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Temperature effects on positronium formation and inhibition: A contribution to the elucidation of early spur processes—IV
Radial. Phys. Chem. Vol. 21, No. 5, pp. 431--438, 1983 Printed in Great Britain.
0146-5724/83/050431-08503,00/0 Pergamon Press Ltd.
TEMPERATURE EFFECTS ON POSITRONIUM FORMATION AND INHIBITION: A CONTRIBUTION TO THE ELUCIDATION OF EARLY SPUR PROCESSES--IV METHANOL
AND ETHANOL
SOLUTIONS
J. TALAMONI,? J. CH. ABBE,:~ G. DUPLATRE and A. HAESSLER Centre de Recherches Nucl6aires et Universit6 Louis Pasteur, Division de Chimie et Physique des Rayonnements, B.P. 20-67037 Strasbourg Cedex, France
(Received 3 June 1982; in revised form 11 August 1982) AbstraetmTemperature effects between 173 and 348 K on the inhibition of positronium formation by NO] and CI- in methanol and in ethanol solutions have been investigated. Whilst the inhibition promoted by NO], a total inhibitor, is temperature independent, this is not the case for CI-, a limited inhibitor. However, different effects are observed in the two solvents and the role of the solvation states of the e- (and of the • ÷) is discussed. I. I N T R O D U C T I O N
temperature effects was restricted to the pure solvents.
WITHIN THE general framework of the spur model of Ps formation "'2) it is important to determine the very nature of the various reactive species in the spur, essentially the solvation states of the positron and of the electron. Information has been gained along this line by investigating the inhibition of Ps formation using some selected solutes in various solvents, more specifically through the observation of temperature effects in water, ¢3) ethylene glycol~4) and water-glycerol mixtures. ~5) However, the solvation times of e- and, very likely too, of e ÷ in these three solvents are very short at room temperature and, with the possible exception of the water-glycerol mixtures for which little is known, the solvation processes cannot be delayed to a significant extent by lowering the temperature. In this last respect, the normal alcohols appear as better candidates for such investigations.(6.7) This paper reports on measurements made in methanol (MeOH) and in ethanol (EtOH) solutions in the temperature range 173-348 K using both the positron lifetime spectroscopy (LS) and the Doppler broadening of the annihilation lineshape (DBARL) techniques. Part of these experiments has already been performed ~s'9) using exclusively the LS technique, but the investigation of the
2. E X P E R I M E N T A L The experimental devices and data processing were the same as described previously. "°-m The resolutions of the LS and DBARL set-ups were 300 ps and 1.4 keV, respectively. The intensities, /~, and lifetimes, ¢~, are given with subscripts i = 1, 2, 3 ascribed to /7 .Ps, e~ and o .Ps, successively. The superscript ,,0,, refers to values in the pure solvents. The DBARL data are expressed in terms of the full width at half maximum of the annihilation line, fwhm (in keV). After correction of the experimental curves for the detector resolution,"z) the deconvoluted curves relative to the pure solvents were resolved into the sum of gaussians which are described by the full widths at half maximum, Fi, and are characteristic of the momentum distribution associated with each positronic species. These Fi values were then used in conjunction with the LS data and the resolution function of the detector to calculate the annihilation lineshape in the solutions for comparison with the experimental measurements. This procedure allows one to verify the overall consistency of the experimental results obtained using both measuring techniques. MeOH and EtOH were pure grade from Merck and were used without further purification. The solutes investigated were NH4NO3(m and NaCI. All samples were degassed by the freeze-thaw technique. The temperatures were adjusted to within +1 K using a liquid nitrogen cryostat.-
?On leave of absence from Instituto de Fisica e Quimica de Sao Carlos, Universidade de Sao Paulo, Brazil. ~Author to whom all correspondence should be addressed.
3. R E S U L T S 3.1 Pure solvents Tables 1 and 2 give the values of the various 431
432
J. TALAMONI et TABLE I.
LS
al.
PARAMETERS FOR PURE SOLVENTS
T Solvent
MeOH
EtOH
TABLE 2.
DBARL
(=k)
(ns)
(ns)
203
20.6
3.00
0. 40
294
22.0
3.20
0.42
343
23.3
3.40
0.43
173
23.3
2.80
O. 36
223
22.8
3.05
0.38
294
22.4
3.20
0.42
348
22.0
3.40
0.43
PARAMETERS: EXPERIMENTAL FULL WIDTH AT HALF MAXIMUM AND INTRINSIC F
Solvent
MeOH
EtOH
(K)
(keY)
b FI (keV)
(keV)
(keV)
203
2.78
0.89
2.66
2.00
294
2.71
0.89
2.66
1.88
343
2.67
0.89
2.66
1. "/9
T
fwhm a
(K)
I 73
2.73
0. 86
2.63
2.15
223 294
2.70
0.86
2.65
2 . O0
2.69
0.84
2.65
I . 79
348
2.67
0.87
2.66
1.74
a) -4-0.01 k e y ; b) 4 . 0 . 0 3 k e y ; c) 4-0.02 keV ;
parameters derived from LS and D B A R L measurements. As previously found, (H) a broad component with intensity of 2.8% and I" = 5.7 keV appeared in the deconvoluted D B A R L spectra and was systematically corrected for. The variations of I3°, the intensity associated to the long lifetime component, with temperature T are shown in Fig. 1. Attempts to resolve the LS spectra into four components as suggested by Smith and Beling (9) were unsuccessful despite a significantly better resolution (0.28ns) of our experimental equipment. An examination of these data leads to several comments: (i) the variations of 13o with T in the two alcohols are in good agreement with those reported by Byakov et al. (s) T h e most salient feature is that as in water, (3) ethylene glycol(`) and water-glycerol mixtures, (5) 13o increases with increasing T in MeOH, at a rate of 0.019 abs% K - ' , while the reverse is observed in EtOH (and in propanol) although the variation is very small, 0.007 abs% K - ' ; (ii) in both alcohols, ~2° and ~'3°
d) 4- 0 . 0 4 k e V
increase with T, as observed by Tao (") and by Smith and BelingCg); although the solutions were degassed and the measurements were performed repeatedly on different experimental set-ups, our values of ~'3° are surprisingly lower, by about 10%,
~ 2/, 23
22
21
20
do
T (K) FIG. 1. Variations of 130 in methanol (O) and in ethanol (A) with temperature, T.
Temperature effects on positronium formation and inhibition in alcoholic solutions than those recently determined (9"'s) (in undegassed solutions, z3° is 2.75 ns); (iii) although the derived F+ parameters are somewhat less reliable than in water, due to the fact that it is not possible to use a strong Ps oxidizing and inhibiting solute to determine separately F2, u~) it appears that the main difference between the solvents bears on F3. This latter is also most sensitive to T changes, decreasing with increasing T, in contrast to Fi and F2 which are not significantly temperature and solvent dependent.
433
> E29
2B
2~
3.2 N O ; solutions In both alcohols, 1/I3 varies linearly with concentration C at all the temperatures investigated. The variations of 13 vs C are, therefore, not reported. They are well described by the following empirical expression
a
o12
oi~
o:6
o18
,io c (M)
2.8oj
Q
h =/3°(1 + kC) where k is the solute inhibition constant. Within the experimental uncertainties, the derived k values, given in Table 3, are temperature independent. The variations of fwhm with C at the various temperatures in MeOH and in EtOH are shown in Figs. 2(a, b), respectively. The values of fwhm increase with C and tend to the asymptotic values fwhm 2 characteristic of the lone e $olv + component. Using the I'+ determined in the pure solvents and the/3 ° and k values derived from LS, it is possible to calculate the variations of fwhm with C. <"'m The resulting calculated curves are drawn as solid lines in the figures; they are well representative of the experimental variations. This confirms that HOg acts by the capture of e-, and not of • +, unless the F value associated with the hypothetical (e+NO;) bound state would be very close to l'z.
TABLE 3.
LS INHIBITION PARAMETERS FOR NOi
Solvent
T (K)
MeOH
EtOH
k
(M °I )
203
I • OS
294
! . 08
343
I • 06
1 73
1.15
294
1.12
348
0.98
2.75
2 7(
~
exp err.
b 2 fi~,
o
o.6s
o.io
o.~s
o~o
o.~s c (M)
FIG. 2. Variations of fwhm with the concentration of NO; (a) in methanol at 203 K, t-l, 294 K, O, and 343 K, A. (b) In ethanol at 173 K, I-I, 294 K, ©, and 348 K, A.
3.3 C1- solutions The LS and D B A R L results are shown in Figs. 3 and 4. The variations of 13 with C measured in LS show a rapid decrease until about 0.2 M and then a levelling off to a plateau value. As in other solvents, ('6> CI- thus appears to lead to a limited inhibition and the variations of 13 vs C are well described using the following empirical expression
I3= r3°/__L_ + 1-/) ~I+KC where f is the fraction of Ps liable to inhibition and K is the (partial) inhibition constant. The parameter values derived from the fitting are g/yen in Table 4.
434
J. TALAMONIet al. TABLE 4. L S INHIBITION PARAMETERS FOR C I -
Solvent
MeOH
EtOH
T
K
(K)
IM-II
203
4.9
0.06
1.3
19.4
294
20.3
2.2
19.8
343
43.6
0.10 0.15
3.5
19.8
173
18.4
0.15
3.5
223
17.8
0. I I
2.5
294
15.0
O. 08
1.8
348
20.0
O. 07
1.5
19.8 20.3 20.6 20.5
2
2
6
o
o2
o~
03
o{
o.s C(M)
l
i
23
b
~)exp err i
22 :Y~
"~'- ~ 20
;
.....
t- .....
*-
....... o ........ o-
Or2
Oa4
(i-O t~
fq
016
018 c (M)
FIG. 3. Variations of 12 with the concentration of CI- (a) in methanol at 203 K, r-l, 294 K, ©, and 343 K, A. (b) In ethanol at 173 K, rl, 223 K, A, 294 K, ©, and 348 K, 0.
In Me©H, K and f increase with T while ( 1 /')/3° remains constant. This confirms the visual observation that the plateau reached at high C is temperature independent. The activation energy for K, E r , calculated on the basis of the equation K = K ° exp ( - E r / k T ) , where k is the Boltzmann constant, is E r = O . 1 1 e V (with K ° = 2 . 9 × 10~M-'), which coincides with that for the viscosity, E~ = 0.11 eV. Qualitatively, these effects of T are similar to those observed in ethylene glycol ") and in water-glycerol mixtures.(s) In comparison with water, there appears to be some difference since K is constant with T in this latter solvent. Compared to the other solvents, the main difference in EtOH lies in the decrease of f, and the correlated decrease of l i f t , with increasing T. Due to the very low value of f at and above 294 K, the derived K values are liable to rather large errors; a more precise determination would demand numerous measurements in the low concentration range. The D B A R L data show that the fwhm's follow the same trend in both alcohols, decreasing with C at all values of T. Resolving the deconvoluted spectra (corrected for the detector resolution) fixing the values of F~, F2 and F3 known for the pure solvents and the intensities 11 and 13 derived from LS, a new component (hereafter referred to by subscript 4) appears with a temperature independent F4 value of 2.35 keV in MeOH and 2.39 keV in EtOH, and with concentration dependent associated intensity,/4. T h e / 4 vs C variations at the various temperatures and for the two alcohols are shown in Fig. 5. In all cases, /4 increases with C and tends to asymptotic values around 70%. Based on arguments developed by Dupl&tre et al., °m) the maximum value of 14 expected, supposing that all free e*,©iv in the solutions can eater into the formation of a bound state with Cl-, is 14
m a x
-1-(1 m
/)41313 ° .
435
Temperature effects on positronium formation and inhibition in alcoholic solutions
~
GO
exp e r r
2 7!
I
26~ ~
D 01
02
03
05 C(M)
04
260 ~O 30 2C
~
b
exp e r r
2.
10
i
0
01
02
~s
03
C(M)
O1
02
03
0,¢
05
06 C(M}
Fio. 5. Variations of 14, the intensity of the e + bound state, with the concentration of Cl- (a) in methanol at 203 K, D, 294 K, ©, and 343 K, A. CO) In ethanol at 173 K, El, 223 K, &, 294 K, ©, and 348 K, Q.
~
E
exp
2.71
err.
b
0
noted that this situation is different from that obtained in water and in ethylene glycol at 294 K. The variations of 14 vs C have been tentatively fitted using the following empirical expression (':)
2 .G!
L = I~'~K4C/(1 ÷ K4C)
2,6C
O ~5
2.55
o
o'.2
o',
o.'~
olg
c(~
FIG. 4. Variations of fwhm with the concentration of Ci(a) in methanol at 203 K, F], 294 K, O, and 343 K, A. Co) In ethanol at 173 K, F-I,223 K, A, 294 K, O, and 348 K, O. From Table 4, it may be seen that I ~ "~ should be close to 73% in all cases. Taking into account the experimental uncertainties in the determination of I,, the agreement between the experimental and predicted I ~ a~ values is quite good. It may be
where K4 is a constant related to the process of formation of the (e÷Cl -) bound-state. The values derived from the adjustment are given in Table 5. The corresponding calculated curves, drawn as solid lines in the figures are well representative of the experimental variations. In MeOH, the activation energy associated to /£4 correlates well with E~ and with EK. In EtOH, no change in K , is observed between 173 and 294 K and it is equal to K at 173 and 223 K; the correlation is less apparent at 348 K, where K4 increases drastically, but K at this temperature is poorly defined. 4. D I S C U S S I O N The main objective of the present experiments was to obtain new information on the mechanisms of the Ps formation and inhibition processes. Therefore, we shall restrict the discussion to the variations of the Ps yields and to the temperature
436
J. TALAMONI et aL TABLE 5. D B A R L PARAMETERS: I~ ''x AND K4 FOR CiT
misx I/4
K4
(K)
(%)
(M - I )
203
76.0
10.9
29/4
72.0
77. 6
343
71.0
233.0
S o t vent
MeOH
EtOH
I 73
73. 6
18.4
223
73. 6
17.8
29/4
?4. !
18.0
3/48
71.3
99. 0
effects. The changes in the intrinsic parameters such as the lifetimes and momentum distributions will not be considered. For the lifetime variations and their correlation with surface tension, reference is made to the papers by Tao ~14) and by Smith and Belong. ~9)However, one should note that • 3° increases with T, contrary to the variations observed in water. °~ As stressed in our previous papers, the observation of important temperature effects on the probability of Ps formation and on the inhibition appears to be in contradiction with the fundamental hypothesis of the hot atom models as developed by Bartal and Ache ~'6) and by Para and Lazzarini. ~SJ The discussion will therefole be made with reference to the spur model as originally proposed by Mogensen ~" and as extended in Ref. 2. It should be recalled that in the latter version of the model the role of the solvation states of the reactive species, essentially e ÷ and e-, is emphasized and it is proposed that Ps formation involves presolvated particles, either quasi-free or localized; in the former, this point is not specified and attention is drawn to the possible implication of e + (or e-) delocalization over several solute molecules. By extrapolating some of the pulse-radiolysis data, it can be estimated that the formation of the fully solvated electron, with maximum absorption at about 600nm, takes 1 n s c6'7) o r significantly more ¢~9)at 173 K. Therefore, as already underlined by Byakov et al., ~8~this species cannot be involved in the formation of Ps at this temperature, as the lifetimes of p . Ps (0.12 ns), the presence of which is evidenced by DBARL, and of e + ,o~v (0.36 ns) are much shorter. Considering the very small variation of 13° when heating up to 348 K in EtOH and the fact that 13° is nearly the same in MeOH as in EtOH at 294 K, it may be inferred that efo,v is not
likely to participate in Ps formation in these solvents, whatever the temperature. It may be noted that if solvated e- (and e +) were involved, Ps would be formed during the time of observation, with a yield which increases as T decreases; as a consequence, the resolution of the LS spectra into a sum of decaying exponentials would become less and less representative. No such phenomenon has ever been experimentally observed. Nevertheless, the particles responsible for the formation of fraction fI3 ° of Ps must be supposed to be associated with the solvent to some extent in order to account for the effects of the temperature on this fraction and on the K and K4 constants related to partial inhibition. It is possible that those electrons from which f13 ° originate are the IR absorbing electrons detected in pulse radiolysis experiments whose formation occurs rapidly, within the resolution time of the equipments of about 20 ps. "9~ From our collected results, the inhibitors may be classified according to their ability to suppress Ps formation either totally or only in part, at high enough concentration. Additional information comes from the observation that changes in T do not modify significantly the inhibiting power of solutes belonging to the first category in contrast to what is observed with solutes of the second category. This is readily interpreted if it is supposed that the total inhibitors scavenge quasifree particles and the limited inhibitors localized particles. That N O ; reacts in aqueous solutions with pre-solvated electrons has recently received strong support from picosecond laser photolysis experiments, ~2°~ corroborating previous pulseradiolysis results. ~2~) However, it should be noted that the ratio of the inhibition constants, k(H20)/k(MeOH or EtOH), is around 3.5 while
Temperature effects on positronium formation and inhibition in alcoholic solutions that corresponding to the C37 is 1. A better agreement, at least as far as the drop of reactivity on passing from aqueous to alcoholic solutions is concerned, is observed when comparing with the reaction rate constants for solvated electrons, since k(ea4 + S)/k(e~o~v + S) is about 8. In the case of limited inhibitors, the present measurements confirm the two main conclusions drawn from previous experiments in other polar solvents: (i) only the fraction fI3 ° of the Ps relative yield is sensitive to changes in T; (ii) most generally, the partial inhibition constant is temperature sensitive, and the related activation energy is close to that associated to the viscosity. These observations reinforce the hypothesis that that fraction of Ps arises from the reaction of particles associated with the solvent but, for the reasons given above, not fully solvated. However, questions remain concerning the peculiar behaviour of E t O H (and propanoi) as compared to the other solvents namely, the decrease of 130 when T increases. At the moment, two interpretations can be considered. (i) The formation of Ps depends, in part, on the yield of e produced by the ionization of the solvent and is in competition with several reactions, in particular with that of geminate recombination. In alcohols, several experiments (22"23) have shown that while the yields of free e~tv are T-independent, the total yields of e~ojv and the yields of geminate e~o~v increase when T decreases. However, these variations do not seem to explain the opposite variations of I3 with T in M e O H and EtOH. (ii) A broad absorption band at 1300 nm is observed in the normal alcohols containing an aliphatic chain which is attributed to electrons loosely bound to the aliphatic g r o u p s , t7"24) It may be suggested that these electrons would contribute to Ps formation in E t O H and not in M e O H and that this contribution decreases with increasing T. On this hypothesis, the particles responsible for the formation of fraction/13 o in M e O H could be different from the IR absorbing ones. As an alternative, it may be supposed that the same kind of electron is involved in the formation of/13 ° in M e O H and in EtOH, but that the electron has a much less transient existence in EtOH. The/130 fraction and the K(K4) constants arise from the competition between various processes in the spur; (2~ the variations with T of the related rate constants can depend upon the solvent, resulting in different behaviours of t h e / I 3 °, K and K4 parameters with T. It is worth noting that delaying the solvation time more substantially in E t O H than in M e O H upon cooling, as is proposed, would result, in the
437
former case, in an increase not only of f13° but also of K a n d / ( 4 as is experimentally found. CONCLUSION The present experiments confirm the previous observations on the influence of the temperature on the inhibition processes. However the data in EtOH (and propanol) reveal peculiar behaviour as compared to the other solvents studied so far. Very significantly, the radiolysis experiments reveal a strong transient IR absorption band in these alcohols containing an aliphatic chain. It seems that these loosely-bound electrons may be responsible for peculiarities observed for these solvents. Acknowledgement--J. Talamoni is grateful to CNPq (Brazil) for financial support.
REFERENCES I. O. E. MOGENSEN,J. Chem. Phys. 1974, 60, 998. 2. J. CH. ABBE, G. DUPLATRE,A. G. MADDOCK, J. TALAMONI and A. HAESSLER, J. Iaorg. Nuci. Chem. 1981, 43, 2603. 3. J. TALAMONI, J. CH. ABBE, G. DUPLATRE and A. HAESSLER, Radial Phys. Chem. 1982, 20, 275. 4. J. TALAMONI,J. CH. Anne, G. DUPLATRE and A. HAESSLER, Chem. Phys. 1981, $$, 13. 5. C. D. JONAH, J. CH. ABnE, G. DUPLATRE and A. HAESSLER, Chem. Phys. 1981, 58, 1. 6. L. GILLES, J. E. ALDRICH and J. W. HUNT, Nature 1973, 243, 70. 7. L. GILLES, M. R. BONO and M. SCHmDT, Can. J. Chem. 1977, $5, 2003. 8. V. M. BYAKOV,V. I. GRAFUTIN,O. V. KOLDAEVA,E. V. MINAICHEV,F. G. NICHIPOROV,YU. V. OBUKHOV and O. P. STEPANOVA,Chem. Phys. 1977, 24, 91. 9. F. A. SMITHand C. D. BF.LINO,Chem. Phys, 1981, 55, 177. 10. A. G. MADDOCK, J. CH. Anne and A. HAESSLER, Chem. Phys. 1976, 17, 343. 11. G. DUPLATRE, J. CH. ABBE, A. (3. MADDOCKand A. HAESSLER, ./'. Chem. Phys. 1980, 72, 89. 12. G. DUPLATRE, J. CH. Anne, J. TALAMONI, J. C. MACI-IADOand A. HAESSLER, Chem. Phys. 1981, $7, 175. 13. J. Cn. ABeE, G. DUPLA~E, A. G. MADDOCKand A. HAESSLER, Radiochem. Radioanal. Lett. 1979, 38, 303, 14. S. J. TAO, J. Chem. Phys. 1972, 56, 5499, 15. P. JANSEN and O. E. MOGENSEN, Chem, Phys. 1977, 25, 75. 16. J. CH. ABBE, G. DUPLATRE, A. G. M A D D O C K and A. HAESSLER, Radiat. Phys. Chem. 1980, I$, 617. 17. L. J. BARTAL and H. J. ACHE, RadiodL Acta 1973, 19, 49. 18. A. FOGLIO PARA and E. LAZZARINI, J. Inorg. Nu¢l. Chem. 1980, 42, 475. 19. W. J. CHASE and J. W. HUNT, J. Phys. Chem. 1975, 79, 2835. 20. J. M. WIESENFELD and E. P. IPPEN, Chem. Phys. Lett. 1980, 73, 47.
438
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21. J. W. HUNT, Advances in Radiation Chemistry (Edited by M. Burton and J. L. Magee), Vol. 5. Wiley, New York, 1976. 22. J. H. BAXENDALE and P. WARDMAN, Chem. Comm. 1971, 429.
23. V. N. BELENSKY and L. T. BUGAENKO, Radiochem. Radioanal. Lett. 1977, 30, 39. 24. J. H. BAXENDALE and P. WARDMAN, J. Chem. Soc., Farad. Trans. I, 1973, 69, 584.
0146-5724/83/050431-08503,00/0 Pergamon Press Ltd.
TEMPERATURE EFFECTS ON POSITRONIUM FORMATION AND INHIBITION: A CONTRIBUTION TO THE ELUCIDATION OF EARLY SPUR PROCESSES--IV METHANOL
AND ETHANOL
SOLUTIONS
J. TALAMONI,? J. CH. ABBE,:~ G. DUPLATRE and A. HAESSLER Centre de Recherches Nucl6aires et Universit6 Louis Pasteur, Division de Chimie et Physique des Rayonnements, B.P. 20-67037 Strasbourg Cedex, France
(Received 3 June 1982; in revised form 11 August 1982) AbstraetmTemperature effects between 173 and 348 K on the inhibition of positronium formation by NO] and CI- in methanol and in ethanol solutions have been investigated. Whilst the inhibition promoted by NO], a total inhibitor, is temperature independent, this is not the case for CI-, a limited inhibitor. However, different effects are observed in the two solvents and the role of the solvation states of the e- (and of the • ÷) is discussed. I. I N T R O D U C T I O N
temperature effects was restricted to the pure solvents.
WITHIN THE general framework of the spur model of Ps formation "'2) it is important to determine the very nature of the various reactive species in the spur, essentially the solvation states of the positron and of the electron. Information has been gained along this line by investigating the inhibition of Ps formation using some selected solutes in various solvents, more specifically through the observation of temperature effects in water, ¢3) ethylene glycol~4) and water-glycerol mixtures. ~5) However, the solvation times of e- and, very likely too, of e ÷ in these three solvents are very short at room temperature and, with the possible exception of the water-glycerol mixtures for which little is known, the solvation processes cannot be delayed to a significant extent by lowering the temperature. In this last respect, the normal alcohols appear as better candidates for such investigations.(6.7) This paper reports on measurements made in methanol (MeOH) and in ethanol (EtOH) solutions in the temperature range 173-348 K using both the positron lifetime spectroscopy (LS) and the Doppler broadening of the annihilation lineshape (DBARL) techniques. Part of these experiments has already been performed ~s'9) using exclusively the LS technique, but the investigation of the
2. E X P E R I M E N T A L The experimental devices and data processing were the same as described previously. "°-m The resolutions of the LS and DBARL set-ups were 300 ps and 1.4 keV, respectively. The intensities, /~, and lifetimes, ¢~, are given with subscripts i = 1, 2, 3 ascribed to /7 .Ps, e~ and o .Ps, successively. The superscript ,,0,, refers to values in the pure solvents. The DBARL data are expressed in terms of the full width at half maximum of the annihilation line, fwhm (in keV). After correction of the experimental curves for the detector resolution,"z) the deconvoluted curves relative to the pure solvents were resolved into the sum of gaussians which are described by the full widths at half maximum, Fi, and are characteristic of the momentum distribution associated with each positronic species. These Fi values were then used in conjunction with the LS data and the resolution function of the detector to calculate the annihilation lineshape in the solutions for comparison with the experimental measurements. This procedure allows one to verify the overall consistency of the experimental results obtained using both measuring techniques. MeOH and EtOH were pure grade from Merck and were used without further purification. The solutes investigated were NH4NO3(m and NaCI. All samples were degassed by the freeze-thaw technique. The temperatures were adjusted to within +1 K using a liquid nitrogen cryostat.-
?On leave of absence from Instituto de Fisica e Quimica de Sao Carlos, Universidade de Sao Paulo, Brazil. ~Author to whom all correspondence should be addressed.
3. R E S U L T S 3.1 Pure solvents Tables 1 and 2 give the values of the various 431
432
J. TALAMONI et TABLE I.
LS
al.
PARAMETERS FOR PURE SOLVENTS
T Solvent
MeOH
EtOH
TABLE 2.
DBARL
(=k)
(ns)
(ns)
203
20.6
3.00
0. 40
294
22.0
3.20
0.42
343
23.3
3.40
0.43
173
23.3
2.80
O. 36
223
22.8
3.05
0.38
294
22.4
3.20
0.42
348
22.0
3.40
0.43
PARAMETERS: EXPERIMENTAL FULL WIDTH AT HALF MAXIMUM AND INTRINSIC F
Solvent
MeOH
EtOH
(K)
(keY)
b FI (keV)
(keV)
(keV)
203
2.78
0.89
2.66
2.00
294
2.71
0.89
2.66
1.88
343
2.67
0.89
2.66
1. "/9
T
fwhm a
(K)
I 73
2.73
0. 86
2.63
2.15
223 294
2.70
0.86
2.65
2 . O0
2.69
0.84
2.65
I . 79
348
2.67
0.87
2.66
1.74
a) -4-0.01 k e y ; b) 4 . 0 . 0 3 k e y ; c) 4-0.02 keV ;
parameters derived from LS and D B A R L measurements. As previously found, (H) a broad component with intensity of 2.8% and I" = 5.7 keV appeared in the deconvoluted D B A R L spectra and was systematically corrected for. The variations of I3°, the intensity associated to the long lifetime component, with temperature T are shown in Fig. 1. Attempts to resolve the LS spectra into four components as suggested by Smith and Beling (9) were unsuccessful despite a significantly better resolution (0.28ns) of our experimental equipment. An examination of these data leads to several comments: (i) the variations of 13o with T in the two alcohols are in good agreement with those reported by Byakov et al. (s) T h e most salient feature is that as in water, (3) ethylene glycol(`) and water-glycerol mixtures, (5) 13o increases with increasing T in MeOH, at a rate of 0.019 abs% K - ' , while the reverse is observed in EtOH (and in propanol) although the variation is very small, 0.007 abs% K - ' ; (ii) in both alcohols, ~2° and ~'3°
d) 4- 0 . 0 4 k e V
increase with T, as observed by Tao (") and by Smith and BelingCg); although the solutions were degassed and the measurements were performed repeatedly on different experimental set-ups, our values of ~'3° are surprisingly lower, by about 10%,
~ 2/, 23
22
21
20
do
T (K) FIG. 1. Variations of 130 in methanol (O) and in ethanol (A) with temperature, T.
Temperature effects on positronium formation and inhibition in alcoholic solutions than those recently determined (9"'s) (in undegassed solutions, z3° is 2.75 ns); (iii) although the derived F+ parameters are somewhat less reliable than in water, due to the fact that it is not possible to use a strong Ps oxidizing and inhibiting solute to determine separately F2, u~) it appears that the main difference between the solvents bears on F3. This latter is also most sensitive to T changes, decreasing with increasing T, in contrast to Fi and F2 which are not significantly temperature and solvent dependent.
433
> E29
2B
2~
3.2 N O ; solutions In both alcohols, 1/I3 varies linearly with concentration C at all the temperatures investigated. The variations of 13 vs C are, therefore, not reported. They are well described by the following empirical expression
a
o12
oi~
o:6
o18
,io c (M)
2.8oj
Q
h =/3°(1 + kC) where k is the solute inhibition constant. Within the experimental uncertainties, the derived k values, given in Table 3, are temperature independent. The variations of fwhm with C at the various temperatures in MeOH and in EtOH are shown in Figs. 2(a, b), respectively. The values of fwhm increase with C and tend to the asymptotic values fwhm 2 characteristic of the lone e $olv + component. Using the I'+ determined in the pure solvents and the/3 ° and k values derived from LS, it is possible to calculate the variations of fwhm with C. <"'m The resulting calculated curves are drawn as solid lines in the figures; they are well representative of the experimental variations. This confirms that HOg acts by the capture of e-, and not of • +, unless the F value associated with the hypothetical (e+NO;) bound state would be very close to l'z.
TABLE 3.
LS INHIBITION PARAMETERS FOR NOi
Solvent
T (K)
MeOH
EtOH
k
(M °I )
203
I • OS
294
! . 08
343
I • 06
1 73
1.15
294
1.12
348
0.98
2.75
2 7(
~
exp err.
b 2 fi~,
o
o.6s
o.io
o.~s
o~o
o.~s c (M)
FIG. 2. Variations of fwhm with the concentration of NO; (a) in methanol at 203 K, t-l, 294 K, O, and 343 K, A. (b) In ethanol at 173 K, I-I, 294 K, ©, and 348 K, A.
3.3 C1- solutions The LS and D B A R L results are shown in Figs. 3 and 4. The variations of 13 with C measured in LS show a rapid decrease until about 0.2 M and then a levelling off to a plateau value. As in other solvents, ('6> CI- thus appears to lead to a limited inhibition and the variations of 13 vs C are well described using the following empirical expression
I3= r3°/__L_ + 1-/) ~I+KC where f is the fraction of Ps liable to inhibition and K is the (partial) inhibition constant. The parameter values derived from the fitting are g/yen in Table 4.
434
J. TALAMONIet al. TABLE 4. L S INHIBITION PARAMETERS FOR C I -
Solvent
MeOH
EtOH
T
K
(K)
IM-II
203
4.9
0.06
1.3
19.4
294
20.3
2.2
19.8
343
43.6
0.10 0.15
3.5
19.8
173
18.4
0.15
3.5
223
17.8
0. I I
2.5
294
15.0
O. 08
1.8
348
20.0
O. 07
1.5
19.8 20.3 20.6 20.5
2
2
6
o
o2
o~
03
o{
o.s C(M)
l
i
23
b
~)exp err i
22 :Y~
"~'- ~ 20
;
.....
t- .....
*-
....... o ........ o-
Or2
Oa4
(i-O t~
fq
016
018 c (M)
FIG. 3. Variations of 12 with the concentration of CI- (a) in methanol at 203 K, r-l, 294 K, ©, and 343 K, A. (b) In ethanol at 173 K, rl, 223 K, A, 294 K, ©, and 348 K, 0.
In Me©H, K and f increase with T while ( 1 /')/3° remains constant. This confirms the visual observation that the plateau reached at high C is temperature independent. The activation energy for K, E r , calculated on the basis of the equation K = K ° exp ( - E r / k T ) , where k is the Boltzmann constant, is E r = O . 1 1 e V (with K ° = 2 . 9 × 10~M-'), which coincides with that for the viscosity, E~ = 0.11 eV. Qualitatively, these effects of T are similar to those observed in ethylene glycol ") and in water-glycerol mixtures.(s) In comparison with water, there appears to be some difference since K is constant with T in this latter solvent. Compared to the other solvents, the main difference in EtOH lies in the decrease of f, and the correlated decrease of l i f t , with increasing T. Due to the very low value of f at and above 294 K, the derived K values are liable to rather large errors; a more precise determination would demand numerous measurements in the low concentration range. The D B A R L data show that the fwhm's follow the same trend in both alcohols, decreasing with C at all values of T. Resolving the deconvoluted spectra (corrected for the detector resolution) fixing the values of F~, F2 and F3 known for the pure solvents and the intensities 11 and 13 derived from LS, a new component (hereafter referred to by subscript 4) appears with a temperature independent F4 value of 2.35 keV in MeOH and 2.39 keV in EtOH, and with concentration dependent associated intensity,/4. T h e / 4 vs C variations at the various temperatures and for the two alcohols are shown in Fig. 5. In all cases, /4 increases with C and tends to asymptotic values around 70%. Based on arguments developed by Dupl&tre et al., °m) the maximum value of 14 expected, supposing that all free e*,©iv in the solutions can eater into the formation of a bound state with Cl-, is 14
m a x
-1-(1 m
/)41313 ° .
435
Temperature effects on positronium formation and inhibition in alcoholic solutions
~
GO
exp e r r
2 7!
I
26~ ~
D 01
02
03
05 C(M)
04
260 ~O 30 2C
~
b
exp e r r
2.
10
i
0
01
02
~s
03
C(M)
O1
02
03
0,¢
05
06 C(M}
Fio. 5. Variations of 14, the intensity of the e + bound state, with the concentration of Cl- (a) in methanol at 203 K, D, 294 K, ©, and 343 K, A. CO) In ethanol at 173 K, El, 223 K, &, 294 K, ©, and 348 K, Q.
~
E
exp
2.71
err.
b
0
noted that this situation is different from that obtained in water and in ethylene glycol at 294 K. The variations of 14 vs C have been tentatively fitted using the following empirical expression (':)
2 .G!
L = I~'~K4C/(1 ÷ K4C)
2,6C
O ~5
2.55
o
o'.2
o',
o.'~
olg
c(~
FIG. 4. Variations of fwhm with the concentration of Ci(a) in methanol at 203 K, F], 294 K, O, and 343 K, A. Co) In ethanol at 173 K, F-I,223 K, A, 294 K, O, and 348 K, O. From Table 4, it may be seen that I ~ "~ should be close to 73% in all cases. Taking into account the experimental uncertainties in the determination of I,, the agreement between the experimental and predicted I ~ a~ values is quite good. It may be
where K4 is a constant related to the process of formation of the (e÷Cl -) bound-state. The values derived from the adjustment are given in Table 5. The corresponding calculated curves, drawn as solid lines in the figures are well representative of the experimental variations. In MeOH, the activation energy associated to /£4 correlates well with E~ and with EK. In EtOH, no change in K , is observed between 173 and 294 K and it is equal to K at 173 and 223 K; the correlation is less apparent at 348 K, where K4 increases drastically, but K at this temperature is poorly defined. 4. D I S C U S S I O N The main objective of the present experiments was to obtain new information on the mechanisms of the Ps formation and inhibition processes. Therefore, we shall restrict the discussion to the variations of the Ps yields and to the temperature
436
J. TALAMONI et aL TABLE 5. D B A R L PARAMETERS: I~ ''x AND K4 FOR CiT
misx I/4
K4
(K)
(%)
(M - I )
203
76.0
10.9
29/4
72.0
77. 6
343
71.0
233.0
S o t vent
MeOH
EtOH
I 73
73. 6
18.4
223
73. 6
17.8
29/4
?4. !
18.0
3/48
71.3
99. 0
effects. The changes in the intrinsic parameters such as the lifetimes and momentum distributions will not be considered. For the lifetime variations and their correlation with surface tension, reference is made to the papers by Tao ~14) and by Smith and Belong. ~9)However, one should note that • 3° increases with T, contrary to the variations observed in water. °~ As stressed in our previous papers, the observation of important temperature effects on the probability of Ps formation and on the inhibition appears to be in contradiction with the fundamental hypothesis of the hot atom models as developed by Bartal and Ache ~'6) and by Para and Lazzarini. ~SJ The discussion will therefole be made with reference to the spur model as originally proposed by Mogensen ~" and as extended in Ref. 2. It should be recalled that in the latter version of the model the role of the solvation states of the reactive species, essentially e ÷ and e-, is emphasized and it is proposed that Ps formation involves presolvated particles, either quasi-free or localized; in the former, this point is not specified and attention is drawn to the possible implication of e + (or e-) delocalization over several solute molecules. By extrapolating some of the pulse-radiolysis data, it can be estimated that the formation of the fully solvated electron, with maximum absorption at about 600nm, takes 1 n s c6'7) o r significantly more ¢~9)at 173 K. Therefore, as already underlined by Byakov et al., ~8~this species cannot be involved in the formation of Ps at this temperature, as the lifetimes of p . Ps (0.12 ns), the presence of which is evidenced by DBARL, and of e + ,o~v (0.36 ns) are much shorter. Considering the very small variation of 13° when heating up to 348 K in EtOH and the fact that 13° is nearly the same in MeOH as in EtOH at 294 K, it may be inferred that efo,v is not
likely to participate in Ps formation in these solvents, whatever the temperature. It may be noted that if solvated e- (and e +) were involved, Ps would be formed during the time of observation, with a yield which increases as T decreases; as a consequence, the resolution of the LS spectra into a sum of decaying exponentials would become less and less representative. No such phenomenon has ever been experimentally observed. Nevertheless, the particles responsible for the formation of fraction fI3 ° of Ps must be supposed to be associated with the solvent to some extent in order to account for the effects of the temperature on this fraction and on the K and K4 constants related to partial inhibition. It is possible that those electrons from which f13 ° originate are the IR absorbing electrons detected in pulse radiolysis experiments whose formation occurs rapidly, within the resolution time of the equipments of about 20 ps. "9~ From our collected results, the inhibitors may be classified according to their ability to suppress Ps formation either totally or only in part, at high enough concentration. Additional information comes from the observation that changes in T do not modify significantly the inhibiting power of solutes belonging to the first category in contrast to what is observed with solutes of the second category. This is readily interpreted if it is supposed that the total inhibitors scavenge quasifree particles and the limited inhibitors localized particles. That N O ; reacts in aqueous solutions with pre-solvated electrons has recently received strong support from picosecond laser photolysis experiments, ~2°~ corroborating previous pulseradiolysis results. ~2~) However, it should be noted that the ratio of the inhibition constants, k(H20)/k(MeOH or EtOH), is around 3.5 while
Temperature effects on positronium formation and inhibition in alcoholic solutions that corresponding to the C37 is 1. A better agreement, at least as far as the drop of reactivity on passing from aqueous to alcoholic solutions is concerned, is observed when comparing with the reaction rate constants for solvated electrons, since k(ea4 + S)/k(e~o~v + S) is about 8. In the case of limited inhibitors, the present measurements confirm the two main conclusions drawn from previous experiments in other polar solvents: (i) only the fraction fI3 ° of the Ps relative yield is sensitive to changes in T; (ii) most generally, the partial inhibition constant is temperature sensitive, and the related activation energy is close to that associated to the viscosity. These observations reinforce the hypothesis that that fraction of Ps arises from the reaction of particles associated with the solvent but, for the reasons given above, not fully solvated. However, questions remain concerning the peculiar behaviour of E t O H (and propanoi) as compared to the other solvents namely, the decrease of 130 when T increases. At the moment, two interpretations can be considered. (i) The formation of Ps depends, in part, on the yield of e produced by the ionization of the solvent and is in competition with several reactions, in particular with that of geminate recombination. In alcohols, several experiments (22"23) have shown that while the yields of free e~tv are T-independent, the total yields of e~ojv and the yields of geminate e~o~v increase when T decreases. However, these variations do not seem to explain the opposite variations of I3 with T in M e O H and EtOH. (ii) A broad absorption band at 1300 nm is observed in the normal alcohols containing an aliphatic chain which is attributed to electrons loosely bound to the aliphatic g r o u p s , t7"24) It may be suggested that these electrons would contribute to Ps formation in E t O H and not in M e O H and that this contribution decreases with increasing T. On this hypothesis, the particles responsible for the formation of fraction/13 o in M e O H could be different from the IR absorbing ones. As an alternative, it may be supposed that the same kind of electron is involved in the formation of/13 ° in M e O H and in EtOH, but that the electron has a much less transient existence in EtOH. The/130 fraction and the K(K4) constants arise from the competition between various processes in the spur; (2~ the variations with T of the related rate constants can depend upon the solvent, resulting in different behaviours of t h e / I 3 °, K and K4 parameters with T. It is worth noting that delaying the solvation time more substantially in E t O H than in M e O H upon cooling, as is proposed, would result, in the
437
former case, in an increase not only of f13° but also of K a n d / ( 4 as is experimentally found. CONCLUSION The present experiments confirm the previous observations on the influence of the temperature on the inhibition processes. However the data in EtOH (and propanol) reveal peculiar behaviour as compared to the other solvents studied so far. Very significantly, the radiolysis experiments reveal a strong transient IR absorption band in these alcohols containing an aliphatic chain. It seems that these loosely-bound electrons may be responsible for peculiarities observed for these solvents. Acknowledgement--J. Talamoni is grateful to CNPq (Brazil) for financial support.
REFERENCES I. O. E. MOGENSEN,J. Chem. Phys. 1974, 60, 998. 2. J. CH. ABBE, G. DUPLATRE,A. G. MADDOCK, J. TALAMONI and A. HAESSLER, J. Iaorg. Nuci. Chem. 1981, 43, 2603. 3. J. TALAMONI, J. CH. ABBE, G. DUPLATRE and A. HAESSLER, Radial Phys. Chem. 1982, 20, 275. 4. J. TALAMONI,J. CH. Anne, G. DUPLATRE and A. HAESSLER, Chem. Phys. 1981, $$, 13. 5. C. D. JONAH, J. CH. ABnE, G. DUPLATRE and A. HAESSLER, Chem. Phys. 1981, 58, 1. 6. L. GILLES, J. E. ALDRICH and J. W. HUNT, Nature 1973, 243, 70. 7. L. GILLES, M. R. BONO and M. SCHmDT, Can. J. Chem. 1977, $5, 2003. 8. V. M. BYAKOV,V. I. GRAFUTIN,O. V. KOLDAEVA,E. V. MINAICHEV,F. G. NICHIPOROV,YU. V. OBUKHOV and O. P. STEPANOVA,Chem. Phys. 1977, 24, 91. 9. F. A. SMITHand C. D. BF.LINO,Chem. Phys, 1981, 55, 177. 10. A. G. MADDOCK, J. CH. Anne and A. HAESSLER, Chem. Phys. 1976, 17, 343. 11. G. DUPLATRE, J. CH. ABBE, A. (3. MADDOCKand A. HAESSLER, ./'. Chem. Phys. 1980, 72, 89. 12. G. DUPLATRE, J. CH. Anne, J. TALAMONI, J. C. MACI-IADOand A. HAESSLER, Chem. Phys. 1981, $7, 175. 13. J. Cn. ABeE, G. DUPLA~E, A. G. MADDOCKand A. HAESSLER, Radiochem. Radioanal. Lett. 1979, 38, 303, 14. S. J. TAO, J. Chem. Phys. 1972, 56, 5499, 15. P. JANSEN and O. E. MOGENSEN, Chem, Phys. 1977, 25, 75. 16. J. CH. ABBE, G. DUPLATRE, A. G. M A D D O C K and A. HAESSLER, Radiat. Phys. Chem. 1980, I$, 617. 17. L. J. BARTAL and H. J. ACHE, RadiodL Acta 1973, 19, 49. 18. A. FOGLIO PARA and E. LAZZARINI, J. Inorg. Nu¢l. Chem. 1980, 42, 475. 19. W. J. CHASE and J. W. HUNT, J. Phys. Chem. 1975, 79, 2835. 20. J. M. WIESENFELD and E. P. IPPEN, Chem. Phys. Lett. 1980, 73, 47.
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21. J. W. HUNT, Advances in Radiation Chemistry (Edited by M. Burton and J. L. Magee), Vol. 5. Wiley, New York, 1976. 22. J. H. BAXENDALE and P. WARDMAN, Chem. Comm. 1971, 429.
23. V. N. BELENSKY and L. T. BUGAENKO, Radiochem. Radioanal. Lett. 1977, 30, 39. 24. J. H. BAXENDALE and P. WARDMAN, J. Chem. Soc., Farad. Trans. I, 1973, 69, 584.