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Atomic Physics with Intense X-rays at LCLS

Atomic Physics with Intense X-rays at LCLS. Philip H. Bucksbaum, University of Michigan, Ann Arbor, MI Roger Falcone, University of California, Berkeley, CA Richard R. Freeman, University of California, Davis, CA Kenneth Kulander, LLNL, Livermore, CA

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Atomic Physics with Intense X-rays at LCLS

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  1. Atomic Physics with Intense X-rays at LCLS Philip H. Bucksbaum, University of Michigan, Ann Arbor, MI Roger Falcone, University of California, Berkeley, CA Richard R. Freeman, University of California, Davis, CA Kenneth Kulander, LLNL, Livermore, CA Linda Young, Argonne National Laboratory, Argonne, IL

  2. Fundamental Science The LCLS, as a high-intensity high-energy photon source, provides a unique opportunity to study fundamental aspects of x-rays interacting with atoms, ions, molecules, and clusters Foundation for all experimental planning The understanding of x-ray–atomic physics interactions is central to experimental designs at the LCLS, as well as all nextgeneration x-ray sources. Dual Motivation to Perform Atomic Physics Studies

  3. The LCLS will Reach Regime that are Currently Unobtainable • Current laser-atom process at I ≥ 1014 W/cm2 amplitude of free e- • Field modulates the atomic potential at visible laser frequency • Outer e- has time to tunnel free: • 2Up > Ip where Up µ (Il2)2 • Strong interaction between free e- and ion core is of interest ion core • LCLS-atom process at I ≥ 1014 W/cm2 • Field modulates the atomic potential at x-ray laser frequency • e- do not have time to tunnel free • Important processes are with deeply bound core e-

  4. New Fundamental Processes will be Observable Experiment 1: Multiple ionization sufficiently rapid to form hollow atoms Experiment 2: Multiphoton ionization yielding absorption below the edge Experiment 3: Giant Coulomb explosions of clusters

  5. The Experimental Setup for all these Experiments is the Same Charged particle detector • Tunable LCLS X-ray detector • Detectors • Charge state spectrometer • Electron energy spectrometer • Ion recoil detector • X-ray fluorescence detector • Atom or cluster source

  6. 3x1017 photons/cm2 LCLS detector Ne source (1011Ne/cm2) Experiment 1: Multiple ionization forming hollow atoms Ionization: G= 1012s-1 Auger: G= 4x1014s-1 75 events/pulse

  7. Neon will display the effect well Relatively high photoionizations Non-corrosive monatomic sample Simple, well understood spectrum Relatively long Auger decay rate (2.5 fs) Auger relaxation > 100 x radiative fluorescence Multiple Ionization Forms Hollow Atoms Ne Photoionization n=2 n=1

  8. Possible Ionization Processes for LCLS Interacting with Ne • Photoionization: • Ne + hn>870eVNe+*(K) + e • Auger Decay: • Ne + hn>870eVNe+*(K) + e Ne2+*(LL) + e • Ne3+* + 2e • Sequential multiphoton ionization: • Ne + hn>870eVNe+*(K) + e + hn>993eVNe2+*(KK) + e Ne3+ + e •  Ne4+ + 2e •  Ne5+ + 3e •  … • Ne + hn>870eVNe+*(K) + e + hn>993eV Ne3+*(KLL) + e • Direct multiphoton ionization: • Ne + 2 hn>932eVNe2+*(KK) + 2e LCLS only

  9. One Photon or Two? An Extremely Difficult Question for Multielectron/multiphoton Systems • The intensity of the LCLS makes numerous processes possible/probable • For example: (KL) double vacancies are possible: • Ne +hn>910ev Ne2+*(KL) + 2e (10%) • Ne3+ + e (9.5%) • Ne4+ + e (0.5%) • Experimentally background signals of this type can be rejected by electron spectroscopy • Calculationally simulations of the LCLS atom interactions and the core relaxations are necessary • LCLS will allow the study of detailed multiphoton atomic core processes

  10. Experiment 2: Focused beam experiments LCLS Saturation: photoionization rate equals the Auger decay rate Kr source Kirkpatrick-Baez mirror pairs (demagnification factor of ≈ 100) detector 2x106 events/pulse • Focusing permits observation of two-photon photoabsorption

  11. 2-Photon Absorption in Kr Kr energy levels Kr photoabsorption n = 4 1700 ev n=3 n = 3 n=2 n = 2 n = 1 hn 2hn 2 LCLS photons with hn>850eV Schematic of Kr ionization process

  12. 2-Photon Absorption – Detecting Events • Excitation mechanism  Kr +2hn>850eVKr+*(L) + e • Detection signatures: radiation and2x106 1.5 keV e-/pulse • Kr+*(L) + e  Kr+*(MM) + e(1.5keV) • Kr+*(L) + e  Kr+*(M)+hn1.5keV radiation particles

  13. Huge enhancements associated with single photon resonances S. A. Novikov, J. Phys. B. 33 (2000) 2-photon rate exceeds 1-photon rate! Rate can be affected by coherence and enhancement due to correlation Neon 2-photon cross-section 10+16 2 10+10 10+4 2 s1s (10-52 cm4/s) 10-2 4 DE1s-2p 3 10-8 884 872 858 844 830 Photon energy (eV) Theory of Resonant 2-Photon Processes Requires Data Only LCLS Can Provide • 2-photon ionization couples to an intermediate state • 1s22s22p6 1s2s22p6np 1s2s22p6+e-

  14. Experiment 3: Intense X-ray beam interacting with clusters • LCLS • focused and unfocused detector • Detectors • Charge state spectrometer • Electron energy spectrometer • Ion recoil detector • cluster source

  15. Xe clusters (109 atoms) Each atom exposed to the unfocused beam will undergo: ~1 ionization event (1031 photons/cm2/s x10-19 cm2 x10-13 s)  the ionization will saturate The dominant relaxation mechanism is Auger decay Therefore, each ionized atom creates 2 or more electrons The cluster becomes a ball of charge with ~ 109 ions Yields fast electrons, fast ions, and x-rays Cluster Explosion Experiment with Unfocused Beam • Due to x-ray penetration the Coulomb explosion >> conventional lasers

  16. Focusing the LCLS beam to 0.01 m Each atom in the cluster will be classically-ionized nearly 104 times over The atom will continue to ionize, as the ~0.1 fs Auger rates are ~ 1000 times faster than the ionization rate Thus, each atom will ionize until it strips down to the core level of the initial ionization event Understanding these processes in detail is central to the imaging of bio-molecular samples Cluster Explosion Experiment with Focused Beam Lysozyme molecule irradiated by LCLS

  17. Fundamental Science LCLS is a unique opportunity to study new fundamental multiple photon x-ray phenomena. Foundation for all experimental planning Ionization and cluster dynamics are central to experimental designs at the LCLS, as well as all next generation x-ray sources. Summary: Dual Payoff From Atom Studies

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