Communicationsto the Editor
425
~ ~ dYields ~ of~ ydrated ~ ~ Electrons ~ i atc30 to 1000 ~ ~ c ~ ~ e after c ~ n Energy d s Absorption Publication costs assisted by i1.S. Atomic Energy Commission
Actual data
,401
Sir: The yield. of hydrated electrons (eaq-) has been studied by numerous experiments a t microsecond times.l nder homogeneous conditions, when eaq- escapes the spur, most experiments agree that the yield is 2.7 & 0.1 eaq- per IGO-eV absorbed energy.2 The initial tight cluster of reactive species (spur) is formed by the ionizing radiation and an unknown fraction of these species recombines before the spur relaxes into a homogeneous distribution. The initial yield of eaq- before it escapes the spur is an extremely critical parameter of the diffusion kinetic theories of the spur.3.4 It is also important in deciding between mechanisms which give rise to the yields of radiation products in concentrated solutions and the unusual competition behavior a t these high c o n c e n t r a t i ~ n s . ~ Direct measurements of electron absorption to determine the initial G value of the hydrated electron before it expands from the spur are difficult since one requires time resolution to be%er than 1 nsec, and a dose measurement in the same spiltial region as that where the absorption spectrum is being measured. Signal-to-noise limitations preclude spreading and scattering the electron beam to make it uniform over a large irradiation cell. We have directly measured G(e,,-) by two entirely different techniques a t times shorter than 1 nsec and at 30 psec and obtain a value 4.0 f 0.2. In Table I we list the G values with the different techniques and dosimeters. Only a simplified sketch of the experimental techniques will be given here, and complete details of the two measurements will be published later in separate papers. The stroboscopic pulse radiolysis (SPR) apparatus at Toronto6 was used to give the value 4.0 0.2 a t 30 psec. The details of the SPR system are described in other papers.6-8 This system observes the kinetics of eaq- between the linac fine st.nia:ture pulses spaced a t 350-psec intervals. The 40-MeV elect^^^ beam both irradiates the sample, and forms its own analyzing light. At Argonne a single fine structure pulse was used, coupled with fast detection equipment. Tht: measurement for times less than 1 nsec med a sampling osciiloscope (SO) with a rise time of 25 psec (Tektronix 7000 series with a S-4 sampling head) and a F4014 photoidode (YX'T), with either a S-4 or S-20 photocathode. Two pulsed-ion lasers were used to analyze the signals a t wavelengths of 514.5 or 647.1 nm. The data were accumulated b:y using a multichannel analyzer. The photodiode has a r,isetime of less than 100 psec (some overshoot makes t ! settling ~ time longer, but less than 1 nsec). The most vexing problem with the determination of G(eaq-) was measuring the dose which produced the observed optical absorption. Four different dosimeters were used, tvvo with SO-PR system and two with the SPR systern. Table I shows the excellent agreement between all four methods;. '!?his consistency underscores the confidence in the measu:red value of G(eaq-). With the SC-PR system, 0.01 M air-saturated thiocyanateg and the oxygen-saturated "super Fricke" dosimeters were used~IoFor the thiocyanate dosimeter, the same analyzing light beam (argon ion laser a t 514.5 nm) was
*
The eaq- signal in H20 using t h e stroboscopic pulse radiolysis (SPR) technique is shown. There is little decay of t h e signal during the time window from 30 to 350 psec which agrees with the observed decay rate of lo* sec-' indicated by the expression, G(ea,-) = 2.7 + 1.4 exp(-108t). A simulated s p u r decayi2 using the calculations of Kuppermann3 shows that a large discrepancy occurs. The 8 and 4% are the percentage deviations from flatness referred to one step as loo%, and are not 8 and 4% decay of t h e total eaq: concentration. As seen in the numbers at left the total absorption is about 50%. Further, in any step the decay is a composite of the decays of electrons of different ages (see ref 6-8).
Figure 1.
used to measure both (SCN)z- and eaq-. The super Fricke was irradiated in a cell comparable to the dimensions of the electron beam. With the SPR system, both the hydrated electron dosimeter and a Perspex dosimeterll were used. The solvated electron dosimetry was done by conventional PR using a He-Ne laser as an analyzing light source through the same irradiation volume (delineated by 3-mm apertures). The eaq- absorption was measured a t 100 nsec and a G value of 2.7 was assumed. The Perspex doiiirnetry was done by irradiating a block of six Perspex slabs spaced to correspond to the length and density of the sample cell. Again 3-mm apertures delineated the analyzing volume and were used with the spectrophotometer to determine the change in absorbance and hence the dose. The yield after 30 psec (SPR) and before 1 nsec (SOPR) can both be called initial yields since there is very little observable decay of eaq- between 30 and 1000 psec. Using SPR, there was no observed decay of eaq- over the 30-350-psec interval as shown in Figure 1.12The SO tech(1) E. M. Fielden and E. J. Hart, Radiat. Res.. 32, 564 (1967). (2) E. J. Hart and M. Anbar, "The Hydrated Electron," Wiley-lnterscience, New York, N . Y., 1970. (3) A. Kuppermann, "Radiation Research," G .Silini, Ed., North Holland Publishing Go., Amsterdam, 1967. (4) H. A. Schwarz, J. Phys. Chem., 73, 1928 (1969). (5) P. L. T. Bevan and W. H. Hamili, Trans. Farauay Soc., 66, 2533 (1970). (6) M . J. Bronskill, W. B. Taylor, R. K. Wolf{, and J. W. Hunt, Rev. Sci. Instrum., 41,333 (1970). (7) M. J. Bronskill, R. K . Wolff, and J. W. Hunt, J. Chem. Phys., 53, 4201 (1970). (8) J. E. Aldrich, P. Foldvary, J. W. Hunt, W . B. Tayior, and R. K. Wolff, Rev. Sci. Instrum., 43, 991 (1972). (9) E. M. Fielden and N. W. Holm, "Manual on Radiation Dosimetry," N. W. Holm and R. J . Berry, Ed., Marcel Dekker, New York, N . Y., 1970, p 288. (IO) H. Fricke and E. J. Hart, "Radiation Dosimetry," Vol. 2. F. H, Attix and W. C. Roesch, Ed., Academic Press, New York, N. Y., 1966, p 226. (11) R. I . Berry and C. H. Marshall, Phys. Med. Bioi., 14, 585 (1969). (12) R. K. Wolff, M. J. Bronskill, J . E. Aldrich, and J. W. Hunt, d. P h p . Chem., submitted for publication. The Journal of Physical Chemistry, Vol. 77, No. 3, 1973
Communications to the Editor
426 TABLE I: Initiisl Yields of Hydrated Electrons Dosimetry
Pulse radiolysis techniques
(a) Sampling (SO -PR: 1. Ab:*olb,te
krads
X, nm
(1) 2.18
514.5 514.5
Type
(SCN)z-
(2) 2.40
Super Fricke G = 2.9 at 30 nsec
2. Relative
(b) Stroboscopic JSPR) 1. Absolute 2. Relative
0.3-3
HX-Perspex G = 2.7at
Earliest times
