Lyoluminescence from gamma-irradiated NaCl

0146-5724/89 $3.00+ 0.00 Copyright © 1989 Pergamon Press plc

Radiat. Phys. Chem. Vol. 34, No. 5, pp. 729-738, 1989 Int. J. Radiat. Appl. Instrum., Part C Printed in Great Britain. All rights reserved

REVIEW LYOLUMINESCENCE FROM GAMMA-IRRADIATED NaC1 C. D. KALKAR Department of Chemistry, University of Poona, Pune-411 007, India

(Received 13 July 1988; receivedfor publication 1 February 1989) A~tract--Lyoluminescence is the emission of visible light during the dissolution of irradiated organic and inorganic solids in pure water. This phenomenon has attracted many researchers of various disciplines like radiation physics, radiation chemistry and physical chemistry. Amongst the alkali metal halides sodium chloride crystals have been extensively used for lyoluminescence study during last two decades. The object of the review is to understand various parameters which control the light yield during the dissolution of irradiated NaCl in water.

on the practical aspects of lyoluminescence in dosimetry. Recently, Avotinsh et al. (1984) have published a monograph on lyoluminescence in Russian. The present review deals with various factors which influence the light yield during the dissolution of y-irradiated NaC1 in water and aqueous solutions of activators.

INTRODUCTION

Luminescence is a conversion of energy, stored someway within matter, into radiation. The energy can be stored in irradiated NaC1 crystals in the form of F and V centres. A visible light is observed if the measurement of luminescence and irradiation of NaC1 crystals are made at room temperature. Some of the V centres that possess energy more than their potential barriers at room temperature become EXPERIMENTAL ASPECTS mobile and combine with electron traps to give Selection of polycrystalline NaCI emission of light known as room temperature luminescence (RTL; Arnikar et al., 1975). An alternaThe stability of F-centres and colouration tive mode of releasing at room temperature the produced in crystals by high-energy radiation deenergy stored in the irradiated NaCl crystals is to pends on the energy barrier against the trapped simply dissolve it in water. electrons. Amongst inorganic solids, the brownishA phenomenon of emission of light during the yellow colouration in NaC1 crystals is stable at room dissolution of alkali chlorides treated with cathode temperature indefinitely. Single crystals of NaC1 had rays was first observed in 1895 by Wiedemann and been used in the past extensively in researches in Schmidt who named it as lyoluminescence (LL). solid-state physics, so that their relevant properties After nearly 60 years, interest in the study was revived are well-known. Therefore, it is convenient to use by the work of Ahnstr6m and Ehrenstein (1959) and NaC1 crystals to the study of LL. of Westermark and Grapengiesser (1960). These workers observed the emission of light during Measurement of light yield dissolution of a number of substances such as The usual method of recording the light emitted glucose, sorbitol, saccharose, glycine and NaCl, LiF, during the dissolution of irradiated NaCl crystals is KI which were previously irradiated by X- or ~-radi- by using a photomultiplier coupled to an electrometer ation or by high energy electrons or fast neutrons. with a digital display. This was found to give a linear Thus, both organic and inorganic solids were found response and was advantageous over the photon showing LL. counting method of Dekker and Morrish (1950). A more systematic approach to the study of The apparatus (Fig. 1) consists of a photomultilyoluminescence emerged only from 1970. Arnikar et plier tube mounted horizontally on two supports in aL (1970a) reported the emission of a faint bluish- front of a cell, in a light-proof wooden box. The cell green glow when y-irradiated alkali halides were is in the form of a semicircular cylinder having an dissolved in distilled water. The glow was seen only optically flat surface facing the photocathode. The when the eye could acclimatize to total darkness. outer surface of the semicircular cell is surrounded They proposed the name for the glow as aqualu- with silver mirror which acts as reflector. The irradiminescence (AL). ated salt can be dropped exactly at the centre of the Ettinger and Puite 0982) have reviewed the work cell by using a sample holder. The uniform stirring is R.P.C. 34/5---A

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Review

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Power7

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Fig. 1. Front view of LL measurement assembly. effected by a magnetic stirrer kept exactly at the centre below the cell. The whole assembly is kept in a dark room. Khedekar et al. (1981) have found an 80% increase in the total light yield with this type of set-up as compared to a vertical set-up (Kannan, 1979) in which the cell is mounted on the top of the photomultiplier kept in a vertical position. Samples of 0.5 g NaC1 powder (40-60 mesh) were prepared in an glass envelope and wrapped in black paper. These samples were irradiated to a pre-determined dose by using a 6°Co V-source. The samples were stored in a desiccator for a sufficiently long time to allow RTL to decay completely (Arnikar et al., 1975). The total LL intensity was measured by carefully adding the irradiated salt in the cell containing a fixed volume of water. L L emission spectrum

The method of recording the emission spectrum is either by using a spectrograph (Arnikar and Kalkar, 1977) or a photomultiplier-oscilloscope assembly (Arnikar et al., 1970b) with a set of interference filters. The latter method has been used by earlier workers (Avotinsh et al., 1984; Atari et al., 1973) for recording the LL emission spectra. These spectra need corrections both for the photomultiplier response and absorption due to interference filters. It is also not an easy task to record the LL emission spectrum on a photographic plate since the LL glow is of short duration and feeble in intensity. Atari (1980a) constructed sophisticated equipment having a gallium arsenide photocathode detector with a flat response from 200 to 930 nm and an optical multichannel analyser system as a spectrometer. This type of equipment is sensitive to detect doses of 0.01 Gy given to the substances and the spectrometer with silicon target detector could resolve with + 4 nm. The details of generating and measuring the LL intensity and its spectrum are reported earlier by

Arnikar and Kalkar (1977), Arnikar et al. (1970b), Atari (1980a), Atari et aL (1973) and Avotins et al. (1975).

RESULTA SND DS ICUSSO IN When 1 g of the irradiated NaCI was dropped into a quartz cell containing 50 ml of distilled water it gave a pulse glow which was recorded on an oscilloscope (Arnikar et al., 1970c) operated on a time base of 6 s; a typical oscilloscopic pattern of the LL glow is shown in Fig. 2. It has been observed that the maximum LL intensity develops in about 0.2 s. The decay time of about 2-3 s varies with the amount of salt added and with the rate of dissolution at constant temperature. A complete study of LL from irradiated NaCI was carried out by Ahnstr6m (1965). The phenomenon of lyoluminescence has raised several questions about the light emission process occurring

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Fig. 2. Oscilloscopic trace of LL from y-irradiated NaC1 with a full time base of 6 s.

Review during dissolution of irradiated solids in water. Is it due to fluorescence or phosphorescence? Which species is responsible for emission of light? What is the mechanism of luminescence? A literature survey clearly shows that the mechanism of the light emission process can be classified depending upon the nature of the solid. Broadly speaking, we have two types of solids: (i) inorganic solids and (ii) organic solids. Mechanism o f Lyoluminescence Inorganic solids

Mittal (1970, 1971) had proposed that a type of excited species ( H 2 0 + . . . e - ) is produced in the triplet state during dissolution of 7-irradiated NaC1 crystals in water. Such species could be produced either via an exchange type interaction with lowenergy electrons liberated on dissolution or via an intermolecular transfer process from triplet excitons generated by radiation in NaC1. The observed emission is a phosphorescence due to the transition T~~S0. Tiliks et al. (1968, 1970) have explained their data obtained on LL of irradiated NaC1 or KC1 in the presence of T1÷ ions on the basis of energy transfer from an excited triplet state of water. A similar view was developed independently by Chertok and Mikhalchenko (1971). The exciton theory appears attractive because it is easy to imagine trapped energy being directly transfered to water. Another approach to the mechanism is via formation of the hydrated electron. Alkali halides are known to develop colour centres (Etzel and Allard, 1959) upon exposure to 7-rays. During destruction of the crystal structure the F-centres first react within a few nanoseconds with water dipoles to form hydrated electrons: F-centre H20' e~q

(1)

During such a short time, only some of the hole centres may get hydrated and the rest still remain as a part of the disintegrating crystal. The next step is the recombination of hydrated electrons with hole centres at solid-water interphases giving rise to luminescence: CI~- + e~q~Cl~q + (Cl~q)* (Cl~q)* ---}Cl~q + hv

(2) (3)

Ahnstr6m (1962) and Arnikar et al. (1970a) published their observations proposing the above mechanism for the light-emission process. The mechanism proposed by Ettinger et al. (1977) is based on recombination of electrons and V: centres which has subsequently been supported by Atari (1980a). Earlier, Walker (1968) has envisaged the irradiated crystal as a possible source for obtaining hydrated electrons. The experiments of Gopinathan et aL (1972) conclusively proved Walker's observation that

731

when ~-irradiated NaC1 crystals dissolve in water, hydrated electrons are formed. Kalkar and Doshi (1986) and Kalkar et al. (1988) have used the stored energy in irradiated NaCI to convert nitrate into nitrite. Total quenching of LL was observed in aqueous nitrate solution simply because the hydrated electrons were utilized in the reduction of nitrate rather than in the light emission process. Therefore, the formation of the hydrated electron is an important step during the dissolution process followed by recombination of the electrons and hole centres at the interphase between solid and solution. Organic solids

Free radicals are generally formed as a result of ),-radiolysis (Pshezhetskii et al., 1974) of organic substances. Ahnstrfm (1961) had suggested the involvement of free radicals in the LL mechanism of organic solids. Many features of LL can be explained by the Russel-Vassil'ev scheme (Russel, 1955; Vassil'ev, 1967). Formation of singlet oxygen (Khan and Kasha, 1964, 1970), perhydroxyl, superoxide anion (Bielski, 1977) and carbonate radical (Stauff et al., 1973) have been proposed to account for the luminescence character of organic solids. All these investigations show that the mechanism of luminescence depends upon the nature of the organic solid; however, the light emitting species have not yet been positively identified. Factors Affecting the L L Intensity

The energy is stored in the alkali halides crystals in the form of colour centres. There are various techniques of producing colour centres in alkali halides, like additive colouration, electrolytic coiouration, electric discharge and radiation-induced colouration. The concentration of colour centres produced on irradiation in alkali halides depends on various factors, i.e. water of hydration, crystal temperature, dose absorbed by alkali halides, storing time of irradiated solids, type of radiation and decay of RTL associated with irradiated alkali halides. The light emission occurs when the stored energy is released during the dissolution of irradiated solid in an appropriate solvent. This depends on the nature of the solvent, temperature of the solvent, presence of dissolved gases in the solvent, pH of solution, particle size of the irradiated crystals, solubility of the salt in the solution and rate of dissolution of the irradiated salt in the solvent. Water o f hydration

Arnikar et al. (1971 a) studied the LL intensity from alkaline earth halides and alkali sulphates and carbonates and found that hydrated substances like BaCI2, 2H20 and Na2SO4' 10H20 do not lead to colouration on intense 7-irradiation nor do they show any light emission on dissolution in pure water; on the other hand, their dehydrated forms developed

732

Review formomide

DimethyL

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N-methyL

I I diomine

EthyLene

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Fig. 3. Oscilloscopic pattern of solvoluminescence from y-irradiated NaC1 in DMF, NMF, MeCN and EDA with a full time base of 4 s. visible colouration and also emitted light on dissolution. It was thought that the water molecules present in the crystal lattice might help the migration of electrons from trap sites to hole centres in a kind of tunnelling effect. Thus colouration and emission of light during dissolution of hydrated salt, both are absent. Solvent

Limited data are available in the literature on the LL of irradiated NaC1 in solvents other than water. The only reason is that the alkali halides, except lithium salts, are insoluble in most of the organic solvents. I (unpublished) have recorded the oscilloscope pattern of irradiated NaCI in organic solvents (Fig. 3) at about 80-120°C. However, there were several difficulties in maintaining such a high temperature very close to the photomultiplier tube. Arnikar et al. (1977) studied the light yield from 7-irradiated LiC1 in various non-aqueous solvents. They proposed the name solvoluminescence (SL) for the emission of light in organic solvents. Khedekar et al. (1985) extended the studies on the solvent effect on LL of LiC1 and sought generalization based on the observation of SL intensity. A plot of SL intensity versus solubility of LiCl salt in various organic solvents is shown in Fig. 4. Solvents like cyclohexanone, pyridine, isopropanol, ethanol and methanol showed very weak SL emission. This is simply because of electron-scavenging processes being predominant in these solvents. A linear increase in SL intensity with solubility is observed in nitrobenzene, methyl cyanide and ethylene diamine while solvents like cyclohexanol, N-methyl acetamide, N-methyl formamide and dimethyl formamide showed intermediate behaviour.

Temperature o f the solvent

The effect on the total light yield with increasing temperature of the solvent has been studied by Gikas (1973) and Atari and Ettinger (1974a). A three-fold increase in LL intensity has been reported when the temperature was raised from 20 to 60°C. This is because the solubility of salts varies with temperatures and hence the total light yield increases at higher temperatures. Dose and storage time

The concentration of colour centres depend upon the amount of dose absorbed by the crystal. The growth rate of F-centre studied with dose by Mador et al. (1954) and Gordon and Nowick 0956) showed that the colouration of alkali halides is a two-stage

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Fig. 4. SL intensity as a function of the solubility of LiCI in different solvents.

Review

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process. The initial fast process is one in which the vacancies present inherently in the crystal are filled. This is followed by vacancies produced by ionizing radiation trap-released electrons to form F-centres. The LL response of irradiated NaC1 studied by Atari et al. (1973) as a function of dose is shown in Fig. 5. The LL intensity reaches a saturation value above 10 kGy dose which is very close to the saturation dose value for NaCI crystals reported by Arnikar et al. (1971b). Freshly irradiated NaCI crystals show RTL which decays exponentially at room temperature (Arnikar et al, 1974). The decay of y-irradiated NaCI and the corresponding LL intensity as a function of time of storing (Arnikar et al., 1978a) is shown in Fig. 6. Once the RTL is over then there is no change in LL intensity. One way of getting a reproducible LL intensity is to allow RTL to decay completely. Avotins et al. (1984) proposed a mathematical method for separation of the intensities of LL and RTL of irradiated crystals from the total glow observed during dissolution of freshly-irradiated solids.

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Isothermal and photo-annealing

Arnikar et al. (1972,1976a) studied the LL intensity of irradiated NaCI crystals under isothermal annealing conditions. A plot of LL intensity versus time of annealing is shown in Fig. 7. The fraction of trapped electrons remaining in the crystal is a measure of LL intensity on dissolution. The process of isothermal annealing follows the first-order rate law given by I / I o = exp(-k~ t)

(4)

where k~ is the first-order rate constant and I is the LL intensity at time t. The kinetic data were used to calculate the energy barriers of irradiated alkali halides by using the Arrhenius equation

( )

k I = A exp - ~ - ~

(5)

where T is the annealing temperature, k the Boltzmann constant and A the frequency factor. Energy barriers for some of the alkali halides measured by the LL techniques are given in Table 1. Arnikar et al. (1975, 1979) studied the regeneration of RTL by F-light and the corresponding changes in the LL intensity of 7-irradiated NaC1 crystals. They made an important observation that the LL intensity was at a maximum when RTL was at a maximum and that it decreased to a minimum when RTL had Table I. Energy barrier of trapped electrons in y-irradiated alkali halides Alkali halides E (eV) NaCI (brownish yellow) l.l NaCI (grey) 1.05 NaBr 0.53 KCI 0.16 KBr 0.26

734

Review 200

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Fig. 8. A plot of LL intensity as a function of photo-annealing time. wholly decayed. A plot of LL intensity vs time of photo-annealing is shown in Fig. 8. The LL intensity decreases with photo-annealing time. It has been observed that the freshly-irradiated NaC1 single crystal (yellow form) changes to blue due to the F ~ M centre transformation (Fig. 9). F-centre concentration decreases during the formation of F-aggregates by the photo-annealing process. The decreases in LL intensity can be correlated to the loss of F-centres. However, the M-centres in photoannealed NaC1 crystals may be reacting with water in a different way during the dissolution process: [e- + e-]a q u2o H2 + 2 O H -

(6)

The pH of the resulting solution after dissolution of irradiated photoannealed NaC1 (blue form) showed an increase of 0.3 pH in water as compared to that of the solution obtained by dissolution of irradiated NaC1 having only F-centres (yellow form).

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Mesh size

Fig. 10. Variation in LL intensity with mesh size of NaCI crystal.

down as the dissolved ions accumulate very close to the crystal surface preventing the further dissolution. Under such circumstances the process of dissolution becomes diffusion-controlled (Arnikar et al., 1976b). Usually, the dissolution is effected during LL measurements by using a mechanical stirrer which increases the total light yield (Avotinsh et al., 1978a, b, 1979).

Particle size of the crystal The light yield depends on the particle size of the crystals. Takavar (1977) and Srirath (1980) have shown that the light yield increases with mesh size for organic solids. Similarly, crushing of NaC1 crystals increases the LL intensity (Atari et al., 1973). The

Dissolution of irradiated crystals

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When a crystal of NaCi comes into contact with a stationary solvent, the surface of the crystal dissolves instantaneously and the rate of dissolution slows

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Review 28

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LL Emission Spectra o f Alkali Halides

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Fig. 12. Variation of LL intensity with quantity of salt added. variation of LL intensity with mesh size of NaC1 crystals studied by myself (unpublished) is shown in Fig. 10. Within a very narrow range of particle size the LL intensity linearly increases with increasing mesh size. Crushing a NaCi crystals produces additional dislocations which enhance the ability of the crystal to produce colouration (Seitz, 1954). When crushed crystals are dissolved in water, the recombination of electrons and hole centres proceeds at a faster rate because a large surface area of crystal is exposed to the fresh solvent. Dissolved gases

The effects of dissolved gases on LL intensity resulting from dissolution of irradiated NaC1 has been studied by Arnikar et al. (1978a). A slight increase in LL intensity was observed when water was saturated with N2 or H 2 gas, while dissolved 05 and SO s quench the LL. Eriksson (1962) also found the quenching of LL from NaCI in the presence of dissolved O:. p H of the medium

The effect of pH on the LL intensity of alkali halides was studied by Arnikar et al. (1971b) (Fig. 11). They obtained an optimum value of pH as 3.2 for NaBr, 9.8 for KBr and 4.0 for NaC1. However, in the case of NaCI, a neutral pH value has been reported by Atari and Ettinger (1974a). It is interesting to note that the maximum LL intensity is observed over a pH range in which hydrated electrons are stable. Effect o f the amount o f salt added

Arnikar et aL (1971b) studied the variation in LL intensity with the amount of irradiated alkali halides added to 50ml of distilled water (Fig. 12). The observed LL intensity tends to saturation at 5 x 10-2mol for NaC1, 4 x 10-2 for NaBr, 2.5 x 10 -2 for KBr and 4.5 x 10 -5 for KCI. The saturation stage differs with the salt added. It may depend upon the

The mechanism of LL proposed by Ahnstr6m (1965), Arnikar et al. (1970a), Atari (1980a) and Ettinger et al. (1977) involves the reaction of e~q with the hole centres at the solid-water interphase. If this is true then all alkali metal chlorides should have the same emission band. Avotinsh et al. (1977) reported the view that the position of the peak does not depend on the nature of the cation. However, I observed a shift in the LL emission band in chlorides from higher energy to lower energy with change in metal ion from lithium to cesium (Table 2; Kalkar, 1983). I explained LL emission bands on the basis of lattice enthalpy. In alkali metal chlorides an electron trapeed at the F-centre is surrounded by six metal ions. During dissolution the F-centres released become hydrated. Some of the hydrated electrons which travel a few ~ngstr6ms away from the crystal surface escape recombination with hole centres and may be destroyed by producing radical or ionic reactions: e~q~H + OH

eaq+ 02"*02 + H20

(7)

(8)

These reactions are responsible for the low light yield in LL. The rest of the electrons will be captured by hole centres at the surface of the disintegrating crystal producing excited molecules which eventually lead to the emission of light. For example, in the case of the NaCI crystal, there is a high probability of association of Na + ions with excited species during recombiantion process at the surface of the interphase giving molecular excitation: CI~- + e~q... Na + --*Cl~q+ N a + . . . (Cl-)*q

(9)

The disruption of the crystal lattice and association of excited species with oppositely-charged ions are important steps in dissolution of irradiated crystals. The net effect is that the band position is a characteristic of the salt used for dissolution. Role of Activators

There are two more factors which govern the LL intensity. First is the type of radiation used for colouration of alkali halides. Data on LL intensity obtained by dissolution of salts irradiated with different types of particles like neutron, electron, X-ray etc. are missing. The second important factor is the presence of impurities either in the irradiated salt or water in which the salt is dissolved. Let us consider the presence of small amount of impurities in the solvent itself. The impurities which enhance the light yield are known as activators. These activators are classified under (i) fluorescent dyes, (ii) chemiluminescent materials and (iii) inorganic metal ions.

Review

736

Table 2. The energy of LL emission band and lattice enthalpies of alkali metal chlorides LL emission band Alkali metal chloride

Lattice enthalpy (kcal mol i)

LiCI NaCI KCI RbCI CsCI

202.0 185.9 169.4 164.0 155.9

(nm)

(eV)

582 512 459 [452] ~ 432

2.13 2.4 2.7 2.74 2.87

~Bracket indicates the expected value from lattice enthalpy.

Fluorescent dyes

Ahnstr6m (1965) studied the light yield when NaC1 crystals irradiated to different doses were dissolved in water containing fluorescein. The LL emission spectra of fluorescent dyes and dye lasers are recorded in Kalkar (1983) and Kalkar et al. (1984) using a conventional technique. Two different mechanisms were proposed to explain the LL emission from the dye. Ahnstr6m (1965) assumed that the light emission takes place by transfer of excitation energy from (Cl,q)* to the fluorescent dye. Atari (1980b) and Atari and Ettinger (1974b) also considered that the energy transfer takes place from excited halogen to the fluorescent molecule. In the case of additively coloured crystals, it was postulated (Atari, 1980b) that a reaction between dye and hydrated electron leads to the excitation of dye molecule. The formation of reduced species of the dye molecules have been well established by means of pulse radiolysis (Priitz and Sommermeyer, 1967; Priitz et al., 1966). Treatment of fluorescein and eosin-Y with irradiated NaCI showed no change in the concentration (Kalkar, 1983) of these dyes. On the other hand, pyronin-G and rhodamin-B showed a decrease in the dye concentration after LL was over (Kalkar, 1983; Kalkar et al., 1984). This clearly indicates that there is a reduction of dye molecules which may or may not regenerate after LL is over as suggested in Kalkar (1983). Therefore, recombination of oxidised and reduced species of dye (D) may be an alternate way of generating LL: Dox + Dred--.D*q~D + hv'

(10)

Atari (1980b) has reported the same LL emission bands in fluorescein and eosin for both additively coloured as well as y-irradiated alkali halides. Usually the fluorescent dye molecules give LL emission spectrum which closely resembles its characteristic fluorescence spectrum. In other words, the fluorescence is due to external excitation by photons while LL is due to internal excitation as a result of reactions of colour centres with dye molecules. Chemiluminescent materials

Most extensively studied chemiluminescent material for LL study is luminol (5-amino-2,3dihydrophthalazine-l,4-dione). When irradiated

alkali halides are dissolved in alkaline luminol solution an enhancement of 105-106 fold in LL intensity is observed (Atari and Ettinger, 1974a) as compared to LL in pure water. The light yield was found to be independent of V-centres present in alkali halides while Chertok et al. (1974) found 4-10 times higher LL yields in the presence of V-centres. According to Atari (1980b) the mechanism of LL is similar to that of the chemiluminescence (CL) of luminol as suggested by White and Bursey (1964). Formation of hydrogen peroxide is considered by Bugaenko et al. (1973) to be the first step during dissolution of irradiated alkali halides in water: 0 2 q- eaq--+O 2

2H20 + 202 ~ H 2 0 2 + 2 O H - + 02

(11) (12)

luminol + H202 + O2-+(3-APA)*-+(3APA) + hv (13) where (3-APA) represents 3-aminophthalate ion which is the emitting species produced during oxidation of luminol. Kalkar et al. (1985) have established that oxidation of luminol does not occur during LL of luminol by irradiated NaC1 crystals. According to these authors, the colour centres react with dinegative ions of luminol represented by L 2 in the following manner: L2q + Cl~--+Laq + 2Claq

(14)

L~q + e~q--+(LZ.q)*~L]q + hv(425 nm)

(15)

Even though same emission band occurs in both cases the mechanism of LL and CL are different. Metal ions

Most of the earlier work on luminescence of alkali halide crystals is on thallium-doped crystals. In solution luminescence of T1÷ ion is extensively studied because it forms the basis of thallium determination (Fukuda, 1964). Kallmann et al. (1956) found that the light output increases extensively when irradiated NaCI crystals are dissolved in dilute TICI solution. Westermark and Grapengiesser (1960) found a 200-fold LL emission when the aqueous solution contained 0.005 gl -~ of T1CI. A 60-fold enhancement in LL intensity has been reported by Arnikar et al. (1971a) at 10 -6 M TIC1 concentration. The LL emission spectrum recorded by Arnikar and Kalkar (1979) for irradiated NaC1 in the presence of T1 + ions consisted of a broad symmetrical band extending from 390 to 546 nm with a maximum around 436 nm and a half-band width of 86 nm. LL emission spectra in the presence of T1+ ions recorded by Atari (1980b) had a maximum in the range 460--480nm for both additively coloured and Yirradiated alkali halides. The mechanism of LL in the presence of T1 ÷ ions is given by Lelievre and Adloff (1964) after spectropho tometrically identifying the TICI~-

Review

737

150

100 £3

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50

010-8

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I 10-6

I t0-5

10-4

Concentration of Tt ÷ (M)

Fig. 13. Variation of LL intensity with concentration of TI + ions.

complex as the emitting species. Atari (1980b) had represented a chloro-complex of thallium by a general formula which is excited in the following way: (Cl~q) + T I C I ~ - " ~ A * ~ A

+ hv(436 nm)

(16)

where n = 4 and .4 represents the excited chiorocomplex. In other words, this mechanism is based on an energy transfer process. Kalkar (1984) proposed that the first step is the reduction of the thallium ion by e~q (rate constant being 10 l° M -l s -l) followed by recombination with the hole centre. Kalkar (1984) obtained a broad shoulder type of distribution for concentration dependence o f LL intensity (Fig. 13) using TI2 SO4 instead of the less soluble T1C1 salt. The concentration dependence as well as the spectral distribution suggest the probability of two excited species being involved in LL, one being (Tl+)*q and the other a chloro-complex T1CI3-. Monovalent and divalent metal ions also show enhanced LL when irradiated NaCI crystals are dissolved in their aqueous solutions (Kalkar, 1984; Kalkar and Ramani, 1981). Westermark and Grapengiesser (1960) did not observe amplification of LL intensity when irradiated NaCI was dissolved in water containing lanthanide ions. As against this, Ettinger and Anunuso (1981) observed an enhancement in LL intensity in the case of rare earth ions. Hence it is necessary to extend the study of LL using various metal ions in order to understand their behaviour with colour centres in aqueous solution. Finally it is now well-established that certain substances like N 2 0 , I - , N O 3 , alcohol, acetone etc., if present in solution totally diminish the LL intensity. These substances are known as quenchers. The stored energy in the irradiated alkali halides can be utilized in effecting oxidation (Kalkar et al., 1983) reduction (Kalkar et al., 1988), redox (Arnikar et al., 1978b) and bio-redox reactions (Arnikar et al., 1982) in situ

by simple dissolution process. The chemical effects induced by irradiated solids during the dissolution process in aqueous solution is itself a new field of research. In general, the study of LL under various conditions may throw some light on our understanding of physico-chemical processes occurring at the solid-liquid interface.

REFERENCES

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