Radar. Phys. Chem. Vol. 33, No. 3, pp. 211-217, 1989 hat. J. Radiat. Appl. Inrtrwn. Part C Printed in Great Britain. All rights reserved

OM-5724/89 $3.00 + 0.00 Copyright 0 1989 Pergamon Press plc

PULSE RADIOLYSIS INVESTIGATIONS ON THE OXIDATION OF IODOBENZENE

IN AQUEOUS SOLUTIONS

Hlw MOHAN and P. N. MOCBRTHY Chemistry Division, Bhabha Atomic Research Centre, Bombay 400085, India

(Receiued 8 March 1988; in revised form 4 May 1988)

Abstract-In the pulse radiolysis of N,O saturated basic aqueous solutions of iodobenzene a transient light absorbing species (.L,_ = 325 nm), attributable to iodohydroxycyclohexadienyl radical has been observed. Its G value and extinction coefficient have been calculated to be 4.53 and 4480 dm3mol-’ cm-’ respectively. Pulsed conductivity measurements and stable product analysis showed that the decay of the species gives rise to I- with a G value of 0.85. The rate constant for the reaction of OH radical with iodobenzene has been determined to be 2.9 x lo9 dm’mol-’ s-l.

In 0, saturated acidic aqueous solutions of iodobenzene, OH radicals have been shown to oxidize iodobenzene to C,H,I+ with J,,,_ at 305 and 630 nm. The G value and extinction coefficient (630 nm) are respectively 2.0 and 5350 dm’ mol-’ cn- ‘. It de-cays by first order kinetics (Q~ = 3 ps) to give I, [G(I,) = 0.31. The rate constant for the reaction of OH radicals with iodobenzene to form C6HSI+ has been determined to be 4.6 x IO9 dm’mol-’ SK’. Cl, has also been found to oxidize iodobenzene to C6HSI+ with a bimolecular rate constant of 3.5 x lo8 dm’mol-’ s-‘. The radical cation of iodobenzene, C,H,I+, is a good oxidant and oxidizes I- with rate constant of 8.2 x 109dm3mol-’ s-l.

INTRODUCTION

Investigations on the reactions of the primary species produced on y-radiolysis of water with halogenated organic compounds have been the subject of consid- erable interest in recent years (Kaster and Asmus, 1973; Lilie et al., 1972; Shankar et al., 1969; La1 et al., 1986; Asmus, 1973). The site of attack of OH radicals and subsequent reactions of the resulting species depend on the nature of halogen and its position with respect to other groups in the compound (Kiister and Asmus, 1973; Lilie et al., 1972; Shankar et al., 1969; La1 et al., 1986; Asmus, 1973). With the combined use of optical and conductivity pulse radiolysis tech- nique, solute radical cations (monomer and dimer) have been identified as the transient intermediates on reaction of OH radicals with alkyl iodides in aqueous solutions (Asmus and Janata, 1982; Asmus, 1984; Mohan and Asmus, 1987). The reactions of OH radicals with iodobenzene are also interesting from the point of view of the mechanism of decomposition of the OH adduct. The present studies have been undertaken with the objective of determining the conditions under which iodobenzene could undergo one electron oxidation reactions in aqueous solu- tions.

EXPERIMENTAL.

The solvent used was “nanopure” deionized water (conductivity = 0.6 @). Iodobenzene was of Fluka, puris grade (purity > 99%). All other chemicals used were of the highest purity and were used as such. The pulse radiolysis experimental set up employing 25 ns

pulses of 7 MeV electrons from a linear accelerator has been fully described elsewhere (Guha et al., 1987). A double cell arrangement for simultaneous optical and conductivity measurements was used (Asmus and Janata, 1982; Asmus, 1984). The absorbed dose per pulse was estimated by simultaneous optical and conductivity measurements of (SCN); ions formed in N,O saturated aqueous solutions of KSCN (0.5 mmol dm-‘, pH = 4.0). The extinction coefficient of (SCN), ion was taken to be equal to 7200 dm’ mol-’ cm-’ at 500 nm and equivalent con- ductivity of (SCN); /H& ion pair A = 360 W’ cm2 equiv-’ [315 and 45 R-’ cm’equiv-’ for H& and (SCN); respectively] (Baxendale et al., 1968; Landolt-BBmstein, 1960). The absorbed dose per pulse was equal to 16 Gy. The measurements of the irradiation induced changes in the conductivity are based on the equation (1) (Asmus and Janata, 1982; Asmus, 1984).

AV, =

where AV, is the voltage observed as a result of formation or destruction of charges in the solution, V is the applied voltage between two electrodes in the cell (10 V), R, is the load resistance (lk a), Kc is the cell constant (0.12 cm-‘), C, is the concentration (mol dm-‘) of the i th charged species produced as a result of the pulse, zi is the net change in its charge (l for one electron oxidation) and A, is the equivalent conductivity (Q-’ cm2 equiv-‘) change after the irradiation. Steady state experiments were carried out

211

212 HARI MOHAN and P. N. M~~RTHY

-10 i / I / I I 0 200 400 600 800 1000

Time (ps)

70 r

9 0 1 Time (ns)

250 350 450 550 650

Wavelength (nm)

Fig. 1. Pulse radiolysis of N,O saturated aqueous solution of iodobenzene @H = 6.0): (a) transient optical absorption spectrum; absorption-time curves; (b) decay at 325 nm; and (c) formation at 325 nm.

with a cobalt-60 y-source whose dose rate was 25.0 Gy min’.

For estimation of I, and II, y-irradiated samples were extracted with equal volumes of hexane. I, was estimated as I; by extracting the hexane layer with aqueous KI and determining the absorbance of 1, at 352 nm (t = 25,800 dm3 mol-’ cm-‘) (Allen et al., 1952). I- was estimated from the aqueous extract directly at 227 nm (6 = 14,000dm3 mol-’ cm-‘) (Habersbergerova, 1977). Solutions were generally saturated with N,O gas to convert e; to OH radicals via reaction (2).

N20 + e&,+ OH + OH- + N, (2)

The yield of oxidizing OH radicals in such solutions corresponds to G = 6.0.

RESULTS AND DISCUSSIONS

Reactions of OH radicals with iodobenzene at pH > 6.0

Figure l(a) shows the transient optical absorption spectrum obtained immediately after pulse radiolysis of N20 saturated aqueous solution of iodobenzene (0.4 mmol dmm3, pH = 6.0). This exhibits an absorp- tion band at 325 nm, which could not be observed in presence of OH radical scavenger (t-butanol), indi- cating it to be due to reaction of OH radicals with iodobenzene. The intensity of the transient absorption band (325 nm) was found to increase

with increasing concentration of iodobenzene (0.050.4 mmol dme3). The transient absorption spectrum was similar to the one observed on pulsing N,O saturated aqueous solution of benzene (pH = 6.0) and assigned to hydroxycyclohexadienyl radical formed by addition of OH radical to benzene (Dorfman et al., 1962). Therefore, the transient ab- sorption spectrum in the case of iodobenzene [Fig. l(a)] may also be assigned to OH adduct, C6H,I.0H. Similar argument has been used for the assignment of the transient absorption band formed on reaction of OH radicals with monofluorobenzene (Koster and Asmus, 1973).

After characterizing the transient absorption band at 325 nm to be due to C,H,I.OH, it is important to determine its extinction coefficient and G value. The competition scavenger technique, using equimolar concentration (0.4mmol dmm3) of KSCN as a standard solute, was used to evaluate the extinction coefficient. On this basis, the c,~~“,,, for C,H,I .OH has been calculated to be 4480 dm3 mol-’ cm-’ and the G(C,H,I.OH) = 4.53.

Figures 2(a) and 2(b) show the conductivity-time curves obtained on pulse radiolysis of N,O saturated water and aqueous solution of iodobenzene (0.4 mmol dmm3) respectively at pH = 6.0. The increase in the conductivity corresponding to 15 mV signal was observed, which indicates the formation of ionic species. OH radicals have been shown to add to all the 6 carbon atoms of fluorobenzene with practically

Pulse radiolysis investigations 213

20-

IO- (b)

2 O-

<

6 _I-

- -lO-

iij 10- (0)

o-

-10 -

I I I I I -10 10 SO 90 130 170

Time (ps)

Fig. 2. Conductivity-time curves obtained on pulse radioly- sis of N,O saturated @H = 6.0): (a) water; and (b) aqueous

solution of iodobenzene.

equal probability (Asmus, 1984). Considering almost equal electron inductive effect (a) of fluorine and iodine in benzene, the OH radicals may also be considered to have equal probability for reaction with all the 6 carbon centres in iodobenzene (Finar, 1973). The following two pathways for reaction of OH radicals with iodobenzene could be possible.

The reaction of OH radicals with carbon centres bonded to H atom only would lead to the formation of iodohydroxycyclohexadienyl radical [path (3a)l decaying by a pure second order kinetics via dis- proportionation and combination reactions. On the other hand, reaction of OH radicals with carbon centres attached to the iodine atom [path (3b)] would stabilize via HI elimination in analogy with similar observations with monofluorobenzene and chloro- trifluoro methane (Kiister and Asmus, 1973; Lilie et al., 1972). The formation of H+ and I- would be by a first order process. The phenoxy radical, (C,H,O),

has &,,, at 400nm (Land and Ebert, 1967). Due to low yield (m 16%) and low extinction coefficient (2.20 x lo3 dm3 mol-’ cm-‘) of C,HsO formed via process (3b), its distinct band at 400 mn could not be observed [Fig. l(a)]. The formation of H+ and I- would lead to increase in the conductivity of the solution after pulse radiolysis. By the use of equation (1) and A=360R-‘cm*equiv-’ (AI-+nH+= 45 + 315 = 360 R-’ cm* equiv-‘), G(I-) was estimated to be 1.20. In steady state y-radiolysis experiments, I- is identified as a product with G(I-) = 0.88, which is close. to the value determined by conductivity measurements. The combined yield of C,H, 1. OH and I- becomes equal to 5.7, which is close to G(OH) in N,O saturated aqueous solutions.

Figure l(b) shows the decay curve (S, E 325 nm) obtained on pulse radiolysis of NzO saturated aqueous solution of iodobenzene (0.4 mmol dm-), pH = 6.0). The species decays predominantly by second order kinetics with 2k/c = 0.75 x lo6 cm s-‘. The reason for being not a pure second order decay may be due to the contribution of process (3b). The increase in the conductivity, steady state detection of I- and mixed order decay of 325 nm band strongly suggest the contribution of process (3b).

The rate constant for the reaction of OH radicals with iodobenzene was determined by pulse radiolysis competition kinetics method using KSCN as the standard solute (K,,,- + oH = 1.1 x 10’” dm3 mol-’ s-‘) and the value was found to be equal to 3.1 x lo9 dm3 mol-’ s-‘. The rate constant was also determined from the formation kinetics (Fig. lc) at 325 nm. The growth of absorbance at 325 mn was pseudo first order from which the bimolecular rate constant was calculated to be equal to 2.7 x lo9 dm3 mol-’ s-l, which agrees with the above competition kinetic value within the limits of experi- mental errors.

The nature of the transient absorption spectrum and the decay kinetics of the transient band (325 mn) observed on pulse radiolysis of NzO saturated aque- ous solution of iodobenzene (0.4mmol dme3, pH = 10.0) were similar to those shown in Fig. 1. However, conductivity-time curves obtained in this case were different from those obtained at pH = 6.0 [Figs 2(a) and (b)]. Figures 3(a) and (3b) show the conductivity-time curves obtained on pulse radiolysis of N,O saturated water and aqueous solution of iodobenzene (0.4 mmol dm-)) respectively at pH = 10.0. Process (3a) would not contribute to conductivity changes. In presence of OH- @H = lO.O), HA formed via process (3b) would be neutralized and the net change in conductivity should show a decrease equivalent to A = 125 R-’ cm* equiv-I(-AOH-+nI-=-170+45=-125R-’ cm* equiv-I). The net decrease in the conductivity of 1OmV corresponds to G(I-) = 2.31. In steady state experiments at pH = 10.0, G(II) was found to be 0.85, which is lower than the value estimated by conductivity.

Reaction of OH radicals with iodobenzene at pH < 6.0

Figure 4(a) shows the transient optical absorption spectrum obtained on pulse radiolysis of N,O saturated aqueous solution of iodobenzene (0.4 mm01 dme3, pH = 3.0). Two transient bands, one with J._ at 630 nm and another at 325 nm with a shoulder at 280nm were observed. Again the transient bands could not be observed in presence of OH radical scavenger (t-butanol), indicating the bands to be due to reaction of OH radicals with iodobenzene. Figures S(a) and (b) show the conductivity-time curves obtained on pulse radiolysis of N,O saturated water and aqueous solution of iodobenzene (0.4 mm01 dm-3, respectively at

214 HARI MOHAN

$I/

-10 10 50 90 130 17O Time (ps)

Fig. 3. Conductivity-time curves obtained on pulse radioly- sis of N,O saturated: (a) water (pH = 10.0); and (b) aqueous

solution of iodobenzene (pH = 10.0).

and P. N. MCKJRTHY

pH = 3.0. A net decrease in conductivity of 10 mV was observed. OH radicals are electrophilic in nature and have been shown to oxidise alkyl iodides and other organic compounds to radical cations at pH < 4.0 (Mohan and Asmus, 1987; BonifaEE and Asmus, 1976; Asmus et al., 1987; Kishore et al., 1988). In the case of iodobenzene, it is possible that OH radicals may also oxidize it to C, HSI+ according to reaction (4).

C,H,I +OH + H+dC,H,I+ + H,O. (4)

The net decrease in the conductivity of the solution after the pulse radiolysis would be equal to A = -265W’cm*equiv-’ (-nH&+nC6HSI+= -315+50= -265R-‘cm2equivm’). The specific conductance of C6HSI+ has been assumed to be equal to 50 W’ cm2 equiv-I, since the conductance of most of the monovalent ions of organic compounds is in the range of 4&6On-’ cm2 equiv-’ (Landolt-B6rnstein, 1960). The transient band at 630 nm [Fig. 4(a)] has been assigned to C,HSI+. The conductivity signal of 10 mV corresponds to G(CsH,It) = 1.1. Simultaneous optical absorption measurement at 630nm gave a value of

20 -

10 -

0 --

-10 -

-20 -

- 30 -

-40 -

-50 -

-60 -

-70 ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ 0 s 10 -

m 0 -’

-10 -

-20 -

-30 -

-40 -

-50 -

-60 -

-?O ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ 0 20 40 60 00 100 120 140160 160 200

Time Q.4

Fig. 5. Conductivity-time curves obtained on pulse radioly- sis of N,O saturated; (a) water (pH = 3.0); and (b) aqueous

solution of iodobenzene @H = 3.0).

5890dm3mol-‘cm-’ for ~~~,,~,,,for C,H,I+. Thecom- petition scavenger method using KSCN as the stan- dard solute gave a somewhat lower value of 4800 dm3 mol-’ cm-‘.

Figure 4(b) shows the absorption-time (2 = 630 nm) curve obtained on pulse radiolysis of N,O saturated aqueous solution of iodobenzene (0.4 mmol dme3, pH = 3.0). It was observed to decay

450 550

Wavelength (nm)

Fig. 4. Pulse radiolysis of N,O saturated aqueous solution of iodobenzene (pH = 3.0); (a) transient optical absorption spectrum; absorption-time curves; (b) decay at 630 nm; and (c) formation at 630 nm.

Pulse radiolysis investigations 215

60

50 I:

5: E ‘0 h 07

40

30

20 (cl

10

L

IdI

0

-10 0 5 10 15 20 25 30 35 40 45 50

-250 450 550

Wavelength (nm)

650 750

Fig. 6. Pulse radiolysis of 0, saturated aqueous solution of iodobenzene: (a) variation of absorbance (630 nm) with pH; (b) transient optical absorption spectrum (PH = 1.5); absorption-time curves; (c) decay

at 310 nm; and (d) decay at 630 nm.

by first order kinetics with t,,2 = 3 ps. The 325 nm band was observed to decay with kinetics different from that of the 630 nm band [Fig. 4(b)] and 325 nm

band observed in Fig. l(a). This suggests that the 325 nm band observed at pH = 3.0 [Fig. 4(a)] is due to a species other than C6HSI+ and CsHsI.OH. It may be due to mixture of both the species.

The rate constant for the reaction of OH radicals with iodobenzene forming C6HSI+ was determined from the formation kinetics [Fig. 4(c)]. The bi- molecular rate constant calculated from the pseudo first order rate is 3.4 x IO9 dm3 mol-’ s-l.

At pH=3.0, G(C,H,I+) was found to be 1.1, which is much lower than the combined yield of C,H,I.OH and I-. Also the mixed order decay of the 325 nm band (Fig. 4a) shows that all the OH radicals are not able to oxidize CsHSI to CsH,I+ at pH = 3.0. Due to the inherent limitation of the conductivity technique at pH < 3.0, the variation in the yield of CsHSI+ with pH could not be studied (Asmus and Janata, 1982). However, such experiments could be carried out by optical absorption measurements at 630 nm as a function of pH of the solution. Since at lower pH, e; also reacts with H& in addition to its reaction with N,O, the total G(OH) would not re- main constant in N,O saturated solutions at pH < 3.0. This difficulty was overcome by carrying out experiments in 0, saturated solutions throughout the pH range. Figure 6(a) shows the variation in the absorbance of the 630mn band observed in O2 saturated aqueous solutions of iodobenzene (0.4 mmol dme3) as a function of pH. A pK of 2.6 is observed. Figure 6(b) shows the transient optical absorption spectrum observed on pulse radiolysis of O2 saturated aqueous solution of iodobenzene (0.4 mmol dm-3, pH = 1.5). The transient bands at

R.P.C. 33,M

305 and 630 nm were observed. These bands are assigned to C,H,I+. The entire spectrum was ob- served to decay by first order kinetics with t,,, = 3 ps [Figs 6(c) and (d)]. The reason for observing the shoulder at 280nm and mixed order decay of the 325 run band [Fig. 4(a)] could be due to overlap of the absorption bands of C,H,I . OH (325 nm) and CsHSI+ (305nm) formed at pH = 3.0. But at pH = 1.5, the only transient species formed would be C6HSI+.

Taking the average value of E~)~,,,,, of C,Hs I+ to be 5350 dm3 mol-’ cm-‘, the G value was estimated to be 2.0 which, within the limits of experimental error is equal to G(OH) in Oz saturated solutions. Steady state y-radiolysis of 0, saturated aqueous solutions of iodobenzene (0.4 mmol dm-3, pH = 1.5), showed absence of I-. I, was identified as a product [G(I,) = 0.311. It could be formed via the following sequence of reactions.

C,H,I+--&,H: + I; (5)

I + I+I,. (6)

If this was the only pathway for the decay of CgH51f, then the G(1,) should be equal to l/2G(C6HSIt). Lower G(1,) suggests the existence of some other pathway for the decay of CsHsI+.

Photoelectron spectroscopy data on pure io- dobenzene shows absorption bands of its molecular cation at 1.78 and 3.5eV, which are close to the optical absorption bands at 1.97 eV (630 nm) and 4.06eV (305 nm) observed on pulse radiolysis of O2 saturated aqueous solution of this compound [Fig. 6(b)] (Turner et al., 1970). The difference in the position of absorption bands could be due to the fact that optical absorption bands in aqueous solutions

216 HARI MOHAN and P. N. M~~RTHY

0.05 -

450 550

Wavelength (nm)

Fig. 7. Pulse radiolysis of 0, saturated aqueous solution (PH = 1.5): (a) transient optical absorption spectrum of Cl; ; (b) transient optical absorption spectrum of Cl; + iodobenzene; absorption-time curves

(370 nm); (c) of Cl, and (d) Cl; + iodobenzene.

would be from fully relaxed states which have under- gone reorientation of electrons and solvation of the cation.

The rate constant for the reaction of OH radicals with iodobenzene (pH = 1 S) forming C6H,If as determined from the formation kinetics at 630 nm, is 5.0 x lo9 dm3 mol-’ s-‘.

Oxidation of iodobenzene by other species

Attempts to oxidize iodobenzene by other ox- idizing species such as Br; and T12+ were not success- ful indicating that the oxidation potential of C6HSI+ > 2.OV. Cl; is also a strong oxidant with oxidation potential of 2.3 V (Henglein, 1980). Figure 7(a) shows the transient optical absorption spectrum obtained immediately after pulse radiolysis of O2 saturated aqueous solution of Cl- (20 mmol dm-3, pH = 1.5). The transient band (1 = 370 nm) was observed to decay by second order kinetics with 2k/c = 2.7 x 106cm SK’ [Fig. 7(c)]. In presence of iodobenzene (0.4 mmol dme3), the transient decay was faster and pseudo first order with rate constant = 1.4 x lo5 S-I [Fig. 7(d)]. Since the concen- tration of iodobenzene was 50 times lower, OH radical would exclusively react with Cl- and not with iodobenzene. Therefore, the faster decay of Cl; band would be due to the oxidation of iodobenzene by Cl, accordingly to reaction (7)

Cl; + C6HSI-K,H,I+ + 2Cl-, (7)

with a bimolecular rate constant of 3.5 x 108dm3 mol-’ s-l. Figure 7(b) shows the transient optical absorption spectrum obtained in presence of 0.4 mmol dm-3 iodobenzene. Transient bands at 330 and 630nm were observed. Since C6HSI+ shows absorption bands at 305 and 630 nm [Fig. 6(b)], it is possible that the 305 nm band of C6HSI+ and 345 nm band of Cl; may overlap to give the band at 330 nm.

The rate constant for the formation of C,HSI+ by Cl; could not be estimated accurately from the formation kinetics at 630 nm due to very low absorp- tion.

Oxidation reactions of C&I +

The fact that iodobenzene could be oxidized by Cl; to its radical cation, C6HSI+, and not by T12+ and Br2- suggest it to be a good oxidant with oxidation potential of 2.0-2.3 V. The oxidation of I- by C6HSIf was studied by observing the decay of C, H, I+ (630 nm) with increasing concentration of 1. , keeping I- < C,H,I to ensure primary oxidation of C,H,I by OH radicals. The bimolecular rate constant of 6.4 x 109dm3mol-’ SS’ was evaluated (C,H,I = 0.6 mmol dme3, I- =O-0.06mmoldm-3 in O2 saturated aqueous solutions at pH = 1.5). Simulta- neous with the faster decay of C,HSI+ band, the transient absorption at 385 nm (12) was observed to grow and the bimolecular rate constant was deter- mined to be 1.0 x 10”dm3 mol-' s-‘. Due to limited solubility of iodobenzene, the oxidation of I- by C,HSI+ could not be investigated at higher concen- trations.

Acknowledgements-The initial experiments were carried out at Hahn-Meitner-Institute Berlin. Our sincere thanks are due to Professor K.-D. Asmus, HMI Berlin, for his help in the initial stages of the work and for giving the dual conductivity cell along with the monitoring device for use at BARC. Our thanks are also due to Drs R. M. Iyer and J. P. Mittal for their encouragement and support of this work.

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Turner D. W., Baker C., Balker A. D. and Brundle C. R. (1970) Molecular photo electron spectroscopy, p. 292. Wiley-Interscience, New York.