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Few-body quantum dynamics in strong fields:

Max-Planck-Institut für Kernphysik. 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.

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Few-body quantum dynamics in strong fields:

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  1. Max-Planck-Institut für Kernphysik 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

  2. Outline • Experimental set-up • Single ionisation of atoms • Multiple ionisation of atoms • Molecular fragmentation

  3. MCP Ions Z(ToF)  Y(jet direction) X(laser beam propagation) B E, Laser Helmholtz coils Spherical mirror Supersonic gas jet Spectrometer: • Background pressure 2x10-11 mbar • Target density 108-109 cm-1 • Extraction voltage 1 V/cm; • Ion-electron coincidence MCP electrons Laser (Ti: Sapphire): Photon energy 1.55 eV (l = 800 nm), pulse length 23 fs, Intensity I  1014-1016 W/cm2, repetition rate 3 kHz Experiment: „Reaction Microscope“ Momentum resolution: ΔP|| < 0.02 a.u. Ultrashort pulses: 6-7 fs

  4. Reaction Microscope

  5. Single ionisation of atoms

  6. Ek Ip - ionisation potential Up = I/42 - ponderomotive potential Ek Ionisation rate: Ek= N ħ  - Ip* Electron energy: ħ Due to AC Stark shiftIp*  Ip + Up Ip* Resonant Nonresonant Single ionisation of atoms Keldysh parameter  > 1: Multiphoton (Above Threshhold) Ionisation

  7. Ip - ionisation potential Up = I/42 - ponderomotive potential 5000 4000 3000 Minimum at ultra–low energies: counts 2000 Ne,1015 W/cm2 8000  = 0.42 6000 1000 counts 4000 1) Tunneling through the lowered barrier 0 2) Classical oscillating motion in the laser field -1,0 -0,5 0,0 0,5 1,0 Coulomb interaction with the parent ion? 2000 P, a.u. 0 K. Dimitriou et al, TU Vienna -3 -2 -1 0 1 2 3 Pion||, [a.u] Single ionisation of atoms Keldysh parameter  < 1: Tunnel ionisation Transverse momentum distribution 2-step process:

  8. 20000 2.1 PW/cm2 15000 1.0 PW/cm2 counts 1.5 PW/cm2 10000 5000 0.6 PW/cm2 0 -3 -2 -1 0 1 2 3 P||, [a.u] Ion momentum distribution:He, 23fs : 0.31 – 0.58

  9. : 0.3 – 0.67 7000 2.0 PW/cm2 1.5 PW/cm2 5000 counts 1.0 PW/cm2 0.6 PW/cm2 3000 0.4 PW/cm2 1000 -3 -2 -1 0 1 2 3 P||, [a.u] Ion momentum distribution:Ne, 23fs

  10. : 0.29 – 1.1 8000 1.5 PW/cm2 6000 0.8 PW/cm2 counts 0.5 PW/cm2 4000 0.25 PW/cm2 2000 0.12 PW/cm2 0 -2 -1 0 1 2 P||, [a.u] Ion momentum distribution:Ar, 23fs

  11. 1.5 PW/cm2 6000 4000 1.0 PW/cm2 0.6 PW/cm2 2000 0.4 PW/cm2 0 0 2 4 6 8 10 12 14 16 18 20 Electron energy [eV] Electron energy spectra:Ne, 23 fs counts No ponderomotive shifts observed!

  12. Z(ToF) 0.6 0.6 0.6  = 0.58 He 0.6 PW/cm2   Y(jet direction) 0.4 0.4 0.4 0.2 0.2 0.2 X(laser beam propagation) 0 0 0 P|| = Pz - momentum along laser polarisation Ne 0.4 PW/cm2  = 0.67 P [a.u.] P = (Px2 + Py2)1/2 Ar 0.25 PW/cm2  = 0.73 Area where the spectrometer has no resolution in the transverse direction -1.0 -0.8 –0.6 –0.4 –0.2 0 0.2 0.4 0.6 0.8 1.0 P [a.u.] Two-dimensional electron momentum distributions

  13. Z(ToF) 0.6 0.6 0.6   Y(jet direction) 0.4 0.4 0.4 0.2 0.2 0.2 X(laser beam propagation) 0 0 0 P|| = Pz - momentum along laser polarisation P = (Px2 + Py2)1/2 Area where the spectrometer has no resolution in the transverse direction Two-dimensional electron momentum distributions 0.25 PW/cm2  = 0.45 He 1.0 PW/cm2 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.]

  14. 0.6 0.6 0.6 23 fs Ne 1.0 PW/cm2  0.4 0.4 0.4 0.2 0.2 0.2 0 0 0 23 fs Ne 0.4 PW/cm2 P [a.u.] 6-7 fs Ne 0.4 PW/cm2 -1.0 -0.8 –0.6 –0.4 –0.2 0 0.2 0.4 0.6 0.8 1.0 P [a.u.] Two-dimensional electron momentum distributions Ultrashort pulses No resonance-like structures resolved!

  15. 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

  16. Double and multiple ionisation of atoms

  17. wt Double and multiple ionisation of atoms Features of strong-field ionisation 1014 – 1015 W/cm2 E(t) = E0sin(wt) • Field (tunnel) ionisation • Recollision • Drift momentum related to phase pd = (qE0/w)cos(wt) = 2q (Up)1/2 cos(wt)

  18. pion|| 2q(Up)1/2 pion|| 0 pion|| Mechanisms for strong-field double ionisation sequential nonsequential recollision (e,2e) recollision-excitation subsequent tunnelling

  19. 4(Up)1/2 He, Ne, Ar: strong-field double ionisation sequential V. B. L. de Jesus et al. JPB 37 (2004) L161

  20. Ionization: Lotz-type formula Excitation: Van Regemorter formula 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

  21. Ne4+ Ne3+ 23 fs Ne2+ 4(Up)1/2 6(Up)1/2 8(Up)1/2 1.5 PW/cm2 P / a.u. 2.0 PW/cm2 P / a.u. P / a.u. P / a.u. Multiple ionisation Sequential

  22. 23 fs Ar3+ Ar4+ 0.3 PW/cm2 6(Up)1/2 1.2 PW/cm2 1.2 PW/cm2 8(Up)1/2 0.5 PW/cm2 1.5 PW/cm2 1.5 PW/cm2 0.8 PW/cm2 2.0 PW/cm2 2.0 PW/cm2 P / a.u. P / a.u. P / a.u. Sequential

  23. Y2+ / Y+ Y3+ / Y2+ Y3+ / Y+ Y4+ / Y2+ Y4+ / Y+ Multiple ionisation of Ar: ion yield ratio Y4+ / Y3+

  24. nonsequential Drift momentum 2n(Up)1/2 Ne  Ne+ Nen+ Field ionisation Recollision (e,ne) sequential / nonsequential Ar  Arm+ Arn+ Field ionisation Recollision (e,(nm+1)e) Mechanisms for strong-field multiple ionisation (2n 2.52(m 1))(Up)1/2 Feuerstein et al. JPB 33 (2000) L823

  25. Ar  Ar2+ Ar4+ Ar  Ar2+ Ar3+ Ar  Ar3+ Ar4+ 0.3 PW/cm2 6(Up)1/2 1.2 PW/cm2 1.2 PW/cm2 8(Up)1/2 0.5 PW/cm2 1.5 PW/cm2 1.5 PW/cm2 0.8 PW/cm2 2.0 PW/cm2 2.0 PW/cm2 Role of excited states? Sequential Ar  Arm+ Arm+* Arn+ Field ionisation Field ionisation Recollision excitation P / a.u. P / a.u. P / a.u.  life time (pulse duration)

  26. Ar2+ 0.5 PW/cm2 23 fs 6-7 fs P / a.u. Ar3+ 1.2 PW/cm2 Ar4+ 1.2 PW/cm2 P / a.u. P / a.u. Lifetime of excited states? - Pulse duration dependence

  27. Y2+ / Y+ Y3+ / Y2+ Y3+ / Y+ Y4+ / Y2+ Y4+ / Y+ Multiple ionisation of Ar: ion yield ratio 23 fs 6-7 fs Y4+ / Y3+

  28. 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 excitationduring 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

  29. 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?

  30. 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 @ R = 5 – 10 a.u. (CREI) H(1s) + H(1s) • Recollision • - e,2e H2 - excitation with subsequent field ionisation

  31. 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

  32. CE CE 25 fs 0.2 PW/cm2 0.3 PW/cm2 0.5 PW/cm2 10 fs 0.5 PW/cm2 counts 6 fs 0.2 PW/cm2 0.5 PW/cm2 0.8 W/cm2 Time-of–flight [ns] From short to ultrashort pulses: non-coincident spectra Dissociation H2+ 2 w 1 w

  33. Recollision CREI counts (log scale) -20 0 20 40 40 20 0 -20 -40 P2 || [a.u.] P1 || [a.u.] From short to ultrashort pulses: coincident spectra 23 fs Due to momentum conservation true coincidence events lie near the P1 ||= - P2 ||diagonal!

  34. Recollision regions, where false coincidences can not be excluded 6 fs -20 0 20 40 40 20 0 -20 -40 Sequential ionisation? P2 || [a.u.] counts (log scale) P1 || [a.u.]

  35. 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

  36. 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?

  37. Max-Planck-Institut für Kernphysik Acknowledgment Claus Dieter Schröter Robert Moshammer (Head of the group) Artem Rudenko Karl Zrost Vitor Luiz Bastos de Jesus

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