John T. Costello National Centre for Plasma Science & Technology (NCPST) and School of Physical Sciences, Dublin City University www.physics.dcu.ie/~jtc & john.costello@dcu .ie Photoabsorption Spectroscopy & Imaging with Laser-Plasma X-VUV Continua (Atomic Photoionization with LPP) Interchannel interaction in photoionization of atoms and photodissociation of molecules, Riezlern, Austria July 12, 2005
Photoabsorption Spectroscopy & Imaging with Laser-Plasma X-VUV Continua (Atomic Photoionization with LPP). John T. Costello National Centre for Plasma Science & Technology (NCPST) and School of Physical Sciences, Dublin City University www.physics.dcu.ie/~jtc & john.costello@dcu .ie. - PowerPoint PPT Presentation
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John T. CostelloNational Centre for Plasma Science & Technology (NCPST) and School of Physical Sciences, Dublin City Universitywww.physics.dcu.ie/~jtc & [email protected]
Photoabsorption Spectroscopy & Imaging with Laser-Plasma X-VUV Continua
(Atomic Photoionization with LPP)
Interchannel interaction in photoionization of atoms and photodissociation of molecules, Riezlern, Austria July 12, 2005
Part I - Laser Plasma 'Line-Free' Continuum Sources Origin, Brief History & Update
Part II - Dual Laser Plasma Experiments - Some Case Studies X-VUV Photoabsorption Spectroscopy VUV (Monochromatic) Photoabsorption Imaging
Part III - Next Steps Atomic Photoionization Photoionization of Atoms in Intense Laser Fields - ‘Pump
Probe’ Experiments with X-VUV FELs
Collaborators and ContributorsPicosecond X-VUV Continuum SourcesRAL - E Turcu & W ShaikhQUB - C Lewis, R O'Rourke and A MacPheeDCU - O Meighan and C McGuinness
EUV Absorption SpectroscopyRostov - P Demekhin, B Lagutin and V SukhorukovDCU - P Yeates, A Neogi, C Banahan, D Kilbane, P van Kampen and
E Kennedy
VUV Photoabsorption Imaging Facility - VPIFPadua -P Nicolosi and L Poletto DCU - J Hirsch, K Kavanagh, E Kennedy & H de Luna
DESY ‘Pump Probe’ Project Hasylab- J Feldhaus, E Ploenjes, K Tiedke, S Dusterer & R TreuschOrsay- Michael Meyer, Denis Cubyannes & Patrick O'Keefe, DCU- E Kennedy, P Yeates, J Dardis and P Orr (QUB)
'Colliding Plasmas'DCU - K Kavanagh, H de Luna, J Dardis and M Stapleton
Academic Staff (4): John T. Costello, Eugene T. Kennedy (now VPR), Jean-Paul Mosnier and Paul van Kampen (on sabbatical)
Post Doctoral Fesearchers (5): Dr. Deirdre Kilbane (PVK/JC) Dr. Hugo de Luna (JC)Dr. Mark Stapleton (JC)Dr. Jean-Rene Duclere (JPM) Dr. Pat Yeates (ETK)
Current PhD students (6): Caroline Banahan (PVK/JC)Kevin Kavanangh (JC) Adrian Murphy (JC)John Dardis (JC) Rick O'Hare (JPM), Eoin O’Leary (ETK)
Visiting PhD students: Domenico Doria (Lecce) and Philip Orr (QUB)
The LPP node comprises 6 laboratory areas focussed on pulsed laser matter interactions (spectroscopy/ imaging)
Laser-Plasma/Atomic Phys-NCPST
Funded by:SFI - Frontiers and InvestigatorHEA - PRTLI and North-SouthIRCSET - Embark & BRGSEnterprise Ireland - BRGSEU - Marie Curie and RTD
Part I - Laser Plasma Continua
Laser Plasma Source Parameter Range
Target
Lens
Laser Pulse 1064 nm/0.01 - 1 J/ 5ps - 10 ns
Spot Size = 50 m (typ.)
: 1011 - 1014 W.cm-2
Te : 10 - 1000 eV
Ne: 1021 cm-3
Vexpansion 106 cm.s-1
Emitted -Atoms,Ions,
Electrons,Clusters,
IR - X-ray Radiation
PlasmaAssisted
Chemistry
Vacuum orBackground Gas
What does a laser plasma look like ?
PLASMAGENERATION
PLASMAEXPANSION
FILMGROWTH
Target
IncidentLaserbeam
Expanding PlasmaPlume
Substrate
Intense Laser Plasma Interaction
S Elizer, “The Interaction of High Power Lasers with Plasmas”, IOP Series in Plasma Physics (2002)
Laser Produced ‘Rare Earth’ Continua -Physical Origin, History & Update
Laser Plasma Rare Earth Continua
P K Carroll et al., Opt.Lett 2, 72 (1978)
What is the Origin of the Continuum ?
Continua emitted from laser produced
rare-earth (and neighbouring elements)
plasmas are predominantly free-bound in
origin and overlaid by Unresolved Trans-
ition Arrays (UTA*) containing many
millions of lines which share the available
oscillator strength.
* J. Bauche, C. Bauche-Arnoult & M. Kalpisch, Phys. Scr 37, 659 (1988)
But why is no line emission observed ?Line emission is due to complex 4d-4f arrays in (typically) 7 - 20 times ionized atoms
4dn5sq5ps4fm 4dn-15sr5pt 4fm+1, q+s = r+t
Furthermore 4f/5p and 4f/5s level crossing gives rise to overlapping bands of low lying configurations, most of which are populated in the ~100 eV plasma
Result - the summed oscillator strength for each 4d - 4f (XUV) and 5p - 5d (VUV) array is spread out over a supercomplex of transitions producing bands of unresolved pseudo continua (so called ‘UTA’) superimposed on the background continuum
Even expectedly strong lines from simple 4f - 4f arrays are washed out by plasma opacity
There are up to 0.5 million allowed transitions in LS couplingover the ~10 eV bandwidth of a UTA
In fact this is a lower bound since many additional LS forbidden transitions are ‘switched on’ by the breakdown in LS coupling here - G O’Sullivan et al., J.Phys.B 32, 1893 (1999)
Brief History/ Highlights of Laser Plasma Rare-Earth’ Continua -1990
1. First report of line free continua - P K Carroll et al., Opt.Lett 2, 72 (1978)
2. First full study/ applications - P K Carroll et al., Appl.Opt. 19, 1454 (1980)
3. VUV Radiometric Transfer Standard - G O’Sullivan et al., Opt.Lett 7, 31 (1982)
4. Absolute Calibration with Synchrotron - J Fischer et al, Appl.Opt. 23, 4252 (1984)
6. First Study for XUV lithography - D J Nagel et al., Appl.Opt. 19, 1454 (1980)
7. XUV Reflectometer - S Nakayama et al., Physica Scripta 41, 754 (1990)
8. First Industrial Application - DuPont - Insulator Band Structure
VUV Reflectance Spectroscopy - R H French, Physica.Scripta 41, 404 (1990) -
System subsequently made available commercially from ARC
For a review of the early years (1,2) and more recent work (3)including applications in photoabsorption spectroscopy see :1. J T Costello et al., Physica Scripta T34, 77 (1991)2. P Nicolosi et al., J.Phys.IV 1, 89 (1991)3. E Kennedy et al., Radiat. Phys. Chem 70, 291 (2004)
Recent Developments in LP Continua Ipsec LPLS (RAL/QUB/DCU)
O Meighan et al., Appl.Phys.Lett 70, 1497 (1997)
O Meighan et al., J.Phys.B:AMOP 33, 1159 (2000)
Summary - LP Continuum Light Sources
1. Table-top continuum light source now well established
It's a plasma with an ionization balance-how do we know that we have Y3+ say ?
4p - nd, Epstein and Reader , J. Opt. Soc. Am 72, 476 (1982)4s - 5p assigned using Clark et al., J. Opt. Soc. Am. B 3, 371 (1986)
What could theory tell us ?
Xn q-value
What about cross sections ?
'Mirroring Resonances'
2q2 0 complete cancellation - no resonances
An update from Aarhus ! Bizau, West & Kilbane (DCU)
Kr-like ions - Summary 1. It is clear that the ‘Fano’ profile parameters for the main 4s – np resonances in each spectrum are very sensitive to degree of ionization and that complex doubly excited resonances persist (at least in the early members of the isoelectronic sequence).
2. Computed cross sections show good agreement with measured spectra.
3. Rescaling the Coulomb interaction is needed to better fit the 4s-5p resonance in Sr2+
4. We observe that the complex doubly excited resonances straddling the first 4s-5p in Kr moves to higher photon energy blending with higher energy 4s-np (n>6) resonances and that the 4s-5p drops below the 4p threshold for Y3+
7. For Y3+ the resonance q values become quite large and the spectrum consists mainly of almost symmetric absorption features
8. We see that a number of features are suppressed in Rb+ and Sr2+ since they are built from exactly (or almost exactly) cancelling 'mirroring resonances'
'duplicity of the 3d orbital' - "valence-like by energy but inner shell-like by radial distribution" (Dolmatov, JPB 29, L687 1996)
Mn2+, 3p63d5 + h 3p5(3d6 + 3d54s) Mn+, 3p63d54s + h 3p5(3d64s + 3d54s2)
3p-subshell photoabsorption - Mn2+
3p-subshell photoabsorption - Mn2+
Overall we see a good match withDolmatov at least at the low energyside of the main resonance
But why does the experimentaltrace fall off much more quicklythan the theory -
Excitation from metastable states ?
Metastables and their effect on 3p-subshell photoabsorption of iron group ions - Mn3+
Mn3+, 3p63d4 + h 3p5(3d5 + 3d44s)
Mn+
Mn2+
Mn3+
What do simple Cowan code calculations give ?
First StepCompute 'cross sections' for photoabsorption from ground and low-lying terms of Mn3+ (3d4) - 5D, 3P, 3F, 3G, and 3H
What do you get ?
Next StepTo 'reproduce' the experimental plasma spectrum, take a weighted sum over each such cross section
Vary the temperature and compare with expt.
Mn ions - Summary 1. Clear that we are going to have problems with excited state absorptionsince we have a hot sample. In fact the plasma temperature at will vary from ~10 eV at 20 ns to 2 eV at 150 ns and hence we will have significant populations of low lying 3dn and 3dn-14s states
3. Same is also true of the ion beams used in the synchtotron experiments
4. So we need a combined atomic physics and plasma physics approach -some codes available (HULLAC) but complex and expensive
5. Contrast with Kr-like ions - closed shell - stable - ions tend to converge and stay on this favoured configuration - nice to work with
6. Same problem will occur in 4d, 5d, 4f and 5f metals
7. However, electronic structure of Mn2+ and Mn3+ ions important in manganites (recent Nature papers), biology (DNA),.....
VUV Photoabsorption Imaging
Part II - Section B (short)
Collaboration between DCU & Univ. Padua
VUV Absorption Imaging Principle
Pass a collimated VUV beam through the plasma sample and measure the spatial distribution of the absorption. Convertto 'Equivalent Width' images and extract column density (NL)maps - see Rev.Sci. Instrum. 74, 2992 (2003) for details
Io(x,y,,t)
Sample
I(x,y,,t)
VUVCCD
€
I =I0e−σ n(l )dl∫
Hirsch et al, J.Appl.Phys. 88, 4953 (2000) - QUB/DCU Collaboration
VUV Photoaborption Imaging
Time resolution: ~10 ns (200 ps - EKSPLA)
Spectral range: 10 - 35 eV (120 - 35 nm)
VUV bandwidth: 0.025 eV@25 eV
(50m slits)
Spatial resolution: ~120 m (H) x 150 m (V)J Hirsch, E Kennedy, J T Costello, L Poletto & P Nicolosi Rev.Sci. Instrum. 74, 2992 (2003)
1. VUV light can probe the higher (electron) density regimes not accessible in visible absorption experiments
2. The refraction of the VUV beam in a plasma is reduced compared to visible light with deviation angles scaling as 2
3. Image analysis is not complicated by interference patterns since the VUVcontiuum source has a small coherence length
4. VUV light can be used to photoionize ions - simplified equation of radiative transfer (no bound states).
5. Fluorescence to electron emission branching ratio for inner shell transitions can be 10-4 or even smaller => almost all photons are converted to electrons
Ca+ plasma plume velocityexperiment: 1.1 x 106 cms-1
simulation: 9 x 105 cms-1
Ba+ plasma plume velocityexperiment: 5.7 x 105 cms-1
simulation: 5.4 x 105 cms-1
Delay (ns)
Plu
me
CO
G P
ositi
on (
cm)
1. VUV light can probe the higher (electron) density regimes not accessible in visible absorption experiments
2. The refraction of the VUV beam in a plasma is reduced compared to visible light with deviation angles scaling as 2
3. Image analysis is not complicated by interference patterns since the VUVcontiuum source has a small coherence length
4. VUV light can be used to photoionize ions - simplified equation of radiative transfer (no bound states).
5. Fluorescence to electron emission branching ratio for inner shell transitions can be 10-4 or even smaller => almost all photons are converted to electrons
Advantages of using a VUV beam
What do we extract from I and Io images ?
€
A=log10(I0(x,y,t,λ)dλ∫I (x,y,t,λ)dλ∫ )Absorbance:
€
WE = [1−e−σ (λ)NL]∫
€
WE =Δλ[I0 −I ]dλ∫I 0dλ∫
⎛
⎝ ⎜
⎞
⎠ ⎟
EquivalentWidth:
d
You can also extracts maps of column density,e.g.,Singly Ionized Barium
Since we measure resonant photoionization, e.g.,
Ba+(5p66s 2S)+h Ba+*(5p56s6d 2P) Ba2+ (5p6 1S)+e-
h = 26.54 eV (46.7 nm) and
the ABSOLUTE VUV photoionization cross-section
for Ba+ has been measured:
Lyon et al., J.Phys.B 19, 4137 (1986)
We should be able to extract maps of column density -
'NL' = ∫n(l)dl
Maps of equivalent width of Ba+ using the 5p-6d resonance at 26.55 eV (46.7 nm)
dl
Convert from WE to NLCompute WE for a range of NL and fit a function f(NL) to a plot of NL .vs. WE
Collisionality () will be determined by both the mean free path(ii) and colliding plasma experimental scale length' (L).
= D/ii = D/Cstii = D.ii/Cs
Expect to strong stagnation for counter streaming plasmas where the mfp is small in comparison to the ablation front separation - Rambo and Denavit, Phys. Plasmas 1 4050 (1994)
Cs= Ion sound speed,ii = ion-ion collision frequency
What have we learned to date ?Strong stagnation in table top colliding plasmasdue to large value of the collisionality parameter ()
Degree of confinement/ hardness of the stagnationlayer can be controlled by designing the value of
Stagnation layer becomes quite uniform after 100s nsand so looks attractive for investigation as alternativepulsed laser materials deposition source
For this reason it also looks a good bet for atomic physics experiments using laser probing/excitation.....
Part III - Next steps in fundamental photoionization studies ?
Atoms and Molecules in Laser Fields1. Attosecond pulse generation/ HHG
3. Atoms, Molecules, Cluster & Ions in Intense Fields
(Multiple-Photon and Optical Field/Tunnel-Ionization)
Free Electron Laser at Hasylab, DESY, Hamburg
'Laser-like' radiation in the VUV and EUV
Free electron radiation sources
Bending magnet, broad band
NW x bending magnet
NU2 x bending magnet
NU2 x Ne x bending magnet
NU , NW = # magnetic periods
Ne = # electrons in a bunch
1=u/22(1+K2/2)
Josef Feldhaus, DESY, Hamburg
Schematic layout of a SASE FEL
Experimental Hall
LINAC Tunnel
Timetable EUV FEL - TTF2February 2004: - complete linac vacuum
- install photon diagnostics in FEL tunnel
Mar.-July 2004: - injector commissioning- completion of LINAC
Aug.-Dec. 2004: - LINAC and FEL commissioning with short bunch trains- installation of first two FEL beamlines (~20 µm focus direct beam and high resolution PGM)
Jan.-March 2005: - commissioning of first FEL beamlines and gas ionisation monitor- photon beam diagnostics
Spring 2005: - first user experiments
Xe PES FEL (June 24th) -
Femtosecond X-VUV + IR Pump-Probe Facility,Hasylab, DESY
OPA
Pump-probe experiments in the gas phase (project: II-02-037-FEL)
Participating groups:HASYLAB, Hamburg, GermanyJ. Feldhaus, S. Dusterer, R. Treusch, Kai Tiedke, Elke Ploenjes
LIXAM, Orsay, France M. Meyer, D. Cubannes
NCPST, Dublin City University, Ireland J. T. Costello, Philip Orr (QUB), P Yeates
M. Meyer et al, LIXAM, Orsay, France
Two subsets of experiments
II-ADirect photoionization in a non-resonant laser field
II-BResonant photoionization in a resonant laser field
Let's first look at II-A -
‘Direct photoionization' in a non-resonant laser field*
*Slides provided by Patrick O’Keefe and Michael Meyer,
Ponderomotive streaking of the ionization potential as a method for measuring pulse durations in the XUV domain with fs resolution
E.S. Toma, H.G. Muller, P.M. Paul, P. Berger, M. Cheret, P. Agostini, C. LeBlanc, G. Mullot, G. CheriauxPhys. Rev. A 62, Art. No. 061801 (2000)
Ar 3p6
Ar+ 3p5
VUV
IRe-presence of IR:- shift of IP- broadening of PES peaks- sidebands
Test-experiments at LLC: M. Meyer, P. O’Keefe (LURE), A. L’Huillier (LLC)fs-laser system: Ti:Saph. 800 nm, 50 fs, 1 kHz
VUV --> HHG, T ≈ 30 fs, 1 kHz, IR --> up to 0.5 mJ --> 1-10 TW/cm2
PES: magnetic bottle spectrometer- high angular acceptance- high energy resolution for Ekin < 10 eV
Tfs
Ar 3p6
Ar+ 3p5
VUV
IR
e-
Cross correlation experiments using high order harmonics
E = 15.8 eV
-50
0
50
H21H19H17H15H13 H23
5 10 15 20
Ekin (eV)
Generation(HHG)
(laser) = 800nm H11 = 17 eV H13 = 20 eV H15 = 23 eV
: :
But also very interesting are:
Type IIB-Experiments-
'Resonant photoionization' in an intense/resonant fields
=>Study intensity controlled autoionization !!
Proposed (approved) experiment at the FEL
Exp.: Two-photon double-resonant excitation FEL : h = 65.1 eV (tunable) Laser : = 750 - 800 nm (tunable)
Coupling of He Doubly Excited States
He 1s2
2s2p
2s3d
Intense Laser
VUV
20 fs (34 meV)
A. I. Magunov, I. Rotter and S. I. StrakhovaJ. Phys. B32, 1489 (1999)
H. Bachau, Lambropoulos and ShakeshaftPRA 34, 4785 (1986)
2s2p 1P – 2s3d 1D
Bachau, Lambropoulos and Shakeshaft, PRA 34, 4785 (1986)
Laser on Resonance (2 = 0)& scan the XUV photon energy
1s2(1S) + hXUV -{2s2p (1P) + hLaser
<=> 2s3d (1D)}
Could this be done with a laser plasma X-VUV source
and a table top OPA ?
In principle YESYou just cross the sample with intense laser (OPA) and weak XUV beams
Need wavelength selection and high (average) X-VUV intensity
Count rate low - ~ 1 ion/laser shot for He with Volint ~ 10 -3 cm-3
Answer
But - the Ca+ 3p-subshell resonances have:1. Cross sections up to 2500 MB .vs. < 0.1MB for He2. Excitation widths up to 100 meV3. A VUV excitation energy (31 eV)
0
500
1000
1500
2000
2500
28 29 30 31 32 33 34
Ca+. ALS Measurement. 5 meV per point.
SigRaw (MB)
Photon Energy (eV)
Scheme- Ca+:
3p64s (2S) + hXUV (33.2 eV)
{3p53d4s (2P) + hLaser (3 eV) <=> 3p53d5p (2D)}
Exploratory study in DCU - Summer 2005
Scheme- Ca+:
Photoionization SummarySingle VUV - X-ray photon photoionization (and concomitant correlation) in atoms and ions is still a rich source of physics
Photoionization of atoms (much less so ions) in intense IR/VIS laser fields is now well established also (MPI .vs. Tunnelling)
New things to so - Cross-over of the above two ?Atoms in intense VUV/XUV (high frequency) fields - first result - Nature 2002
Resonant/ non-resonant photoionization of atoms in intenseresonant/non-resonant laser fields
Why bother ? Pushing limits - exploring new spaces - new science & technol.
Ideas that come to mind at this workshop
FEL Opportunities1. PIFS on weak resonances2. PIFS on ion beams - Kr-like ions
DLP1. Exploring Xe-like, 5s-subshell excitation
Table Top PIFS1. PIFS with laser plasma source2. Or with tuneable (upconverted) VUV