Time-resolved Chemical Imaging with infrared Lasers
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Transcript of 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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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
Extract Photo-recombination cross sections from HHG— based on results from TDSE
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4-cycle pulse
Electron wave Packets “derived” from HHG
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Photoionization crs derived from HHG by comparing Ar vs H
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Model for molecules
),()( ),(),()(~),( kii ekeWNdw
W: Returning electron wave-packet
σ: Photorecombination cross section
θ: Alignment angle (for molecule)
k: Electron momentum, k2/2=ω-Ip
W is largely independent of target for targets with similar Ip
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Cooper minimum
Different lasers are usedPhoto-recombination can be extracted with high accuracy!
PhaseCross section
Cooper minimum
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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
)()()()(
SFA
PWA
exactSFASW SS
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800 nm, 10 fs (FWHM), 2x1014 W/cm2
Discrepancy by 2-3 orders of magnitude here
Lewenstein model is good here
Improved Lewenstein modelor Scattering-wave Strong-Field Approximation (SW-SFA)
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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
PWA: Plane-wave approx.Exact (with scattering waves)
Fainstein et al
Electron energy (eV)
PWA
Electron energy (eV)
0o
30o
45o
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SW-SFA results
SW-SFA is much better than SFA!
SFA
TDSE for H2+: D. Telnov
3x1014W/cm2, 20-cycle, 800 nm
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Angular dependence of HHG
SW-SFA TDSE (parallel)
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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 R’s
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HHG depends on R’s even for nonaligned molecules
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R’s can be extracted from nonaligned data
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R’s 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