Ultrafast Dynamics of Water

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Ultrafast Dynamics of Water
9 eV
60 fs
540 eV, 2 ps

• Erzsi Szilagyi

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Why study water?
http//www.unitedhealthgroup.com
http//www.biosci.ohio-state.edu/prg/protein1.gif
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Why are we STILL studying water?

• Water is an anomaly!
• Examples of anomalous behavior
• Ice floats liquid denser than solid
• Large specific heat

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Anomaly Ice Floats

• Ice floats implying liquid denser than solid
• During melting, H-bonds break and bend and
  structure undergoes partial collapse.

Ice Hexagonal Crystal Structure
Unit Cell Tetrahedral
http//www.lsbu.ac.uk/water
D. Nordlund, Thesis, 2004
Low packing efficiency
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Anomaly Large Specific Heat

• Heat water
• Increased movement of water molecules
• Breaking of H-bonds
• ? Increase in kinetic energy of water.

Illustrated in higher boiling point than
extrapolated.
http//www.lsbu.ac.uk/water/explan2.htmlmelt
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Why is water an anomaly?

• Hydrogen bonding
• Versatile
• Many angles and lengths

Bond Strengths Covalent 5 eV Hydrogen 0.5
eV van der Waals 0.1 eV
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Character of Hydrogen Bond

• Attraction between O-HO
• Electrostatic
• Interaction b/w molecules
• Cooperative
• Results in water molecules being closer

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Water Dynamics
1 ps - Hydrogen bonds break and reform in liquid
H20 due to atomic motion
100 ps Water molecule surrounded by completely
different molecules
10 fs - O-H bond stretch
100 as
10 fs
1 fs
100 fs
1ps
10ps
100ps
1ms
1ns
10ns
100ns
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How do we investigate water?

• Infrared spectroscopyVibration ? Structure
• DiffractionRadial distribution function ? Angle
  averaged distances
• Molecular dynamics ? Microscopic picture
• X-ray Absorption Spectroscopy ? Electronic
  structure

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X-Ray Absorption Spectroscopy
For O K-edge, nonzero for orbitals with
p-character.
D. Nordlund, Thesis, 2004
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X-Ray Absorption Spectroscopy
Molecular Orbital Diagram of Water
XA Spectrum of Water
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X-Ray Absorption Spectroscopy
Intensitydensity of states with p-character
Structurefrom convolution of absorbing orbitals
EnergyE required for absorption from 1s orbital
Indicative of molecular and environmental
electronic structure
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Pump-Probe

• Technique for time-resolved information
• Time resolution limited by pulse duration

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Femtosecond Pump Laser

• FemtoLasers Femtosource Scientific XL 500
• 800nm wavelength
• 500nJ per pulse
• 5.1 MHz repetition rate
• lt60fs pulses
• Synchronizable
• Small footprint!!

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X-Ray ProbeThe Neighborhood Synchrotron

• Electrons driven by RF signal
• Buckets 2ns wide
• Pulse width 2 ps
• Independent buckets
• Normally 0.3mA (6E-13 Coulombs), but up to 5mA
  (10E-11 Coulombs)
• Not all buckets filled
• We synchronize to signal from SPEAR3.

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What do the signals look like?
Camshaft Pulse
Laser
X-Rays
2.5 MHz
5.2 MHz
Counts
Multibunch
Time (ns)
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Sample Considerations

• Fewer than 200 ns between pulses
• Need fast samples
• Either recover or replacement between pulses
• 20 µm spot ? 100 m/s sample velocity
• Flow samples in a liquid jet

20 mm
30 mm
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Radiation Chemistry _at_ SPEAR3 Understanding
Ionization Dynamics
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Radiation Chemistry _at_ SPEAR3 Understanding
Ionization Dynamics
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Water Timescales for Radiation Chemistry
300 fs - translational motion of H2O molecules
surrounding a solvated electron
100 fs - localization of electrons in
pre-existing traps
1 ps - Hydrogen bonds break and reform in liquid
H20 due to atomic motion
500 as - time it takes for excited electron to
de-affiliate with its original H2O molecule
ms transient radicals
ps geminate recombinations between hydrated
electrons and H30 or OH-
10 fs - O-H bond stretch
100 as
10 fs
1 fs
100 fs
1ps
10ps
100ps
1ms
1ns
10ns
100ns
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What will we look for?
Energy Scans
Species Present
Fixed delay time
Peaks indicate presence of radiation species
Laffon et al. J Phys Chem, 125, 204714 (2006).
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What will we look for?
Energy Scans
Time Scans
Fixed delay time
Fixed energy
Shows changes at an energy as function of time.
Peaks indicate presence of radiation species
Laffon et al. J Phys Chem, 125, 204714 (2006).
Huse et al. PCCP (2009).
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Probing H-Bond Network _at_ ALS
OH Stretch
Pump the OH Stretch at 3 mm Probe O
K-edgesensitive probe of hydrogen bonding
Estimate temperature change 10K ? Initial
pressure jump of 8 MPa
Huse et al. PCCP (2009).
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Probing H-Bond Network _at_ ALS
Timescale for heating 5 ps after OH-stretch
excitation Timescale for volume expansion (lower
limit) 270 ps
Static Energy Spectrum
Time Spectrum
Pre
Post
Post

• Note difference between pre- and post-edge
  absorption.
• Attributed to elevated internal pressure
• ? pre- post-edge regions probe of internal P

Huse et al. PCCP (2009).
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Summary

• Importance of studying water
• Laser-pump x-ray-probe
• Examples
• 1) Species resulting from ionization
• 2) Hydrogen bonding network in liquid water

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Future Directions

• Actually doing the solvated electron pump-probe
  measurements described here
• LCLS
• Intense, coherent x-ray source
• Extremely short pulses (lt30 fs)
• ? Need new sample from shot-to-shot

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Acknowledgements

• Aaron Lindenberg
• Dennis Nordlund
• Tim Miller
• Lindenberg Lab
• ALS Team