MECHANISM OF RADIATIOX-ISDUCED LUMINESCEXCE
1783
The Mechanism of Radiation-Induced Luminescence from Scintillators in Cyclohexane
by Robert R. Hentz and Ronald J. Knight Department of Chemistry and the Radiation Laboratory,l University of Xotre Dame, Notre Dame, Indiana (Received h'ovember SO, 1967)
46656
The radiation-induced luminescence from scintillators (p-terphenyl and 2,5-diphenyl-lJ3,4-oxadiazole)was studied in deaerated cyclohexane solutions containing an electron scavenger. The scavengers SFG,NzO, COz, CzH5Br,n-C3H7ClJand benzyl acetate were used. A study also was made of the effect of these solutes and the scintillators on nitrogen yields obtained in radiolysis of cyclohexane solutions of N20. Over the concentration ranges studied with y radiation, the six solutes have no effect on the intensity of luminescence produced by uv excitation of the scintillators. However, all six solutes do quench scintillator luminescence produced by y irradiation of the cyclohexane solutions and in a manner very similar to the effect of the electron scavengers and scintillators on N2 yields from the ?-irradiated X20 solutions. Relative specific rates of electron capture are calculated using a model for radiation-induced ionic reactions in liquids of low dielectric constant; values obtained from the luminescence-quenchingand NZO-protection studies are compared. It is concluded that radiation-induced scintillator luminescence in cyclohexane is explicable in terms of a sequence of ionic reactions. The scintillator captures electrons and positive charge in competition with recombination of the sibling cationelectron pairs produced in the solvent. Scintillator excitation occurs in the neutralization of a scintillator anion by a solvent or scintillator cation and in the neutralization of a scintillatorcation by an electron.
Introduction Studies of radiation-induced luminescence from scintillators in various solvents have shown that different mechanisms are involved in aromatic and alkane solvents.2 I n the case of benzene solutions, e.g., it has been established that scintillator excitation occurs via energy transfer from the 'Beu state of the solvent to the scintillator. However, in cyclohexane solutions the mechanism of scintillator excitation has not been established although a number of mechanisms have been suggested: (1) (energy transfer from an excited cyclohexane moleculej4 ( 2 ) charge transfer from a solvent cation and subsequent n e ~ t r a l i z a t i o n , ~and - ~ (3) nonlocalized deposition of energy followed by preferential localization in the scintillator.6 Burton2 recently has reviewed evidence against the participation of excited cyclohexane molecules in the sensitization of scintillator luminescence. From the absence of structure in the absorption spectrum of cyclohexane,s it was inferred that the lifetime of the excited state is less than lo-" sec; measurement of decay times of scintillator luminescence in X-irradiated cyclohexane solutions gives T < 0.3 X sec for the lifetime of excited cyclohexane in the liquid phase.5 By combination of 7 < 0.3 X sec with Q' valuesg and y' specific rates are obtained of k > 10l2M-' sec-' and k > 3 X 10" M-' sec-' for excitation transfer to scintillators and for quenching of a cyclohexane excited state by CCl,, respectively. Because such specific rates are considerably greater than the maximum value expected for an excitation-transfer process (-5 X 10'0 j7
X - l sec-'), it was argued2 that a mechanism involving excitation transfer is untenable. Much recent ~ o r k ~ O indicates -'~ that ionic processes play a significant role in the radiation chemistry of alkanes. Such work suggests that ionic processes may play an important role in the radiation-induced luminescence of scintillators in alkane solutions. The usual scintillators qualify as scavengers of electrons and of positive charge (by charge transfer from solvent cations). Thus, luminescence may be a consequence of the neutralization of scintillator cations and anions. To test such a mechanism, the radiation-induced luminescence of scintillators has been studied in cyclohex(1) The Radiation Laboratory of the University of Notre Dame is operated under contract with the U. S. Atomic Energy Commission. This is AEC Document N o . COO-38-582. (2) M. Burton, Mol. Cryst., in press, provides a recent review of the data and arguments with references to the pertinent literature. (3) 5. Lipsky and M. Burton, J . Chem. Phys., 31, 1221 (1959). '. Laor and A. Weinreb, ibid., 43, 1565 (1965). (4) 1 (5) M. Burton, A. Ghosh, and J. Yguerabide, Radiation Res. Suppl., 2,462 (1960). (6) C. R. Mullin, &I. A. Dillon, and M.Burton, J . Chem. Phys., 40, 3053 (1964). (7) 11.Burton, Discussions Faraday SOC.,36, 7 (1963). (8) L. W.Pickett, M. Muntz, and E. M.McPherson, J . Am. Chem. SOC.,73, 4862 (1951). (9) M. Burton, Ill. A. Dillon, C. R. Mullin, and R. Rein, J . Chem. Phys., 44,2236 (1964). (10) G. Scholes and M.Simic, Nature, 202, 895 (1964). (11) F. Williams, J . Am. Chem. Soc., 86, 3954 (1964); J. W. Buchanan and F. Williams, J. Chem. Phys., 44,4377 (1966). (12) P. J. Dyne, Can. J . Chem., 43, 1080 (1965). (13) G. R. Freeman, J . Chern. Phys., 46,2822 (1967).
Volume 72>S u m b e r 5
M a y 1968
ROBERT R. HENTZAND RONALD J. KNIGHT
1784 ane solutions containing asolute previously characterized as an electron scavenger; six such solutes were used. For comparison, a study was made of the effect of these same solutes and of the scintillators on nitrogen yields obtained in radiolysis of cyclohexane solutions of the electron scavenger Y2O.'0 Correlations were sought between the kinetic parameters of luminescence quenching and S z Oprotection.
Experimental Section Materials. Fisher Spectrograde cyclohexane was passed through a 5-ft column of silica gel, subjected to two partial freezings, dried over sodium, and distilled from sodium with a Nester-Faust spinning-band column; only the middle fraction was used, which contained negligible amounts of unsaturated impurities M ) as shown by both ultraviolet spectrophotometry and gas chromatography with a flameionization detector. Scintillation grade p-terphenyl (PTP) from Eastman Organic Chemicals was recrystallized twice from benzene. The following chemicals were used as received : scintillation grade 2,s-diphenyl1,3,4-oxadiazole (PPD) from I< and I< Laboratories, puriss grade benzyl acetate from Aldrich Chemical Co., ethyl bromide from Columbia Organic Chemicals, and Eastmaii Red Label n-propyl chloride. Natheson Co. NzO and SF6 were passed through S a O H pellets and stored on the vacuum line. These gases and COz, obtained from Air Reduction, were purified on the vacuum line just prior t o sample preparation. Air was removed by pumping on the gas condensed in a trap at 77°K; water was retained in the trap as it was allowed to warm from 77 to 193"K, and the more volatile gases distilled into another trap at 77°K. This procedure was repeated several times. Procedures. Solutions (IO ml) were pipetted into 78 =k 2-ml vessels which consisted of a 50-ml Pyrex flask fitted with a side arm for connection to the vacuum line, a break-seal, and a tube of 12 cm length and 1.6 cm diameter in which the solution was irradiated. The solutions were degassed by at least four cycles of freeze (77"K), pump, and thaw. Such gaseous solutes as K20, SFG,and COz were added by condensation of the desired amount onto the sample at 77°K; the amount of gas was determined by measurement of the pressure (with a calibrated Series 17477 Consolidated Vacuum Corp. gauge) in a 380-ml volume at room temperature. The cells mere sealed and those containing gases were shaken thoroughly and allowed to stand overnight prior t o irradiation. The concentration of a gas in solution was calculated from the known amount of gas in the vessel, the volumes of vessel and solution, and the following Ostwald solubility coefficients: p(S20)14 = 2.62 at 23", ,8(SFB)16= 1.22 at 25", and p(COz)16 = 1.72 at 2 5 " . Solutions were irradiated at room temperature in the Sotre Dame 10-kc W o facility at a dose rate to the The Journal of Physical Chemistry
Fricke dosimeter solution, based on G(Fe3+) = 15.6, of 1.51 X 1 O I 8 eV g-' min-'. Dose to the cyclohexane solutions was determined by correction for the electron density of the solution relative to that of the dosimeter. After irradiation, Nzand H2 were removed by three cycles of freeze (77"K), pump, and thaw using a Toepler pump to collect the gas in a closed loop for subsequent injection into a gas chromatograph. Separation of the gases mas achieved with a 0.25-in. X 5-ft column packed with activated 30-60 mesh 5A molecular sieve and operated at room temperature with helium as carrier gas; a Gow-Mac 9677 matched-thermistor detector was used. Calibrations were made with each series of analyses by injection of known amounts of pure gas measured in calibrated volumes of the Toepler pump. The apparatus and procedure for measurement of relative luminescence intensities from y-irradiated scintillator solutions have been described;" 20-ml samples were prepared in 35 f 1 ml cylindrical vessels by the procedure described for preparation of solutions for radiolysis. Relative luminescence intensities from uv-excited scintillator solutions were measured with a Cary spectrophotometer fitted with a fluorescence attachment;'* ~ 4 - m lsamples were prepared, as previously described, in quartz cells 1 cm square and -6 cm in length. A Cary Nodel 14-R was used for absorption spectrophotometry.
Results On the basis of preliminary investigations, the following six electron scavengers were chosen for study as quenchers of the radiation-induced luminescence of scintillators in cyclohexane: SFfi,Ig,liT20,10,20 C02,21 CzHsBr,22 n-C1H,C1,22,23 and benzyl a ~ e t a t e . ~ Over ~-~~ the concentration ranges studied with y excitation, the six solutes have no effect on the intensity of luminescence produced by direct excitation of PTP or P P D at 3130 8. However, all six solutes do quench scintillator luminescence produced by y irradiation of the deaerated cyclohexane solutions. This quenching effect (14) S. Sato, R. Yugeta, K. Shinsaka, and T. Terso, Bull. Chem. SOC. J a p a n , 39, 156 (1966). (15) G. Archer and J. H. Hildebrand, J . Phys. Chem., 67, 1830 (1963). (16) J. Gjaldbaeck, Acta Chem. Scand., 7, 534 (1953) (17) M. Burton, P. J. Berry, and S. Lipsky, J . Chim. Phys., 52, 657 (1955). (18) M . A. Dillon and AT. Burton, "Pulse Radiolysis," Academic Press, London, 1965, p 260. (19) R. N. Compton. L. G. Christophorou, G. S. Hurst, and P. IV. Reinhardt, J . Chem. Phys., 45, 4634 (1966); B. H. AIahan and C . E. Young, ibid., 44, 2192 (1966). (20) R. K. Curran and R . E. Fox, ibid., 34, 1590 (1961); G. J. Schula, ibid., 34, 1778 (1961). (21) P. AT. Johnson and A. C. Albrecht, ibid., 44, 1845 (1966). (22) J. P.Guarino, M. R. Ronayne, and W. H. Hamill, Radiation Res., 17,379 (1962). (23) L. J. Forrestal and W. H. Hamill, J . A m e r . Chem. Soc., 83, 1535 (1961). (24) J. A. Ward and W. H. Hamill, ibid., 87, 1853 (1965).
MECHANISM OF RADIATIOK-INDUCED LUMINESCENCE I
I
I
1785
I
0.6 80
& 0.4 \
t5
i
i
5. r
40
0.2
0
2 [p-Terphenyl], mM.
4 0
0
Figure 1. The effect of K2O and benzyl acetate on the radiation-induced luminescence from p-terphenyl in deaerated cyclohexane solutions: 0, p-terphenyl alone; @, 5 X 1M N20; 0 , 0.12 M N20; 0, 10-8 1M benzyl acetate; 0 , 7 x 10-2 M benzyl acetate; U, 0.28 M benzyl acetate.
0
10
20
Solute concentration.
on the concentration of Figure 2. Dependence of (l~/I)z an electron scavenger (units are 4 m M for n-CaH7Cl and m&f for the other solutes) in y irradiation of deaerated solutions of 10-2 M P P D in cyclohexane: a, SFB; 0, benzyl acetate; 0, C2HjBr; V, N 2 0 ; 0, COZ; Ll, n-CaH7Cl.
is illustrated in Figure 1 by plots of relative luminescence intensity, I ( I , for solutions without quencher), vs. concentration of PTP for solutions containing dif-
0.2
0.4
0.6
[Solute], M.
Figure 3. The effect, in deaerated cyclohexane, of high concentrations of a n electron scavenger on nitrogen yields from 10-2 M N20 solutions (solid symbols) and on luminescence from 10-2 M P P D solutions (open symbols) and 3.5 X 10-3 ill PTP solutions (partially filled symbols): 0,benzyl acetate; 0, CpHaBr; V, N20.
ferent concentrations of ,R20 or benzyl acetate. I n Figure 2, results obtained for low concentrations of each of the six solutes in M PPD solutions are plotted as (10/1)2 us. scavenger concentration; similar plots were obtained for PTP solutions with each of the four solutes studied-SFs, NzO, CzH5Br, and benzyl acetate. The significance of such a plot is considered in the Discussion. Results obtained at higher scavenger concentrations in P P D and P T P solutions are shown in Figure 3. The reaction of hydrated electrons with XzO to produce nitrogen is fast, ICz5 = 5.6 X lo9 M-' sec-', whereas the corresponding reaction of H with NzO is comparatively slow, ICz6 = 1oj M-' sec-'. Such considerations have led to the use of NzO as a scavenger for electrons produced in the y irradiation of hydrocarbons.1°J4 Use of NzO has the virtue that an easily measured product is formed in the electron scavenging reaction; however, interpretation of the Nz yields involves certain complications which are considered in the Discussion. In Figure 4, nitrogen yields obtained at low concentrations of each of six solutes in M N 2 0 solutions are plotted as ( G o / G )us. ~ concentration of the added solute; G and Go denote the (25) J. P. Keene, Radiation Res., 22, 1 (1964); E. J. Hart and E. M. Fielden, Advances in Chemistry Series, No. 50, American Chemical Society, Washington, D. C., 1965, p 253. (26) G. Czapski and J. Jortner, .Vatwe, 188,50 (1960). Volume 72, Number 5 M a y 1968
1786
ROBERTR. HENTZAXD RONALD J. KXIGHT ignored), sibling ion pairs have a probability of recombination given by 1 - f, (in which f, represents the escape probability2*), and a characteristic lifetime for recombination which can be represented as the reciprocal of a specific rate IC,; Le., those sibling ion pairs that recombine can be treated as excited molecules with a specific rate of decay k,. From electricalconductivity measurements that give G f i = 0.1 for free ion pairsz9 (that escape recombination) and using an estimated Gi = 3 for total ion pairs,11-13 a value of -97% is calculated for the percentage of all sibling cation-electron pairs that recombine. With the general model presented, it is possible to derive an equation for Go/G in a solution in which two scavengers, Sl and Sz,compete for the electrons and scavenging by SI gives a measured product with yields of Go and G in the absence and presence of SZl respectively. Thus
G = GIt [kiSi/(h& 0
10 Solute concentration.
20
Figure 4. Dependence of (Go/G)2on the concentration of an electron scavenger or scintillator (units are 4 mM for n-C,H7C1 and mM for the other solutes) in y irradiation of deaerated solutions of M N20 in cyclohexane: A, P P D ; (3, SFe; 0, C 0 2 ; Dl benzyl acetate; 0, CIHJBr; 0 ,n-C3H7C1.
100-eV yields of N2from NzO solutions with and without added solute, respectively. Similar plots were obtained with PTP as solute at lower NzO concentrations. Results obtained at higher concentrations of the added solutes in M N20 solutions are shown in Figure 3. At M N20, a value of Go = 1.7 was obtained which was independent of dose up to at least 1.1 X lozoeV ml-l; all yields were determined at doses of 5.6 X 1019eV ml-’ or less.
GI
+ kzSz) I + Ilc~Sl/(k,+ ~CISI + ~ z SIF& J
1
(1)
in which F, = F,’(l - f7), S denotes molarity of the scavenger, and k1 and k2 are specific rates of the reactions
+ SI e- + SZ
e-
+
SI-
(1)
+
Sz-
(2)
It is assumed that scavenger concentrations are large enough to preclude combination of fyee electrons and cations at the dose rate used. Because
Discussion Nitrogen-Yield Suppression. I n recent years, a useful model has been emerging for interpretation of the behavior of ionic species produced by high-energy radiation in liquids of low dielectric constant.11-13,27 The general features of such a model are as follows. Electrons having a wide range of initial energies (from essentially zero to values of -1 MeV in most cases) are generated in the liquid by high-energy radiation; such electrons lose their energy to the medium and are thermalized at various distances, related to their initial energies, from their sibling cations. Thus, a distribution function exists which gives the fraction of sibling cation-electron pairs for which the separation at thermalization lies between r and r dr; such a function is represented by F,’dr. Under the influence of their mutual Coulombic fields (the complications associated with spurs of more thap ope ion pair being
+
The Journal of Physical Chemistry
(27) A. Hummel and A. 0. Allen, J. Chem. Phys., 46, 1602 (1967). (28) L. Onsager, Phys. Rev., 54,554 (1938), givesf, = e x p ( - e * / e k T r ) , in which e is the charge on the electron and e is the dielectric constant. (29) A. 0. Allen and A. Hummel, Discussions Faraday Soc., 36, 95 (1963); G.R. Freeman, J . Chem. Phys., 39,988 (1963). (30) A. A. Scala, S. G. Lias, and P. Ausloos, J . Amer. Chem. Soc., 88, 6701 (1966); P. Ausloos, A. A. Scala, and S. G. Lias, ibid., 89, 3677 (1967).
MECHANISM OF RADIATIOK-INDUCED LUMINESCENCE concentrations) that Gb a(k18l Ic2SZ)'/' gives
+
=
( G O / G ) ~= 1
a(klS1)'/' and G,
+ kzSz/klSl
=
(IV)
The results obtained (cf. Figure 4) conform reasonably well to eq IV. Such conformity is not considered a confirmation of the model; the results conform about as well to a Stern-Volmer plot. If applicability of eq IV to the results for 10-2M SZO solutions is assumed, specific rates for eS can be calculated relN20) = 1. Such IC values are preative to k(esented in Tablie I.
+
+
+
S) for Various Solutes Table I : Values of k ( e Relative to k(eN20) = 1
+
Solute
PPD SF.5
coz
Benzyl acetate CIHsBr
PTP n-CzHTC1
-_-_ N2Oa 2.9 2.5 2.0 2.0 1.1 1.0 0.062
k(e-
+ S)------
PPDa
1.4 2.7 0.34 1.6 1.3
.. 0.084
PTPa ,.
2.3
.. 1.7 1.1 1.8
..
a The column hleaded NQOgives values obtained from results on the suppression o:f G(N2)in N10 solutions; the columns headed PPD and PTP give values obtained from the results on luminescence quenching in P P D and PTP solutions, respectively.
The value of' G/Go is about 0.06 at 0.6 M of either C2H5Bror benzyl acetate (cf. Figure 3)) which corresponds to G = 0.1 (Go = 1.7). If essentially all electrons are scavenged at 0.6 M of a good scavenger,la and assuming that each electron scavenged by NzO gives -1.6 molecules of X2,31,32 G = 1.6 Gtk1S1/ M , and SZ = (klS1 k2X~). For Gr = 3, SI = 0.6 M , values OF G equal to 0.07 and 0.04 are calculated for C2H5Brand benzyl acetate, respectively, using the relative specific rates of Table I ; thus, the mechanism indicates that processes other than electron scavenging make little or no contribution to the measured G = 0.1 a,t 0.6 M SZ. Luminescence Quenching. As shown in Figures 1-3, electron scavengers quench radiation-induced scintillator luminescence in cyclohexane solutions in a manner very similar to the effect of such scavengers and the scintillators on N2 yields from irradiated cyclohexane solutions of S20. Such a similarity suggests that a common precursor, namely the electron, is involved in scintillator luminescence and formation of Nz from N20. There is evidence for the formation of both aromatic cations and anions, including those of PTP, in irradiated nonpolar glasses at 77'K33>a4 and in cyclohexane at room1 temperature. 35 Arai and Dorfmana6 report k = 7.2 X lo9 M-l sec-l for reaction of PTP
+
1787 with the solvated electron in ethanol. Consequently, aromatic scintillators (I.P. = 8-9 eV) should scavenge both electrons, as indicated by the effect of PTP and PPD in N2O solutions (Figure 4), and positive charge in irradiated cyclohexane (I.P. = 9.88 eV). There is also abundant evidence that charge neutralization processes involving aromatic cations or anions or both (produced by photoionization,37 high-energy or ~hernically~~) i r r a d i a t i ~ n,a8, ~ ~ele~trochemically,~~ give rise to luminescence. Therefore, the general ionic model, which was used for interpretation of N2yield suppression by electron scavengers in irradiated NzO solutions, can be applied plausibly to the quenching of radiation-induced scintillator luminescence by electron scavengers in cyclohexane solutions. Scintillator ions are formed in reactions 1 and 3
c++ s1 ---it c + s1+
(3) in which C denotes cyclohexane and S1 is now the scintillator. As in the case of K20, reaction 1 competes with the recombination at a specific rate k, of sibling cation-electron pairs and scavenges all free electrons (-3y0 of the total). However, reaction 3 (in addition to scavenging all free solvent cations) is in competition with either the recombination of sibling cation-electron pairs at a specific rate k , or the recombination of sibling cation-scavenged electron(anion) pairs at a specific rate k,'; Le., account is taken of the possibility that an electron and anion may have appreciably different mobilities whereas those of C+ and S1+ are essentially equal. Scintillator luminescence, then, may be a consequence of one or more of the reactions 4-6. S1+ e- +Sl* (4)
+
c++ s1- +c + Sl* Sl+ + s1- ---it s1 + Sl*
(5)
(6) Thus, luminescence is quenched by an electron scavenger S2 whose product anion is such as to preclude luminescence in any of the possible charge-neutralization processes. (31) See Table T'II of R. R. Hentz, D. B. Peterson, S.B. Srivastava, H. F. Barzynski, and M.Burton, J . Phys. Chem., 70,2362 (1966). (32) J. ,M. Warman, Nature, 213,381 (1967). (33) J. P. Guarino and W. H. Hamill, J. Amer. Chem. SOC., 86, 777 (1964); N. Christodouleas and W.H. Hamill, ibid., 86, 5413 (1964); T. Shida and W. H. Hamill, J . Chem. Phys., 44,2375 (1966). (34) B. Brocklehurst and R. D. Russell, Nature, 213, 65 (1967). (35) G. Scholes, 31.Simic, G. E. Adams, and J. W. Boag, ibid., 204, 1187 (1964); J. P. Keene, E. J. Land, and A. J. Swallow, J . Amer. Chem. SOC.,87,5284 (1965). (36) 8. Arai and L. M.Dorfman, J . Chem. Phyls., 41,2190 (1964). (37) G. N. Lewis and D. Lipkin, J . Amer. Chem. Soc., 64,2801 (1942); T.V. M.McClain and A. C. Albrecht, J . Chem. Phys., 43, 465 (1965). (38) D. W.Skelly and T.V. H. Hamill, ibid., 43,3497 (1965). (39) E. A. Chandross and J. TV. Longworth, J . Amer. Chem. SOC.,87, 3259 (1965); K. S. V. Santhanam and A. J. Bard, ibid., 87, 139 (1965). (40) E. A. Chandross and F. I. Sonntag, ibid., 86, 3179 (1964); A . Weller and K. Zachariasse, J . C h e n . Phys., 46,4984 (1987).
Volume 73, .\'umber
6
May 1068
ROBERTR. HEKTZAND RONALD J. I ~ I G H T
1788
An equation analogous to eq I can be derived for t'he luminescence yield, G(proportiona1 to I ) , in the presence of electron scavenger Sa
in which /3 represents the probability that the neutralization process gives rise to luminescence. The equation for Gois obtained when Sa = 0. There are three cases in which eq V can be converted into eq VI (Go/G)'
=
IO/^)^ = 1
+
k2Sz/k1S1
(VI)
by the procedure used in conversion of eq I into eq IV; case A: Pj = 0, p4 = ps, k, = kr', kl = ka; case B: p4 = 0, p5 = ps; case C: p4 = p5 = 0 6 and [k38l/(kr
+
k3S1)
1 [kr/(kT + k.181 + k2s!Zi1
