV. This strongly indicates efficient nonradiative relaxation of the excitation and strong electron-phonon coupling. 

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. 

The extent to which 4 j-varies with temperature at higher temperatures is an indication that nonradiative processes may contribute to the relaxation of the higher electronic states. This can arise because of coupling of the states to the lattice phonons so that the excitation energy is dissipated in the form of heat. Evidence exists that such a nonradiative process... 

Carrier relaxation due to both optical and nonradiative intraband transitions in silicon quantum dots (QDs) in SiOa matrix is considered. Interaction of confined holes with optical phonons is studied. The Huang-Rhys factor governing intraband multiphonon transitions induced by this interaction is calculated. The new mechanism of nonradiative relaxation based on the interaction with local vibrations in polar glass is studied for electrons confined in Si QDs. 

Nonradiative relaxation between the 4f states of lanthanide ions can occur by the simultaneous emission of several phonons. The multiphonon emission rate decreases exponentially with the energy gap AE to the next-lower level ... 

Compared to oxide glass matrices, such as silicates with a phonon energy at Wp = 1100cm the value of Wp 500 cm in fluoride glasses corresponds to much lower nonradiative relaxation and consequently higher quantum efficiency for laser emission. 

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