FEL Hamburg 2008 1

Time-Dependent Calculations of Strong Field Ionization and High Harmonics Generation

Anatoli KheifetsAnatoli Kheifets and Igor Ivanov

Interplay between strong field effectsand many-electron correlations

Igor Bray

Centre for Antimatter-Matter Studies

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Motivation

• Strong Field Ionization – New XUV sources

• FEL’s (FLASH, SCSS, LCLS etc)• High harmonic generation

– Fundamental importance• Interplay of correlations and strong field

• High harmonic generation– Race for the “table-top synchrotron”

• Coherent, compact, intense, tunable, source of XUV radiation• Neutral gases or ionized plasmas of noble gas atoms

• Attosecond control– Steering of coherent wave packets.– Time-resolved atomic physics

FEL Hamburg 2008 3

Number of cycles

One-electron targetsLi, K, Rb

Time-DependentPhotoionization(,2e), (2,2e)

HG, photoionziation

(n, ’), (n, e)

∞

1

10

100

Coherent control

HHG (N, ’)

Two-electron Targets He, H─

Many-electron Targets Ne, Ar, Xe

Time-dependent theory

road map

Target complexity

FEL

Lasers

XUVbursts

Time-IndependentPhotoionization(,2e), (2,2e)

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• Theoretical model

– Time-dependent Schrödinger equation• Basis based integration vs coordinate space integration

– Field-free atomic states• One-electron HF states

• Two-electron CCC states

– Discretization of continuum• Pseudostates vs continuum states

Outline

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• Applications

– Two-photon double ionization of He• Total ionization cross-section

– DC field assisted double ionization of He and H-

• Field stabilization• DC tunneling ionization

– Four-photon ionization of Li• MOT based experiment• Photoelectron momentum distribution

– Harmonics generation from Li/Li*, K Rb• Ground state vs excited state• Resonant enhancement of HG

Outline

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Theoretical Model

Field on:

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Theoretical Model

Field off:

CCC expansion for two-electron continuum

Coulomb wavePseudostate

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Theoretical Model

Harmonics Generation:

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3.5 x 1014 W/cm2

Integrated TPDI Cross-section of He

2 or

ders

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Integrated TPDI Cross-section of HeTheory vs. experiment:

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F=0F=0.01au

F=0.02au

F=0.03auω=90eV

ω=85eV

He

DC field assisted double ionization of He

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ω=15 eV

H-

DC field assisted double ionization of H-

DC tunneling

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Four-photon ionization of Li

MOT experiment

MOT-transition

4-photon ionization from 2s3-photon ionization from 2p

Jochen Steinmann, Max-Planck-Institut für Kernphysik, Heidelberg, ISIAC 2007

25fs @795nm

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Tunneling-ionization- high intensity- classical field picture- field ionization in quasistatic laser field

Multiphoton-ionization-low intensity, E-field of laser weak compared to atomic field- perturbative regime

IP

ground state

E el

Vs=Ub

Over-the-barrier-ionization- highest intensities- complete suppression of the Coulomb-barrier of bound state-Atomic system dominated by external field

2IP4 I

11

Adiabaticity parameter (Keldysh 1964)

Regimes of strong field ionization

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Lithium atom in intense laser field

Jochen Steinmann, Max-Planck-Institut für Kernphysik, Heidelberg, ISIAC 2007

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Calculation:Li(2s), 25 fs, 800 nm, 3x1011 W/cm2

Experiment:Li(2s), 30 fs, 775 nm, 150 mW

Photoelectron momentum distribution

Momentum space multi-photon

rings:

Px2+Pz

2=N ─ IP, N≥4 for Li 2s

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Scaled approach to HHG:Ncut-off=(Ip+3.17Up)/» 1

= (Ip/Up)1/2≈ 1

3.5 m 0.5 TW/cm2

0.78 m 100 W/cm2

From noble gases in NIR

To alkaline metals in MIR

Resonant Enhancement of Harmonics Generation

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2s @ 8x1011W/cm2

Ncut-off ≈ 24

2p @ 2x1011W/cm2

Ncut-off ≈ 12

Harmonic spectrum of Li @ 330 fs, 3500 nm

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Harmonic spectrum of Li @ 330 fs, 3500 nm

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Harmonic spectrum of K @ 150 fs, 1750 nm

5-photon 4s-4f transition (0.7 eV)

3d

9th5th

7th

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Harmonic spectrum of Rb @ 200 fs, 2000 nm

4-photon 5s-4d transition (0.6 eV)

3d

9th

5th 7th

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Conclusion

• Basis-based solution of TDSE applied to• Photoionization

– (2,2e) on He

– (DC,2e) on He and H ─

– (4,e) on Li

• HHG– Li/Li* K. Rb Ncut-off ≈10 (optical regime)

• Future plans– Very High Harmonics Generation

• Ncut-off ≈100 XUV regime

• Laser cooled He* and Ne*

– Uttosecond Pulse Generation / Attosecond control– Two-color / Pump-Probe