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

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

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

Electron Radiolysis 5 nm

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

H2OH2O H 2 O*H 2 O + + e - (H 3 O + + OH) + (e - ) aq (H + OH) 1 ps Nonhomogeneous spur recombination ns G(molecules/100eV) (e-) aq H 3 O + OH H H 2 H 2 O 2 Time

Model Requires: 1)Chemical Mechanism 2)Reaction rates 3)Diffusion coefficients 4)Low energy electron scattering coefficients 5)ASSUMPTIONS about the chemical action of the low energy electrons Stochastic Spur Simulation

Electron Accelerators for Pulse Radiolysis

Short pulse electron radiolysis

Hydrated Electron Spur Decay Kinetics at 25 o C Need to make the connection from subpicosecond to microseconds

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

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 -1 cm -1, G(e-)= 4e-7moles/J, 0.1cm path  mOD absorbance **************path length less than 0.1cm is impractical due to S/N limitation****************

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

How is the xray energy absorbed? Compton scattering Photoelectric Effect

X ray photon absorption will give: A) 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

OD  8000eV

Assume 1mm pathlength, 1mm diameter spot  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) What kind of signal can we expect?

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

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

Femtosecond photolysis experiments

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

Exponential dependence on scavenger concentration not understood

Workshop questions and answers FEL: 1.Temporal resolution as short as possible, minimum jitter 2.Time-zero measurement is important 3.Spectral jitter not very important 5.Ideal repetition rate >120Hz 6.FEL photon energy probably 7-8kV 8.Spot size not critical

Workshop questions and answers Optical laser: 10.Broadest possible tunable probe light range is desirable, > 10E6 photoelectrons/pulse detected. 11.Duration should be shorter than FEL 12. For pump/bleach/probe experiments, amplified Ti:sapphire fundamental or second harmonic can be used

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?