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Research Opportunities in Radiation-Induced Chemical Dynamics

Scientific Opportunities for Studying Laser Excited Dynamics at the LCLS:. Research Opportunities in Radiation-Induced Chemical Dynamics. David Bartels Notre Dame Radiation Laboratory. Radiation-induced chemistry. X-ray (photoelectric effect) Gamma (Compton scattering)

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Research Opportunities in Radiation-Induced Chemical Dynamics

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  1. Scientific Opportunities for Studying Laser Excited Dynamics at the LCLS: Research Opportunities in Radiation-Induced Chemical Dynamics David Bartels Notre Dame Radiation Laboratory

  2. Radiation-induced chemistry X-ray (photoelectric effect) Gamma (Compton scattering) Electron (coulomb perturbation) Positive Ion (coulomb perturbation) Neutron (=positive recoil ion) medium Energy deposition Products(vs. time) G(X,time) = moles product/energy absorbed

  3. Radiation-induced chemistry The goal is to elucidate the mechanism and kinetics of chemical change induced by ionizing radiation. Can LCLS help achieve this goal?

  4. Electron Radiolysis 5 nm

  5. Absorption Spectrum of Water “The complete optical spectrum of liquid water measured by inelastic x-ray scattering” Hayashi et al., Proc. Nat. Acad. Sci,2000,97, 6264

  6. H2O Time H2O* H2O+ + e- (H + OH) (H3O+ + OH) + (e-)aq 1 ps Nonhomogeneous spur recombination 100-1000ns (e-)aq H3O+ OH H H2 H2O2 G(molecules/100eV) 2.7 2.7 2.9 0.6 0.45 0.65

  7. Stochastic Spur Simulation • Model Requires: • Chemical Mechanism • Reaction rates • Diffusion coefficients • Low energy electron • scattering coefficients • ASSUMPTIONS about • the chemical action of the • low energy electrons

  8. Electron Accelerators for Pulse Radiolysis

  9. Short pulse electron radiolysis

  10. Hydrated Electron Spur Decay Kinetics at 25oC Need to make the connection from subpicosecond to microseconds

  11. The picosecond barrier in electron radiolysis Relativistic electrons speed in sample—  C (speed of light in vacuum) Phase velocity of probe light in sample—  C /η (i.e. refractive index) Walkoff for 0.1cm sample = 0.1cm/3E10cm/s *.5= 1.6ps

  12. Density of excitation in picosecond electron pulse radiolysis Energy deposition by relativistic charged particles in water occurs at rate of 1.9E6 eV/ cm of travel Assume 1nC of charge in pulse = 6E9 electrons  1.1E16eV/cm = .0017J/cm Assume 0.1cm diameter spot = .008 cm^2 • .0017/ .008 = 0.22 J/cc (per nanocoulomb) Assume solvated electron extinction coefficient of 20,000 M-1cm-1, G(e-)= 4e-7moles/J, 0.1cm path  0.176 mOD absorbance **************path length less than 0.1cm is impractical due to S/N limitation****************

  13. What will LCLS do for this problem? Photon energy 800eV 8000eV Repetition rate 120Hz 120Hz

  14. How is the xray energy absorbed? Photoelectric Effect Compton scattering

  15. X ray photon absorption will give: • A deep hole at the origin, probably followed by Auger decay with emission of electron • B) Electron with kinetic energy in the kilovolt range This means the geometrical distribution of transients will be different from electron or gamma excitation, and the corresponding recombination will be different (probably faster).  Another constraint for modeling of track chemistry

  16. OD  4 @ 8000eV

  17. What kind of signal can we expect? Assume 1mm pathlength, 1mm diameter spot .00078 cc 2.2 e12 photons of 8000eV energy  3.2 e-3 J/ pulse Average dose of approximately 4 J/ cc/ pulse Assuming a G-value of 4 e-7 moles/J, the concentration of transient species would be 1.6 e-6 M In a 1mm pathlength, the transient OD for solvated electrons (extinction coefficient=20,000 M-1 cm-1) would be: 2e4 * 1.6e-6 * 0.1 = 3.2 e-3 OD per shot (at 120Hz)

  18. Temperature Jump Average dose of approximately 4 J/ cc/ pulse for the 8000eV xrays Assuming most of the energy goes into heat, DT = 4J/cc *1cal / 4.18J /(1cal/cc/deg) = 1.0 degree C

  19. Optical transient absorption experiment Liquid jet sample Diode array Spectrometer Xray pump Optical probe (e.g. white light)

  20. Femtosecond photolysis experiments

  21. Pump/probe and Pump/bleach/probe in Hydrocarbons Magnetic field effect

  22. Exponential dependence on scavenger concentration not understood

  23. Workshop questions and answers FEL: • Temporal resolution as short as possible, minimum jitter • Time-zero measurement is important • Spectral jitter not very important • Ideal repetition rate >120Hz • FEL photon energy probably 7-8kV 8. Spot size not critical

  24. Workshop questions and answers Optical laser: • Broadest possible tunable probe light range is desirable, > 10E6 photoelectrons/pulse detected. • Duration should be shorter than FEL • For pump/bleach/probe experiments, amplified Ti:sapphire fundamental or second harmonic can be used

  25. Workshop questions and answers Sample environments: Liquid jet surface must be accessible to FEL beam – How? Solid samples need to be refreshed between shots – How?

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