摘要:

The application of narrow band dye lasers to high resolution spectroscopy within the inhomo-geneously broadened zero-phonon lines of colour centres is reviewed. Most work so far has centered on the use of holeburning which is exhibited by the majority of centres examined. This technique has been used to make detailed Stark and Zeeman effect measurements. In addition, optical coherent transients have been used to determine dephasing times and the results of optical phase switching and free decay experiments are discussed.

ISSN:0033-7579    

DOI:10.1080/00337578308218622

出版商:Taylor & Francis Group    

年代:1983

数据来源: Taylor

摘要:

A review is given of recent developments in the study of magnetic (localized) photo-excitations in ionic solids. Emphasis will be on three topics: (i) the importance of magnetic properties in the structural characterization of defects, (ii) photo-chemical reactions as revealed from magnetic interactions in point defects, and (iii) energy and phase relaxation dynamics in photo-excited colour centres as probed from magnetic transients. In our survey we will focus on defects photo-excited to phosphorescent triplet states. In discussing a number of dynamical processes, some attention is given to the application of (optical-microwave) coherence spectroscopy which holds considerable promise for future work.

ISSN:0033-7579    

DOI:10.1080/00337578308218623

出版商:Taylor & Francis Group    

年代:1983

数据来源: Taylor

摘要:

Large, asymmetric atomic relaxations have been shown to play a crucial role in the structure and properties of several point defects in oxide materials. Examples include trapped hole centers in alkaline-earth oxides and E1′ and E4′ oxygen-vacancy centers and peroxy-radical defects in silicon dioxide. Schirmer's “bound small polaron” model, applied in particular to the alkaline-earth oxide defects, and model treatments of the E1′ center in SiO2 by Yip, Griscom and Fowler clearly illustrate the important spectroscopic consequences of such atomic relaxations. In fact, such effects had been incorporated in Lüty's classic model of the Type II FAcenter in alkali halides. Edwards and Fowler have recently applied MNDO and MINDO/3 quantum-chemistry approaches to the E1′, E4′, and peroxy radical defects in SiO2. These calculations generally corroborate suggested models and bear as well on possible creation mechanisms. Large relaxation effects are likely to be important in many other defects in oxide materials.

ISSN:0033-7579    

DOI:10.1080/00337578308218624

出版商:Taylor & Francis Group    

年代:1983

数据来源: Taylor

摘要:

The paper summarizes progress made in analysing defect structures. For this electron spin resonance has long been a standard tool. However, its applicability is often very limited due to its insufficient power to resolve hyperfine structures, necessary to determine defect structures. Nuclear magnetic resonance provides high resolution, but insufficient sensitivity. The combination of both, electron nuclear double resonance (ENDOR), turned out to be the most powerful technique. However, complicated systems with low symmetry or with several types of defects present simultaneously usually yield very complex ENDOR spectra which cannot be analysed using “conventional” methods. Considerable progress was made by improving the experimental methods including computer assistance, processing of the data and novel ENDOR measurement techniques. The application and advantages of methods like ENDOR-induced ESR and DOUBLE ENDOR are demonstrated using several recent examples.

ISSN:0033-7579    

DOI:10.1080/00337578308218625

出版商:Taylor & Francis Group    

年代:1983

数据来源: Taylor

摘要:

The influence of defect-lattice coupling on energy structure and recombination processes at localized defects is discussed. The emphasis is put on those processes in which the charge state of the defect changes. Among the topics discussed are: extrinsic self-trapping, negativeUdefects, metastable defect states, multiphonon recombination and recombination induced defect reactions.

ISSN:0033-7579    

DOI:10.1080/00337578308218626

出版商:Taylor & Francis Group    

年代:1983

数据来源: Taylor

摘要:

This paper describes techniques for the direct measurement of configurational relaxation times in color centers. The first such measurements, on theFa(II) center in KCl:Li, yield a zero-temperature lifetime of 13 psec. Measured temperature dependence of the lifetime indicates that the relaxation is dominated by a phonon mode of energy ∼47 cm−1. The possibility of extending the measurements to other centers is discussed.

ISSN:0033-7579    

DOI:10.1080/00337578308218627

出版商:Taylor & Francis Group    

年代:1983

数据来源: Taylor

摘要:

Hydrogen atoms occupying interstitial tetrahedral sites are produced by U.V. excitation of OH−impurities in alkali halides. A single narrow Raman line is associated with this U2center and polarization studies confirm the A1→T2nature of the vibrational transition. In alkali halides containing also heavier halogen ion impurities, the interstitial hydrogen can be trapped next to them and a singly perturbed U2center of C3vsymmetry as well as a doubly perturbed U2center of C2vsymmetry are produced. Their identification was based on a careful analysis of polarized Raman studies on both randomly oriented and preferentially oriented centers. RbBr:I∼:OH∼ is a typical example: the fundamental A1→T2of the pure U2center is at 330 cm−1; the singly perturbed U2center yields a A1→A1Raman line at 284 cm−1and a A1→E transition at 345 cm−1. The doubly perturbed center exhibits a single line at 269 cm−1. A qualitative discussion of these results is attempted.

ISSN:0033-7579    

DOI:10.1080/00337578308218628

出版商:Taylor & Francis Group    

年代:1983

数据来源: Taylor

摘要:

The spin-orbit interaction of FA-type centres (FA,Z2+) has been studied experimentally and theoretically. It is found to have a greater anisotropy than anticipated from early experiments on FA(Na) centres. Nearest-neighbour impurities with sufficiently large spin-orbit interaction may dominate the effect to yield both negativeandpositive spin-orbit energies depending on the relative orientation of the centres and the measuring light beam.

ISSN:0033-7579    

DOI:10.1080/00337578308218629

出版商:Taylor & Francis Group    

年代:1983

数据来源: Taylor

摘要:

EPR experiments on photoholes in both AgCl :Cu and AgCl :Pd show the existence of an activation energy barrier, of height near 1.8 meV, in the transition to the localized self-trapped state. The self-trapped hole then migrates athermally via the small-polaron band, for temperatures below 30 K, and by phonon-assisted hopping for temperatures above 35 K. In the hopping régime, the activation energy is 61 meV, suggesting that the binding energy is about 0.1 eV. From the pre-exponential factor, one estimates that the electron transfer integral is about 1% of the energy of the pertinent acoustical phonon. Experiments on AgX:Cu in which 1% of the halide is Br−show that all of the self-trapped holes ultimately migrate to sites with 2 bromide neighbors. Upon annealing, these disappear with complex kinetics and an activation energy of about 180 meV. In AgCl :Pd, Fe, the temperature-dependence of the efficiencies of production of trapped photocarrier centers is determined and correlated with the dynamics of migration of the self-trapped hole and the cation vacancy.

ISSN:0033-7579    

DOI:10.1080/00337578308218630

出版商:Taylor & Francis Group    

年代:1983

数据来源: Taylor

摘要:

In a currently accepted model for formation of the photographic latent image, the photoefficiency is determined by the relative cross sections for electron capture at a silver site and at a bound hole. Comparison with sensitivity data shows that these cross sections are nearly identical. The two types of competing centers have sets of shallow coulombic levels with equal initial cross sections, but re-escape of the electron from these levels should be more probable than transition to the deep level, and it is not clear why the final capture probabilities should be equal for the two very different centers.

ISSN:0033-7579    

DOI:10.1080/00337578308218631

出版商:Taylor & Francis Group    

年代:1983

数据来源: Taylor