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Few-body quantum dynamics in strong fields From
"simple" single ionisation to exploding molecular
clocks
Bernold Feuerstein, Artem Rudenko, Karl Zrost,
Vitor L. B. de Jesus, Claus Dieter Schröter,
Robert Moshammer and Joachim Ullrich
Max-Planck-Institut für Kernphysik,
Saupfercheckweg 1, 69117 Heidelberg
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Outline

• Experimental set-up

• Single ionisation of atoms

• Multiple ionisation of atoms

• Molecular fragmentation

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Experiment Reaction Microscope
Momentum resolution ?P lt 0.02 a.u.
Ultrashort pulses 6-7 fs
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Reaction Microscope
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Single ionisation of atoms
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Single ionisation of atoms
Keldysh parameter
? gt 1 Multiphoton (Above Threshhold) Ionisation
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Single ionisation of atoms
Keldysh parameter
? lt 1 Tunnel ionisation
Transverse momentum distribution
2-step process
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Ion momentum distribution He, 23fs
? 0.31 0.58
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Ion momentum distribution Ne, 23fs
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Ion momentum distribution Ar, 23fs
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Electron energy spectra Ne, 23 fs

counts
No ponderomotive shifts observed!
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Two-dimensional electron momentum distributions
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Two-dimensional electron momentum distributions
0.25 PW/cm2
He 1.0 PW/cm2
? 0.45
Ne 1.0 PW/cm2
? 0.42
P? a.u.
Ar 1.0 PW/cm2
? 0.36
-1.0 -0.8 0.6 0.4 0.2 0 0.2
0.4 0.6 0.8 1.0
P?? a.u.
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Two-dimensional electron momentum distributions
Ultrashort pulses
No resonance-like structures resolved!
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Single ionisation Conclusions

• Smooth transition from multiphoton to tunneling
  ionisation

• Target dependence near zero momenta
• Minimum for He and Ne, maximum for Ar

• No ponderomotive shifts observed
  resonance-like structures
• Contribution of resonant processes can explain
  the absence
• of ponderomotive shifts

• Rich structures in two-dimensional electron
  momentum spectra

• Multiphoton features of the process are washed
  out
• for a few-cycle pulse

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Double and multiple ionisation of atoms
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Double and multiple ionisation of atoms
Features of strong-field ionisation
1014 1015 W/cm2
E(t) E0 sin(wt)

• Field (tunnel) ionisation

• Recollision

pd (qE0/w)cos(wt) 2q (Up)1/2 cos(wt)

• Drift momentum related to phase

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Mechanisms for strong-field double ionisation
sequential
nonsequential
recollision (e,2e)
recollision-excitation subsequent tunnelling
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He, Ne, Ar strong-field double ionisation
sequential
V. B. L. de Jesus et al. JPB 37 (2004) L161
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Influence of the atomic structure a simple model
Cross sections for
Initial phase average
V. B. L. de Jesus et al. JPB 37 (2004) L161
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Multiple ionisation
Sequential
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Sequential
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Multiple ionisation of Ar ion yield ratio
Y4 / Y3
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Mechanisms for strong-field multiple ionisation
(2n ?2.52(m ?1))(Up)1/2
Feuerstein et al. JPB 33 (2000) L823
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? life time (pulse duration)
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Lifetime of excited states? - Pulse duration
dependence
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Multiple ionisation of Ar ion yield ratio
23 fs
6-7 fs
Y4 / Y3
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Double and multiple ionisation Conclusions

• First systematic study of ion momentum
  distributions for strong-field
• double and multiple ionisation of noble gases
  (He, Ne, Ar)

• Core excitation during recollision dominates
  nonsequential double
• ionisation for He and Ar

• Recollision (e,ne) is the dominating mechanism
  for creation of
• Ne2, Ne3 and Ne4 ions (double-hump
  structure)

• Multiple ionisation mechanism for argon is more
  complex
• most likely combined sequential and
  nonsequential processes
• enhanced double-hump structure for ultrashort
  pulses
• indicates importance of core excitations

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Molecular fragmentation
Confusion reigns when Sir James Dwighton is
murdered... Luckily, his broken clock tells the
tale -- or does it?
What do broken (Coulomb-exploded) molecular
clocks tell us? Does confusion reign also here?
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Hydrogen molecular potential curves in a strong
laser field
Fragmentation channels
Single ionisation (SI) H2 ? H2 e-
2ppu
H H
Dissociation H2 ? H H0
H H(2p)

• 1- and 2-photon net absorption

• recollision - excitation

2psu
H H(1s)
Double ionisation (Coulomb explosion, CE) H2 ?
H H e-
1w
Dressed states
2w
1ssg
3w
H2

• Sequential (field) double ionisation (SDI)
• enhanced _at_ R 5 10 a.u. (CREI)

H(1s) H(1s)

• Recollision
• - e,2e

H2
- excitation with subsequent field ionisation
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H2 (D2) as a molecular clock
Principle of a molecular clock based on the
propagation of electronic (recollision) and
nuclear wavepacktes
H. Niikura et al. Nature 417 (2002) 917, 421
(2003) 826
But
works only if the fragmentation path can be
identified
Recent progress
A.S. Alnaser et al. PRL 91 (2003) 163002
Experiment coincident detection of emitted
protons
Theory comprehensive model including
recollision-excitation and ionisation
X.M. Tong, Z.X. Zhao and C.D. Lin PRL 91 (2003)
233203 PRA 68 (2003) 043412
? recollision-excitation is the dominating
mechanism for both dissociation and double
ionisation channels producing high-energy
fragments
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From short to ultrashort pulses non-coincident
spectra
H2
Dissociation
2 w
1 w
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From short to ultrashort pulses coincident
spectra
23 fs
Due to
momentum conservation true
coincidence events lie near the P1 - P2
diagonal!
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6 fs
-20 0
20 40
40 20 0 -20 -40
Sequential ionisation?
P2 a.u.
counts (log scale)
P1 a.u.
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Molecular fragmentation Conclusions

• Dynamics of the H2 fragmentation depends
  drastically
• on the pulse duration

• Charge-resonant enhanced ionisation (CREI) is
  suppressed for 6 fs

• Coincidence measurements provide a method to
  distinguish
• dissociation and double ionisation
  contributions within the
• same energy range

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Open questions and outlook

• Single ionisation
• More detailed measurements with well-controlled
  few-cycle pulses
• Other targets, broader range of ?, molecules,
  atomic hydrogen
• Ultrashort pulses absolute phase effects

• Multiple ionisation
• Towards higher and lower intensities (transition
  to sequential regime /
• threshold effects fpr recollision
• More on correlated electron dynamics
• Ultrashort pulses absolute phase effects

• Molecular fragmentation
• Origin of low-energy Coulomb explosion peaks
• dependence on temporal pulse shape
• Branching ratios for different fragmentation
  channels
• Electron dynamics breakdown of
  Born-Oppenheimer approximation?

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Acknowledgment
Claus Dieter Schröter
Robert Moshammer (Head of the group)
Artem Rudenko
Karl Zrost
Vitor Luiz Bastos de Jesus