Time-resolved Chemical Imaging with infrared Lasers

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Time-resolved Chemical Imaging with infrared
Lasers

• Electron diffraction and X-ray diffraction
  cannot be used for time-resolved imaging at the
  femtoseconds level
• Can use IR lasers to probe molecular structure?
• First needs to identify the role of molecular
  structure in laser-induced phenomena electron
  momentum spectra and HHG
• Retrieve the molecular structure (inverse
  scattering)

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• Tomography of Molecular Orbitals
• HHG from molecules via rescattering/recombination
• HHG depends on the target HOMO orbital
• Retrieve HOMO orbital from HHG via Tomography

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Validity of the plane wave approximation
not adequate for typical returning electrons
PWA Tomographic imaging of Itatani et al
Nature 2004
(HHG)TDSE(WP) (crs)exact (HHG)SFA(WP) (crs)PWA
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Extract Photo-recombination cross sections
from HHG based on results from TDSE
Model HHG (wave packet) x (photo-recombination
cross section) -- Electron wave packet is
determined by the driving laser only --- Compare
two atomic systems with identical ionization
potential Neon vs Scaled atomic hydrogen --
or from strong field approximation
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Electron wave Packets derived from HHG
4-cycle pulse
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Photoionization crs derived from HHG by comparing
Ar vs H
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Model for molecules
W Returning electron wave-packet s
Photorecombination cross section ? Alignment
angle (for molecule) k Electron momentum,
k2/2?-Ip W is largely independent of target for
targets with similar Ip
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Phase
Cross section
Cooper minimum
Cooper minimum
Photo-recombination can be extracted with high
accuracy!
Different lasers are used
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Ne 1064 nm, 10.3 fs (FWHM), 2x1014 W/cm2
Wave-packet from the Lewenstein model is good!
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• Current SFA model not adequate (even for atoms!)
• For molecules, the interference minimum positions
  not correctly predicted by SFA
• Our strategy use the wave-packet from SFA or
  TDSE for system with similar ionization potential

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Improved Lewenstein model or
Scattering-wave Strong-Field Approximation
(SW-SFA)
800 nm, 10 fs (FWHM), 2x1014 W/cm2
Discrepancy by 2-3 orders of magnitude here
Lewenstein model is good here
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Example HHG from H2
Collaborators D. Telnov, Russia (TDSE for
H2) P. Fainstein R. D. Picca, Argentina
(photoionization cross section) M. Lein, Germany
(TDSE for H2, high intensity)
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Photoionization cross section
Exact (with scattering waves) Fainstein et al
PWA Plane-wave approx.
PWA
0o
30o
45o
Electron energy (eV)
Electron energy (eV)
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SW-SFA results
3x1014W/cm2, 20-cycle, 800 nm
SFA
SW-SFA is much better than SFA!
TDSE for H2 D. Telnov
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(No Transcript)
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Angular dependence of HHG
TDSE (parallel)
SW-SFA
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Retrieving molecular structure from HHG spectra
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Retrieving Interatomic distances from HHG for
linear molecules

• We test the method using HHG generated from SFA
• The fitting method is very efficient and requires
  less data alignment and intensity
• effect of isotropic molecules and phase matching
• extract structure from dipole moment deduced from
  HHG

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Dependence of HHG vs interatomic distances
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Variance vs tested range of Rs
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HHG depends on Rs even for nonaligned molecules
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Rs can be extracted from nonaligned data
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Rs can be extracted from the photoionization
cross sections
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other issues

• effect of propagation in the medium (in progress)
• extension to polyatomic molecules
• first test within the SFA model
• efficient codes for calculating dipole matrix
  elements from molecules