Nonradiative processes in crystals

Grinberg M, Mandelis A, Fieldsted K, Othonos A (1993) Spectroscopy and analysis of radiative and nonradiative processes in Ti Al203 crystals. Phys Rev B 48 5922... 

Two configurational coordinate models, presented in Figure 11.13 and 11.14, are sufficient to allow the interpretation of the temperature dependences of the Cr3+ fluorescence in crystal materials qualitatively, even quantitatively to an extent of sufficient precision for thermometric applications, as shown in the cases of CnLiSAF, alexandrite, and ruby. In high-field-strength host crystals, two mechanisms, the thermal repopulation of the 473 and 2E states and the nonradiative process, dominate the... 

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). 

At present it is universally acknowledged that TTA as triplet-triplet energy transfer is caused by exchange interaction of electrons in bimolecular complexes which takes place during molecular diffusion encounters in solution (in gas phase -molecular collisions are examined in crystals - triplet exciton diffusion is the responsible annihilation process (8-10)). No doubt, interaction of molecular partners in a diffusion complex may lead to the change of probabilities of fluorescent state radiative and nonradiative deactivation. Nevertheless, it is normally considered that as a result of TTA the energy of two triplet partners is accumulated in one molecule which emits the ADF (11). Interaction with the second deactivated partner is not taken into account, i.e. it is assumed that the ADF is of monomer nature and its spectrum coincides with the PF spectrum. Apparently the latter may be true when the ADF takes place from Si state the lifetime of which ( Tst 10-8 - 10-9 s) is much longer than the lifetime of diffusion encounter complex ( 10-10 - lO-H s in liquid solutions). As a matter of fact we have not observed considerable ADF and PF spectral difference when Sj metal lo-... 

F.K. Fong, Nonradiative processes of rare-earth ions in crystals 317... 

Any discussion of nonradiative processes and line broadening must at some time make a reference to azulene. It is not our aim to review the wealth of spectroscopy that has been carried out on azulene over the past few years, but rather to describe some recent and very pertinent work by Hochstrasser and Li 48> on linewidths for the Si S0 (at 14652) and S2 S0 (at 28048 cm-1) transitions. The linewidths were measured at 1.2 K for azulene and azulene-ds in a naphthalene host crystal. 

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. 

The relatively low efficiency of T2 A2 luminescence in glasses is also reflected by much smaller decay times than in crystals in which the quantum efficiences are almost 100% at room temperature. The reason for low quantum efficiencies in glasses arises from high nonradiative relaxation of the excited T2 level which competes with radiative processes. The theory of nonradiative relaxation of Cr in glasses is quite complicated ... 

In crystals where the probabilities of nonradiative decay processes are smaller (the latter takes place in luminescent crystals with a large quantum luminescence yield), the lifetime for singlet-excitons in pure crystals can be of the order of 10-9 s. For triplet-excitons this time can be a few orders of magnitude larger (for example, the lifetime of a triplet exciton in anthracene is of order 10-4 s). The characteristic time of exciton scattering by phonons is of the order of picoseconds and thus usually is much less than its radiative lifetime. This means that generally one may assume that during the exciton s lifetime thermodynamic equilibrium of excitons and phonons is established. 

However, the process of free excitons binding in a deep local state need not be taken into consideration if no account is taken of the processes of nonradiative decay of single excitons, whereby the energy A goes over into the phonon energy. In crystals, where the quantum yield of exciton luminescence is close to unity (for instance, in anthracene crystals), the nonradiative decay of excitons cannot be realized within the exciton lifetime (otherwise we cannot regard the number of excitons in the crystal in consideration of collective processes as specified). 

F.K. Fong, Nonradiative processes of rare-earth ions in crystals 317 J.W. O Laughlin, Chemical spectrophotometric and polarographic methods 341 S.R. Taylor, Trace element analysis cf rare earth elements by spark source mass spectroscopy RJ. Conzemius, Analysis of rare earth matrices by spark source mass spectrometry 377 37D. E.L. DeKalb and V.A. FasseL Optical atomic emission and absorption methods 405 37E. A.P. D Silva and V.A. Fassel, X-ray excited optical luminescence of the rare earths 441 F.W.V. Boynton, Neutron activation analysis 457... 

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