Transcript Slide 1

Multiphoton (Inner-Shell) Ionization
in Intense EUV FEL Fields
TPI of Xe (93 eV) and Kr (46 eV)
John T Costello
National Centre for Plasma Science & Technology (NCPST)/
School of Physical Sciences, Dublin City University
www.ncpst.ie
www.physics.dcu.ie/~jtc
VUVX – SLAC / July 19 – 21, 2010
Collaboration
XFEL: M. Meyer & P. Radcliffe
PTB (Berlin): A. A. Sorokin (now at IOFFE & DESY) & M. Richter
DESY (Hamburg): K. Tiedke, S. Düsterer, W. Li, J. Feldhaus & P. Juranić
Orsay: D. Cubaynes
Queen’s University Belfast: Hugo W. van der Hart (T)
Crete: P. Lambropoulos (T)
Oulu/GSI: S. Fritzsche (T)
DCU: V. Richardson & J. T. Costello
Thanks to AG Photon (R Treusch et al.) & AG Machine (M Yurkov et al.)
VUVX – SLAC / July 19 – 21, 2010
Outline of Talk
1. FLASH (One slide)
2. Comments on ionization in intense laser fields
3. Setup for Photoelectron Spectroscopy @ FLASH
4. Case Study 1: 2-Photon 4d-Ionization of Xe (ATI)
5. Case Study 2: Resonant 2-Photon 3d-Excitation/ Auger
Decay in Kr
6. Proposal: (Optical) Coherent Control of Auger (and
Fluorescence) State Decay………..
7. Some conclusions
VUVX – SLAC / July 19 – 21, 2010
DCU Laser Plasma/ AMOP Group
6 laboratory areas focussed on pulsed laser matter interactions
(NIR – X-ray/ 30fs – 30 ns, spectroscopy/ imaging/ PLD)
Academic Faculty (5): John T. Costello, Eugene T. Kennedy (Emeritus), Jean-Paul
Mosnier, Lampros Nikolopoulos (T) and Paul van Kampen
Funded by:
Current Postdocs (3):
Dr. Patrick Hayden, Dr. Sateesh Krishnamurty
and Dr. Subhash Singh
SFI - Frontiers and Investigator
HEA – PRTLI (Kit)
IRCSET (People)
EU - Marie Curie (People)
Current PhD students (8 + 1 + 2):
Jack Connolly, Leanne Doughty, Brian Doohan, Colm Fallon, Eanna Mac Carthy,
Mossy Kelly, Vincent Richardson, Jiang Xi, Damien Middleton, open position (with
LN), Ricarda Laasch (Univ. Hamburg) and Nadia Gambino (Univ. Catania)
Recent PhDs: Caroline Banahan, Kevin Kavanagh, Eoin O’Leary, John Dardis,
Padraig Hough, Conor McLoughlin, Rick O’Haire, Dave Smith & Tommy Walsh
Recent Postdocs: Deirdre Kilbane, Hugo de Luna, Jofre Gutieriez-Pedrogosa,
Brendan Doggett, Subo Chakraborti and Jean-Rene Duclere
VUVX – SLAC / July 19 – 21, 2010
FLASH:
Key Performance Indicators
Wavelength – 4.5 nm to 60 nm
Pulse Energy – 20 to 70 µJ
Pulse Length – ~10s fs
Photons per Pulse ~1013
Single and Multi-Bunch Mode
Ackermann et al., Nature Photonics 1 336 (2007)
What are the USPs of XFELs in AMOP ?
•
Ultra-dilute targets
•
Photo-processes with ultralow cross-sections
•
Pump and probe experiments (EUV + EUV or EUV + Opt.)
•
Single shot measurements
•
Few-photon single and multiple ionization processes
NB1: Makes inner-shell electrons key actors in non-linear
processes for the first time
NB2: Re-asserts primacy of the photon over field effects !
VUVX – SLAC / July 19 – 21, 2010
Keldysh - Ionization Regimes
Multiphoton Ionization
>>1

Tunnel Ionization Field Ionization
~2
 <<1
IP
where UP  9.31014 I Wcm2 2m eV
2Up

VUVX – SLAC / July 19 – 21, 2010

Keldysh - Ionization Regimes
Multiphoton Ionization
>>1
Tunnel Ionization Field Ionization
~2
 <<1
Intensity/ Wavelength
Photon Energy
VUVX – SLAC / July 19 – 21, 2010
Keldysh - Ionization Regime
Multiphoton Ionization
>>1
Ti-Sapphire in the NIR
Non-Pertubative (TI) Regime
Tunnel Ionization Field Ionization
~2
 <<1
FLASH in the EUV Pertubative (MPI) Regime:
So these non linear photoionization processes will involve
predominantly few photons and few electrons.…
Note - ion yield scales with intensity as In
VUVX – SLAC / July 19 – 21, 2010
What’s really important about NLO/S
with EUV/X-ray Lasers ?
• Importantly - EUV/X-ray FELs bring inner shell electrons into the nonlinear interaction of radiation with matter for the first time…..
• So Autoionising states (with femtosecond lifetimes) can play a key role
in the process…. This will lead to a complex dynamical interaction
between X-ray excitation and decay which means that simple ‘Single
Active Electron - SAE’ models will no longer suffice……See e.g., papers
by Robin Santra et alia (e.g., PRA 76 033416 2007)
• Self-consistent atomic structure and dynamics models that can combine
and capture the physical competition between pumping and rapid
(mainly) non-radiative decay of small quantum systems, along with a
gamut of other parasitic/competitive non-linear (e.g., ATI) and correlative
processes (e.g., shake up/down) in intense EUV/X-ray fields are now
needed for matter from atoms to macromolecular systems…….
VUVX – SLAC / July 19 – 21, 2010
General EUV / XFEL AMOP Refs
1. Photoionization Experiments with the Ultrafast XUV Laser FLASH
J. T. Costello, J Phys Conf Ser 88 Art No 012057 (2007)
2. Experiments at FLASH
C. Bostedt et al., Nucl. Inst. Meth. in Res. A 601 108 (2009)
3. Non-linear processes in the interaction of atoms and molecules with
intense EUV and X-ray fields from SASE free electron lasers (FELs),
N. Berrah et al., Journal of Modern Optics (2010)
(DOI:10.1080/09500340.2010.487946)
VUVX – SLAC / July 19 – 21, 2010
3. Photoelectron Spectroscopy Setup
hn = 93 eV (13.5 nm –
EUVL Wavelength)
•High quality, Si/Mo multilayer mirror employed –
spot size at focus ~ 4μm
→ 1016Wcm-2 (IOF-Jena)
•Gas Monitor Detector
93 eV
(GMD) provides shot-toshot measure of FEL pulse
intensity
•0.65m Electron TOF spectrometer - shaped
magnetic field to maximise
collection efficiency
VUVX – SLAC / July 19 – 21, 2010
4. Xe ionization in intense XUV fields
Motivation - Sorokin, Bobashev, Richter et al., PTB, PRL 2007 – IONS !!
VUVX – SLAC / July 19 – 21, 2010
One Photon Ionisation at hn = 93 eV
Electrons !!!
•Single shot spectrum….
•For low intensities (<1014
W.cm-2), one photon
processes are dominant
•Salient features – spin orbit
split 4d photoelectron line +
Auger electron spectrum
•Not shown – 5s-1 and 5p-1
lines at higher KEs
VUVX – SLAC / July 19 – 21, 2010
Two Photon Inner Shell Ionisation in Xe
•Using MBES, first evidence of two
photon inner shell ionisation, (in this
case) of 4d electron –
Xe + 2hv → Xe+ 4d9 + e•‘Retardation field’ applied to suppress
low KE electrons (one photon processes)
– hence electrons detected are due
solely to multiphoton events
•Energetically –
2 × (93) eV – 118 eV = 68 eV
•Yield scales quadratically, n=1.95 ± .2
VUVX – SLAC / July 19 – 21, 2010
Two Photon Inner Shell Ionisation in Xe
1. However, from Sorokin et al. PRL 99
(2007) 213002 one may conclude that the
FEL field produces and interacts with a
highly ionized target.
2. Xe+ has four 4d-1 ionization thresholds at
71.6 eV, 72.9 eV, 74.9 eV, and 76.2eV yield photolines with KE from 110 to
115eV. However, Xe+ appears only
weakly in the ion spectra even at very
high FEL intensity.
3. 4d-1 from higher charge states also
possible – outside KE region of interest
4. Additionally, two photon O-shell ionisation
cross section are weak at 93 eV for XE
ions, even for Xe4+ & Xe5 with nearby
resonances: J-M. Bizau et al………..
VUVX – SLAC / July 19 – 21, 2010
Two Photon Inner Shell Ionisation in Xe
1. R-Matrix (H.W. Van der Hart) – one
and two photon 4d emission cross
sections
2. Dominant process is one photon
ionization – 93 eV can remove next 4d
as well - or maybe excite 4p – 4d.
Removal of the second 4d electron may
lead to excitation over a wide range of
states
3. Accurate calculation requires a far more
rigorous description of the atomic
structure than at present
4. Estimated two photon 4d-1 emission is
~1% of total at ~ 7 x 1015 W.cm-2
VUVX – SLAC / July 19 – 21, 2010
Two Photon Inner Shell Ionisation in Xe
V Richardson et al. PRL 105 013001 2010
5. Resonant two photon Excitation of Kr
1. To date we have looked at a non-resonant
two photon process (sort of ATI really)
2. FEL are wavelength tunable, so one can also
explore resonant two photon processes
VUVX – SLAC / July 19 – 21, 2010
Kr - Resonant Two Photon Excitation
1. Kr 3d104s24p6 (1S0) + 2 x hn (46
eV) -> 3d94s24p64d (J=0,2) i.e.,
3d - 4d two photon excitation
hn = 46 eV (~27 nm)
2. Of course there is a direct
ionization path and the usual
interference results - manifested
as asymmetric resonance
profiles (Fano/ Fano-Mies)
3. But here the 3d94s24p64d (J=0,2)
resonance undergoes Auger
decay to Kr+ on a femtosecond
timescale - similar to the FLASH
pulse duration - so competition
between excitation and decay
(ergo, in addition to simple ATI,
this case makes for an intriguing,
problem for theory)..
Meyer et al., PRL 104 213001 (2010)
VUVX – SLAC / July 19 – 21, 2010
94d) 2 Photon Resonance Auger
Kr -(3d
Resonant Two Photon Excitation
MBES Photoelectron spectrum - ~ 5 x 1014 W.cm-2
Auger
ATI
VUVX – SLAC / July 19 – 21, 2010
94d) 2 Photon Resonance Auger
Kr -(3d
Resonant Two Photon Excitation
Theoretical Spectra – Stefan Fritzsche (GSI & Oulu)
VUVX – SLAC / July 19 – 21, 2010
94d) 2 Photon Resonance Auger
Kr -(3d
Resonant Two Photon Excitation
Ionization rates – P. Lambropoulos, Crete
VUVX – SLAC / July 19 – 21, 2010
94d) 2 Photon Resonance
Kr
(3d
Kr - Resonant Two Photon Excitation
M Meyer et al., PRL 104 213001 (2010)
6. Next step:X-ray coherent control ?
Resonant ATI
Neon: (Very) Simplified
Energy Level Scheme
or RATI
Ne - 1s-13p(1P1)
AIS
Auger Emission
Rabi-Flopping
Ne+ - 2p5(2P1/2,3/2)
Ne -
2p6(1S
0)
GS
VUVX – SLAC / July 19 – 21, 2010
Next steps: X-ray coherent control ?
VUVX – SLAC / July 19 – 21, 2010
Next steps: X-ray coherent control ?
Another proposed scheme: Bozek (SLAC), Cavalieri (CFEL), Coffee (SLAC),
Costello (DCU), Di Mauro (OSU), Duesterer (DESY-FLASH), Hastings (SLAC), Kelly
(DCU), Kennedy (DCU), Kienberger (MPQ/TU-M), Nikolopoulos (DCU – Theory)
Meyer (XFEL), Radcliffe (XFEL) and Tschenscher (XFEL)
VUVX – SLAC / July 19 – 21, 2010
Ne Auger Splitting for 908 eV Pump
Ne + hn (908 eV) -> Ne+ (k-1) + e- -> Ne2+ (2p4) + hn (pump, 908 eV)
Irradiance @ 908 eV:
3.5 x 1016 W.cm-2
Calculation –
Lampros Nikolopoulos (DCU)
VUVX – SLAC / July 19 – 21, 2010
X-ray
Next steps:
AugerX-ray
& Fluorescence
coherent control
Ctrl….
?
Tune the LCLS to a photon energy of 908 eV in order to optically pump ground state
(1s2) Ne2+ ions to 1s-13p states to control the Auger decay of core ionized Ne atoms,
formed within the same X-ray pulse.
1. Ne (1s22s22p6 1S0) + hn  Ne+ (1s2s22p6 2S1/2) + ewhich will decay predominantly via the emission of an electron (Auger decay):

Ne2+ (1s22s22p4 1,3LJ) + ebut also via photon emission (fluorescence decay)

Ne+ (1s22s22p5 2P1/2,3/2) + hn
However, since 908 eV represents the excitation energy for the Ne2+ transition:
Ne2+ (1s22s22p4 1D2) + hn  Ne2+ (1s2s22p43p 1F3)
Rabi flopping will occur on this transition at high X-ray pump intensity and so the Auger
spectral profile will be altered from the normal unhindered decay case. At sufficiently
high intensity one can induce an easily measurable Rabi splitting in the Auger line
profile. Also as the Ne+ (1s-1) state can decay to Ne+ (1s22s22p5 2P1/2,3/2) one can pump
on the Ne+ (1s – 3p) transition at 891 eV which will strongly modify the X-ray
fluorescence profile, evidenced by the appearance of a significant Rabi splitting in it.
VUVX––SLAC
SLAC // July
2010
VUVX
July19
19– –21,
21,
2010
Ne Auger Splitting for 908 eV Pump
Ne + hn (908 eV) -> Ne+ (k-1) + e- -> Ne2+ (2p4) + hn (pump, 908 eV)
Irradiance @ 908 eV:
3.5 x 1016 W.cm-2
Calculation –
Lampros Nikolopoulos (DCU)
VUVX – SLAC / July 19 – 21, 2010
Ne Fluor Splitting for 891 eV Pump
Ne+ (1s-13p) -> Ne+ (2p5 2P3/2,1/2)
Irradiance @ 891 eV:
3.5 x 1016 W.cm-2
Calculation –
Lampros Nikolopoulos
VUVX – SLAC / July 19 – 21, 2010
Summary
• First detection of a so-called ‘above threshold ionization’ (ATI) twophoton process in an inner electron shell.
• The strength and the nature of the 4d → εf resonance may open up, at
high irradiance, additional ionization channels, namely the simultaneous
multiphoton / multi-electron from the inner 4d shell, ‘inside-out ionization’
• Kr - first step on the road to resonant NL processes with EUV/X-rays….
REMPI at X-rays…..
• Next step - Optical pumping / coherent control at X-rays @ XFEL/LCLS
• Also – optical pumping on atomic ions to produce spectrally narrowed
laser lines in the X-ray…………………
Xe - Richardson et al. PRL (July 2 – 2010), Kr - Meyer et al., PRL (May 28 - 2010)
VUVX – SLAC / July 19 – 21, 2010