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Pulse Radiolysis of Supercritical Water. 1. Reactions between Hydrophobic and Anionic Species †

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Pulse Radiolysis of Supercritical Water. 1. Reactions between Hydrophobic and Anionic Species †
  Pulse Radiolysis of Supercritical Water. 1. Reactions between Hydrophobic and AnionicSpecies † Jason Cline, Kenji Takahashi, ‡ Timothy W. Marin, Charles D. Jonah, and David M. Bartels* Chemistry Di V  ision, Argonne National Laboratory, Argonne, Illinois 60439 Recei V  ed: September 18, 2002 Reaction rates of solvated electrons with oxygen and with sulfur hexafluoride were measured in hydrothermaland supercritical water using transient absorption spectroscopy and electron pulse radiolysis. Under alkalineconditions, the reaction of hydrogen atoms with hydroxide ions to generate solvated electrons was also observedin the presence of the SF 6  scavenger. At temperatures below 300  ° C, the rate constants for scavenging by O 2 or SF 6  follow Arrhenius behavior but become increasingly dependent on water density (pressure) at highertemperatures. Above 100  ° C, the rate constant for the H reaction with OH - falls well below the numbersextrapolated from the Arrhenius behavior in the one atmosphere liquid. At a fixed temperature above thewater critical temperature (380  ° C,  T   /  T  c ) 1.01), rate constants for all three reactions reach a distinct minimumnear 0.45 g/cm 3 . We propose an explanation for this behavior in terms of the potential of mean force separatingan ion (OH - or (e-) aq ) from a hydrophobic species (H, O 2 , or SF 6 ) in the compressible fluid. The data alsoreveal an increasing initial yield of atomic hydrogen relative to solvated electrons as water density decreases.The initial yield of H appears to surpass that of solvated electrons when the water density is below 0.6 g/cm 3 at 380  ° C. I. Introduction Commercial nuclear reactors provide a source of heat, usedto drive a “heat engine” (steam turbine) to create electricity.A fundamental result of thermodynamics shows that thehigher the temperature at which any heat engine is operated,the greater its efficiency. Consequently, one obvious way toincrease the operating efficiency for future nuclear powerplants is to heat the water of the primary cooling loop tohigher temperatures. Current pressurized water reactors run atroughly 300  ° C and 100 atm pressure. Designs under consid-eration would operate at 450  ° C and 250 atm, i.e., well abovethe critical point of water. 1 - 3 This improves the attainableefficiency by about 30% and is successfully used in manyconventionally fired power plants. In the context of nuclearpower, however, a major unanswered question is what changesoccur in the radiation-induced chemistry in water as thetemperature and pressure are raised beyond the critical pointand what this could imply for the limiting corrosion processes 3 - 5 in the materials of the primary cooling loop. This manuscriptrepresents the first in a series that will attempt to providefundamental data (yields and reaction rates) to answer thisquestion.Quite apart from the practical motivation for this project, thestudy of free radical reactions in the supercritical water regimeis interesting in light of recent work on reaction rate theoryand spectroscopy in compressible supercritical fluids. 6 - 15 Theprimary free radicals generated by radiolysis, (e - ) aq , OH, andH, are respectively ionic, dipolar, and hydrophobic in nature.Their recombination and scavenging reactions can be expectedto highlight the effects of clustering (i.e., local density enhance-ments) both in terms of relative diffusion and static or dynamicsolvent effects on the reaction rates. The temperature andpressure effect on the solvated electron spectrum is anothersubject of fundamental interest which will be addressed in afuture paper. 16 The radiolysis of water by neutrons, recoil ions, gammaphotons, and high-energy electrons can be represented byEnergy tends to be deposited in isolated spurs and tracks sothat recombination of the reactive transients occurs on ananosecond time scale in competition with diffusive escape. 17 - 22 A chemical reaction of critical importance in nuclear-reactorcoolant is the reaction of the hydroxyl radical with hydrogento produce hydrogen atoms and water:This is the only reaction that can occur sufficiently quicklyto convert the oxidizing radical, OH, into the reducing radical,H, before OH reacts in an oxidizing reaction. The reaction of OH as an oxidizing radical must be prevented to suppressoxidative corrosion in the primary heat transport system.When a sufficient quantity of excess hydrogen gas has beenadded to the reactor coolant, the net production of oxygen andhydrogen peroxide becomes essentially zero. 4,23 Other reactions † Work performed under the auspices of the Office of Science, Divisionof Chemical Science, US-DOE under Contract No. W-31-109-ENG-38.Additional funding was provided under Nuclear Energy Research InitiativeGrant No. M9SF99-0276.* To whom correspondence should be addressed. ‡ Permanent address: Division of Quantum Energy Engineering, Hok-kaido University, Sapporo 060-8628, Japan. H 2 O 9 8  radiation (e - ) aq , H + , H, H 2 , OH, H 2 O 2 , HO 2  (or O 2 - )(1)OH + H 2 f  H + H 2 O (2) 12260  J. Phys. Chem. A  2002,  106,  12260 - 1226910.1021/jp0270250 CCC: $22.00 © 2002 American Chemical SocietyPublished on Web 12/03/2002  occurring include the following: 17,24 and at alkaline pHBasically, all of the species in (1) can react with each other,but some reactions are much more important (faster) than others.Reaction 2 is the overall rate-determining reaction in theproduction of oxygen or peroxide. Reactions 2 and 10, or 2,12, and 7, constitute chain reactions that can consume hydrogenperoxide, which prevents significant formation of O 2 - , H 2 O 2 ,and O 2 , in quantities that could otherwise exacerbate degradationof materials in the heat transport system.Direct measurement of the chemistry in reactor cores isextremely difficult, if not impossible. The extreme conditionsof high temperature, pressure, and radiation fields are notcompatible with normal chemical instrumentation. There are alsoproblems of access to fuel channels in the reactor core. For thesereasons, theoretical calculations and chemical models have beenused extensively, by all reactor vendors and many operators, tomodel the detailed radiation chemistry of the water in the coreand the consequences for materials. Data required to model thischemistry up to 300  ° C (the operating temperature of pressurizedwater reactors or PWRs) have been collected and summarizedby Elliot and co-workers at AECL for both light 24 and heavywater. 25 For many of the dominant reactions, data have beenmeasured up to 200 or 250  ° C, and some measurements havebeen taken to 300  ° C, but significant gaps in the databaseremain. In particular, the recombination reaction 4 of twosolvated electrons shows a very strange temperature dependenceabove 150  ° C that has not been explained. 25,26 Reaction 12 hasonly been measured up to 95  ° C, 27,28 and we will show in thisstudy that one cannot extrapolate to higher temperatures usinga simple Arrhenius expression.In the following sections, we study directly the reactions of solvated electrons with scavengers O 2  and SF 6 . The intensevisible absorption of the solvated electron is very easy tomeasure in a pulse radiolysis/transient absorption experiment.Oxygen is both easy to add to water and must be removed forother experiments, so we begin our studies with an investigationof the temperature and pressure dependence of reaction 11. Thereaction with SF 6  was chosen on a more practical ground.To extract second-order reaction rate coefficients, it isnecessary to determine absolute extinction coefficients for somespecies under supercritical conditions. Our plan has been to useSF 6  as a scavenger for solvated electrons and directly measureboth the transient absorption and resulting chemical product.In aqueous solution, SF 6  is chemically inert and highly stableover wide ranges of temperature, 29 except in the presence of solvated electrons, 30 This selective stability makes it a suitable scavenger for solvatedelectrons in the aggressive environment of supercritical water.The stable ionic products can be measured by conductivity andion chromatography in line with the radiolysis flow cell.We measured the reaction rates of solvated electrons withSF 6  over a range of temperature and pressure conditions and inthe presence of hydroxide to neutralize the acid product. Toour surprise, reaction 12 was immediately visible as a secondexponential tail on the electron decay kinetics, because therelative initial yield of H atoms in supercritical water is muchhigher than in ambient conditions. In this paper, we thereforereport reaction rates for reactions 11 - 13 as a function of watertemperature and density, as well as relative initial yields of Hand (e - ) aq . II. Experimental Section The experiment is shown in schematic form in Figure 1. Thekinetics of solvated electrons were measured by pulse radiolysisand transient absorption spectroscopy in a special high-pressure,high-temperature flow cell described previously. 31 The opticalwindows are sapphire, and the cell body is Hastelloy C-276alloy.At the cell, solvated electrons (and other transient species)were generated using 4 and 8 ns pulses from Argonne ChemistryDivision’s 20 MeV electron linac. The dose to water was 15 - 20 Gy per 30 ps or 4 ns pulse; pulse amplitude was constant to5% or better during a daylong experiment but varied betweendays. Because of the difficulty with all-metal connections tothe cell, the charge per pulse was not measured for relativedosimetry.The analyzing light was generated by a focused 75 wattxenon-arc lamp, which was pulsed for approximately 300  µ s,timed such that the linac pulse coincided with the flattest partof the lamp pulse. Wavelength selection was achieved using40 nm bandwidth interference filters. The choice of wavelengthvaried with water temperature and density. At higher temper-atures and lower densities, the solvated electron spectrum isred-shifted. 26,32,33 In general, for most elevated-temperature OH + OH f  H 2 O 2  (3)(e - ) aq + (e - ) aq f  2OH - + H 2  (4)OH + O 2 - (or HO 2 ) f  OH - + O 2  (5)(e - ) aq + OH f  OH - (6)(e - ) aq + H 2 O 2 f  OH + OH - (7)(e - ) aq + H + f  H (8)(e - ) aq + H  f  H 2 + OH - (9)H + H 2 O 2 f  OH + H 2 O (10)(e - ) aq + O 2 f  O 2 - (11)H + OH - f  (e - ) aq  (12) Figure 1.  Experimental configuration for supercritical water radiolysistransient absorption. (e - ) aq + SF 6 f  F - + SF 5  (13a)SF 5 + 4H 2 O f  SO 3 ) + 5F - + 7H + + OH (13b) Pulse Radiolysis of Supercritical Water. 1.  J. Phys. Chem. A, Vol. 106, No. 51, 2002  12261  conditions, we used 1000 nm; the kinetics at 200  ° C and belowwas measured at 700 nm.Transmitted light was measured using an EG&G FND100Qsilicon photodiode for 700 nm light and a Germanium PowerDevices GAP520 InGaAs diode for 1000 nm light. Diodephotocurrents during the lamp pulse were typically in the 5 - 50mA range. The signal was acquired using a Tektronix TVS645Adigital waveform analyzer (1 GHz bandwidth, 5 GHz sampling)controlled via GPIB through extensions to Wavemetrics’ IGORprogram. Data were taken using a 250 MHz bandwidth filter(1.2 ns rise time). The DC offset and gain were routinelyadjusted to use most of the 8-bit resolution for the transientsignals. For most conditions, averaging six trace “sets” was morethan sufficient to provide a clear signal down to the 10 - 3 ODlevel (see Figure 2), where each “set” consists of a signal traceminus a dark trace (closed shutter) to subtract RF noise fromthe linac. For short acquisitions ( e 1.6  µ s), the baseline lightwas assumed to change linearly with time; for longer intervals,the full baseline lamp shape was recorded and used to computeOD( t  ).The GAP520 InGaAs diode was found to have a secondaryresponse that distorts the kinetic measurements on a one-microsecond time scale. (A secondary response is also foundin the FND100 silicon diode for wavelengths longer than 800nm.) For pure first-order kinetics, this requires a relatively minorcorrection, but it becomes more important in the analysis of biexponential kinetics. Measurements from the GAP520 at 1000nm were compared with reliable measurements from time-correlated absorption experiments and measurements with anFND100Q photodiode at a state point (45  ° C, 1 bar) where thesolvated electron spectrum is stable and well-known, and thespur kinetics are well-characterized down to a picosecond timescale. 34 Comparison of transmission curves allowed generationof an approximate impulse response function (a delta functionand an exponential decay). 35 The GAP520 has a secondaryresponse with a time constant of approximately 800 ns, whoseintegral amounts to 10% of the integrated current signal. Duringcurve fitting, each model curve was convolved with thisinstrument response before comparison to the experimental data.Two water reservoirs, sparged at atmospheric pressure withAr and O 2  or SF 6 , respectively, fed two independent AlltechHPLC pumps. As described in Takahashi et al., 31 extra HPLCpulse dampeners were connected to each pump outlet, to ensuresmooth flow. Both flow streams proceeded to a mixing tee andthen through a preheater to the optical cell. From the cell, flowproceeded through a water-cooled shell-and-tube heat exchanger,where it was cooled to room temperature. Pressure let-downwas effected using either a back-pressure regulator or a lengthof PEEK plastic or stainless steel capillary tubing (see Figure1). System pressure was monitored at the exit of the cool-downheat exchanger using a high-sensitivity piezoelectric transducerof 2.5 psig calibrated accuracy (Omega PX02 series). Temper-ature was monitored using thermocouples immersed in the flowat the exit of the preheater and within the cell. Overall systemstability was roughly  ( 0.5  ° C, and  ( 1 bar for supercriticalconditions.Scavenger concentration was modified by changing the ratioof the flow rates of the feed pumps. The concentrations of SF 6 and O 2  in the water were computed using the equilibrium dataof Cosgrove and Walkley 36 with the flow fraction of thescavenger-saturated stream and the fluid density as computedfrom the NBS Equation of State. 37 Flow rate was kept constantfor any given state point at values ranging from 2.5 to 6.0 mL/ min; these low, constant flow rates ensured that (a) the systemwas isobaric and (b) the temperature in the flow cell wasconstant, whereas (c) radiolysis products were not allowed toaccumulate in the once-through flow cell. (No more than threeor four beam pulses could irradiate the cell in the flushing time,which is insufficient to affect the pseudo-first-order kineticsunder study.) In general, flows of 6 mL/min were used whenthe back-pressure regulator was in place; for capillary tubing,the flow rate was 2.5 - 4.0 mL/min as required to set thepressure.Ar-sparged, ultrapure (18 M Ω -cm) water flowed through onepump during system heat-up. A second pump head was primedto feed SF 6 - or O 2 -saturated water. For SF 6  experimentsemploying alkaline conditions, after heat-up, the feed wasswitched to Ar- and SF 6 -sparged potassium hydroxide solutions.Special care was taken to reduce or eliminate O 2  and CO 2 contamination of the solutions. Precautions against O 2  contami-nation were particularly important for alkaline-solution experi-ments, as it seemed that if any trace of residual oxygen remainedin the system when switching to alkaline feeds, the sapphirewindows were etched, dramatically reducing the transmittedlight.At each P - T state point, the scavenger concentration wasset initially low and then increased. This precaution was takenbecause we found that the system plumbing could act as ascavenger reservoir. For example, although responses to increas-ing flow of SF 6 -saturated water were immediate, we found itcould take up to an hour after shutting off the SF 6 -saturatedwater to flush it all out, as determined from observation of thesolvated electron lifetime; a similar but much less dramatic effectwas seen with oxygen (i.e., the flush out was several timesfaster). In contrast, salt solutions were readily flushed throughwithin a minute. Because the system was always at pressures g 100 bar, at no time were the gas solubilities expected to fallbelow the molal saturation concentration for 1.0 atm at 25  ° C.Additionally, the use of zero-dead-volume HPLC fittingseliminated the possibility of a vapor reservoir developing inthe upstream system; there was no second phase visible in theoptical cell.Eventually, it was realized that actual concentrations of SF 6 in the irradiation cell were not as high as expected (see below)and that the discrepancy became more severe as the flow rateof the SF 6 -saturated water decreased. SF 6  delivery to the cellwas tested by collecting water from various points in the flowsystem in a glass syringe. The syringe was irradiated to convert Figure 2.  Sample fitted data and pseudo-first-order plot for scavengingof solvated electrons by O 2 . The conditions were 370  ° C and 204 bar,in the subcritical vapor phase (0.165 g/cm 3 ). 12262  J. Phys. Chem. A, Vol. 106, No. 51, 2002  Cline et al.  all SF 6  to fluoride and sulfite via reaction 13. Then the fluorideconcentration was measured with ion chromatography. Thisdiagnostic showed that SF 6  was being stored in or lost throughthe oxygen-impermeable plastic tubing used between thereservoir and pump and also in the HPLC pulse dampenerswhose construction includes a plastic diaphragm. All stainlesssteel tubing and a different dampener arrangement have sinceresolved the problem. Although we make a correction for thisconcentration problem, the SF 6  rate constants presented beloware somewhat more approximate than the other numbers wereport. We should emphasize that the SF 6  itself is entirely stableunder the supercritical water conditions we have explored. Noconductivity change in the (nearly 18Mohm-cm) neutral watereffluent could ever be detected to indicate a thermal breakdownof the SF 6  to hydrofluoric acid and sulfite. No similar retentionfor oxygen was found.Most of the SF 6  scavenging experiments described belowwere performed in the presence of 0.5 or 1.0 millimolal KOH.Addition of the electrolyte to the solution raised concerns of whether density changes due to electrostriction would besignificant and also whether the KOH was fully dissociated.No partial molal volume data could be found for KOH or NaOHat 380  ° C and 250 bar, to address the electrostriction issue. Theliterature does show that, in supercritical water, OH - behavesvolumetrically as Cl - ; 38 we thus examined the behavior of NaClto estimate behavior of KOH. In the work of Majer and Wood, 39 we find that at 280 bar and 378  ° C the infinite-dilution partialmolar volume of NaCl is - 1059 cm 3 mol - 1 , which for molalitiesof 10 - 3 produce only a change in volume of   - 0.05% at 280bar and 378  ° C. Although it cannot be ruled out that at lowerdensities the effect will become stronger, it must increase bytwo orders of magnitude to influence our results significantly.The degree of dissociation is estimated after the approach of Ho et al. 38 Using the extended form of the Debye - Hu¨ckellimiting law, and assuming a continuum dielectric fluid, weestimate an activity coefficient of approximately 0.79, whichresults in a degree of dissociation within 0.1% of unity at 0.5g/cc. The available data extend only to 0.4 g/cc, and at lowerdensities, we might expect some association to affect rateconstant  k  12 . Our available data for two KOH concentrations(1.0 and 0.5 molal) show no effect on  k  12  for the range of densities studied (greater than 0.1 g/cm 3 ). III. Results and AnalysisOxygen.  An extensive survey of solvated electron scavengingby oxygen (reaction 11) was carried out for different  T  ,  P conditions in the water critical region. All of the fitted rateconstants, with estimated errors (one standard deviation) arelisted in Table 1S of the Supporting Information. A typical dataset is illustrated in Figure 2, for scavenging at 370  ° C and 204bar in the high-density vapor phase (density 0.158 g/cm 3 ). TheGAP 520 diode was used at 1000 nm for detection. The fittedcurves shown include the small correction for secondary dioderesponse mentioned above. In general, the fitting was startedfrom approximately 20 ns after the pulse to avoid distortionsfrom linac noise and from spur recombination chemistry.Results for the measurements at 250 bar and some lowerpressures are plotted in the Arrhenius format in Figure 3. Below300  ° C the reaction rate is very insensitive to the pressure. TheArrhenius curve adapted from the compilation of Elliot 24 fromroom temperature to 200  ° C is also superimposed for compari-son. Clearly, the agreement of our measurements with previouswork is quite good. Along the 250 bar isobar, a strong dip inreaction rate is visible in the critical region, with a minimum at380  ° C.The phase-point behavior is more clearly represented inFigure 4, where we plot rate constants for 360, 370, 380, and400  ° C as a function of the water density. The error bars dependstrongly on the density as indicated because in the compressibleregion it is more difficult to maintain constant temperature andpressure. A very sharp drop in the rate constant is obvious aswe proceed from high density down to about 0.45 g/cm 3 on agiven isotherm. Proceeding along the 380  ° C isotherm, the rateincreases again down to the lowest densities measured. Althoughthe intermediate densities are not available for the subcriticalisotherms at 360 and 370  ° C, the dense vapor (0.10 - 0.16 g/cm 3 )at these temperatures shows the same behavior as the super-critical 380  ° C reaction rate. The isochoric activation energyon this low-density side is apparently quite small. The high-density side is not available at 400  ° C because the largepressures required exceed the capabilities of the pumps. Therate constants in the compressible region around 0.3 g/cm 3 aresignificantly higher at 400  ° C than at 380  ° C, and the overalleffect of density is less at the higher temperature. Figure 3.  Arrhenius plot of reaction 11 at 250 bar and lower pressures,compared to existing literature. 25 The rapid dip in rate at 380  ° Cillustrates the effect of the compressible solvent on the reaction rate. Figure 4.  Effect of density on rate constant of reaction 11 at variousnear-critical temperatures. Except where shown explicitly, the errorsare on the order of the symbol size. Pulse Radiolysis of Supercritical Water. 1.  J. Phys. Chem. A, Vol. 106, No. 51, 2002  12263  The error bars in Figure 4 correspond to the error from thecurve fit as well as uncertainties in density. The majority of the error stems from uncertainties in the temperature andpressure coupled with large isothermal compressibilities. Forinstance, the error propagation from measurements  P  and  T   todensity is computed through thermodynamic properties:where  κ T  is the isothermal compressibility of the fluid, and the σ  i ’s are the estimated standard errors of the measured quantities.The error bars extend to a width of 2 σ   (i.e., the interval is mean-(  x )  (  σ  ). Because of increasing fluctuations near the criticaldensity ( F) 0.32 g cm - 3 ) and finite instrument response time,the magnitude of the errors may be systematically underesti-mated at these points. SF 6 .  As explained in the Introduction, we measured thescavenging of SF 6  in alkaline solution to avoid any problemswith buildup of hydrofluoric acid product. Figure 5 comparesthe kinetics observed in neutral and alkaline solution at 380  ° Cand 300 bar (0.534 g/cm 3 ). In the absence of SF 6 , the electronabsorption decays to baseline within 200 ns in neutral solutionbut lasts nearly a microsecond in 10 - 3 molal KOH solution.This comparison clearly illustrates the importance of reaction8 in the electron decay under these conditions. 40 Addition of 1.2 × 10 - 4 molar SF 6  causes the electron to decay to the baselinewithin about 50 ns in neutral solution. In 1.0 millimolal KOHsolution, the solvated electron concentration quickly decays toapproximately 10% of its initial value and then persists with adramatically slowed decay.Figure 6 illustrates the qualitative behavior of the scavengingkinetics in alkaline solution above 300  ° C. Within a fewnanoseconds after the pulse (some spur decay is apparent), thedecays are biexponential. Addition of SF 6  scavenger shortensthe initial (fast) decay time and reduces the amplitude of thesignal at long time. The time constant for the slow decay isunaffected. Under alkaline conditions, reaction 12acts as a secondary source of solvated electrons. This sourcebecomes dominant after the scavenger has depleted most of theinitial (e - ) aq  and the residual absorption is the result of competition between source and sink. Because reaction 12 ismuch slower than the reaction of the electron with SF 6 , the slowdecay directly gives the rate of reaction 12. The rate of thisreaction depends on the concentration of OH - and is indepen-dent of the concentration of SF 6 , as can be seen in Figure 6.The relative magnitude of the slowly decaying componentincreases at lower water densities. This shows that the initialyield of H atoms increases relative to the yield of (e-) aq , as willbe discussed below.At very low densities at 380  ° C, we discovered a dynamicshift of the solvated electron spectrum that made the apparentdecay rate at 1000 nm slower. In Figure 7, it is clear that at144 bar (0.066 g/cc) the spectrum of the solvated electron shiftsdynamically to the blue in the first 200 ns after the electronpulse. This effect disappears at pressures above 190 bar (0.107g/cm 3 ). We assume the spectral shift is due to ion pairing of the electron with counterions; a detailed analysis will bepublished in the future. Because of the severity of the effect,short-time scale data at pressures 190 bar and below (at 380 ° C) have been excluded from the present analysis of alkalinesolutions.Preliminary analyses fitted the data at a given dose and SF 6 concentration assuming a biexponential form. The slow com-ponent of the biexponential decay appears to be independentof dose. The fast component of the biexponential decays in Figure 5.  Effect of KOH on decay rate of solvated electrons insupercritical water (380  ° C, 300 bar, 0.534 g/cc). In the presence of 1.2  ×  10 - 4 M SF 6  scavenger, the long tail on the kinetics in alkalinesolution is due to conversion of H atoms to solvated electrons. σ  F 2 ) ( κ T  /V) 2 [ σ  P2 + ( ∂ P/  ∂ T) V2 σ  T2 ] (14)H + OH - f  (e - ) aq + H 2 O (12) Figure 6.  Log plot of data for reactions of solvated electrons and SF 6 in alkaline water at 350  ° C and 304 bar (0.646 g/cc) following 8 nselectron pulses ([KOH] ) 10 - 3 mol kg - 1 ; [SF 6 ] ) 1.6 × 10 - 4 - 2.5 × 10 - 4 mol L - 1 ). Figure 7.  Transient spectral shift of (e - ) aq  in alkaline water vapor(380  ° C, 144 bar, 0.066 g/cc). A pure decay is seen at 1250 nm, but atshorter wavelengths, first a rise and then a decay is seen. 12264  J. Phys. Chem. A, Vol. 106, No. 51, 2002  Cline et al.
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