Rost

					           mpipks!
                                    Dynamics of clusters
Finite !                          under intense laser pulses
Systems!



                                          Jan Michael Rost
                Max-Planck-Institute for the Physics of Complex Systems, Dresden &
                       Max Planck Advanced Study Group @ CFEL, Hamburg



                     Ulf Saalmann
                                    

                                                         project leader
                     Ionut Georgescu
                                 

                                                                postdoc
                     Christian Gnotdke
                               

                                                                 predoc
                     Alexey Mikaberidze
                              

                                                                 predoc
                     Vitali Averbukh
                                 

                                                          distinguished
                                                                      

                                                             PKS fellow

                     Christian Siedschlag

                     Ranaul Islam

           mpipks!
                             New light
Finite !
Systems!


                                     Lasers:    Synchrotrons:
                             monochromatic      monochromatic
                              single photon     single photon
                          “long” wavelength     “short” wavelength

                              Short & intense   Synchrotrons 2nd & 3rd generation:
                                 laser pulses   monochromatic, more intensity
                           “long” wavelength    single photon
  ‘intelligent light’:           multiphoton    “short” wavelength
  coherent control            high harmonics
  pulse shaping          femtosecond pulses
  laser cooling               Attosecond pulses    Free electron lasers:
  constructive noise               (from intense   VUV and X-ray
                                high harmonics)    Intense pulses
           mpipks!
                             New light - new insight
Finite !
Systems!


      -Inducing and monitoring Lasers:          Synchrotrons:
                                                 -Time resolved
                            monochromatic       monochromatic
       ultrafast                                   X-ray dynamics
                             single photon      single photon
       non-equilibrium dynamics
                         “long” wavelength       -Single molecule
                                                “short” wavelength      imaging
      - transient dissipative
       dynamics             Short & intense     Synchrotrons 2nd & 3rd generation:
                                 laser pulses   monochromatic, more intensity
                           “long” wavelength    single photon
  ‘intelligent light’:           multiphoton    “short” wavelength
  coherent control            high harmonics
  pulse shaping          femtosecond pulses
  laser cooling
  constructive noise     Attosecond pulses      Free electron lasers:
                              (from intense     VUV and X-ray
                           high harmonics)      Intense pulses
                          Why clusters?


  Share atomic (finite size)

  and solid state (atom density ρ=1022/cm3) properties

  Ideal “matter” to learn about light-matter interaction
   with XFEL beams:

   ➜  Can be produced in beams
      (crossed – beam experiments)

   ➜  Probe the high spatial energy density of the light
   Rare gas clusters: XeN
closed shell configurations
     Major energy absorption mechanisms


  “Optical slingshots” (a few very fast electrons)
   (IR 800nm)

  Plasma formation &
   collective resonant absorption (800nm)

  Inverse Bremsstrahlung (IBS)
   (VUV 100nm)

  Electron delocalization (weak absorption)
   (XUV and X-ray <3nm)

  Non-equilibrium plasma dynamics
          Strong field physics (minimal coupling):
    Classical electric field – non-relativistic – non-linear
    A(t) = F/ω cos ωt


    H = 1 ( p − A) 2 + V = 1 p 2 + V +  +
                                       pA             A2
                                                      1
        2                  2 
                                                   
                                                      2

                              HS       H SF                F2
                                              HF   ≡ Up =               ➜  ponderomotive
                                                          4ω 2              potential
                                                                   F
                                               xω =   ∫   A dt =        ➜  quiver amplitude
                                                                   ω2
€

      F 2 ≈ 3⋅1014 W/cm2
                                   €
      λ/ω [nm/eV]   Up            xω [a0]
      800 / 1.2     21 eV         50
€     100 / 12.7    0.3 eV        0.8
        3 / 350     0.3 meV       7 10-4
                                                                           cluster of 1000
                                                                           Ar atoms
                    How to absorb energy?
     Interrupt oscillating electron motion with collisions

 
                                                  p0

 




 
 Energy absorption in extended quantum systems
           (clusters, quantum dots…)

 




 




 


                                          L
Saalmann & Rost, PRL 100, 133006 (2008)
        Analogy to gravitational slingshots
                              … see Startrek
                              or NASA
                              (e.g., Mars expeditions)
  as

                      Here:   gravitational
                              effect from planet

                                   velocity of space craft
Laser assisted energy absorption in
  rare gas clusters:            .


Ar1000
              Ar33000

              Ar1700
         Xe9093


    Xe1151
Schematic cluster evolution under a laser pulse in time
        (U Saalmann, Ch Siedschlag and JMR, TopicalReview, J. Phys. B 39, R39 (2006))
     phase I: single ionization of atoms (only intensity dependent!)
                    phase II: critical expansion between t0 and tc:
                           phase III: relaxation    cluster grows
                                                    ion density decreases
R                                                   electron density changes




                                                                  t0: every 2nd atom
                                                                      on average ionized
R0                                                                tc: optimum absorption

                   t0         tc                            T               t
    Resonant absorption by collective electron motion


                  intensity



                                   resonant light absorption
Average charge




                                       when plasma frequency
                                       of cluster ω = ωL



                                atom


                 pulse length    Experiment:
                                 cluster of ~20 Platinum atoms
                                 (Köller etal., PRL 82, 3783 (1999))
     Xe923



     ion charge &
     charging rate




velocity vCM electron CM
inside the cluster,
amplitude along
laser polarization




phase φ between
electron CM and
driving laser field
          Electron CM:
Driven damped harmonic oscillator
      (Saalmann & Rost, PRL 91, 223401(03))
          Electron CM:
Driven damped harmonic oscillator
      (Saalmann & Rost, PRL 91, 223401(03))
          Numerical realization:
MD simulations (tree codes) with quantum rates
                 Light pulse parameter space
ω [eV]
                 Intensities: 1012 - 1015W/cm2

  1000
                                                       800 nm Ti:Sa laser

                                                       VUV FEL
   100
                                                       attosecond /HH

    20

    12

    1.5


          0.25   0.5   1            100   200
                           T [fs]
                                           (pulse length)
     Major energy absorption mechanisms


  “Optical slingshots” (a few very fast electrons)
   (IR 800nm)

  Plasma formation &
   collective resonant absorption (800nm)

  Inverse Bremsstrahlung (IBS)
   (VUV 100nm)

  Electron delocalization (weak absorption)
   (XUV and X-ray <3nm)

  Non-equilibrium plasma dynamics
FLASH: The DESY FEL
  not a table top machine!!




Courtesy: J Ullrich
FLASH: The DESY FEL
 
     not a table top machine!!
      Realization:

     2000: TTF1 - first experiments with 98 nm light

     2007: TTF2 - light down to 6 nm light

     2013: X-FEL with hard X-rays

     2009: LCLS (Stanford) 8 keV, 1/100 intensity




  intensity:             ~1015 . . . 1017 W/cm2
  photon energies: ~10 . . . 10.000 eV (100...0.1 nm)
Courtesy: J Ullrich
  pulse length:    ~ 50 fs . . . 5 fs . .?
     The revolution:
X-ray Free Electron Lasers
                                 wavelength (nm)




                Same frequencies as
                 attosecond pulses
                       Photon energy (eV)
              XFEL: SASE principle
  (Self Amplification by Stimulated Emission)




  Synchrotron




3rd generation



          FEL
   FLASH, Hamburg: Ion spectrum of Xenon clusters
                       (Xe, 100fs, 7x1013W/cm2@12.7eV)

                                               Theoretical description:




                                              relative abundance
                                                                      Xe
                                                                      80




Wabnitz etal, Nature 420, 482 (02)            Siedschlag & Rost, PRL 93, 043402 (04)

                                     See also Santra etal. PRL 91, 233401 (03)
     Barrier suppression:
up to 8 electrons inner-ionized

            single ion   cluster ions
                          Barrier suppression:
                     up to 8 electrons inner-ionized

t                    t=10 fs
                                  single ion          cluster ions
            t=1 fs
=0
     ω=Ee




                               ●  Electrons “boil off” long after
                                  the pulse is over
                               ➜  inner-ionized electrons form
                                  a plasma and are
                                  heated by inverse Bremsstrahlung (IBS)
                               ➜  the plasma is transiently much higher
                                 charged than seen from the final
                                 ionic distribution
        Energy absorbed into the plasma due to IBS




IBS:


valid for unscreened plasma    Krainov, J.Phys.B 33, 1585 (2002)
                  Why is plasma forma.on so universal ? 

  Energy absorption from light leads to loss of electrons
    ➜  ionic charge builds up
        ➜  bound electrons from surface atoms are field ionized;
             electrons are trapped and form a (quasi-neutral) plasma
        ➜  non-screened surface ions explode


                                                 (MD simulation, Ar147)


      ω=Ee
     Major energy absorption mechanisms


  “Optical slingshots” (a few very fast electrons)
   (IR 800nm)

  Plasma formation & collective resonant absorption

  Inverse Bremsstrahlung (IBS)
   (VUV 100nm)

  Non-equilibrium plasma dynamics through
   almost simultaneous transfer of many photons
   to many electrons
I = 7x1013W/cm2   Total cluster charge
ω = 20 eV
                      “time-resolved”

                                   Definition:
                                   Q/A, where
                                   Q= {sum over electrons j
                                       with energy Ej>0}
I = 7x1013W/cm2   Total cluster charge
ω = 20 eV
                      “time-resolved”

                                  ■  cross over
                                   Definition:    between
                                   direct
                                   Q/A, where
                                    ionization and
                                   Q= {sum over electrons j
                                   evaporation
                                        with energy Ej>0}
                                         over indicates
                                  ■  cross
                                    plasma formation
                                  ■  whyovershooting for
                                    shortest pulse?
                                  ➜  Non-equilibrium
                                  plasma ?
  I = 7x1013W/cm2             Total cluster charge
  ω = 20 eV
                                  “time-resolved”

                                              ■  cross over
                                               Definition:       between
                                               direct
                                               Q/A, where
                                                ionization and
                                               Q= {sum over electrons j
                                               evaporation
                                                    with energy Ej>0}
                                                     over indicates
                                              ■  cross
                                                plasma formation
                                              ■  whyovershooting for
                                                shortest pulse?
                                        t=0            t=50 as      t=500 as
                                              ➜  Non-equilibrium
                                              ω=Ee
                                              plasma ?


?  Non-equilibrium plasma
   observable by attosecond
   pump probe?
         Probe pulse ionizes tightly bound electrons,
           but probes time-dependent screening by
                      plasma electrons




 before the
     probe pulse
             probe pulse

pump pulse
         at t1
               at t2~t1+Tω/4

               250 as pump & probe with I=5x1014W/cm2
         Attosecond resolved non-equilibrium plasma dynamics
      (time of flight measurement of electrons ionized by probe pulse as before)




                                                                 with plasma
                                                   ■  oscillations
                                                     period 2π 31/2ωpl-1 = 770 as
                                                   ➜  non-equilibrium plasma
                                                     dynamics: τcorr~ τrel
                                                   ➜  formation of a
                                                      strongly coupled plasma,
                                                   ■  Coulomb   coupling
                                                      parameter
                                                      Γ = Epot/Ekin ~ 6



Saalmann, Georgescu and Rost, New J. Phys. 10, 025014 (2008)
               250 as pump & probe with I=5x1014W/cm2
         Attosecond resolved non-equilibrium plasma dynamics
      (time of flight measurement of electrons ionized by probe pulse as before)




Saalmann, Georgescu and Rost, New J. Phys. 10, 025014 (2008)
Clusters embedded in Helium droplets


- elegant way to make clusters

- tamper for imaging
                    a He droplet under

                1014 W/cm2@780nm over 20 fs.

what happens? - nothing.

           1014 W/cm2@780nm over 20 fs
  put one (1) Xenon atom into the He droplet (~105 atoms)

     (not sufficient to ionize a Helium atom! )
what happens?    - nothing.

  put a hand full (5) Xe atoms into the He droplet

what happens? the entire He droplet will be stripped
              of all electrons
Ignition of Helium ionization by n Xe seed atoms
         7x1014 W/cm2@780nm over 20 fs.



                      ?
                                           field
           780nm                           ionization




                           Xen@He2500
                   200nm
           13 Xe seed atoms –dependence on droplet size




                        Xe13@Hem


  laser wavelength matters!
  resonant process ?
                                 what happens ...


  fast ignition through Xe seeds in the
   center: field ionization of He

  sustained full ionization even for
   large droplets requires extremely
   efficient (collective) mechanism

  but how: Ωdroplet >> ωlaser

➜  seed in the center + linear laser
   polarization create
   unisotropic (cigar shaped)
   nanoplasma
  ωlaser ~ ΩII < Ωdroplet
                                 what happens ...


  fast ignition through Xe seeds in the
   center: field ionization of He

  sustained full ionization even for
   large droplets requires extremely
   efficient (collective) mechanism

  but how: Ωdroplet >> ωlaser

➜  seed in the center + linear laser
   polarization create
   unisotropic (cigar shaped)
   nanoplasma
  ωlaser ~ ΩII < Ωdroplet
                                 what happens ...


  fast ignition through Xe seeds in the
   center: field ionization of He

  sustained full ionization even for
   large droplets requires extremely
   efficient (collective) mechanism

  but how: Ωdroplet >> ωlaser

➜  seed in the center + linear laser
   polarization create
   unisotropic (cigar shaped)
   nanoplasma
  ωlaser ~ ΩII < Ωdroplet
 Generation and heating of unisotropic nanoplasmas
               in doped He droplets:


  fast and extremely efficient process
   (electrons not nuclei are responsible)
  should be realizable for all seeds with lower Ip than
   embedding material
  new way to create controlled
   microscopic plasmas
   in solids?
                         Summary

  surprising collective effects can be initiated by laser
    excitation of matter, strongly localized in space and time
   800 nm laser, localization supplied by system:
   linear polarization and seed atoms (Xe in He))

  attosecond technique can reveal tranisient dynamics in
   dissipative processes
   VUV excitation (100 nm) and XUV probing of clusters

  almost simultaneous coupling of energy transfer from the
   light pulse to many electrons induces new kinds of
   non-equilibrium dynamics
   attosecond pump-probe scenario in clusters
          Interaction of finite (atomic) systems
                     with laser light

                     Ultracold gases:
                     Cenap Ates . Ivan Liu . Andrei Lyubonko
                     Thomas Pohl & group. Yurii Dumin
                     Alex Jurisch
Time resolved X-ray physics:

                        Thanks!
Ionut Georgescu                                   Attosecond physics:
Christian Gnodtke                                 Paula Riviere
Vitali Averbukh                                   Olaf Uhden
                  Alex Eisfeld . Ulf Saalmann

         Intense field physics:            Single photon
         Alexey Mikaberidze .              absorption:
                 Anatole Kenfack           Ulrich Galster
                                           Jan Roden


           For pre/reprints please visit http://www.pks.mpg.de/~rost

				
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