Transitions energy transfer

Fig. 10. Energy level and upconversion scheme for the Er3+ and Yb3+ codoped system. Full, dashed and curved arrows indicate radiative transition, energy transfer and nonradiative relaxation processes, respectively (redraw after...

This chapter reviews recent studies on energy levels and excited state dynamics of lanthanides (R) in nano-structures, which include R-doped dielectric nano-crystals, implanted nano-particles of semiconductors, coated core-shell nano-particles, nano-tubes and nano-balls stuffed with R ions. New phenomena such as the action of confinement on ion-phonon interaction and its consequences for electronic transitions, energy transfer, and phase transitions are discussed in the light of experimental and theoretical studies reported in the literature. Although the review aims at being comprehensive and covers all the important aspects in the field, emphasis is given to identification and theoretical analysis of various mechanisms for... 

In the case of excitation energy transfer two distinct types of mechanisms can take place. When a weak interaction can occur between the transition moments of the radiative and A A transitions, energy transfer can take place via dipole-dipole interactions. This mechanism occurs frequently in energy transfer processes between a donor in the excited singlet state and an acceptor in the ground state. The rate of energy transfer, itu A, is then described by Eq. (3.5) first derived by Fcirster 1 ... 

Besides the well-known relaxation paths for an excited state (fluorescence emissions, non-radiative transitions, energy transfers), there is a number of other processes which may arise when excited states are highly populated, namely coherent emission either of the stimulated or of the spontaneous type (superfluorescence), excited state absorption with or without energy transfers. 

Brickmann J, Pfeiffer R and Schmidt P C 1984 The transition between regular and chaotic dynamics and its influence on the vibrational energy transfer in molecules after local preparation Ber. Bunsenges. Phys. Chem. 88 382-97... 

Once the excited molecule reaches the S state it can decay by emitting fluorescence or it can undergo a fiirtlier radiationless transition to a triplet state. A radiationless transition between states of different multiplicity is called intersystem crossing. This is a spin-forbidden process. It is not as fast as internal conversion and often has a rate comparable to the radiative rate, so some S molecules fluoresce and otliers produce triplet states. There may also be fiirther internal conversion from to the ground state, though it is not easy to detemiine the extent to which that occurs. Photochemical reactions or energy transfer may also occur from S. ... 

The dynamics of fast processes such as electron and energy transfers and vibrational and electronic deexcitations can be probed by using short-pulsed lasers. The experimental developments that have made possible the direct probing of molecular dissociation steps and other ultrafast processes in real time (in the femtosecond time range) have, in a few cases, been extended to the study of surface phenomena. For instance, two-photon photoemission has been used to study the dynamics of electrons at interfaces [ ]. Vibrational relaxation times have also been measured for a number of modes such as the 0-Fl stretching m silica and the C-0 stretching in carbon monoxide adsorbed on transition metals [ ]. Pump-probe laser experiments such as these are difficult, but the field is still in its infancy, and much is expected in this direction m the near fiitiire. 

C3.4.6 EXCHANGE MECHANISM OF ENERGY TRANSFER IN FORBIDDEN TRANSITIONS... 

Because both spins are in the transverse plane and transition energy levels are matched, energy can be transferred from the protons to the nuclei. In this manner the rate of repolarization is controlled by rather than by Because the protons can interchange energy by spin-diffusion only a single-proton exists and its value is usually on the order of 1 s. As a result the preparation delay can be reduced from 10 s to about 5 s increasing the number of transients, which can be acquired by two or more orders of magnitude. 

In photoluminescence one measures physical and chemical properties of materials by using photons to induce excited electronic states in the material system and analyzing the optical emission as these states relax. Typically, light is directed onto the sample for excitation, and the emitted luminescence is collected by a lens and passed through an optical spectrometer onto a photodetector. The spectral distribution and time dependence of the emission are related to electronic transition probabilities within the sample, and can be used to provide qualitative and, sometimes, quantitative information about chemical composition, structure (bonding, disorder, interfaces, quantum wells), impurities, kinetic processes, and energy transfer. 

Characterization and control of interfaces in the incompatible polymer blends were reported by Fayt et al. [23]. They used techniques such as electron microscopy, thermal transition analysis, and nonradiative energy transfer (NRET), etc. They have illustrated the exciting potentialities offered by diblock copolymers in high-performance polymer blends. 

Energy transfer between organic molecules and transition metal ions. V. L. Ermolaev, E. G. Svesh-nikova and T. A. Shakhveraov, Russ. Chem. Rev. (Engl. Transl.), 1975,44,26-40 (110). 

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