David M. Bartels Hope College, Holland, Michigan, B.A. (’77) Northwestern University, Evanston, Illinois, Ph.D. ('82) Tel. (574) 631-5561 e-mail: bartels.5@nd.edu Fast Kinetics of Radiation-Initiated Chemistry

Scientific Interests

Fast Kinetics of Free Radical Reactions — Free radicals are generated in virtually all radiation-initiated processes, and are responsible for most of the permanent chemical changes. The recombination reactions are often diffusion limited or nearly so, but also depend on pairing of spin to produce stable singlet products. This gives rise to the fascinating Chemically Induced Dynamic Electron Polarization (CIDEP) phenomenon in their time-resolved EPR spectra, and Chemically Induced Dynamic Nuclear Polarization (CIDNP) in NMR spectra of the recombination products, where some lines appear with negative phase due to population inversions.

Radiation Chemistry and Photochemistry of Water — To ionize water molecules in the gas phase requires at least 12.6 eV of energy, but dissociation of water to produce (H+)_aq, (e-)_aq, and OH radicals can be accomplished in liquid water with 6 eV photons in a photochemical event that is still not well understood. What is the nature of electronically excited liquid water, and how can we explain the escape yields of H atoms, OH radicals, and solvated electrons?

Solvent Effects on Reaction Rates in Supercritical Water — Supercritical water is proposed as the coolant for efficient Generation-IV nuclear reactors, and is the medium for an important advanced oxidation technology for hazardous waste destruction. The properties of water change dramatically in the supercritical region as the water density changes continuously between zero and 1 g/cc. The primary free radicals in water – hydrated electrons, H atoms, and OH radicals – are respectively ionic, hydrophobic, and dipolar, providing opportunity to investigate nearly all possible solvent effects using radiolysis excitation. Many strange effects are being found, such as rate constants that decrease as the temperature is raised.

Recent Accomplishments | Top |

Hydrated Electron Spectrum in Supercooled and Supercritical Water — the intense red absorption spectrum of the hydrated electron shifts strongly to the infrared as the temperature is raised. In supercritical water there is a slight red shift as the density decreases. By integrating the spectra we can estimate the rms size of the electron wavefunction and its kinetic energy at different phase points. This was compared with absolute thermodynamic energies derived for hydrated electron solvation. Surprisingly the rms size of the wavefunction barely changes in supercritical water, even though the density changes a factor of six in this study.

Reactions Between Ions and Hydrophobes in Supercritical Water — At 380 °C, several reactions of hydrated electron with hydrophobic molecules, and of H atom with (OH-)_aq, all show a substantial minimum in reaction rate at a density of 0.45g/cc. Because the reactions are not diffusion limited or species-dependent, we postulate the minimum results from a potential of mean force keeping the strongly solvated ion away from the hydrophobe.

Radiolysis Yields in Supercritical Water — Of tremendous practical importance is the radiolysis escape yield (G value), defined as the number of product molecules divided by the total radiation energy deposited.  Our recent measurements of H atom, solvated electron, and molecular hydrogen yields in supercritical water found a minimum in yields at intermediate density and nearly “gas phase” escape yields at 0.1 g/cc water density.  To explain this we need to postulate that almost no geminate recombination occurs, and that the photophysics of the water molecules change dramatically with density.

Small Free Radical Recombinations in High Temperature Water — Near room temperature, recombination of small free radicals like H and OH are nearly diffusion limited in aqueous solution, i.e. once they meet their reaction is certain.  We have been surprised to learn that above about 200C, “barrierless” reactions involving H and OH are no longer limited by diffusion.  Diffusion becomes so fast that the solvent “caging effect” fails to average over all possible angles of approach, and the reaction rate is limited by a “steric effect”.  The great surprise has been that the rates measured in water, where hydrogen bonding was assumed to be important, are identical to the “high pressure limit” rate in the gas phase.  Water is “merely” a very effective third body for energy transfer.

Selected Publications | Top |

T.W. Marin, K. Takahashi, C.D. Jonah, S.D. Chemerisov and D.M. Bartels
Recombination of the hydrated electron at high temperature and pressure in hydrogenated alkaline water
J. Phys. Chem. A 2007 111, 11540-51 link

I. Janik, D.M. Bartels and C.D. Jonah
Hydroxyl radical self-recombination reaction and absorption spectrum in water up to 350 °C
J. Phys. Chem. A 2007 111 1835-43 link

D. Janik, I. Janik and D.M. Bartels
Neutron and beta/gamma radiolysis of water up to supercritical conditions.  1. Beta/gamma yields for H₂, H atoms, and hydrated electrons
J. Phys. Chem. A 2007 111, 7777-86 link

Y. Du, E. Price and D.M. Bartels
Solvated electron spectrum in supercooled water and ice
Chem. Phys. Lett. 2007 438, 234-7 link

D.M. Bartels, K. Takahashi, J.A. Cline, T.W. Marin and C.D. Jonah
Pulse radiolysis of supercritical water. 3. Spectrum and thermodynamics of the hydrated electron
J. Phys. Chem. A 2005 109 1299-307 link

J. Cline, K. Takahashi, T.W. Marin, C.D. Jonah and D.M. Bartels
Pulse radiolysis of supercritical water. 1. Reactions between hydrophobic and anionic species
J. Phys. Chem. A 2002 106, 12260-9 link

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