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J . Phys. Chem. 1989, 93, 2412-2421
HID Isotope Effects in Water Radiolysis. 1. Chemically Induced Dynamic Electron Polarization in Spurs D. M. Bartels,* M. T. Craw, Ping Han, and A. D. Trifunac Chemistry Division, Argonne National Laboratory, Argonne. Illinois 60439 (Received: May 11, 1988; In Final Form: September 12, 1988)
We report pulsed electron paramagnetic resonance (EPR) measurements of atomic hydrogen and deuterium magnetization at 30-50 ns after radiolysis of aqueous solutions with short pulses of 3-MeV electrons. The H and D EPR spectra are strongly polarized by radical pair mechanism chemically induced dynamic electron polarization (RPM CIDEP) generated by spin-dependent recombination reactions in radiolysis spurs. The amplitude of the initial H and D magnetization has been measured as a function of pH (pD) and N 2 0 saturation in both H 2 0 and DzO. The major polarization mechanisms are identified as (a) H + H (or D + D) CIDEP, (b) H (or D) + eq-CIDEP, (c) fast OH thermal relaxation followed by Heisenberg spin exchange with H (or D), and OH, ea; spin exchange followed by reaction of ea; with H30+ (or D30+) to give H (or D). A crude model of the polarization dynamics based on a “diffusion kinetics” description of the spur chemistry is proposed and evaluated. It is concluded that the observations can be accounted for with the standard theories of RPM CIDEP and Heisenberg spin exchange.
I. Introduction The nature of energy deposition in water by high-energy photons and electrons is one of the oldest and, practically speaking, most important problems of radiation chemistry.’-’ It has long been known that such particles leave behind “tracks” consisting of well-separated local regions of high-energy content. The local regions, or “spurs”, generally contain several geminate pairs of free radicals or radical ions, solvated electrons, and electronically excited water molecules created in close proximity. The initial chemistry, naturally enough, is dominated by reactions of those species created near each other in spurs, until diffusion creates a more uniform distribution throughout the solution. A quantitative description of the spur chemistry is exceedingly difficult because of the inherent inhomogeneity of initial concentrations: one must account for diffusion effects within a complex reaction scheme. Even the basic mechanisms of product formation remain somewhat uncertain because relative reaction probabilities depend so strongly on the (unknown) initial distances between reactants and because several of the possible reactions give the same product (Le., H2).3 Recently, we reported the direct observation of “prompt” EPR signals from H and D atoms within 30 ns after radiolysis of H 2 0 and D 2 0 solutions with short pulses of 3-MeV electrom6 The spectra were characterized by the typical low field emission/high field absorption (E/A) pattern of radical pair mechanism (RPM) chemically induced dynamic electron polarization (CIDEP).7-9 Similarly, one can detect chemically induced dynamic nuclear polarization (CIDNP) in the H D molecular product formed in spur reactions of isotopically mixed water.1° The information provided by these experiments is qualitatively new in that the
signals depend on the number and nature of spur reactions rather than on the number of species that survive the spur chemistry. This paper represents the first in a series which will systematically explore the implications of chemically induced spin polarization in the primary products of water radiolysis. Since these signals are intimately related to nuclear properties, differences in the radiolysis of light and heavy water have proven extremely useful in deducing CIDEP polarization mechanisms. In addition, one can easily measure by EPR the isotope effects on atomic H and D formation in spurs. Future papers will address this topic, which bears on several unanswered questions concerning the primary radiolysis events. In the following sections, we first present a brief overview of the mechanism by which low LET (y and relativistic electron) radiation is thought to cause chemical change in water. The major problem areas are outlined, and the utility of magnetic resonance measurements in resolving some of the issues is noted. Section I11 contains an outline of our experimental (pulsed EPR) methods, and section IV details our measurements of prompt H (D) CIDEP as a function of pH (pD) and as a function of argon or N 2 0 saturation. Section V begins with an analysis of CIDEP phenomena to be expected in water spurs, which one must consider for even a qualitative understanding of the experiments. A crude model based on the classical “diffusion kinetics” description of spurs is then developed and evaluated. 11. Background
The generally mechanism of “primary events” in water radiolysis is indicated schematically: HZO
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(1) Farhataziz; Rogers, A. J. Radiation Chemistry, Principles and Applications; VCH Publishers: New York, 1987. (2) Spinks, J. W. T.; Woods, R. J. An Introduction to Radiation Chemistry, 2nd ed.; Wiley-Interscience: New York, 1976. (3) D!aganic, I.; Draganic, 2. D. The Radiation Chemistry of Water: Academic Press: New York, 1971. (4) Anbar, M. In Fundamental Processes in Radiation Chemistry: Ausloos, P., Ed.; Wiley-Interscience: New York, 1968. ( 5 ) Buxton, G. V. In The Study of Fast Processes and Transient Species by Electron Pulse Radiolysis: Baxendale, J. H., Busi, F., Eds.: D. Reidel: Boston, 1981. (6) Bartels, D. M.; Chiu, T. M.; Trifunac, A. D.; Lawler, R. G. Chem. Phys. Lett. 1986, 123, 497. (7) Freed, J. H.: Pedersen, J. B. Adu. Magn. Reson. 1976, 8, 1. (8) Adrian, F. J. Reu. Chem. Intermed. 1979, 3, 3. (9) Syage, J. A. J . Chem. Phys. 1987.87, 1022, 1033. (10) Trifunac, A. D.; Chiu, T. M.; Lawler, R. G. J. Phys. Chem. 1986, 90, 1871.
0022-3654/89/2093-2412$01.50/0
H20*
OH*
+
H*
H2
-
+
( 2 0 H * or
\
-
H202)
H ~ O ++ e-
OH*
+
H30+aq
+
e,
solutions and in Figures 7 and 8 for 0.1 M C1- solutions. Conversion of e,; to H' (DO) on the spur time scale is obvious from the increase in initial signal in acidic solutions (pH
