Clifford C. Klick

Luminescence in Solids

Publisher Summary This chapter discusses luminescence in solids. Luminescence has been an active field of scientific investigation for a period that essentially encompasses the present century. During this period, there have been almost revolutionary changes in the materials, methods, and concepts that distinguish current research in the field from those of the earlier work. There has occurred, first, a realization of the extreme importance of small concentrations of impurities in luminescence. This has led to the development of careful preparative procedures for controlling phosphor compositions and for reducing the concentration of unwanted impurities to a concentration of 10 -6 or less. A second important advance has been the application of the investigative techniques and concepts of solid state physics to the analysis and interpretation of luminescence phenomena. This approach has accelerated rapidly during the last decade. Luminescent systems have been divided into three categories: (a) systems in which the absorption and emission of energy both take place in the same center; (b) systems in which absorption occurs in one center and luminescence is emitted by another center, the energy transfer taking place without accompanying movement of charge carriers; and (c) systems in which the transport of energy by charge carriers is the dominant feature.

On the Luminescence of Divalent Manganese in Solids

It is shown that, under certain conditions, measurements on the emission and absorption of luminescent centers allow the determination of the configurational coordinate curves for the ground and excited states of the centers. The method is used as a first approximation in obtaining configurational coordinate curves for divalent manganese in zinc silicate. These curves offer an explanation of the asymmetry of the manganese emission. Comparison of the energy levels for the manganese ion in the solid and gaseous state leads to an identification of the various levels and to the suggestion that emission in the solid state corresponds to a 4G→6S transition.

Polarized luminescent emission from KCl : Tl at low temperatures

Abstract At temperatures near that of liquid helium, polarized emission is observed from isolated thallium centers in KCl with a polarization P = 0.2. The center responsible for this effect has its maximum emission along the cubic axis. The polarization does not appear at liquid nitrogen temperatures and it is not dependent on the concentration of thallium. Arguments are given which make it seem unlikely that this effect is due to crystal strain or the proximity of the luminescent center to other imperfections. It is possible that there is a splitting of the levels of the excited state of thallium due to the Jahn-Teller effect. Comparison is made of the experimental results with the theory of the Jahn-Teller effect developed by van Vleck and by OPik and Pryce. Difficulties with this interpretation are discussed. In contrast to the thallium center, polarization is not found for luminescent emission from F -centers in KCl at temperatures near liquid helium.

Low Temperature Luminescence of Inorganic Solids

The emission spectra of a representative group of inorganic solid phosphors have been measured at temperatures as low as 4°K. Of the phosphors investigated, only those having “edge” emission show a tendency to exhibit a line spectrum at low temperatures; the ordinary impurity activated materials show little or no change in emission below 100°K. It is improbable that the breadth of emission is due to the interaction of the luminescent center with impurities or lattice defects since neither cold work nor variation in concentrations of impurities affects the breadth of emission. Possible explanations of the broad band, low temperature emission are the existence of a zero-point vibrational energy of a nonphotoconducting luminescent center in its excited state; and, in the case of photoconducting phosphors, the distortion of exciton and conduction levels near the luminescent center.

Thermoluminescence and Color Centers in LiF:Mg

A variety of optical absorption, emission and luminescence excitation spectra have been measured in an attempt to identify the centers involved in the thermoluminescence of commercial LiF:Mg. It is concluded that the principal trapping centers consist of a hole trapped near various groupings of Mg2+ ions and vacancies. The optical absorption bands of these centers occur in the 3100–3800 A region which contains several absorption bands corresponding to different geometries of the centers. It is suggested that the 2200‐A band arises from Mg2+ ion‐vacancy complexes which have captured two holes. During thermoluminescence, holes are transported from traps to emitting centers. The luminescent center appears to be the F center both in an isolated position and when adjacent to a complex involving Mg2+ ions.

Some Optical Properties of Lead-Activated Sodium Chloride Phosphors

Absorption and luminescence phenomena in NaCl:Pb phosphors have been found to be rather complex. Melt-grown NaCl:Pb has an asymmetrical, 2730A-peaked absorption band. The variation of emission spectrum with wave-length of excitation within this band shows that it consists of two poorly resolved absorption bands, one peaking at 2730A and the other at 2900A. At low Pb concentrations, irradiation into the first of these causes a near-ultraviolet emission peaking at 3200A; irradiation into the second causes a visible emission peaking at about 4500A. At high Pb concentrations, irradiation into the first band gives a second near-ultraviolet emission band peaking at 3850A in addition to the one peaking at 3200A. Precipitated NaCl:Pb shows all the above phenomena, and in addition, has an excitation band peaking at 2600A, producing simultaneously a 3300A-peaked emission and a visible emission. The NaCl:Pb phosphors are unstable, deteriorating after a few days at room temperature and more rapidly at 130°. X-irradiation of these phosphors destroys the above absorption and emissions, and gives a print-out effect due apparently to the formation of colloidal Pb. The x-rayed material can be excited by near ultraviolet to give a red emission, peaking at about 6100A. Some suggestions concerning the interpretation of the above phenomena are given.

On the Existence of “Sub-Bands” in the Luminescence Emission Spectrum of Manganese Activated Zinc Silicate*

Failure of the luminescence decay of manganese-activated zinc silicate to follow an exact exponential curve has been cited as evidence for the presence of two different emitting centers with different decay rates. However, the shapes of the spectral emission curves taken at different times after excitation are identical, showing that if two centers exist their emissions are the same. The luminescence emission of this phosphor has also been measured at 4°K and shows a single peak a few hundred angstroms wide. There is little observed change in the emission between 90°K and 4°K. The width of the room temperature emission is therefore the result of the temperature-broadening of an already broad band. No experimental evidence is found for bands predicted on the assumption that emission spectra are formed by the superposition of bands having the shape of Gauss error curves. Theoretical work on the shape of emission and absorption bands is reviewed and does not support the Gauss error curve theory.

Properties of Electron Centers

In this chapter, as in the previous one, there will be a great deal of emphasis on color centers in alkali halides and especially on the F center.* It is surely the best understood and most widely investigated center in ionic crystals. Since it consists of an electron localized at a halide ion vacancy, it is a very simple center. Also, many of the other, more complex centers consist of clusters of two or more F centers. As a result, the F center holds a position in the area of defects in solids which is similar to that of the hydrogen atom in atomic physics. It is the simple system on which new models, theories, and techniques are usually first tried before being extended to more complex problems.

Franck-Condon principle

The generalization that the transition from one energy state of a molecular system to another occurs…

An Apparatus for Automatic Recording of Reflection Spectra

A system permitting the automatic recording of ultraviolet and visible reflection spectra has been constructed, In which a function generating potentiometer, operating in synchronism with the wavelength drive of a conventional spectral reflectance apparatus, attenuates the phototube output to compensate for the spectral distribution of the light source, the wavelength dependence of the monochromator transmission, and the spectral sensitivity of the phototube. The potentiometer is tapped at points corresponding to about 100A intervals from 2200A to 7000A, and the resistances between taps are adjusted to give a constant recorder deflection when MgO is used as a sample. Once the potentiometer has been adjusted in this way, a direct record of spectral reflectance of an “unknown” sample is obtainable in a few minutes.