Above-threshold-ionization (ATI) of atoms in an intense, few-cycle laser pulse

1
Above-threshold-ionization (ATI) of atoms in an
intense, few-cycle laser pulse
Marlene Wickenhauser Collaborators Xiao Min Tong
and Chii Dong Lin
2
Schematic picture
ionization of electron
atom
Ar
laser pulse
Calculation
10 fs 400 - 800 nm

• Electron spectra
• 2D momentum distribution

I 2 x 1014 W/cm2
3
Motivation
Recent experiments MPI
Heidelberg, KSU
e-
5 x 1014 W/cm2
800 nm
Low energy spectra -lots of structure
-even in tunneling regime
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Introduction
Multiphoton ionization
Tunneling ionization
Above-threshold-ionization (ATI)
Keldysh parameter
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Typical ATI spectrum
Absorbed Photons
P. H. Bucksbaum PRA 37 3615 (1988)
12 14 16 18 20
22
h?
h?
ATI peaks
0 0.1 0.2 0.3
Electrons/eV
ponderomotive energy
Ionization potential
0 5 10 15 20
25 30
Energy (eV)
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Outline

1. Theory
2. Energy Spectra
3. 2D electron-momentum distribution
4. Projection on parallel momentum

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Theory
1) Numerical solution of TDSE
-Single active electron approximation
-grid -Split operator method for time propagation
2) Strong field approximation (SFA)
Neglect -Coulomb field on ionized electrons
-Depletion of ground state
-Other bound states
Dipole transition moment
Laser-dressed energy
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Energy spectrum
SFA
TDSE
Energy (eV)
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Electron spectra from a short pulse
No well defined frequency intensity
time
0 0.5 1
P (arb. unit)
0 2 4
6 8
Energy (eV)
10
Redefined Volkov phase
Laser-dressed energy
energy shift averageUp
electron-field coupling
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2D momentum Distribution - SFA
P (a.u.)
0 0.3 0.6
-0.8 -0.4 0 0.4
0.8
P (a.u.)

• ATI peaks
• Subpeaks
• Parity
• Angular momentum

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Comparison with TDSE
0 0.3 0.6
SFA
0 0.3 0.6
TDSE
-0.8 0.4 0 0.4 0.8
P (a.u.)
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Intensity dependence Ar 400 nm
Ip Up
threshold
Channel closing
6 h?
Ar Ip 15.76 eV
1.7 x 1014 W/cm2 Up 2.55 eV
Ip
6 h?
intensity
1.7 x 1014 W/cm2
3.2 x 1014 W/cm2
0 0.3 0.6
-0.8 0.4 0 0.4 0.8
3.9 x 1014 W/cm2
2.4 x 1014 W/cm2
0 0.3 0.6
P (a.u.)
-0.8 -0.4 0 0.4 0.8
-0.8 -0.4 0 0.4 0.8
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Momentum projection
e-
Ne 25 fs, 800 nm, I 4 x 1014 W/cm2
Rudenko et al. J. Phys. B 37 L407 (2004)
atom
0.6
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Explanation for dip in literature

• Rescattering
  J.
  Chen et al, PRA 63 11404(R) (2000)
• Coulomb potential
  K. Dimitriou et al, PRA 70 061401(R)
  (2004)
• Position of ATI peaks (in tunneling regime)
  F. H. M. Faisal et al, J.
  Phys. B 38 L223 (2005)
• Freeman Resonance
  A. Rudenko et al, J. Phys. B 37 L407
  (2004)

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Argon 400 nm 10 fs
Multiphoton
I 1.7 x 1014 W/cm2
I 3.9 x 1014 W/cm2
g 1.76
g 1.13
0 0.3 0.6
0 0.3 0.6
0 0.5 1
0 0.5 1
P (a.u.)
-1 -0.5 0 0.5 1
-1 -0.5 0 0.5 1
P (a.u.)
P (a.u.)
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Argon 800 nm 10 fs
dip
peak
Tunneling
I 1.65 x 1014 W/cm2
I 1.8 x 1014 W/cm2
g 0.89
g 0.85
0 0.3 0.6
0 0.3 0.6
0 0.5 1
0 0.5 1
-1 -0.5 0 0.5 1
-1 -0.5 0 0.5 1
P (a.u.)
P (a.u.)
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Conclusion

• Subpeaks in ATI spectra from short pulses
• Explained structures in 2D momentum distribution
• Dip in parallel momentum
• -Tunneling regime ATI peaks
• -Multiphoton regime Parity of first ATI peak
• -Coulomb effect not relevant
• -Longer pulses Freeman resonances