10720
J. Phys. Chem. 1993,97, 10720-10724
Production of the Hydrated Electron in the Radiolysis of Water with Helium Ions Jay A. Laverne' and Hiroko Yoshida Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana, 46556, and Health and Safety Research Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 Received: June 7, 1993; In Finai Form: August 9, 1993'
The scavenged yield of the hydrated electron has been determined in the radiolysis of water with 2-22-MeV helium ions by measuring the production of ammonia from glycylglycine. Glycylglycine concentrations were 1,O.l,O.Ol, and 0.00 1 M, which correspond to hydrated electron lifetimes of 3 ns to 3 ps, respectively. Differential hydrated electron yields were obtained from the observed energy dependencies, and for 1 M glycylglycine they ranged from 1.76 to 3.09 molecules/lOO eV for 5-20-MeV helium ions, respectively. These values are slightly larger than the corresponding integral yields of 1.5 1-2.20 molecules/ 100 eV, respectively, but smaller than the yield of 3.81 found with fast electrons. In 0.001 M glycylglycine solutions the integral scavenged yield of the hydrated electron varies less than 1% per MeV of helium ion energy. At this concentration it was found that for 5-MeV helium ions the differential hydrated electron yield was 0.145 and the integral yield was 0.098 molecules/ 100 eV. These values are substantially less than the value of 2.47 molecules/ 100 eV found with fast electrons indicating the importance of intratrack reactions with helium ions in the nanosecond to microsecond time scale. It appears that the scavenged yields of hydrated electrons approach constant values with decreasing glycylglycine concentration for all helium ion energies studied. These lower limits are considerably less than the scavenged yields of hydroxyl radicals previously determined in formic acid solutions of comparable scavenging capacity.
Introduction The hydrated electron, eaq-,is probably the most studied of the transient chemical species produced in the radiolysis of water.'.* The temporal dependence of the hydrated electron has been measured in the fast electron pulse radiolysis of and its scavenged yield has been determined with a number of solutes in the y-ray and the fast electron radiolysisof aqueous solutions.s-'O However, relatively little is known about the production of the hydrated electron in the radiolysis of water with heavy particles. The radiolytic yields of radicals are known to decrease with increasing linear energy transfer (LET, equal to the stopping power, -dE/dx) of the irradiating particle because of the increase in importance of intratrack processes over diffusion into the bulk medium.' Detailed experimental data on the yield of the hydrated electron in the heavy ion radiolysis of water will give a better understanding of the track structure of these particles and how it affects the subsequent radiation chemistry. There has been only one reported study in which a scavenger was used to measure the yield of the hydrated electron in the heavy ion radiolysis of neutral water." This work measured the production of Nz from N2O solutions irradiated with 18-MeV deuterons and 12- and 32-MeV helium ions. The yield of the hydrated electron with heavy ions has been determined directly from its absorption in two cases. Burns et aI.l2-I4measured the decay of the hydrated electron in a jet stream of water irradiated with 3-MeV protons. They observed the absorbance decay from 1 to 30 ns and found that it was considerably different than that found with fast electrons. The data were consistent with the concept that the initial yield for the formation of the hydrated electron is the same for all types of particles. The LET of 3-MeV protons is not very high (12 eV/nm), and the local density of species in the track is not much greater than with fast electrons. At about the same time, Sauer et al.15 determined the yield of the hydrated electron as a function of penetration depth for 20MeV deuterons and 40-MeV helium ions by observing the absorbance in a segment of the particle track. These experiments were limited to 10 p s for the direct determination of the hydrated Abstract published in Adounce ACS Abstracts. September 15, 1993.
0022-365419312097-10720$04.00/0
electron yield because of the width of the beam pulses. Obviously, allowances must be made for the substantial decay of the hydrated electron within the duration of the pulse. In addition, at low helium ion energies the particle energy is changing rapidly and interpretation of the observed yield in a track segment of macroscopic dimensions becomes difficult. The data did show that the yield of the hydrated electron decreases by about an order of magnitude from 5 to 50 eV/nm and that at the same LET the yield with helium ions is greater than that for deuterons. It would be helpful for the elucidation of track processes to have information on the yields of the hydrated electron at shorter times and to expand the data with helium ions to include a wider range of energies and LETs. We have determined the scavenged yield of the hydrated electron in aqueous solutions of glycylglycine irradiated with helium ions of 2-22 MeV (LETs of 168-3 1 eV/nm, respectively). The hydrated electron reacts exclusively with the glycylglycine to give ammonia which is easily measured quantitatively. Variation of the ammonia yield with glycylglycine concentration gives an indication of the temporal dependence of the hydrated electron yield in pure water. Integral ammonia yields were measured in 0.001, 0.01,O. 1, and 1 M glycylglycine solutions and the differential yields determined from thedependence on helium ion energy. A comparison of the results for the scavenged yields of the hydrated electron with helium ions and with fast electrons at the same scavenging capacity is made. Experimental Section The irradiations were performed using the FN Tandem Van de Graaff facility of the University of Notre Dame Nuclear Structure Laboratory. The window assembly and irradiation procedure were the same as reported earlier.16J7 Completely stripped 4He ions were used with total beam currents of about 1 nA (charge current = particle current times particle charge, 2).Particle energy was determined to within 0.1% by magnetic analysis, and the energy loss to windows was determined from standard stopping power tables.'* The uncertainty in energy corresponds to an uncertainty in particle range of 0.3 mg/cm2. Absolute dosimetry was performed by collecting and integrating 0 1993 American Chemical Society
Scavenged Yield of the Hydrated Electron the charge from the sample cell and exit window in combination with the particle energy. Total doses were 3.2 X 1019 eV in 20 mL of solution (250 Gy). The samples were irradiated in Pyrex sample cells to which a thin mica window (-6 mg/cm2) was attached. Each sample cell contained a magnetic stirrer which was used throughout the radiolysis. Oxygen was purged from thesolution before irradiation by bubbling with nitrogen or helium and the cell sealed with septa. Glycylglycine (Sigma Chemical Company) was recrystallizedlg and dissolved in triply distilled water. The solutions were irradiated at the natural pH (5.4-5.7) and transferred to sample vials. Ammonia analysis was performed within a few hours of the radiolysis by converting the cationic ammonia to the gas by increasing the pH to 11.5 with an ionic strength adjuster solution of 5 M sodium hydroxide, 0.01 M NazEDTA, and 2 M methanol. Gaseous ammonia was measured using an Orion Ionanalyzer Model EA920 with a Model 95 12ammonia gas probe. Glycylglycine interferes with the ammonia analysis when present above 0.08 M, so the samples were diluted, if necessary, to avoid this problem. The Ionanalyzer was calibrated with standard ammonium chloride solutions which contained the same concentrations of glycylglycine and ionic strength adjuster as the analyzed portion of the irradiated sample. The estimated accuracy for the ammonia determination is &8%.
The Journal of Physical Chemistry, Vol. 97, NO. 41, 1993
10721
[glycylglycine]
80X
-3
30-
.
9)
Helium Ion Energy, E, (MeV) Figure 1. Production of ammonia (G&o(NHp), molecules/ 100 particles) as a function of initial helium ion energy, EO, for glycylglycine concentrations of 0.001,0.01,0.1 and 1 M. The solid lines were drawn using the fitted parameters obtained from Figure 2.
r
3.0
Results and Discussion The mechanism for the radiolysis of glycylglycine was first presented by Garrison and co-workers.20J1 They determined that ammonia is produced exclusively by the reaction of glycylglycine with the hydrated electron and that this reaction is quantitative. Hydroxyl radicals and hydrogen atoms undergo hydrogen atom extraction reactions with glycylglycine, and these reactions do not lead to the production of ammonia. In addition, the rate constants for the reactions of glycylglycine with hydroxyl radicals and hydrogen atoms are less than or equal to that with hydrated electrons22 so cooperative scavenging effects on track processes are negligible. Subsequent investigators using pulse radiolysis techniques reported efficiencies of 80-100% for the production of ammonia in the reaction of the hydrated electron with gly~ylglycine.23~~~ For this reason the reinvestigation of glycylglycine radiolysis using y-rays was performed.25 This study shows that within experimental errors the yield of ammonia produced in 0.001-1 M glycylglycine solutions is equal to the scavenged yield of the hydrated electron as determined using a variety of other solutes. The sample composition and ammonia analysis technique used in this latter work are identical to those employed here. Therefore, it can be assumed that the yield of ammonia in the radiolysis of glycylglycine solutions gives a good representation of the scavenged yield of the hydrated electron. The observed production of ammonia G&o(NH3), molecules/ 100 particles) in the radiolysis of glycylglycine solutions is shown in Figure 1 as a function of the initial helium ion energy, EO. Glycylglycine concentrations were 0.001,0.01,0.1, and 1 M, and the incident helium ion energy to the solution was varied from 2 to 22 MeV. Each data point represents an average of several experiments. The production of ammonia increases with increasing helium ion energy, and at a given helium ion energy the amount of ammonia observed increases with increasing glycylglycine concentration. Helium ions having an energy of 22 MeV have a range in water of only 41 mg/cm2 (410 pm). In all of the experiments the helium ions were completely stopped in the solutions so it is the track-averaged or integral ammonia, Go, yields which were measured. The radiation chemical yields of scavenged hydrated electrons can be obtained for each data point on Figure 1 by dividing by the appropriate energy, and the results are shown in Figure 2. Also shown in this figure are the data of Appleby and Schwarz for the production of N2 in 0.7 m M NzO solutions
0
10
15
20
25
30
Helium Ion Energy, E, (MeV) Figure 2. Track-averaged scavenged yields of the hydrated electron as a function of initial helium ion energy, EO,forglycylglycineconcentrations of 0.001,0.01,0.1, and 1 M. The data points (X) are obtained from ref 11. The solid lines were obtained by a linear-least-squares fit to the data for each of the glycylglycine concentrations.
irradiated with 12- and 32-MeV helium ions." The scavenging capacity of that system corresponds to a glycylglycine concentration of about 0.02 M. The agreement between the two sets of data is good. At all glycylglycine concentrations there is an increase in the scavenged hydrated electron yield with increasing helium ion energy. This increase appears to be linear with helium ion energy over the range studied. A first-order polynomial was fit to the data for each of the glycylglycine concentrations using a linearleast-squares technique.26 The resultant lines are drawn in Figures 1 and 2, and they appear to agree well with the data. At the zero helium ion energy intercept the yields of scavenged hydrated electrons are computed to be 0.052, 0.165, 0.611, and 1.317 molecules/ 100 eV for 0.001-1 M glycylglycine, respectively. Over the same range of glycylglycine concentrations the values of Go increase a t a rate of 0.0093,0.0191,0.0223, and 0.0444 units per MeV of helium ion energy, respectively. These results suggest that at low scavenger concentrations the yields of scavenged hydrated electrons vary with helium ion energy by less than 1% per MeV of energy. However, the temporal dependence of the hydrated electron yields in the particle track is much more sensitive to helium ion energy (see below). Differential or track segment yields (Gi = d(G&o)/dEo) are better suited than the integral yields for comparison between
10722 The Journal of Physical Chemistry, Vol. 97, No. 41, 1993
3.0 0,
0.0 0
5
15
10
25
20
30
Helium Ion Energy (MeV) Fipre3. Variation of the track segment or differential scavenged yields of the hydrated electron with helium ion energy for the various
glycylglycine concentrations. The solid lines were drawn using the fitted parameters obtained from Figure 2. An.
>^ m
3.5
.
3.0
.
.
0
2.5
.
.
.
.
.
.
.
.
.
.
.
v1
m
3 2.0 '
u
-
i 1 . 5 1
/
[glycylglycine] 1M
05 00
0
50
100
150
200
250
300
LET in Water (eV/nm)
Figure 4. Variation of the differential scavenged yields of the hydrated electron with helium ion LET for thevarious glycylglycineconcentrations. The solid lines were drawn using the fitted parameters obtained from Figure 2 . The data points (0) are obtained from ref 15.
different particles and with model calculations. The differential hydrated electron yield at a particular helium ion energy and glycylglycine concentration is equal to the slope of the tangent to the appropriate curve of Figure 1. Uncertainty in the data makes the direct determination of these yields difficult, and so the differential yields are obtained from the derivatives of the fitted curves drawn through the data. The variation of the differential yields for the hydrated electron with helium ion energy are shown in Figure 3. As the helium ion energy approaches zero the differential yields and the integral yields approach the same value. With increasing helium ion energy the differential yields arelarger than the integral yields. At even higher energies beyond the scope of the present work the differential and integral yields will again approach the same ~ a l u e . 2 ~ Many studies have shown that radiation chemical yields are not uniquely determined by LET.1,2Js-17 Radiation chemical processes are dependent on the local track structure which is determined by a number of parameters such as particle type, energy, and secondary electron spectra. However, for a given type of particle, LET is a reasonable parameter to show the effects of particle track structure on radiation chemical yields. The differential scavenged hydrated electron yields are presented in Figure 4 as a function of helium ion LET. At all glycylglycine concentrations the yields of the hydrated electron decrease with increasing LET in water. The absolute value of this decrease is more pronounced at higher glycylglycine concentrations although
LaVerne and Yoshida the relative change is about the same for all concentrations. With increasing LET the initial concentration of the hydrated electron increases and intratrack combination processes are more dominant than the scavenging reactions. All of the chemical reactions are competing with diffusion of the hydrated electron into the bulk medium as the track structure relaxes by diffusion. Eventually the track structurewill disappear as the hydrated electron becomes homogeneously distributed in the bulk medium. For initial spherical distributions, as found with fast electrons, the concentration of hydrated electrons will reach a constant value at about 1 ps and further recombination processes are unlikely. The radiation chemical yield of the scavenged hydrated electron will no longer be dependent on scavenger concentration. The variation of the radiation chemical yield with scavenger concentration has received a considerable amount of attention from theory and experiment.lV2 A nice description of the scavenger concentration dependence of the fraction radicals which escape the particle track was given in the early work of Burton and Kurien.28 That work was one of the first to show the effects of particle track structure on this fraction. To a first approximation high-LET particles have a more cylindrical initial track geometry and radical combination processes are never negligible even at long times.29 In reality this case may not be observed because most particle tracks are not perfect cylinders since they can contain a significant contribution from secondary electrons. Certainly, track ends and clustering as the track dissipates will also lead to a noncylindrical geometry. The competition between radical combination and radical diffusion into the bulk medium makes the temporal variation of the hydrated electron yield very dependent on the particle track structure. This temporal variation is manifested in the variation of the scavenged yield with scavenger concentration. There is a procedure involving Laplace transforms for converting the scavenger concentration dependence of the scavenged yield of radicals into the temporal variation of the radicals in pure ~ a t e r . This ~ ~ ,procedure ~~ has been shown to work very well for the radiolysis of water with low-LET particle^.^' It may also work with higher LET particles,32 but the procedure is not sufficiently established to apply here. Therefore, one can only look at the time-averaged yield of the hydrated electron. Hydrated electrons are scavenged by glycylglycine with a rate constant, k, of 3.0 X lo8 M-l s-1.22 The average lifetime of the hydrated electron with respect to glycylglycine is equal to 1/(k[glycylglycine]) where [glycylglycine] is the glycylglycine concentration. In this work the average lifetime of the hydrated electron varied from 3 ns to 3 ps as theglycylglycineconcentration changed from 1 to 0.001 M. Over the lifetimes studied here the largest change in the scavenged hydrated electron yield occurs at the higher helium ion energies. Most of the temporal variation of the hydrated electron with lower energy, higher LET, helium ions occurs at times shorter than the scope of the present work. The data in Figure 4 show that at a given LET the change in the yields of the hydrated electron with time (scavenger concentration) gets smaller a t longer times (smaller scavenger concentration). Also shown in Figure 4 are the results of Sauer et al.ls for neat water at neutral pH. Their results represent the differential hydrated electron yields at 10 ps. It appears that the progression of the present results agree very well with the data of Sauer et al. The present results extend over a much larger LET range, and they give an indication of the temporal variation of the hydrated electron. It should be noted that the work of Sauer et al. also contained results using benzophenone as a scavenger which they determined was representative of the yield of the hydrated electron at about 1 ps. However, extraction of these results actually requires a diffusion-kinetic analysis which contains a number of assumptions.l~'3 More importantly, those experiments were performed at very high pH where the yields of the hydrated electron are much different than in neutral water.
The Journal of Physical Chemistry, Vol. 97, No. 41, 1993
Scavenged Yield of the Hydrated Electron
8
7
0
3
a 0 E
Y
2
n
S
1 o5
1 06
10’
1o8
1 09
10’0
k [glycylglycine] (s.‘)
Figure 5. Differential scavenged yields of the hydrated electron as a function of the scavenging capacity of glycylglycine (equal to the glycylglycine concentration times the scavenging rate constant, k). The solid points are from the present work for 5-, lo-, 15- and 20-MeV helium ions with LETSof 93, 57, 42, and 33 eV/nm, respectively. The data are for y-rays, ref 25, and the dashed line is from an empirical points (0) equation obtained by fitting all of the available data for the scavenged yield of the hydrated electron with fast electrons and y-rays, ref 31.
A comparison of the scavenged yields of the hydrated electron obtained with fast electrons and with 5-, lo-, 15-, and 20-MeV helium ions is presented in Figure 5. This figure shows the variation of the scavenged hydrated electron yields with the scavenging capacity of glycylglycine, that is the rate constant, k, times the glycylglycine concentration. The data for fast electrons are those of Yoshida,25 and the dashed line is the empirical equation given by LaVerne and Pimblott.31 The equation was obtained by fitting all of the reported experimental data for the scavenging of hydrated electrons in the radiolysis of aqueous solutions with fast electrons or y-rays. It is apparent for fast electron radiolysis that the results with glycylglycine as a scavenger agree well with the other scavenger studies. As discussed above the yield of the scavenged electron is independent of glycylglycine concentration at low concentrations. At higher glycylglycine concentrations the scavenging reactions of the hydrated electron are competing more effectivley with radical combination reactions in the track. Extrapolation of the yields beyond the maximum glycylglycine concentration leads to the value of 4.8 molecules/ 100 eV for the initial yield of the hydrated electron.31 The work of Burns and c o - ~ o r k e r s ~ with 2 - ~ ~protons seems to indicate that the initial yield of hydrated electrons is the same for all incident radiation. Differences in the track geometry for the various types of particles will lead to different temporal yields for the hydrated electron. A comparison of the variation in the temporal dependencies between fast electrons and helium ions can be seen in Figure 5 since the average lifetime of the hydrated electron is just the inverse of the scavenging capacity. For 20MeV helium ions the scavenged hydrated electron yield is about the same as that for fast electrons above a scavenging capacity of 109 s-1. Below this value the scavenged yield of the hydrated electron continues to decrease with decreasing scavenging capacity for helium ions whereas it approaches a constant value for fast electrons. A t a scavenging capacity of 3.0 X 105 s-1 (0.001 M glycylglycine) the scavenged hydrated electron yield with 20MeV helium ions is 0.423 molecules/ 100 eV, which is considerably less than the value of 2.47 found with fast electrons. With increasing LET the deviation between the scavenged hydrated electron yield for fast electrons and helium ions occurs at increasingly earlier times. At a scavenging capacity of 3 X lo* s-1 (1 M glycylglycine) the scavenged hydrated electron yield with 20-MeV helium ions is 8 1% of that found with fast electrons, while with 5-MeV helium ions it is only 46% of the fast electron
10723
value. The nonhomogeneous spatial distribution of hydrated electrons initially present in the spur produced by the fast electron is virtually gone by 100 ns as shown by the near invariance of the scavenged yield with scavenging capacities up to about 107 s-1. This dependence is not the same for the helium ions studied here where it is observed that the scavenged hydrated electron yield varies considerably with scavenging capacities of lo5 to lo9 s-1. It does appear that the absolute differences in the scavenged hydrated electron yields are getting smaller with decreasing scavenging capacity for all energy helium ions. More work needs to be done at lower scavenging capacities, but this lower limit for the scavenged hydrated electron yield seems to be becoming independent of the scavenger concentration and would therefore correspond to the residual yield of hydrated electrons after the track structure has disappeared. The scavenged yield of the hydroxyl radical has been determined previously with helium ions of 2-25-MeV energy using formic acid as a ~cavenger.3~ Formic acid has a rate constant for the reaction with hydroxyl radical of 1.3 X lo8 M-1 s-l 32 which is only slightly smaller than the rate constant for the scavenging of hydrated electrons by glycylglycine. The differential scavenged yields of hydroxyl radicals with 1 M formic acid are 1.16 and 3.10 molecules/lOO eV for 5- and 20-MeV helium ions, respectively. These yields are very similar to the values of 1.76 and 3.09 molecules/ 100 eV determined here for the scavenged yields of hydrated electrons with 1 M glycylglycine. However, at lower scavenging capacities corresponding to 0.001 M formic acid the differential scavenged hydroxyl radical yields are 0.43 1 and 1.51 molecules/ 100 eV for 5- and 20-MeV helium ions, respectively. Thesevalues areconsiderably larger than the scavenged hydrated electron yields of 0.145 and 0.423 molecules/lOO eV determined with 1mM glycylglycine solutions. Preliminary diffusion-kinetic calculations have been performed to examine the major processes occurring in high-LET tracks.32 These calculations have found that there is considerably more reaction of the hydrated electron with hydrogen peroxide in the tracks of helium ions than in the spurs produced by fast electrons. This reaction is expected to occur on the time scales studied in the present experiments, and it produces hydroxyl radicals. The net effect is a much lower yield of hydrated electrons compared to hydroxyl radicals at long times in the tracks of heavy ions. This process can have very important consequences in the heavy ion radiation chemistry and radiation biology of aqueous solutions. Acknowledgment. We thank Dr. C. P. Browne of the Notre Dame Nuclear Structure Laboratory for making the facilities available to us. The Nuclear Structure Laboratory is supported by the National Science Foundation. One of us (H.Y.) was supported by the Office of Health and Environmental Research of the Department of Energy under contract DE-ACO5840R21400 with Martin Marietta Energy Systems, Inc. The research described herein was supported by that Office and the Office of Basic Energy Sciences of the Department of Energy. This contribution is No. NDRL-3614 from the Notre Dame Radiation Laboratory. References and Notes (1) Allen, A. 0. The Radiation Chemistry of Water and Aqueous Solutions; Van Nostrand: New York, 1961. (2) Farhataziz;Rodgers, M. A. J., Eds.Radiation Chemistry. Principles and Applications; VCH Publishers: New York, 1987. (3) Bronskill, M. J.; Wolff, R. K.; Hunt, J. W. J . Chem. Phys. 1970,53, 4201. (4) Buxton, G. V. Proc. R . SOC.London, A 1972, 328, 9. ( 5 ) Jonah, C. D.; Hart, E. J.; Matheson, M. S. J . Phys. Chem. 1973,77, 1838.
(6) Jonah, C. D.; Matheson, M. S.;Miller, J. R.; Hart, E. J. J . Phys. Chem. 1976,80, 1267. (7) Sumiyoshi,T.;Tsugaru, K.;Yamada,T.;Katayama, M. Bull. Chem. SOC.Jpn. 1985, 58, 3073.
10724 The Journal of Physical Chemistry, Vol. 97, No. 41, 1993 (8) Dainton, F. S.; Logan, S. R. Trans. Faraday Soc. 1965, 61, 715. (9) Asmus, K.-D.; Fendler, J. H. J . Phys. Chem. 1968,72,4285. (10) Balkas, T. I.; Fendler, J. H.; Schuler, R. H. J . Phys. Chem. 1970, 74, 4497. (11) Appleby, A.; Schwarz, H.A. J . Phys. Chem. 1969,73, 1937. ( 12) Burns, W. G.; May, R.; Buxton, G. V.; Tough, G. S. Faraday Discuss. Chem. Soc. 1977,63, 47. (13) Burns, W. G.; May, R.; Buxton, G. V.; Wilkinson-Tough, G. S. J . Chem. Soc., Faraday Trans. 1 1981,77, 1543. (14) Rice, S. A.; Playford, V. J.; Burns, W. G.; Buxton, G. V. J . Phys. E I S . 1240. _ . 1982. _ (li)’Sauer:M.C., Jr.;Schmidt, K.H.;Hart,E. J.;Naleway,C.A.; Jonah, C. D. Radiat. Res. 1977,70, 91. (16) Laverne, J. A,; Schuler, R. H. J. Phys. Chem. 1987,91, 5770. (17) Laverne, J . A.: Schuler, R. H. J . Phys. Chem. 1987,91, 6560. (18) Ziegler, J. F.; Biersack, J. P.; Littmark, U. Thestopping Power and Ranre of Ions in Solids: Pernamon: New York. 1985. (79) -Yoshida, H.; Bolch,b. E.; Jacobson, K. B.; Turner, J. E. Radiat. Res. 1990,121, 257. (20) Willix, R. L. S.;Garrison, W. M. Radial. Res. 1967,32, 452. (21) Garrison, W. M.;Sokoi, H. A.; Bennett-Corniea, W. Radiar. Res. i9i3,53, 376. (22) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. J . Phys. Chem. ReJ Data 1988,17, 513.
Laverne and Yoshida (23) Simic, M.; Neta, P.; Hayon, E. J . Am. Chem. Soc. 1970,92,4763. (24) Faraggi, M.; Tal, Y. Radiat. Res. 1975,62, 347. (25) Yoshida, H. Radial. Res., in press. (26) Bevington, P. R. Data Reduction andError Analysisfor the Physical Sciences; McGraw-Hill: New York, 1969. (27) In the present analysis it is assumed that at a given glycylglycine concentration GOis defined by the function a + bE, where E is the helium ion energyandaand barefittedconstants. ThedataofFigure 1 isthendetermined by aE + bE2. From the definition of differential yields one gets G,= a 2bE. Such a formalism gives the differential yield as twice the integral yield at high energies whereas in reality the two should approach the same value. This result is a shortcoming of the choice of the initial function, and it shows that in general a polynomial can be used to fit the data only over a limited energy range. (28) Burton, M.; Kurien, K. C. J . Phys. Chem. 1959,63, 899. (29) Magee, J. L.;Chatterjee, A. Theoretical aspectsofradiationchemistry. In ref 2, p 137. (30) Warman, J. M.; Asmus, K.-D.; Schuler, R. H. J . Phys. Chem. 1970, 74, 4497. (31) Laverne, J. A.; Pimblott, S. M. J . Phys. Chem. 1991,95, 3196. (32) Pimblott, S. M.; Laverne, J. A. Unpublished work. (33) Naleway, C. A.; Sauer, M. C., Jr.; Jonah, C. D.; Schmidt, K. H. Radiat. Res. 1979,77, 47. (34) Laverne, J. A. Radial. Res. 1989,118, 201.
+
