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The Journal of Physical Chemistry, Vol. 82, No. 20, 1978
Table I also shows the values calculated for G(Fe3+)from eq I and I1 using G(H) = 3.0, G(0H) = 2.6, and G(H,O,) = 0.9 for 52 MeV/cm 2H+,and G(H) = 2.2, G(0H) = 1.8, and G(H,O,) = 1.1 for 220 MeV/cm 4He2t. These “primary” G values are the best we can estimate in terms of being in agreement with the results obtained in this work and at the same time being in agreement with past work, especially with regard to the expectation that G(H) should be somewhat greater than G(OH)2. However, using the experimental values in Table I to solve eq I-V for G(H), G(OH), and G(H202),one obtains G(H) = 2.85, G(OH) = 3.1, and G(H202)= 0.85 from the 2H+results, and G(H) = 1.95, G(OH) = 2.15, and G(H202)= 1.15from the 4He2+results. That there is a discrepancy is apparent, and the reason is that application of eq I-V to the experimental data is not a very sensitive method of separately determining G(H) and G(0H). In fact, this can be seen by making a standard analysis of the error index for the quantity G(H)/G(OH) in terms of estimated errors in
quantities are-13.3 f 0.5,0.444 f 0.030, 7.7 f 0.3, and 0.779 f 0.030, respectively, we find that the value and error index for G(H)/G(OH) is 0.92 f 0.3. (Nearly all of this error results from the error in G(Fe3+)f,tgerated/G(Fe3+)~,aerated.) The ratio calculated using the “more reasonable” values for G(H) and G(0H) of 3.0 and 2.6, respectively, is 1.15, which is within the range of values “allowed” on the basis of the estimated errors. A similar situation exists for the 4He2+results. The following conclusions can be reached on the basis of the results: First, the observed increase in G(H20,) and decrease in G(H) G(0H) with increasing LET agrees with the general conclusions from earlier work using steady radiolysis,2 and with the general theoretical expectation that increasing LET favors the recombination in the track which is responsible for these trends. Secondly, the
+
Schindier et al.
diffusion-kinetics calculations show that better agreement with experiment is obtained if the effects of energetic 6 rays are specifically accounted for using theoretical methods previously developedll to account for variations in G(e,,-) with particle type and LET. These conclusions indicate how the present work has provided information related to the structure of the track. Similar experiments are planned using ion beams from the Bevalac.17 Because of the much higher beam energies, the effects of high energy 6 rays are expected to be quite important, and it is hoped that similar studies of the fast and slow components of Fe3+ will lead to a better understanding of track structure.
Acknowledgment. We thank the operators of the cyclotron, Milan Oselka, George Cox, and Edwin Kemereit, for their contribution to this work. References and Notes (1) (a) Work performed under the auspices of the Division of Basic Energy Sciences of the US. Department of Energy. (b) M. C. S. is a former student of J. E. Willard, to whom this issue is dedicated. (2) A. 0. Allen, “The Radiation Chemistry of Water and Aqueous Solutlons”, Van Nostrand, Princeton, N.J., 1961, pp 32-60. (3) I. 0. DraganiE and Z. D. DraganiE, ”The Radiation Chemistty of Water”, Academic Press, New York, N.Y., 1971, pp 150-155. (4) M. C. Sauer, Jr., K. H. Schmidt, E. J. Hart, C. A. Naleway and C. D. Jonah, Radiat. Res., 70, 91 (1977). (5) W. G. Burns, R. May, G. V. Buxton, and G. S. Tough, Faraday Discuss. Chem. Soc., 63, 47 (1977). (6) R. H. Schuler and A. 0. Allen, Rev. Sci. Instrum., 26, 1128 (1955). (7) E. J. Hart, W. J. Ramler, and S. R. Rocklin, Radiat. Res., 4,378 (1956). ( 8 ) A. R. Anderson and E. J. Hart, Radiat. Res., 14, 689 (1961). (9) W. G. Brown, E. J. Hart, and M. C. Sauer, Jr., Radht. Res., submitted for publication. (10) J. P. Keene, Radiat. Res., 22, 14 (1964). (11) C. A. Nalewav, M. C. Sauer, Jr.. C. D. Jonah, and K. H. Schmidt, Radiat. Res.,.submitted for publication. (12) N. F. Barr and R. H. Schuler, J . Phys. Chem., 63, 808 (1959). (13) R. H. Schuler and A. 0. Allen, J. Am. Chem. Soc., 79, 1565 (1957). (14) See ref 2, pp 34-38. (15) See ref 3, p 136. (16) R. W. Matthews, J . Chem. Soc., Faraday Trans. 1, 73, 526 (1977). (17) M. Javko, A. Chatteriee. J. L. Maaee. M. C. Sauer, Jr.. E. J. Hart, and K. H. Schmidt, work in progress. ’
Studies by the Electron Cyclotron Resonance Technique. 13. Electron Scavenging Properties of the Molecules CCI,F, CCI2F2,CCIF,, and CF, R. Schumacher, H.-R. Sprunken, A. A. Christodoulldes, and R. N. Schindler” Instltut fur Chemie, III, KernforschungsanlageJulich GmbH, D-5170 Julich and Instltut fur Physikalische Chemie, Unlversitat Kiel, 0-2300 Kiei, German Federal Republic (Received January 12, 1978) Publication costs assisted by KernforschungsanlageJulich GmbH
Using the electron cyclotron resonance (ECR) technique the homogeneous gas-phase reaction of free electrons with the molecules CC13F, CC12F2,CC1F3, and CF4 was studied in the low-energy range. The following rate constants in units of cm3 s-l for the attachment process (5“ = 298 K) were obtained: h(CC13F)= (1 f 0.2) X In addition, and an upper limit h(CF$ 4 h(CCl,F,) = (7 f 1) X k(CC1F3) = (7 f 1) X activation energies and energy dependences for the scavenging process were measured. The capture mechanisms are discussed.
I. Introduction The photochemistry of halocarbons and the kinetic behavior of these compounds became the subject of numerous investigations in the last decade, It is felt that still more detailed knowledge on the properties of these 0022-3654/78/2082-2248$01.00/0
compounds, especially of the halomethanes CC13F and CC1zF2, is needed to assert on the effect of the hahcarbons on the earth’s mOne shield.1s2 In the present contribution the electron scavenging properties of the molecules CC13F, CC12F2,CC1F3,and CF4 0 1978 American Chemical Society
The Journal of Physical Chemistry, Vol. 82, No. 20, 1978 2249
Electron Scavenging of CCI3F, CClpFp,CCIF,, and CF,
TABLE I: Rate Constants k , ( T =298 K ) and Activation Energies for the Capture of Thermal Electrons by CC1,F AE, kcal carrier ha, cm3 s-' mol-' gas method 1.0 1.3 1.2 1.2 1.0
x lo-' x lo-' a x 10-7 x lo-' x lo-'
0.1
Ar Ar/CH, N, N, N,
ECR pulse sampling electron swarm electron swarm electron swarm
1.0 1
I
I
I
I
I
0.05
0.10
0.15
0.20
0.25
i
ref this work 9 10 11 12
a Determined from In (KT3") vs. ( l / T ) plot (see ref 9) cm3 s-' for CCl,.26 Value at 20 " C using 12, = 4.1 X measured at 0.2-eV electron energy.
are investigated. The ECR technique was employed to follow the reaction of low-energy electrons with these
halo me thane^.^
11. Experimental Section The experimental setup for the study of the interaction of low-energy electrons with molecules in a flow system by the ECR technique has been previously described.M Using this method rate constants, activation energies, and energy dependences of the capture processes could conveniently be obtained. The rate of the electron loss process as function of scavenger concentration [AB] a t constant reaction time t, and vice versa can be described through eq 1with h, and hobeing the peak-to-peak heights of ECR h0
In - = k,[AB]t, hn signals, with and without the electron scavenger, respectively, and k , the homogeneous two-body electron attachment coefficient. All experiments were carried out in a quartz flow tube with five mixing chambers located a t different distances from the detector. The tube could be heated along the reaction region to measure activation energies. Temperatures up to 180 "C were attained using an oil circulation system. Argon a t a pressure of about 4 Torr was generally used as carrier gas. Changes of the ECR line intensity with electron energy were studied by varying the microwave power in the resonance ~ a v i t y .Electron ~ energies up to 0.3 eV could be reached. No difficulties were encountered in the present kinetic study resulting from changes of ECR line ~ h a p eand/or ~?~ electron mobility6p7on adding scavenger molecules. All plots of In (ho/h,)vs. [AB] with constant t, or vs. t, a t constant [AB] show good linearity. Their extrapolations pass through the origin. This "good" behavior is in agreement with the low polarity of the molecules. The reported dipole moments, p, are smaller than 0.5 D.8
111. Results CCZ3F. The electron loss rate a t room temperature in the presence of CC1,F was studied in the concentration rangez8 from 2.4 to 8.2 X lo9 cmW3.The experimentally determined rate constant was independent of [CC13F] indicating a homogeneous two-body attachment process. From the measured electron decay along the reaction region the k, value was calculated to be (1f 0.2) X lo-' cm3 s-l. The rate constant for this reaction has already been determined previously by several groups"12 using different techniques. A compilation of the data is given in Table I. Reasonably good agreement is found between the values except a swarm experimentlo which appears to be too high for 0.2-eV electrons.
0.30 [eVl
Figure 1. Energy dependence of the relative ECR line intensity for the molecules CC1,F (X), CClpFp(0),and CCIF, (A).
An activation energy for electron capture was not determined for this molecule. Only one measurement was carried out a t 180 "C. The slight increase in the capture rate observed supports a value of -0.1 kcal mol-' given by Wentworth et al.9 The change of relative ECR line intensity as a function of microwave power input into the cavity is shown in Figure 1. The observed dependence is considered to be proportional to the changes of the reactive cross section as a function of energy, Within the applied energy ranging from 0.04 to 0.26 eV the intensity decreases by a factor of roughly 2.5. From the experimental results shown it is suggested that the maxium for the attachment cross section should be located in the subthermal energy range. A similar behavior for the energy dependence was found in swarm experiments with nitrogen" as a buffer gas. The formation of different negative ions was reported in an early mass-spectrometric study13and recently in a crossed beam experiment with potassium atoms.14 In general, care should be exercised to relate results from studies on potassium atom-molecule reactions to those occurring in electron-molecule collisions since completely different processes are involved. Reference13 states that the negative ion C1- is formed in the low electron energy range 0-0.05 eV. The plot given for the C1- current as function of electron energy shows a maximum around 0.2 eV. Although the energy resolution of 0.3 eV was rather low in this study it is suggested that the energy dependence for the appearence of this C1- peak should be related to the changes observed in the present ECR study. The very close resemblance of the C1- formation in CC14taken from the same work further supports this suggestion. In the molecular beam experiment14 the abundance of the negative ions CC13F- and C1- together with other negative ions is given. The ion Cl- was the main product on allowing potassium atoms with kinetic energies in the range of 1-30 eV to react with CC13F. The relative cross section for formation of the parent ion was calculated to be only 0.2%. An electron affinity of EA(CC13F) = (1.1 f 0.3) eV is reported. CC&F2. The rate constant for the capture of thermal electrons by CCl2FZa t room temperature was determined in the scavenger concentration range of 1.5-7.5 X lo1' ~ m - ~ . In this range the electron loss process studied as a function of reaction time a t constant [AB] followed a linear dependence as expressed by eq l. No change of the obtained values using different scavenger concentrations was detected. The calculated value k, = (7 f l) X cm3 s-l has to be considered as the two-body attachment coefficient in the gas phase. For comparison rate constants are listed in Table I1 which were reported by other groups1"l8 using different techniques. Except for the values measured with the electron beam technique,15 a reasonable agree-
2250
The Journal of Physical Chemistry, Vol. 82, No. 20, 1978
TABLE 11: Rate Constants k, ( T = 298 K ) and Activation Energies for the Capture of Thermal Electrons by CCl,F2
k,, cm3 s-' 0.7 X
AE, kcal carrier mol-' gas 4.5 Ar
5.9 x 10-9 3.0 1.9 x 1 0 - 9 ~ 1.7 X 3.4 1.36 x 10-9 1.2 x 10-9
method ECR
TABLE 111: Rate Constants k, ( T = 298 K ) and Activation Energies for the Capture of Thermal Electrons by CClF,
ref this work 15 11 9 16 12
electron beam electron swarm pulse sampling microwave calcd from ref 1 8 -1 x 10-9 N, electron swarm 17 0.6 x 10-9 total ion current 18 0.4 x 10-9 NZ electron swarm 10 Value determined at 0.06-eV electron energy. De) (1/T) plot (see ref 9) at 20 termined from In ( K T 3 / * vs. "C using k, = 4.1 x cm3 s-' for CC1,.26 N2 Ar/CH, C,H,
x IO2 I
I
1
I
Schindler et al.
I
[CCIF,] ( ~ r n - ~ ) Figure 2. Concentration dependence of the electron loss frequency k,[CCIF,] at room temperature from time-dependence measurements.
ment exists with the result from this investigation. From measurements of the temperature dependence of the capture process in the range 20-180 "C at a scavenger concentration of 2.5 X 10l1~ m - an ~ ,activation energy AE = 4.5 kcal mol-l was obtained. Around 30% smaller values (Table 11) were found in pulse samplinggand electron beam experiments.15 The change of the relative ECR line intensity as a function of electron energy is shown in Figure 1. The intensity of the absorption which is considered as a measure for the reactive cross section increases with energy from the thermal value and reaches a flat maximum around 0.05 eV. Swarm studiesll applying electron energies in the range up to 0.6 eV show similar behavior at low energies. A peak at an electron energy of 0.06 eV was indicated. The formation of the negative ion C1- was demonstrated by several groups using different mass spectrometric techniques.14J5J8 As in the case of CC1,F a very small signal was found by Dispert et al. for the parent ion CC12F2-. The relative cross section was calculated to be 0.15%. An electron affinity of EA(CF2C12)= (0.4 f 0.3) eV is given. No other negative fragments were observed under thermal conditions. CC1F3. Studies of the electron disappearence in the presence of CClF3 were carried out in the concentration range 0.3-9 X 1015cm3. The measured rate constants were found to be independent of [CClF,]. No deviation from linearity was observed applying eq 1. The plot ha[AB] vs. [AB] passed through the origin for all temperatures as shown in Figure 2 for room temperature. From the cm3 s-l was obtained. measurements ha = (7 f 1) X This result is in agreement with an earlier determination using a mixture of N2with CH3NH2as carrier gas for which ha = 8 X cm3 s-l was reported.lg The measured rate constant is suggested to correspond to the homogeneous
ha, cms s-'
7x 5x
1044
AE, kcal mol-'
carrier gas
5.7 Ar NZ
method ECR
ref this work 10
electron swarm a . 1 x 10-13 drift-dwell- 20 drift 1.9 x 10-l3a 7.5 Ar/CH, pulse 9 sampling 5.2 x 1 0 4 4 microwave 21 8.1 x 10-14 N,/CH,NH, ECR 19 Determined from In ( K T 3 / *vs. ) (1/T) plot (see ref 9) at 20 " C using ha = 4.1 X lo-' cm3 s-* for CC1,.26
two-body electron attachment process in the gas phase. The rate constant from the present experiments and the h values obtained in other s t ~ d i e s ' ~ -are ~ l given in Table 111. Measurements of the electron decay rate21in pulse irradiated CC1F3at various CClF, pressures with no buffer gas present in the range from 4 to 44 Torr (1.6 X lo1' to 1.4 X lo1* ~ m - showed ~) a two-body electron attachment cm3 s-l. The agreement with a rate constant of 5.2 X of this h value with that obtained in the present work is considered acceptable, in view of the difficulties usually encountered in the studies of poor electron scavengers. An upper limit of 3.1 X cm3 s-l for the thermal attachment rate constant20was reported from drift-dwelldrift experiments at [CC1F3] = 6.4 X 10l6 cm-3 (2 Torr). Also, the electron scavenging efficiency of CC1F3 was studied by the pulse sampling t e ~ h n i q u e which ,~ yields relative rates only. The calculation of an absolute rate cm3 s-l which is a factor constant yields ha = 1.9 X of 2.7 higher than the ha determined in the present work. No explanation can be offered as to why higher attachment rate constants are found in experiments with applied pulsed fields. Since the cross section for the reaction strongly increases with electron energy (see below) such results would be obtained if nonthermal electrons were present. From electron swarm datalo for CC1F3,the attachment rate (YO = 1.52 X 10' Torr-l s-l was calculated at 0.038 eV. This attachment rate corresponds to a k value of 5 X cm3 s-l. In general, the h values derived from ref 10 for poor electron scavengers were found to be much higher than other values existing in the literature. Measurements of the temperature dependence of the electron loss in the presence of CC1F3 were carried out at concentrations of (4.6 f 2) X 1015 at various temperatures between 20 and 180 "C. From the slope of the Arrhenius plot an activation energy of AE = 5.7 kcal mol-l was obtained. A higher activation energy of 7.5 kcal mol-l (Table 111) is reported by Wentworth et al. utilizing the pulse sampling te~hnique.~ The dependence of the relative ECR line intensity which is considered to be proportional to the attachment cross section as a function of the electron energy was followed over a very limited range only (Figure 1). In this region the relative rate increased sharply by a factor of -1.7. Since CClF3 is a rather ineffective scavenger, concentrations up to 6 X 10l6cmT3were necessary to follow the electron decay as a function of energy. Under these conditions the ECR signal line shape started to change with energy increase and thus made the value of In (h,/h,) insignificant for higher energy settings.
The Journal of Physical Chemistry, Vol. 82,No. 20, 1978 2251
Electron Scavenging of CC13F, CC12F2,CCIF3, and CF,
Formation of the negative ion C1- from CC1F3 a t low electron energies was reported,22but others23were unable to detect them mass spectrometrically. The very small negative ion abundance would suggest that CClF3 is a poor electron scavenger. McCormik et al. have previously pointed out a relation between dielectric strength and negative ion yields of a number of halogen containing gases as measured in a mass ~pectrometer.~, CF,. The capture of thermal electrons by CF4 was investigated with the ECR technique using Ar as well as N2as a carrier gas. It was not possible to obtain a reliable k, value for the electron attachment although large concentrations (>IOl5~ m - of ~ )CF4 were employed in both cm3 s-l only systems. Values in the range of k, I indicate that CF4 practically does not capture thermal electrons. Other investigations also suggest that CF4 is indeed a very poor scavenger for thermal electrons. An upper limit of cm3 s-l was reported21 from pulse radiolysis experiments. No negative ions could be observed23in an electron beam study at low electron energies. However ions F- and CF< appeared25at 4.7 and 5.4 eV, respectively. A similar result was reported using the crossed beam technique employing potassium atoms with relative kinetic energies of >7 eV.14 The calculated relative cross sections for the negative ions F-, CF3-, and FT were found to be 94.8, 4.5, and 0.7%, respectively. A negative parent ion was not detected.
IV. Discussion Capture of low energy electrons by Freon molecules RC1 (with R = CC12F, CC1F2, and CF3) is described by the equations eth RCl-*
+ RC1-
RC1-*
(a)
+ (M)
products
(b)
-
Since no external fields are applied in ECR experiments to transport the electrons along the flow tube they will stay in thermal equilibrium with the carrier gas and thus be denoted as eth. The existence of the negative parent ions from CC13F and CC12F2has been demonstrated in the crossed beam experiment with potassium atoms.14 Formation of C1- has been repeatedly reported from CC13F,13 CC12F2,15J8and CC1F3.22 Thermodynamic considerations may be used to speculate on the probability of a nondissociative capture process. The electron affinities for the molecule CC1,F and CC12F214are given as (1.1 f 0.3) and (0.4 f 0.3)eV, respectively. No value was found in the literature for CClF,. Bond energies D(R-Cl-) of (0.7 f 0.3) and (0.1 f 0.3) eV for the above compounds are reported by the same authors. Again, no value is known for CClF3. It can be taken from these data that capture of a free electron would yield vibrationally excited parent negative ions. They will contain an energy in excess of D(R-Cl-) amounting to 0.4 and 0.6 eV, respectively. The ECR experiments described here were carried out at pressures
