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John T. Costello

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.

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John T. Costello

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  1. 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 Interchannel interaction in photoionization of atoms and photodissociation of molecules, Riezlern, Austria July 12, 2005

  2. Outline of Talk 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

  3. Collaborators and Contributors Picosecond X-VUV Continuum Sources RAL - E Turcu & W Shaikh QUB - C Lewis, R O'Rourke and A MacPhee DCU - O Meighan and C McGuinness EUV Absorption Spectroscopy Rostov - P Demekhin, B Lagutin and V Sukhorukov DCU - P Yeates, A Neogi, C Banahan, D Kilbane, P van Kampen and E Kennedy VUV Photoabsorption Imaging Facility - VPIF Padua - 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 Treusch Orsay- 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

  4. Laser-Plasma/Atomic Phys-NCPST The LPP node comprises 6 laboratory areas focussed on pulsed laser matter interactions (spectroscopy/ imaging) 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) Funded by: SFI - Frontiers and Investigator HEA - PRTLI and North-South IRCSET - Embark & BRGS Enterprise Ireland - BRGS EU - Marie Curie and RTD

  5. Part I - Laser Plasma Continua

  6. Laser Plasma Source Parameter Range Vacuum or Background Gas Target Plasma Assisted Chemistry Laser Pulse 1064 nm/ 0.01 - 1 J/ 5ps - 10 ns Lens Spot Size = 50 mm (typ.) F: 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

  7. What does a laser plasma look like ?

  8. Intense Laser Plasma Interaction S Elizer, “The Interaction of High Power Lasers with Plasmas”, IOP Series in Plasma Physics (2002)

  9. Laser Produced ‘Rare Earth’ Continua - Physical Origin, History & Update

  10. Laser Plasma Rare Earth Continua P K Carroll et al., Opt.Lett 2, 72 (1978)

  11. 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)

  12. 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

  13. 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)

  14. Brief History/ Highlights of Laser Plasma Rare-Earth’ Continua -1990 First report of line free continua - P K Carroll et al., Opt.Lett 2, 72 (1978) First full study/ applications - P K Carroll et al., Appl.Opt. 19, 1454 (1980) VUV Radiometric Transfer Standard - G O’Sullivan et al., Opt.Lett 7, 31 (1982) Absolute Calibration with Synchrotron - J Fischer et al, Appl.Opt. 23, 4252 (1984) Photoelectron Spectroscopy - Ch. Heckenkamp et al., J.Phys.D 14, L203 (1981) 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)

  15. Recent Developments in LP Continua I psec 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)

  16. Summary - LP Continuum Light Sources Table-top continuum light source now well established Covers Deep-UV to soft X-ray spectral range Pulse duration can be < 100 ps ! Continuum flux ~ 1014 photons/pulse/sr/nm (0.8J/10ns) Low cost laboratory source - needs greater awareness 6. Recent work on (100 ps) + (6ns) Pre-plasma source - we already see a flux gain of up to X4 with Cu- A Murphy et al., Proc SPIE, 4876, 1202 (2003) Problem of plasma debris for work in clean environments - proposals to solve, Michette, O’Sullivan, Attwood,…

  17. Part II- Dual Laser Plasma Photoabsorption Experiments

  18. Part II - Section A Photoabsorption Spectroscopy of Ions

  19. Photoionization of Atomic Ions Still a lot of work to be done here- Nice review by John West in: J.Phys.B:AMOP 34, R45 (2001) Covers DLP Experiments & Merged Synchrotron + Ion Beams

  20. Dual Laser Plasma Principle Flexible Neutral/ Multiplycharged/ Refractory Elements No tuning required No vapour required Backlighting Plasma Io Both Plasmas I = Ioe-snL Relative Absorption Cross Section sNL =Ln(Io/I) Isonuclear Sequences Isoelectronic Sequences Dx, DT, I(W/cm2)  Species choice

  21. XUV DLP setup at DCU

  22. XUV DLP Specifications • Time resolution:~20 ns (LP Continuum duration) • Inter-plasma delay range:0 - 10 sec • Delay time jitter:± 1ns • Monochromator:McPherson™ 2.2m GI • X-VUV photon energy: 25 - 170 eV • Resolving power:~2000 @25 eV (20mm slits) • ~1200 @ 170 eV • Detector:Galileo CEMA with PDA readout • Spatial resolution:~250 m (H) x 250 m (V)

  23. Some sample DLP case studies Kr-like ions Mn ions

  24. Dublin Have published upwards of 100 papers on DLP photoabsorption experiments on selected atoms and ions from all rows of the periodic table. Motivation - almost always exploration of some 'quirk' of the photoionization process in a many electron atom ! Kr-like ions - Rb+, Sr2+, Y3+

  25. Why Specifically Kr-like Ions ? Electronic Configuration 4s24p6 1. Prototypical high-Z closed shell atom - beyond simple Fano theory 2. 30+ years of research in both single and multiphoton ionization 3. Will the photoionization dynamics (q/G) change (a little or a lot ?) 4. How will current many-electron photoionization theory stand up ?

  26. XUV Photoabsorption along Isoelectronic (Kr-like) Sequence (Rostov/ DCU) 4s24p6 + hnVUV 4s4p6np + 4s64p4nln’l’  Kr+(4s24p5) + e’l A Neogi et al., Phys.Rev.A 67, Art. No. 042707 (2003) P Yeates et al., J. Phys. B: At. Mol. Opt. Phys. 37, 4663 (2004)

  27. 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)

  28. What could theory tell us ? Xn q-value

  29. What about cross sections ?

  30. 'Mirroring Resonances' r2q2 0  complete cancellation - no resonances

  31. An update from Aarhus ! Bizau, West & Kilbane (DCU)

  32. 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+ For Y3+ the resonance q values become quite large and the spectrum consists mainly of almost symmetric absorption features 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'

  33. DLP XUV Photoabsorption along isonuclear sequences - Mn2+ (and Mn3+) Ions Mn+, 3p63d54s + hn  3p5(3d64s + 3d54s2) Mn2+, 3p63d5 + hn  3p5(3d6 + 3d54s) 'duplicity of the 3d orbital' - "valence-like by energy but inner shell-like by radial distribution" (Dolmatov, JPB 29, L687 1996)

  34. 3p-subshell photoabsorption - Mn2+

  35. 3p-subshell photoabsorption - Mn2+ Overall we see a good match with Dolmatov at least at the low energy side of the main resonance But why does the experimental trace fall off much more quickly than the theory - Excitation from metastable states ?

  36. Metastables and their effect on 3p-subshell photoabsorption of iron group ions - Mn3+ Mn3+, 3p63d4 + hn  3p5(3d5 + 3d44s) Mn+ Mn2+ Mn3+

  37. What do simple Cowan code calculations give ? First Step Compute 'cross sections' for photoabsorption from ground and low-lying terms of Mn3+ (3d4) - 5D, 3P, 3F, 3G, and 3H

  38. What do you get ?

  39. Next Step To 'reproduce' the experimental plasma spectrum, take a weighted sum over each such cross section

  40. Vary the temperature and compare with expt.

  41. Mn ions - Summary Clear that we are going to have problems with excited state absorption since 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 Same problem will occur in 4d, 5d, 4f and 5f metals However, electronic structure of Mn2+ and Mn3+ ions important in manganites (recent Nature papers), biology (DNA),.....

  42. Part II - Section B (short) VUV Photoabsorption Imaging Collaboration between DCU & Univ. Padua

  43. VUV Absorption Imaging Principle Hirsch et al, J.Appl.Phys. 88, 4953 (2000) - QUB/DCU Collaboration VUV CCD Sample Io(x,y,l,Dt) I(x,y,l,Dt) Pass a collimated VUV beam through the plasma sample and measure the spatial distribution of the absorption. Convert to 'Equivalent Width' images and extract column density (NL) maps - see Rev.Sci. Instrum. 74, 2992 (2003) for details

  44. 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 (50mm 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)

  45. Advantages of using a VUV beam 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 l2 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

  46. VUV absorption Imaging- Ca+ - 33.2 eV 3p64s (2S) - 3p54s3d (2P)

  47. Plume Expansion Profile - Singly Charged Calcium & Barium Ions Plume COG Position (cm) Delay (ns) Ca+ plasma plume velocity experiment: 1.1 x 106 cms-1 simulation: 9 x 105 cms-1 Ba+ plasma plume velocity experiment: 5.7 x 105 cms-1 simulation: 5.4 x 105 cms-1

  48. Advantages of using a VUV beam 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 l2 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

  49. What do we extract from I and Io images ? Absorbance: l Equivalent Width: dl l

  50. 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+*(5p56s6d2P)  Ba2+ (5p61S)+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

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