Dynamics of Ion Pairing in Dilute Aqueous HCl Solutions by Spectroscopic Measurements of Hydroxyl Radical Conversion into Dichloride Radical Anions.

The rate of formation of dichloride anions (Cl2•-) in dilute aqueous solutions of HCl (2-100 mmol·kg-1) was measured by the technique of pulse radiolysis over the temperature range of 288-373 K. The obtained Arrhenius dependence shows a concentration averaged activation energy of 7.3 ± 1.8 kJ·mol-1, being half of that expected from the mechanism assuming the •OHCl- intermediate and supporting the ionic equilibrium-based mechanism, i.e., the formation of Cl2•- in the reaction of •OH with a hydronium-chloride (Cl-·H3O+) contact ion pair. Assuming diffusion-controlled encounter of the hydronium and chloride ions and including the effect of the ionic atmosphere, we showed that the reciprocal of τ, the lifetime of (Cl-·H3O+), follows an Arrhenius dependence with an activation energy of 23 ± 4 kJ·mol-1, independent of the acid concentration. This result indicates that the contact pair is stabilized by hydrogen bonding interaction of the solvent molecules. We also found that at a fixed temperature, τ is noticeably increased in less-concentrated solutions (mHCl < 0.01 m). Since this concentration effect is particularly pronounced at near ambient temperatures, the increasing pair lifetime may result from the solvent cage effect enhanced by the presence of large supramolecular structures (patches) formed by continuously connected four-bonded water molecules.

Failure of molecular dynamics to provide appropriate structures for quantum mechanical description of the aqueous chloride ion charge-transfer-to-solvent ultraviolet spectrum.

The lowest band in the charge-transfer-to-solvent ultraviolet absorption spectrum of aqueous chloride ion is studied by experiment and computation. Interestingly, the experiments indicate that at concentrations up to at least 0.25 M, where calculations indicate ion pairing to be significant, there is no notable effect of ionic strength on the spectrum. The experimental spectra are fitted to aid comparison with computations. Classical molecular dynamic simulations are carried out on dilute aqueous Cl-, Na+, and NaCl, producing radial distribution functions in reasonable agreement with experiment and, for NaCl, clear evidence of ion pairing. Clusters are extracted from the simulations for quantum mechanical excited state calculations. Accurate ab initio coupled-cluster benchmark calculations on a small number of representative clusters are carried out and used to identify and validate an efficient protocol based on time-dependent density functional theory. The latter is used to carry out quantum mechanical calculations on thousands of clusters. The resulting computed spectrum is in excellent agreement with experiment for the peak position, with little influence from ion pairing, but is in qualitative disagreement on the width, being only about half as wide. It is concluded that simulation by classical molecular dynamics fails to provide an adequate variety of structures to explain the experimental CTTS spectrum of aqueous Cl-.

Radiation-Induced Oxidation Reactions of 2-Selenouracil in Aqueous Solutions: Comparison with Sulfur Analog of Uracil.

One-electron oxidation of 2-selenouracil (2-SeU) by hydroxyl (●OH) and azide (●N3) radicals leads to various primary reactive intermediates. Their optical absorption spectra and kinetic characteristics were studied by pulse radiolysis with UV-vis spectrophotometric and conductivity detection and by the density functional theory (DFT) method. The transient absorption spectra recorded in the reactions of ●OH with 2-SeU are dominated by an absorption band with an λmax = 440 nm, the intensity of which depends on the concentration of 2-SeU and pH. Based on the combination of conductometric and DFT studies, the transient absorption band observed both at low and high concentrations of 2-SeU was assigned to the dimeric 2c-3e Se-Se-bonded radical in neutral form (2●). The dimeric radical (2●) is formed in the reaction of a selenyl-type radical (6●) with 2-SeU, and both radicals are in equilibrium with Keq = 1.3 × 104 M-1 at pH 4 (below the pKa of 2-SeU). Similar equilibrium with Keq = 4.4 × 103 M-1 was determined for pH 10 (above the pKa of 2-SeU), which admittedly involves the same radical (6●) but with a dimeric 2c-3e Se-Se bonded radical in anionic form (2●-). In turn, at the lowest concentration of 2-SeU (0.05 mM) and pH 10, the transient absorption spectrum is dominated by an absorption band with an λmax = 390 nm, which was assigned to the ●OH adduct to the double bond at C5 carbon atom (3●) based on DFT calculations. Similar spectral and kinetic features were also observed during the ●N3-induced oxidation of 2-SeU. In principle, our results mostly revealed similarities in one-electron oxidation pathways of 2-SeU and 2-thiouracil (2-TU). The major difference concerns the stability of dimeric radicals with a 2c-3e chalcogen-chalcogen bond in favor of 2-SeU.

Ultraviolet spectroscopy of pressurized and supercritical carbon dioxide.

Carbon dioxide (CO2) is prevalent in planetary atmospheres and sees use in a variety of industrial applications. Despite its ubiquitous nature, its photochemistry remains poorly understood. In this work we explore the density dependence of pressurized and supercritical CO2 electronic absorption spectra by vacuum ultraviolet spectroscopy over the wavelength range 1455-2000 Å. We show that the lowest absorption band transition energy is unaffected by a density increase up to and beyond the thermodynamic critical point (137 bar, 308 K). However, the diffuse vibrational structure inherent to the spectrum gradually decreases in magnitude. This effect cannot be explained solely by collisional broadening and/or dimerization. We suggest that at high densities close proximity of neighboring CO2 molecules with a variety of orientations perturbs the multiple monomer electronic state potential energy surfaces, facilitating coupling between binding and dissociative states. We estimate a critical radius of ~4.1 Å necessary to cause such perturbations.

Radiation Induced One-Electron Oxidation of 2-Thiouracil in Aqueous Solutions.

Oxidative damage to 2-thiouracil (2-TU) by hydroxyl (•OH) and azide (●N3) radicals produces various primary reactive intermediates. Their optical absorption spectra and kinetic characteristics were studied by pulse radiolysis with UV-vis spectrophotometric and conductivity detection and by time-dependent density functional theory (TD-DFT) method. The transient absorption spectra recorded in the reactions of •OH with 2-TU depend on the concentration of 2-TU, however, only slightly on pH. At low concentrations, they are characterized by a broad absorption band with a weakly pronounced maxima located at λ = 325, 340 and 385 nm, whereas for high concentrations, they are dominated by an absorption band with λmax ≈ 425 nm. Based on calculations using TD-DFT method, the transient absorption spectra at low concentration of 2-TU were assigned to the ●OH-adducts to the double bond at C5 and C6 carbon atoms (3●, 4●) and 2c-3e bonded ●OH adduct to sulfur atom (1…●OH) and at high concentration of 2-TU also to the dimeric 2c-3e S-S-bonded radical in neutral form (2●). The dimeric radical (2●) is formed in the reaction of thiyl-type radical (6●) with 2-TU and both radicals are in an equilibrium with Keq = 4.2 × 103 M-1. Similar equilibrium (with Keq = 4.3 × 103 M-1) was found for pH above the pKa of 2-TU which involves admittedly the same radical (6●) but with the dimeric 2c-3e S-S bonded radical in anionic form (2●-). In turn, ●N3-induced oxidation of 2-TU occurs via radical cation with maximum spin location on the sulfur atom which subsequently undergoes deprotonation at N1 atom leading again to thiyl-type radical (6●). This radical is a direct precursor of dimeric radical (2●).

Ultraviolet charge-transfer-to-solvent spectroscopy of halide and hydroxide ions in subcritical and supercritical water.

The temperature dependence of the vacuum ultraviolet charge-transfer-to-solvent (CTTS) absorption spectra of aqueous halide and hydroxide ions was measured for the first time up to 380 °C in subcritical and supercritical water. With increasing temperature, absorption spectra are observed to broaden and redshift, much in agreement with previous measurements below 100 °C. These changes are discussed alongside classic cavity models of the solvated species, which tie in the configuration of the adjoining polarized medium and its critical role in light absorption for electronic transitions. The data seemingly confirm the validity of the "diffuse" model pioneered by Platzman and Franck and later revised by Stein and Treinin, which has largely gone untested for nearly 60 years due to lack of experimental data in this extended temperature range. A gradual increase in anion cavity size is inferred as a function of increasing temperature while the enthalpy and entropy of hydration are largely unaffected. The changes in solvation properties are considered in the context of recent studies of the ultraviolet spectroscopy of subcritical and supercritical water and historic studies of the CTTS absorption. The "diffuse" polarizable continuum model succeeds in describing the absorption due to lack of well-defined ion hydration shells for these ions. CTTS spectra for iodide in supercritical water show no energy shift as a function of pressure/density, suggesting dielectric saturation of the I- anion by the adjacent H2O molecules at all experimental pressures/densities.

Partial Molar Volume of the Hydrated Electron.

The partial molar volume of the hydrated electron was investigated with pulse radiolysis and transient absorption by measuring the pressure dependence of the equilibrium constant for e-aq + NH4+ ⇔ H + NH3. At 2 kbar pressure, the equilibrium constant decreases relative to 1 bar by only 6%. Using tabulated molar volumes for ammonia and ammonium, we have the result V̅(e-aq) - V̅(H) = 11.3 cm3/mol at 25 °C, confirming that V̅(e-aq) is positive and even larger than the hydrophobic H atom. Assuming on the basis of recent molecular dynamics simulations that the molar volume of the H atom is somewhat less than that of H2, we estimate V̅(e-aq) = 26 ± 6 cm3/mol. The positive molar volume is consistent with an electron that exists largely in a small solvent void (cavity), ruling out a recent model ( Larsen , R. E. ; Glover , W. J. ; Schwartz , B. J. Science 2010 , 329 , 65 - 69 ) that suggests a noncavity structure with negative molar volume.

The selenocyanate dimer radical anion in water: Transient Raman spectra, structure, and reaction dynamics.

The selenocyanate dimer radical anion (SeCN)2 •-, prepared by electron pulse irradiation of selenocyanate anion (SeCN)- in water, has been examined by transient absorption, time-resolved Raman spectra, and range-separated hybrid density functional (ωB97x and LC-ωPBE) theory. The Raman spectrum, excited in resonance with the 450 nm (λmax) absorption of the radical, is dominated by a very strong band at 140.5 cm-1, associated with the Se-Se stretching vibration, its overtones and combinations. A striking feature of the (SeCN)2 •- Raman spectrum is the relative sharpness of the 140.5 cm-1 band compared to the S-S band at 220 cm-1 in thiocyanate radical anion (SCN)2 •-, the difference of which is explained in terms of a time-averaged site effect. Calculations, which reproduce experimental frequencies fairly well, predict a molecular geometry with the SeSe bond length of 2.917 (±0.04) Å, the SeC bond length of 1.819 (±0.004) Å, and the CN bond length of 1.155 (±0.002) Å. An anharmonicity of 0.44 cm-1 has been determined for the 140.5 cm-1 Se-Se vibration which led to a dissociation energy of ∼1.4 eV for the SeSe bond, using the Morse potential in a diatomic approximation. This value, estimated for the radical confined in a solvent cage, compares well with the calculated gas-phase energy, 1.32 ± 0.04 eV, required for the radical to dissociate into (SeCN)• and (SeCN)- fragments. The enthalpy of dissociation in water has been measured (0.36 eV) and compared with the value estimated by accounting for the solvent dielectric effects in structural calculations.

Transient Raman spectra, structure, and thermochemistry of the thiocyanate dimer radical anion in water.

Time-resolved resonance-enhanced Stokes and anti-Stokes Raman spectra of the thiocyanate dimer radical anion, (SCN)2•-, prepared by pulse radiolysis in water, have been obtained and interpreted in conjunction with theoretical calculations to provide detailed information on the molecular geometry and bond properties of the species. The structural properties of the radical are used to develop a molecular perspective on its thermochemistry in an aqueous solution. Twenty-nine Stokes Raman bands of the radical observed in the 120-4200 cm-1 region are assigned in terms of the strongly enhanced 220 cm-1 fundamental, weakly enhanced 721 cm-1, and moderately enhanced 2073 cm-1 fundamentals, their overtones, and combinations. Calculations by range-separated hybrid density functionals (ωB97x and LC-ωPBE) support the spectroscopic assignments of the 220 cm-1 vibration to a predominantly SS stretching mode and the features at 721 cm-1 and 2073 cm-1 to CS and CN symmetric stretching modes, respectively. The corresponding bond lengths are 2.705 (±0.036) Å, 1.663 (±0.001) Å, and 1.158 (±0.002) Å. A first order anharmonicity of 1 cm-1 determined for the SS stretching mode suggests a convergence of vibrational states at an energy of ∼1.5 eV, using the Birge-Sponer extrapolation. This value, estimated for the radical confined in solvent cage, compares well with the calculated gas-phase energy of 1.26 ± 0.04 eV required for the radical to dissociate into SCN• and SCN- fragments. The enthalpy of dissociation drops to 0.60 ± 0.03 eV in water when solvent dielectric effects on the radical and its dissociation products upon S-S bond scission are incorporated in the calculations. No frequency shift or spectral broadening was observed between light and heavy water solvents, indicating that the motion of solvent molecules in the hydration shell has no perceptible effect on the intramolecular dynamics of the radical. The Stokes and anti-Stokes Raman frequencies were found to be identical within the experimental uncertainty, suggesting that the frequency difference between the thermally relaxed and spontaneously created vibrational states of (SCN)2•- in water is too small to be observable.

Vacuum ultraviolet spectroscopy of the lowest-lying electronic state in subcritical and supercritical water.

The nature and extent of hydrogen bonding in water has been scrutinized for decades, including how it manifests in optical properties. Here we report vacuum ultraviolet absorption spectra for the lowest-lying electronic state of subcritical and supercritical water. For subcritical water, the spectrum redshifts considerably with increasing temperature, demonstrating the gradual breakdown of the hydrogen-bond network. Tuning the density at 381 °C gives insight into the extent of hydrogen bonding in supercritical water. The known gas-phase spectrum, including its vibronic structure, is duplicated in the low-density limit. With increasing density, the spectrum blueshifts and the vibronic structure is quenched as the water monomer becomes electronically perturbed. Fits to the supercritical water spectra demonstrate consistency with dimer/trimer fractions calculated from the water virial equation of state and equilibrium constants. Using the known water dimer interaction potential, we estimate the critical distance between molecules (ca. 4.5 Å) needed to explain the vibronic structure quenching.

The nature of the CO2 (-) radical anion in water.

The reductive conversion of CO2 into industrial products (e.g., oxalic acid, formic acid, methanol) can occur via aqueous CO2 (-) as a transient intermediate. While the formation, structure, and reaction pathways of this radical anion have been modelled for decades using various spectroscopic and theoretical approaches, we present here, for the first time, a vibrational spectroscopic investigation in liquid water, using pulse radiolysis time-resolved resonance Raman spectroscopy for its preparation and observation. Excitation of the radical in resonance with its 235 nm absorption displays a transient Raman band at 1298 cm(-1), attributed to the symmetric CO stretch, which is at ∼45 cm(-1) higher frequency than in inert matrices. Isotopic substitution at C ((13)CO2 (-)) shifts the frequency downwards by 22 cm(-1), which confirms its origin and the assignment. A Raman band of moderate intensity compared to the stronger 1298 cm(-1) band also appears at 742 cm(-1) and is assignable to the OCO bending mode. A reasonable resonance enhancement of this mode is possible only in a bent CO2 (-)(C2v/Cs) geometry. These resonance Raman features suggest a strong solute-solvent interaction, the water molecules acting as constituents of the radical structure, rather than exerting a minor solvent perturbation. However, there is no evidence of the non-equivalence (Cs) of the two CO bonds. A surprising resonance Raman feature is the lack of overtones of the symmetric CO stretch, which we interpret due to the detachment of the electron from the CO2 (-) moiety towards the solvation shell. Electron detachment occurs at the energies of 0.28 ± 0.03 eV or higher with respect to the zero point energy of the ground electronic state. The issue of acid-base equilibrium of the radical, which has been in contention for decades, as reflected in a wide variation in the reported pKa (-0.2 to 3.9), has been resolved. A value of 3.4 ± 0.2 measured in this work is consistent with the vibrational properties, bond structure, and charge distribution in aqueous CO2 (-).

Design of an ultrashort optical transmission cell for vacuum ultraviolet spectroscopy of supercritical fluids.

We present the design and characteristics of an ultrathin flow cell optimized for vacuum ultraviolet transmission spectroscopy experiments on supercritical fluids. The cell operates satisfactorily at pressures up to 300 bar and temperatures up to 390 °C. The variable path length concept of the cell allows for optical transmission studies of analytes ranging from dense condensed-phase systems to gas-phase systems. The path length of the cell can be adjusted from hundreds of nanometers to hundreds of micrometers by an exchange of a variable thickness spacer sandwiched between two sapphire windows. In the path length range from nanometers to single micrometers, metal vapor deposited on one or both of the two sandwiched optical windows constitute the spacer. Spacers with thicknesses of 2 µm and greater can be constructed from simple commercially available metal foils. The cell has been used to measure the lowest-lying absorption band of water in both the vapor and condensed phases from room temperature up to and above the critical point. It has also found application in the studies of aqueous ions and nonaqueous liquids including various common organic solvents and carbon dioxide.

The nature of the superoxide radical anion in water.

Vibrational properties of the superoxide radical anion (O2(-●) in liquid water have been experimentally investigated for the first time. The stretching frequency, its shift from the gas-phase to aqueous solution, anharmonicity constant, and the Raman bandwidths provide an insight into the radical-water interactions and the hydration cage. In view of the spectroscopic information obtained in this work, the structural models based on molecular dynamics simulation in solution and gas-phase infrared studies of the water molecules bound to O2(-●) are critically examined.

The early events in the OH radical oxidation of dimethyl sulfide in water.

The time-resolved Raman observation of a prototype of the hetero-atom three electron bonds (-X-OH) that often form on encounter of the OH radical with chemical species in water is reported. In spite of their wide chemical and biochemical importance, no experimental structural information exists, thus far, on any such bond in solution or in the gas phase. The nature of the >S-O bond formed on the reaction of the OH radical with dimethyl sulfide in water, investigated in the present work, would necessitate a reexamination of the existing reaction mechanisms in related biological systems and development of the appropriate computational methods.

Comparison of acid generation in EUV lithography films of poly(4-hydroxystyrene) (PHS) and noria adamantyl ester (Noria-AD(50)).

The mechanism for acid production in phenolic extreme ultraviolet (EUV) lithography films containing triphenylsulfonium triflate (Ph(3)S(+)TfO(-)) acid generator has been investigated by electron paramagnetic resonance (EPR) spectroscopy and by use of the acid indicator coumarin 6 (C6). Gamma radiolysis was substituted for the EUV radiation with the assumption that the chemistry generated by ionization of the matrix does not depend on the ionization source. Poly(4-hydroxystyrene) (PHS) was first investigated as a well-studied standard, after which the water-wheel-like cyclic oligomer derivative containing pendant adamantyl ester groups, noria-AD(50), was investigated. EPR measurements confirm that the dominant free radical product is a phenoxyl derivative (PHS-O(•) or noria-O(•)) that exhibits quite slow stretched exponential recombination kinetics at room temperature. Also observed at 77 K was the presence of a significant hydrogen atom product of radiolysis. The G value or yield of acid production in thin lithography films was measured with the C6 indicator on a fused silica substrate. It was found that a significant amount of acid is generated via energy transfer from the irradiated fused-silica substrate to the Ph(3)S(+)TfO(-) in the films. By varying the film thickness on the substrates, the substrate effect on the acid yield was quantitatively determined. After subtraction of the contribution from the substrates, the acid yield G value in the PHS film with 10 wt % Ph(3)S(+)TfO(-) and 5 wt % C6 was determined to be 2.5 ± 0.3 protons per 100 eV of radiation. The acid yield of noria-AD(50) films was found to be 3.2 ± 0.3 protons per 100 eV.

Dissociative electron attachment to triflates.

Gas phase studies of dissociative electron attachment to simple alkyl (CF(3)SO(3)CH(3)) and aryl (C(6)H(5)SO(3)CF(3) and CF(3)SO(3)C(6)H(4)CH(3)) triflates, model molecules of nonionic photoacid generators for modern lithographic applications, were performed. The fragmentation pathways under electron impact below 10 eV were identified by means of crossed electron-molecular beam mass spectrometry. Major dissociation channels involved C-O, S-O, or C-S bond scissions in the triflate moiety leading to the formation of triflate (OTf(-)), triflyl (Tf(-)), or sulfonate (RSO(3)(-)) anions, respectively. A resonance leading to C-O bond breakage and OTf(-) formation in alkyl triflates occurred at electron energies about 0.5 eV lower than the corresponding resonance in aryl triflates. A resonance leading to S-O bond breakage and Tf(-) formation in aryl triflates occurred surprisingly at the same electron energies as C-O bond breakage. In case of alkyl triflates S-O bond breakage required 1.4 eV higher electron energies to occur and proceeded with substantially lower yields than in aryl triflates. C-S bond scission occurred for all presently studied triflates at energies close to 3 eV.

Solvated electron extinction coefficient and oscillator strength in high temperature water.

The decadic extinction coefficient of the hydrated electron is reported for the absorption maximum from room temperature to 380 degrees C. The extinction coefficient is established by relating the transient absorption of the hydrated electrons in the presence of a scavenger to the concentration of stable product produced in the same experiment. Scavengers used in this report are SF(6,) N(2)O, and methyl viologen. The room temperature value is established as 22,500 M(-1) cm(-1), higher by 10-20% than values used over the last several decades. We demonstrate how previous workers arrived at a low value by incorrect choice of a radiolysis yield value. With this revision, the integrated oscillator strength, corrected by refractive index, is definitely (ca. 10%) larger than unity. This result is fully consistent with EPR and resonance Raman results which indicate mixing of the hydrated electron wave function with solvent electronic orbitals. Oscillator strength appears to be conserved vs temperature.

Carbonate radical formation in radiolysis of sodium carbonate and bicarbonate solutions up to 250 degrees C and the mechanism of its second order decay.

Pulse radiolysis experiments published several years ago (J. Phys. Chem. A, 2002, 106, 2430) raised the possibility that the carbonate radical formed from reaction of *OH radicals with either HCO(3)(-) or CO(3)(2-) might actually exist predominantly as a dimer form, for example, *(CO(3))(2)(3-). In this work we re-examine the data upon which this suggestion was based and find that the original data analysis is flawed. A major omission of the original analysis is the recombination reaction *OH + *CO(3)(-) --> HOOCO(2)(-). Upon reanalysis of the published data for sodium bicarbonate solutions and analysis of new transient absorption data we are able to establish the rate constant for this reaction up to 250 degrees C. The mechanism for the second-order self-recombination of the carbonate radical has never been convincingly demonstrated. From a combination of literature data and new transient absorption experiments in the 1-400 ms regime, we are able to show that the mechanism involves pre-equilibrium formation of a C(2)O(6)(2-) dimer, which dissociates to CO(2) and peroxymonocarbonate anion: *CO3(-)+*CO3(-)<-->C2O6(2-)-->CO2+O2COO(2-) *CO3(-) reacts with the product peroxymonocarbonate anion, producing a peroxymonocarbonate radical *O2COO(-), which can also recombine with the carbonate radical: *CO3(-)+CO4(2-)-->*CO4(-)+CO3(2-) *CO3(-)+CO4(-)-->C2O7(2-).

Pulsed electron beam water radiolysis for submicrosecond hydroxyl radical protein footprinting.

Hydroxyl radical footprinting is a valuable technique for studying protein structure, but care must be taken to ensure that the protein does not unfold during the labeling process due to oxidative damage. Footprinting methods based on submicrosecond laser photolysis of peroxide that complete the labeling process faster than the protein can unfold have been recently described; however, the mere presence of large amounts of hydrogen peroxide can also cause uncontrolled oxidation and minor conformational changes. We have developed a novel method for submicrosecond hydroxyl radical protein footprinting using a pulsed electron beam from a 2 MeV Van de Graaff electron accelerator to generate a high concentration of hydroxyl radicals by radiolysis of water. The amount of oxidation can be controlled by buffer composition, pulsewidth, dose, and dissolved nitrous oxide gas in the sample. Our results with ubiquitin and beta-lactoglobulin A demonstrate that one submicrosecond electron beam pulse produces extensive protein surface modifications. Highly reactive residues that are buried within the protein structure are not oxidized, indicating that the protein retains its folded structure during the labeling process. Time-resolved spectroscopy indicates that the major part of protein oxidation is complete in a time scale shorter than that of large scale protein motions.

Neutron and beta/gamma radiolysis of water up to supercritical conditions. 1. beta/gamma yields for H(2), H(.) atom, and hydrated electron.

Yields for H2, H(.) atom, and hydrated electron production in beta/gamma radiolysis of water have been measured from room temperature up to 400 degrees C on a 250 bar isobar, and also as a function of pressure (density) at 380 and 400 degrees C. Radiolysis was carried out using a beam of 2-3 MeV electrons from a van de Graaff accelerator, and detection was by mass spectrometer analysis of gases sparged from the irradiated water. N2O was used as a specific scavenger for hydrated electrons giving N2 as product. Ethanol-d(6) was used to scavenge H(.) atoms, giving HD as a stable product. It is found that the hydrated electron yield decreases and the H(.) atom yield increases dramatically at lower densities in supercritical water, and the overall escape yield increases. The yield of molecular H2 increases with temperature and does not tend toward zero at low density, indicating that it is formed promptly rather than in spur recombination. A minimum in both the radical and H2 yields is observed around 0.4 kg/dm(3) density in supercritical water.