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Two Colour and Two Photon Ionization Processes in
Intense EUV and Optical Fields at FLASH
John T. Costello National Centre for Plasma
Science Technology (NCPST)/ School of Physical
Sciences, Dublin City University www.physics.dcu.i
e/jtc
' FLASH ' - Free Electron LASer in Hamburg
Ultrafast Imaging Workshop, Ischia May 3, 2009
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Acknowledgements

• LIXAM (Orsay) D. Cubaynes, M. Meyer
• Universite Paris 06 (PMC) R. Taieb, A. Maquet
• DESY (Hamburg) A. Azima, S. Düsterer, P.
  Radcliffe, H. Redlin, W-B Li,
• J. Feldhaus
• PTB (Berlin) A. A. Sorokin, M. Richter
• Moscow State University A. N. Grum-Grzhimailo,
  E. V. Gryzlova,
• S. I. Strakhova
• Queens University Belfast Hugo van der Hart
• Dublin City University J. Dardis, P. Hayden, P.
  Hough, T. Kelly,
• V. Richardson, E. T. Kennedy, J. T. Costello
• Thanks to AG Photon (R Treusch et al.) AG
  Machine (M Yurkov et al.)

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Collaboration - Origin
Collaboration grew out of EU RTD
Project HRPI-CT-1999-50009 Title X-Ray FEL
Pump Probe Facility Partners Orsay, DCU,
Lund, MBI, BESSY DESY
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Outline of the Talk

• FLASH (Brief) Overview
• Atoms Molecules in Intense EUV NIR Fields
• Coherent Photoionization Processes in Superposed
  Fields
• Optically probing fragments in EUV dissociated
  H2
• Two Photon Ionization in Intense EUV Fields
• Next steps

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Part 1 - FLASH Overview
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FLASH Key Performance Indicators
Wavelength range (fundamental) 6 - 60 nm FEL
harmonics (_at_13.7 nm) 3 rd 4.6 nm (270 eV)
5 rd 2.7 nm (450 eV) Spectral width (FWHM)
0.5-1 Pulse energy up to 70 µJ (average),
170 µJ (peak) Pulse duration (FWHM) 10-50
fs Peak power (fundamental) Few GW Average
power (fundamental) 0.1 W (up to 3000 pulses
/ sec) Photons per pulse 1013
Ackermann et al., Nature Photonics 1 336 (2007)
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O/P Profile and Spectral Distribution
FEL output builds up from spontaneous emission
(photon noise) gt SASE Self Amplified
Spontaneous Emission
Spectral Fluctuations
Temporal Fluctuations
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O/P Profile and Spectral Distribution
FEL output builds up from spontaneous emission
(photon noise) gt SASE Self Amplified
Spontaneous Emission
Corrected EUV Spectrum
Raw EUV Spectrum
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Part 2 - Experiments on dilute targets with
FLASH Motivations 1. FLASH
Characterisation 2. Demonstration Experiments 3.
Future (Pathfinding)
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Atoms Molecules in Intense EUV Optical
(800/400 nm) fields

• Photoionization of rare gas atoms dressed by
  intense optical fields
• 2. Optical probing EUV dissociated H2

Summary of AMOP_at_FLASH http//hasylab.desy.de/scien
ce/user_collaborations/amopflash Photoionization
Experiments with the Ultrafast EUV Laser FLASH -
Free Electron Laser in Hamburg J T Costello, J
Phys Conf Ser 88 Art No 012057 (2007) C. Bostedt
et al., Experiments at FLASH, Nucl. Inst. Meth.
in Res. A 601 108-122 (2009)
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FLASH NIR/Vis and EUV Beam Layout
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Two colour ATI/ Laser Assisted PES
Superposition of visible and VUV pulses in a
noble gas jet
Schins et al. PRL 73, 2180 (1994)
Electron Spectrometer
VUV
Visible fs laser pulse
15.76 eV
Ar(IP)
gas jet
E.S. Toma et al. PRA 62 061801 (2000)
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Two colour ATI/ Laser Assisted PES
Experimental Layout at FLASH - (EU-RTD)
P. Radcliffe, et al., Nucl. Instr. and Meth. A
83, 516-525 (2007)
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Two colour ATI/ Laser Assisted PES
Sideband number/intensity depend strongly on
EUV/NIR overlap ? by comparison with theory we
are able to determine relative time delay to
better than 100 fs
550 fs

• 1. New ultrafast EUV-modulated optical-reflectivit
  y methods 2. TEO
• C. Gahl et al., Nature Photonics 2 165-169
  (2008) A. Azima et al., APL.
• T. Maltezopoulos et al., New J Phys 10 Art. No.
  033026 (2008) (In Press 2009)

P. Radcliffe, et al., Nucl. Instr. and Meth. A
83, 516-525 (2007) P Radcliffe, et al., APL 90
131108 (2007)
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Two colour ATI - Soft Photon
One photon cross-section
n photon ATI cross-section
- Classical excursion vector of an electron in a
laser field of amplitude
- Momentum transfer
After a little work.sideband strength is
given by an expression like
Jn - Bessel function (first kind order n)
kn - Shifted wavenumber of the ejected electron
?? - Usual asymmetry parameter
A Maquet and R Taieb, J. Mod. Opt. 54 1847 (2007)
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Two colour ATI - Z Scaling
800 nm Ti-Sa 1.55 eV/ 120 fs
FLASH h? 93 eV 20 ?J/pulse
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Two colour ATI - Z Scaling
Intesnity ratio of n 12 sidebands - Model vs
Experiment
Fitted 800 nm intensity 5 x 1011 W.cm-2
Experiment
Soft Photon
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2 colour ATI - FEL Wavelength Scaling
?FEL 800 nm. SB Ratio - Comparison of SPA with
Experiment
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2 colour ATI - Optical Intensity Scaling
Ne Asymmetry in sideband distribution
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2 colour ATI - Optical Intensity Scaling
Ne Toy SFA Code / Asymmetry in sideband
distribution
Ne Simulation
h?FEL 46 eV
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Atomic Dichroism in Two colour ATI
Strong Polarisation Dependence of Sidebands (Low
Field)
He 1s2 h?XUV ----gt He 1s ?p He 1s2 h?XUV
h?OL ---gt He 1s ?s, ?d
FLASH 13.7 nm, 10-20 fs, 20µJ OL 800nm, 4ps,
400µJ, 6 x 1011 W/cm2
?
Meyer et al., PRL 101 Art. no. 193002 (2008)
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Atomic Dichroism in Two colour ATI - He
Low Optical Laser Field
High Optical Laser Field
?(?) 3Sd (5Ss Sd) cos2? Ss/Sd 1.25 0.3
P. Theory A Grum-Grzhimailo et al.
SPA A Maquet/ R Taieb
Meyer et al., PRL 101 Art. no. 193002 (2008)
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Atomic Dichroism in Two colour ATI - Kr
AD in TC-ATI Ne, Kr, Xe work in progress
FLASH 13.7 nm, 10-20 fs, 20µJ OL 800nm, 4ps, 6
x 1011 W/cm2
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Atomic Dichroism in 2 colour ATI - Ne
Sequential Ionization
Work in progress - but we can begin to think
about studying isonuclear trends.
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Summary - Two Colour ATI

• Demonstrated interference free SBs to high order,
  polarisation control, laser and FEL parameter
  dependencies SBs in atomic and ionic targets
• At low optical intensities 2nd OPT SPA agree
• 3. Beyond He we really need to measure angular
  distributions to try to unravel the l channels
• 4. SPA works well at high intensities but the
  number of open high angular momentum channels is
  a challenge for other approaches such as R-Matrix
  Floquet (HvdH)
• 5. Is there really value in going beyond SPA here
  ? Does the residual ion core atomic structure
  really matter.?

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Optical Probing of H2 Dissociation
Demonstration Experiment
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Optical Probing of H2 Dissociation
1) H2 h?(FEL) --gt H H
lt 100 fs
FEL 13.7 nm, 90.5 eV
2) H h?(las) --gt H e-
h?VUV
Opt.Las. 400 nm, 3.1 eV
H H
H H(nl)
H H(1s)
H(1s) H(1s)
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Optical Probing of H2 Dissociation
13.7 nm (pump) 400 nm (probe)
High Intensity (1.5 mJ/120 fs)
Low Intensity (0.5 mJ/120 fs)
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Optical Probing of H2 Dissociation
Dynamics - more to do - jitter limited.
expt.
fit
risetime 500 fs
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Xe ionization in intense EUV fields
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? 4d two photon direct ionization
FEL only. h? 93 eV
Xe 2h? ? Xe e-
Replace PIS by MBES - First evidence of
inner-shell TPI
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Part 3. Current and Future Interests
Two Colour Resonant Photoionization Processes

• To date we have looked only at one and two colour
  non-resonant processes
• 2. Next phase - FEL tunable and so we can explore
  resonant two colour processes where inner shell
  electrons dominate

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Kr (3d94d) 2 Photon Resonance Auger
Expt. June 2009 (Hopefully) MLM - 3 degree
incidence
Kr 3d9 2D5/2
92.0eV
4d
Auger
?
Kr 4p4 4d, 5d
VUV 46.1 eV
Kr 3d10 4s2 4p6
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Kr (3d94d) DOR-Auger
Kr 3d9 2D5/2
92.0eV
4d
Auger
1.55 eV
5p
?
Kr 4p4 4d, 5d
XUV 13.9 nm
Kr 3d10 4s2 4p6
Next Expt h?FEL (89 eV) h?OL(3.1 eV)
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Experiments post-upgrade
Upgrade - Optical Parametric Amplifier - (fs OPA)
Laser coupling/spectroscopy of autoionization
states - 'Autler-Townes' meets Codling-Madden
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Tunable Optical Laser - Laser Coupling of
autoionization states - 'Autler Townes'
Autoionizing Resonances - He

• New Knobs
• Laser Frequency
• Laser Intensity

Bachau Lambropoulos, PRA (1986) Themelis,
Lambropoulos, Meyer, JPB (2005)
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FLASH - Technical Developments
OPA Upgrade - 0.01 - 1 mJ/ 0.1 - 20
ps Synchronisation - New FIR Undulator
(THz) Seeding with HHs - Full
coherence Stabilisation Synchroniosati
on
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In Conclusion

• 1. To date we have looked only at one and two
  colour non-resonant photoionization processes
• Now - FEL easily tunable - we can explore
  resonant two colour processes where inner shell
  electrons dominate
• 3. Study fragmentation and ionization from
  vibrationally excited/selected wavepackets in
  simple molecules
• Beyond 2009 Seeding, fs jitter, angle resolved
  PES,
• LCLS/XFEL/SPRING-8/NLS..

Ultrafast Imaging Workshop, Ischia May 3, 2009