Laserinduced Coulomb explosion imaging of molecular dynamics

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Laser-induced Coulomb explosion imaging of
molecular dynamics

• Introduction
• Structure determination with CEI
• Dynamics studies with CEI

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Coulomb explosion
MeV
100 Å

• Developed in 1979 ANL
• Interaction times 100 attoseconds --- 1
  femtosecond

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Coulomb Explosion Imaging
Accelerator Coulomb Explosion Imaging
MeV
beam
ANL 1979
Ultrathin
foil
Detector
Laser-induced Coulomb Explosion Imaging
Detector
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Time-Resolved Molecular Imaging
The Objective
To observe the dynamics of polyatomic molecules
Possible approaches Spectroscopy Electron
Diffraction X-Ray Diffraction Coulomb Explosion
Imaging
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Dynamics Pump-Probe Spectroscopy
Weak pump pulse
excites molecule
E
Probe
Pump
?t
Delay allows
time evolution
Probe
Pump
?t
Intense probe
E
pulse ionizes
molecule
Molecule explodes
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Exploding pulse duration and nuclear dynamics
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Generation of ultra-short laser pulses
Hollow fiber 1 m, DI 250 µm
Ar 1.05 atm
Ei 300 mJ 800 nm, 40 fs
Eo 150 mJ 800 nm

• Compression with chirped mirror Tpulses lt 8 fs.
• Focusing to 5 microns I 1016 W/cm2

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Experimental apparatus
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Simplest model --- Coulomb explosion of D2
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Timing ionization - Molecular clock

• (1) Ionisation of D2 starts the clock
• (2) Ionisation of D2 stops the clock
• Kinetic energy of D measures delay between 1 and
  2

Molecular clock
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D2 explosion
40 fs 2x1015 W/cm2
Sub-8fs 2x1015 W/cm2
2.66 fs
Theory 5.22 fs
Sub-8 fs double ionization of D2 Is complete in
less than 4 femtosecondes
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Nuclear wave function density ?(R,t)2
D2
Theory 4 fs
Exp. 8fs
Exp. gt40fs
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Diatomic vs triatomic
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Molecular structure reconstruction from 3D momenta
Experiment Piex
Potential U(Xi) - Coulombic or ab initio
Equations of motion dPi/dt -?U(Xi)/?Xi
Initial conditions Xi(t0) Xi0
Solve Xi(t), Pi(t), Pi? Pi(t ? ?)
Iterative optimization vary Xi0 to minimize
?Pi?- Piex
good fit
Structure Xiopt
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Triple coincidence 3D momentum spectroscopy
Uncorrelated
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Kinetic energy --- D2O
lt 8fs
I 5x1015 W/cm2
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Structure of D2O
Angstrom
Angstrom
Signal (u.a.)

Angstrom
q
(degrees)
DOD
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Structure of SO2
Angstrom
Angstrom
Signal (a.u.)

q
(degrees)
Angstrom
OSO
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Structure of SO2 vs angle in respect to laser
polarization polarisation
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Image distortion

• Dynamics before the explosion
• D2O numeric simulation
• Dependence of ionization rate
• on nuclear coordinates
• D2 numeric simulation
• Angle in respect to the laser field ???
• SO2
• Spatial resolution lt 0.3 Å

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Pump-probe spectroscopy --- observing motion of
protons
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Pump-probe spectroscopy - D2
(2) D D
(1) D2
?t
0 75 fs
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Dynamics ofD2

• Dinamics can be measured with sub-5fs time
  resolution
• 3D momemtum spectroscopy with pump-probe would
  allow transient structure determination

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Pump-probe spectroscopy --- SO2
(2) O2 S3 O2
(1) SO2,2,3
?t
0 330 fs
SO2
O
S
SO
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Symmetric vs. asymmetric dissociation

Kinetic energy correlation as function of delay
should indicate the symmetry
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Dynamics of O-O energy correlation
0 fs
30 fs
15 fs
45 fs
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Dt45 fs
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Confirmation of dissociation channel
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Symmetricdissociation
0 fs
15 fs
30 fs
45 fs
60 fs
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Applications

• Study dynamics of internal rearragement in fast
  photochemical reactions
• Another example excited state proton transfer
  (lt 50 fs)
• Understanding and optimizing coherent control of
  photochemical reactions

UV ?
H-CC-H ? H-CC-H2 ? CH CH ?
C CH2
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• F. Légaré - Université de Sherbrooke
• K. Lee, P. Dooley - McMaster University
• D. Villeneuve, P. Corkum NRC Ottawa

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