Transcript Slide 1

Above-threshold-ionization (ATI)
of atoms
in an intense, few-cycle laser pulse
Marlene Wickenhauser
Collaborators:
Xiao Min Tong and Chii Dong Lin
Schematic picture
ionization
of electron
atom
laser pulse
Ar
 = 10 fs
Calculation:
 = 400 - 800 nm
• Electron spectra
I ~ 2 x 1014 W/cm2
•2D momentum distribution
Motivation
Recent experiments: MPI Heidelberg, KSU
e0.4
5x

1014 W/cm2
E
0.2
800 nm
atom
0
P (a.u.)
A. Rudenko et al. J. Phys. B 37 L407 (2004)
-1.0
-0.5
0
P|| (a.u.)
0.5
1.0
Low energy spectra:
-lots of structure
-even in tunneling
regime
Introduction
Multiphoton ionization
Tunneling ionization
Above-threshold-ionization (ATI)
 1
Keldysh parameter:
 
Ip
2U p
 1
Typical ATI spectrum
Absorbed Photons
18
20
22
0.2
ħω
ATI peaks
0.1
ponderomotive
energy
16
0
Ionization
potential
Electrons/eV
E n  ( I p  U p )
14
0.3
ħω
P. H. Bucksbaum PRA 37 3615 (1988)
12
0
5
10
15
20
Energy (eV)
25
Helium I= 2.3 x 1014 W/cm2

=8
ps,
532 nm
30
Outline
1. Theory
2. Energy Spectra
3. 2D electron-momentum distribution
4. Projection on parallel momentum
Theory
1) Numerical solution of TDSE
-Single active electron approximation
H (t )  T  Veff (r)  E(t )  r
-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
Energy spectrum
Argon
I ~ 1.7 x 1014 W/cm2
 = 400 nm
10 fs
SFA
TDSE
Energy (eV)
Electron spectra from a short pulse
No well defined
frequency & intensity
time
0.5
0
P (arb. unit)
1
E n  ( I p  U p )
0
2
4
Energy (eV)
6
8
Redefined Volkov phase

Laser-dressed energy:
p2
A(t ')2
  dt '(  p A(t ') 
)
2
2
t
electron-field
coupling
energy shift:
average=Up
-No subpeaks
-ATI peaks shifted
Energy (eV)
Argon
0.3
I ~ 1.7 x 1014 W/cm2
 = 400 nm
10 fs
0
P (a.u.)
0.6
2D momentum Distribution - SFA
-0.8
-0.4
0
P|| (a.u.)
0.4
0.8
0
2
•ATI peaks
•Subpeaks
•Parity
•Angular momentum
4
6
Energy (eV)
8
0.6
Comparison with TDSE
0
0.6
0.3
TDSE
0
P (a.u.)
0.3
SFA
-0.8
0.4
0
0.4
P|| (a.u.)
0.8
Intensity dependence Ar 400 nm
Channel closing:
Ip + Up threshold
6 ħω
Ar: Ip = 15.76 eV
1.7 x 1014 W/cm2: Up=
2.55 eV
Ip
1.7 x 1014 W/cm2
3.2 x 1014 W/cm2
0 x 1014
0.4W/cm0.8
2
2.4
3.9 x 1014 W/cm2
0.6 0
0.3
0.6
intensity
-0.8
0.4
0.3
P|| (a.u.)
0
P (a.u.)
6 ħω
-0.8
-0.4
0
0.4
0.8 -0.8
-0.4
0
0.4
0.8
Momentum projection
e-

Ne: 25 fs, 800 nm, I = 4 x 1014 W/cm2
2
4
6
8
atom
Interesting points:
0
P (arb. unit)
10
Rudenko et al. J. Phys. B 37 L407 (2004)
-1.0
-0.5
0
P|| (a.u.)

~ 0.6
0.5
1.0
•
Dip in contrast to ADK
•
Neon, Helium: dip
Argon: peak
Explanation for dip in literature
1. Rescattering:
J. Chen et al, PRA 63 11404(R) (2000)
2. Coulomb potential:
K. Dimitriou et al, PRA 70 061401(R) (2004)
3. Position of ATI peaks: (in tunneling regime)
F. H. M. Faisal et al, J. Phys. B 38 L223 (2005)
4. Freeman Resonance:
A. Rudenko et al, J. Phys. B 37 L407 (2004)
Argon
dip
400 nm
peak
Multiphoton
I = 1.7 x 1014 W/cm2
0.6
I = 3.9 x 1014 W/cm2
 ~ 1.76
 ~ 1.13
0
0
P|| (a.u.)
0.5
0.5
1
10
0
0.3
0.3
0.6
10 fs
-1
-0.5
0
0.5
P|| (a.u.)
1
-1
-0.5
0
P|| (a.u.)
0.5
1
Argon
dip
800 nm
Tunneling
peak
I = 1.8 x 1014 W/cm2
 ~ 0.85
0.3
0.6
 ~ 0.89
0
0
0.5
0.5
1
0
1 0
0.3
0.6
I = 1.65 x 1014 W/cm2
10 fs
-1
-0.5
0
P|| (a.u.)
0.5
1
-1
-0.5
0
P|| (a.u.)
0.5
1
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