T H E
J O U R N A L
OF
PHYSICAL CHEMISTRY Registered in U. 5'. Patent Office 0 Copyright, 1976, by the American Chemical Society
VOLUME 80, NUMBER 12 JUNE 3, 1976
Yield and Decay of the Hydrated Electron from 100 ps to 3 ns C. D. Jonah,. M. S. Matheson, J. R. Miller, and E. J. Hart Chemistry Division, Argonne National Laboratory, Argonne, lllinois 60439 (Received November 3, 1975) Publication costs assisted by Argonne National Laboratory
Energy is deposited by fast electrons in water in small localized volumes called spurs. T h e decay of t h e hydrated electron in a spur has been measured to be about 17% between 100 ps and 3 ns. Half of this decay occurs before 700 ps where previous techniques could not observe any decay. T h e principal reactions of the hydrated electron in t h e spur are presumed to be eaq- eaq-, eaq- H f , and eaq- OH. Scavengers for the hydrogen ion are about equally effective in slowing t h e decay as are scavengers for the hydroxyl radical except for OH- which seems unusually effective. This suggests H+ and O H are distributed similarly with respect to eaq-. Spur decay is also observed for the products of electron capture, Cd+ or RSSR-, where RSSR is cystamine. T h e observations reconcile different methods for measurement of t h e initial hydrated electron yield to 4.6 f 0.2 molecules/100 eV, and combined with our earlier data, provide t h e major features of t h e history of eaq- in t h e spur from 100 ps t o t h e achievement of a homogeneous distribution.
+
Introduction
For several years radiation chemists have been learning about t h e deposition of energy by ionizing radiation.' As experimental progress has been made in unraveling the processes of energy deposition, more and more detail is needed. I t is clear t h a t energy is deposited i n h o m o g e n e o u ~ l y . For ~ - ~ low linear energy transfer (LET) radiation, there are local volumes of energy deposition called spurs. T h e objective (from the view of radiation chemistry) is to know what species are present, their spatial distribution, and t h e reactions taking place between t h e various species. In water, the primary species are thought to be e-, H20+, and H20* (where the asterisk indicates an excited molecule). s) the species are When one reaches chemical times Haq+,OH, eaq-, H, possibly Hz, and a reactive precursor of the hydrated electron, edry-. In pure water, after spur reactions have taken place (>lO-'s), H+, OH, eaq-, H, OH-, Hz, and H202 will be the predominant species. T o understand t h e processes occurring which take us from t h e beginning of t h e chemical state until the end of spur reactions, we need to know the initial yields and time evolution of all of these species. The yields of each of these species have been measured at the homogeneous limit.6 The yield of the hydrated electron has been determined a t 2.7 f 0.1 in the long time s) limit6 T h e decay to this limit for the hydrated electron has been determined by several groups starting less than 5 ns after a
+
+
short pulse of ionizing radiation. All of these data are consistent with a yield of hydrated electrons a t 1 ns of 4.1 f 0.1 molecules/100 eV of energy absorbed. Also some data have been accumulated about the various reactions taking place through studying the time evolution of the hydrated electron with various reactions "turned off" by adding solutes which remove Haq+or OH.436*7Yields a t times only a few nanoseconds after the irradiation pulse have also been determined with solutes added, but the significance of these results is unclear since the changes from pure water are small (5%) and possible errors in the extinction coefficient could outweigh these difference^.^^^^^ Another approach to studying mechanisms is through modeling the chemical reactions using diffusion kinetic^.^,^ Parameters are chosen for the number of radicals in the spur and the distribution of the radicals to give agreement with radiation chemical results. Recently, these calculations have been modifiedg to give good agreement with our data on the time evolution of t h e hydrated electron. We have attempted to obtain further information on spur reaction mechanisms in pure water by studying the decay of the hydrated electron from 0.1 to 3 ns. The form of decay was also studied using Hf and O H scavengers. Ethanol was used as the hydroxyl radical scanvenger, since the reactions are well known and t h e changes in diffusion in ethanol-water are known.lOsllSodium hydroxide, ammonia, and sodium acetate were used as the hydrogen ion scavengers. 1267
m a
C. D. Jonah, M. S. Matheson, J. R. Miller, and E. J. Hart
Using the decay curves determined in this and earlier work4 we have used G values for various times after energy deposition determined by ourselves and others to calculate G values at 100 ps.
1.20€+07
I
i
Experimental Section
The picosecond pulse radiolysis equipment has been previously described in detail.12 T h e Argonne microwave linear accelerator generates pulses of 20-22 MeV electrons 60 times per second. Each pulse consists of a main pulse containing greater than 90% of t h e charge with a width (fwhm) of 30 ps. As focussed for a n experiment a single pulse deposited 1.5 krads (10" eV/g) in the irradiated volume. Approximately 30% of the electron beam is intercepted by a cell containing 1 atm of xenon. T h e Cerenkov light generated by the xenon cell is used as the analyzing light. T h e remainder (70%) of t h e electron beam is used for radiolysis. Both t h e electron beam and the light beam are delayed. The delay of the light beam can be varied so that it reaches the irradiation cell before, during, or after the pulse. By varying t h e delay of t h e light beam t h e transmission of the sample as a function of time is determined. To improve linearity changes were made in t h e previously described light detection system. The integrator-stretcher was replaced by a commercial charge sensitive F E T preamplifier (Tennelec T C 162). The photomultiplier was an RCA C7253A, a five stage side-on photomultiplier. The primary nonlinearity problems were caused by the analyzing light and the light from the sample cell overlapping in time.'* T o test the system, the absorption of the system a t 600 nm was measured in t h e presence of 4 M perchloric acid. I t was assumed that the only absorption at 600 nm would be due to the electron so that any change in the absorption except during t h e decay of the 0 after the pulse) was assumed t o be due to electron ( ~ 1 0 ps photomultiplier nonlinearity. (It might also be due to a transient decay of absorption in t h e cell windows.) T h e total change of the absorption from right after the pulse to 3 ns was less than 0.2% of t h e total transmission from which we infer t h a t the system is very close to linear. All chemicals were used as received since, on the experimental time scale, small amounts of impurities would have a negligible effect. Solutions were prepared with deionized or singly distilled water and were degassed by bubbling with helium for 15 min. Approximately 2 1. of solution were used in a recirculating flow system with a flow rate of 100 ml/min. An average run would last 15 min. This would generate 25-50 pM products a t the end of the run.
1.00Et07 LT
1
1
r
a
t
Ly
NRNESECBNCS
Figure 1. Untransformedoutput of experiment. The signal is proportional to transmission. 100% transmission corresponds to the level portion on the left. The small step 770 ps after the pulse arises from a small satellite Dulse.
I
I
1
1
-
I .oo
0.95
0.90
f 3 M Ethanol
-
IM Acetate t
-
M 3-- \
I
I
0.0
1.0
I
Ethanol
2 .o
I
I
3.0
4.0
NANOSECONDS
Figure 2. Decay of the hydrated electron in water, 3 M ethanol, 1 M ammonia, 1 M sodium acetate, 3 M ethanol plus 1 M sodium acetate, 3 M ethanol plus 1 M ammonia, and 3 M ethanol plus 0.1 M sodium hydroxide.
1
1.00I M NaOH t 3M Ethanol
0.950.90-
Results and Discussion
Figure 1 shows the raw data as plotted by the computer. Figures 2 and 3 show relative concentration plots. All plots are normalized to one a t 100 ps. This is done t o facilitate comparison of t h e decay curves. We do not attempt to compare the yields of absorption, because the presence of added solutes changes the extinction coefficient of the hydrated electron and t h e amount of energy absorbed per unit volume. T h e corrections for these effects are too uncertain to make the comparison of yields useful. T h e curves in Figures 2 and 3 are derived from the data of the type shown in Figure 1 by plotting the log of the optical density as a function of time. T h e results are of the form shown in Figures 5 and 6. The second absorption step is removed by tracing the data and sliding the curve to match t h e sections before and after t h e small pulse. T h e relatively small absorption in the second pulse means almost all of t h e decay is due t o reactions of hydrated electrons produced by the main pulse. Also, by plotting the log of the optical density, The Journal of Physical Chemistry, Vol. 80, No. 12, 1976
OIMNoOH 3 M Ethanol
0.85\Water
0.0
1.0
2.0
3.0
I 4.0
NANOSECONDS
Figure 3. Decay of the hydrated electron in water, 3 M ethanol, 0.1 or 1 M sodium hydroxide, and 3 M ethanol plus 0.1 or 1 M sodium hydroxide.
the effect of the small pulse is decreased even further. T h e vertical axis in Figures 2 and 3 is not linear but is logarithmic which does not vary greatly from linearity over the range. Differences between curves run on different days are less than 0.5%. Previously published experimental data for the decay of the
1269
Yield and Decay of the Hydrated Electron
hydrated electron have shown no decay for the hydrated electron within experimental error in the time windows of T h e approximately 10% decay 20-350 ps or 0.1-1 ns.4,19714 which we measure from 0.1 t o 1ns is within the risetime and settling time of our photodiode system (see, for example, Figure 2 of ref 4). We have also been informed that the 3% change in absorbance which we measure from 100 to 350 ps is within the experimental error of the Toronto group.15 We do not have the capability to go to shorter times so no statement can be made about times earlier than 100 ps. The major reactions of the hydrated electron in the spur are:
I .o
0.8
-
0.7
-
e,,-+
OH
+ OH-
(2)
1
O ’ T 0.5
0.4
3 x 10’0 17
A STROBOSCOPIC P: R. 0 LASER R R .
0.9-
1
,
1
1
1
I l l 1 1
10-10
I
I
I
1
1 / I l l
,
1
I
I
I 1 1 1
Io-a
10-9
10-7
SECONDS
Figure 4. Decay of the hydrated electron in the spur from 100 ps to 40 ns.
T o study the relative distribution of the radicals as originally formed it would be advantageous to study each reaction by itself. By adding scavengers for H + and/or OH reactions 1 and/or 2 may be suppressed. In 3 M ethanol, the half-life of the hydroxyl radical is about 125 ps. The half-life of the hydrogen ion is 5 and 50 ps in 1 and 0.1 M sodium hydroxide and 30 ps in 1M ammonia or sodium acetate.18 Most of the differences between the decay of the electron in the aqueous solutions shown in Figures 2 and 3 occur a t times after the primary products we are attempting to scavenge have been eliminated. In Figure 2 one sees that the decays of eaq- in 3 M ethanol, 1 M ammonia, or 1 M sodium acetate are reasonably similar (&I%of the concentration of the electron). This implies that OH and H+ in the spur react equally with the electron. Since the rate constants for reaction of H+ and OH with the hydrated electron are similar (within a factor of 21, their distributions relative to the electron must be similar. Such a similarity of distribution would be expected for H+ and OH formed by the fast reaction H20+ H2O H30+ OH. (At these short times in the spur, diffusion kinetic calculations show very little difference for the hydrated electron when either H + or OH is scavenged if their distributions are identical despite the difference in rate constants implying that the distribution is the more important effect.) In examining Figure 2, the scavenging of both H+ and OH is more than twice as effective as scavenging only one of these species. This result can be expected, since, if an electron were “fated” t o react with a hydroxyl radical and the radical were not there, it still might react slightly later with the hydrogen ion. We have carried out diffusion kinetic calculations using the parameters of Kuppermanng for water and for hydrogen ion and/or hydroxyl radical scavenging. We find faster decay in our experimental data than in the calculations when scavengers are present. (As graphed in Figure 2 water 0.83 expt, 0.83 calcd; one scavenger 0.86 expt, 0.90 calcd; and two scavengers 0.93 expt, 0.98 calcd. These differences are outside experimental error.) We have not attempted to fit experimental data with the calculations. However, the greater experimental decay might occur in the larger spurs or “short tracks” or it could arise from a lack of a random distribution in the spur. In Figure 3, the effects of sodium hydroxide and ethanol on ea,- decays are shown. The difference between the 0.1 and the 1.0 M sodium hydroxide is probably not significant. Sodium hydroxide is clearly more effective in reducing the enq- spur
+
-
+
TABLE I Yield
Time, ns
Yield at 100 ps
Ref
3.2 4.1 3.6 3.8
10-15
4.4-4.6 4.55 4.8 4.8
2 4 5 19
1
7.5a 2”
Time quoted is from end of a 5-11s pulse. decay than are other H+ scavengers. We have a t present no explanation for this unexpected result. The similarity between the 1 and 0.1 M sodium hydroxide eliminates the possibility that the reaction OH-
+H
-
eaq-
(4)
gives delayed formation of e,,-‘and therefore less apparent decay than is seen with other hydrogen ion scavengers. If G H were 1and if no H were lost through side reactions, a 1%difference a t 3 ns would be expected with 1M NaOH. The 0.1 M NaOH would be l/10 as much. Many possible explanations could be considered for these data. For instance, it could be an excited water molecule reacting with OH- to generate eaqor the OH- could be eliminating a loss pathway (a dry hole?). In Figure 4 we have plotted the relative concentration of the hydrated electron as a function of time from 100 ps to 40 ns using the results of Figure 2 as well as the data from our laser pulse radiolysis system. We have used Figure 4 to estimate Geaq-a t 100 ps from measurements reported in the literature using nanosecond pulse r a d i o l y ~ i s . ~ The * ~ J ~estimates are listed in Table I. There seems to be fair agreement that the G value is 4.6 f 0.2 molecules/100 eV. T h e yield measurements of ref 2 , 5 , and 19 did not correspond to definite times because of the finite pulse widths used. The estimates in Table 1 include small corrections for this problem. We have also had to critically reevaluate the time a t which our value of 4.1 f 0.1 was m e a ~ u r e d Little .~ concern was given to the time the C value was measured when the experiments were done since it was thought there was no decay before 1 ns. The -200 ps mentioned clearly was in error since the settling time was cited as 500 ps. After examining original data we feel 1 ns is a reasonable time. If we used 500 psec the G value would be 4.4. The value from Table I is higher than the value of 4.0 molecules/100 eV a t 30 ps previously determined by Hunt and The Journal of Physical Chemistry, Vol. 80, No. 12, 1976
C. D. Jonah, M. S. Matheson, J. R. Miller, and E. J. Hart
1270
-2.00
1
t h e scavenger data using cadmium in the presence of tertbutyl alcohol where a G value for the dry electron of 5.5 was found.20 We feel that several methods are now in agreement to establish that the yield of hydrated electrons in water is 4.6/100 eV a t 100 ps. T h e only data in disagreement with this value are the measurements of the Hunt group14 and we do not know the reason for t h e difference.
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'.
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Conclusions
. 2 , 2 0 L - - L d A d A.ooA 1 .nu0
0.0
2
3.3c
4 .OC
NRNESCCENDS
Figure 5. Decay of the absorption at 315 nm of Cd+ formed in the irradiation of 1 M cadmium perchlorate. -1 Y O ,
r
'
'
,
,
,
'
'
'
'
,
'
'
7
T--T-----~ 7 -
i i
. , ... ...:...
... ,
N R N ti s r.
c f j N u I,
Figure 6. Decay of the absorption at 410 nm of the cystamine anion formed in the irradiation of 2 M cystamine. coworkers.7J.' Their results are based on measurements using the stroboscopic pulse radiolysis system.14 Later experiments7 measured the yields a t 100 ns of either Cd+ from the irradiation of 1 M CdC104 or of RSSR- from the irradiation of 2 M cystamine. (They also measured the yield at 30 ps and 6 ns but these measurements are dependent on the C: value assumed for the electron a t 30 ps.) These experiments can be interpreted to be consistent with the 4.0 value if one assumes, as they did, that there is no decay of the product species, Cd+ or RSSR-. T o check the validity of their assumptions, we have measured the product decay in l M Cd(C104)2and 2 M cystamine on t h e time scale of 100-3000 ps. These data are displayed in Figures 5 and 6. I t is clear that there is some spur decay. From these data and the data in ref 7 (methods 1 and 3), their scavenger data are consistent with an eaq- yield of 4.6 and a dry electron yield of 5.4. This interpretation is reinforced by
The Journal of Physical Chemistry, Vol. 80, No. 12, 1976
We have determined the G value for the hydrated electron at 100 ps to be 4.6 f 0.2 molecules/100 eV. If we use this value with the data of Wolff e t al.14 we derive a value for the yield of dry electrons of 5.4 molecules/100 eV. We have also established t h e spur decay of eaq- from 100 ps to homogeneous distribution. The decay data have shown t h a t H+ and OH must be relatively similarly distributed relative t o the hydrated electron. These data will act as a new test on theoretical calculations. The decay of the electron is slower in the presence of sodium hydroxide than in the presence of other H+ scavengers. Further work is needed to identify whether any presently suggested explanation is adequate or whether an as yet unknown process is required.
Acknowledgment. Work performed under the auspices of the U S . Energy Research and Development Administration. We are grateful for the assistance of Lee Rawson, Don Ficht, and Benno Naderer who ran the accelerator, when it ran, and got it running when it felt lazy. I t is a pleasure to acknowledge the invaluable assistance of Robert M. Clarke in running these experiments. References and Notes (1) E. J. Hart and M. Anbar. "The HydratedElectron", Wiley-lnterscience,New York, N.Y., 1970. (2) J. K. Thomas and R. V. Bensasson. J. Chem. Phys., 46,4147 (1967). (3) G. A. Kenney and D. C. Walker, J. Chem. fhys., 50,4074 (1969). (4) C. D. Jonah, E. J. Hart, and M. S. Matheson, J. Phys. Chem., 77, 1838 (1973). (5) G. V. Buxton, Proc. R. SOC.London, Ser. A , , 328,9 (1972). (6) I. G. Draganic and 2.D. Draganic. "The Radiation Chemistry of Water", Academic Press, New York, N.Y., 1971. (7) R. K. Wolff, J. E. Aldrich. T. L. Penner, and J. W. Hunt, J. fhys. Chem., 79, 210 (1975). (8)H. A. Schwartz. J. fhys. Chem.. 73, 1928 (1969). (9) "Diffusion Kinetics in Radiation Biology: An Assessment", A. Kuppermann, in "Physical Mechanisms in Radiation Biology", R. D. Cooper and R. W. Wood, Ed., Technical Information Center, Office of Information Services, U S . Atomic Energy Commission, 1974. (10) H. 0. Spivey and T. Shedlovsky, J. Phys. Chem., 71, 2165 (1967). (11) F. Barat, L. Gilies, B. Hickel, and B. Lesigne, J. Phys. Chem.. 77, 1711 (1973). (12) C. D. Jonah, Rev. Sci. Instrum., 46, 62 (1975). (13) J. W. Hunt, R. K. Wolff. M. J. Bronskill, C. D. Jonah, E. J. Hart, and M. S. Matheson, J. Phys. Chem.. 77, 425 (1973). (14) R. K. Wolff. M. J. Bronskill. J. E. Aldrich, and J. W. Hunt, J. fhys. Chem., 77, 1350 (1973). (15) J. W. Hunt, private communication. (16) This value is lower than the generally accepted value, since the ionic strength in the spur is relatively high. (17) M. Anbar. M. Bambenek. and A. B. Ross, Nafl. Stand. Ref. Data Ser., Natl. Bur. Stand., No. 43. (18) M. Eigen. W. Kruse, G. Maass and L. DeMaeyer, frog. React. Kinet., 2,287 (1963). (19) G. V. Buxton, private communication. (20) C.Stradowski, private communication.
