AMO physics with LCLS

Transcription

1 AMO physics with LCLS Phil Bucksbaum Director, Stanford PULSE Center SLAC Strong fields for x-rays LCLS experimental program Experimental capabilities End-station layout PULSE Ultrafast X-ray Summer June 20, 2007 AMO Instrument Lead Scientist: John Bozek (SLAC) AMO Instrument Design Team Leaders: Louis DiMauro (OSU) Nora Berrah (WMU) Linda Young (ANL)

2 2 Atomic Physics at X-ray FEL s LCLS first experiments will extend the physics of this Summer into the domain of hard x-rays Basic atomic physics with short wavelengths and intense laser fields Nonlinear processes Multiple ionization processes in atoms Time-domain experiments with x-rays and atoms Two-laser experiments: sidebands and crosscorrelations

3 3 Strong laser fields wiggle electrons Force in an oscillating field Velocity Position Oscillation amplitude Time-averaged energy (Ponderomotive Potential)

4 4 Stong-field ionization: Classical limit Atomic unit of field F a is the field experience by an electron in the ground state of H in a Bohr orbit 9 F a = v / cm Laser Intensity (averaged over one laser cycle) corresponding to this field strength (independent of ω): I a = W / cm Strong-field ionization happens when the Coulomb barrier is depressed by the binding energy: 2 ( I. P.) Fa 8 Fc = = = v / cm 4 16 Ia 14 2 Ic = = W / cm 2 16

5 5 Keldysh Parameter γ merges these views γ is the tunneling time measured in optical cycles. Multiphoton ionization Tunneling ionization γ > 1 Keldysh parameter: γ < 1 γ = I p 2U p = 1.1for F λ = 800nm

6 6 LCLS parameters for strong-field studies photon energy: 800 ev-8000ev number of photons: /shot pulse energy: 1 mj peak power: 5 GW focused spot size: 1 μm flux: cm -2 s -1 intensity: W/cm 2 period: 2 as number of cycles: 40,000 ponderomotive energy: 25 mev displacement: au temporal coherence: ~1fsec

7 7 LCLS doesn t look strong by these measures Laser γ(ev) I (W/cm 2 ) α 0 (Bohr) Keldysh γ U P (ev) x > Ti:Sapphire x ~ x < x ~3100 <<1 ~210k LCLS (1 micron focus) x > x >>

8 8 Strong field wavelength scaling: frequency (au) Keldysh picture optical frequency γ = (I tunneling frequency p /2U p ) 1/2 λ γ> photon description μλ MPI Ti:sap & Nd excimer field amplitude (au) mir CO 2 tunnel tunnel μλ lcls γ<1 MPI dc-tunneling picture keldysh parameter, γ frequency (au) data compiled based on both electron and ion experiments

9 9 Can the LCLS get to the strong-field regime? impose a Keldysh parameter of one γ 1= (I p /2U p ) 1/2 U p 400 ev (8 800 ev, intensity needed is W/cm 2 (10 14 W/cm 2 ) number of photons is fixed, require tighter focus and shorter pulse τ lcls ~ 10 fs (very possible) thus require a beam waist of 50 nm (in principle possible) 10 1 lcls II 10 1 frequency (au) μλ MPI tunnel field amplitude (au) tunnel μλ MPI keldysh parameter, γ frequency (au)

10 10 Changing strong field regimes inside atoms IR: Low frequency regime VUV FEL: Intense photon source X-RAY FEL: Highly ionizing source -I p -I p -I p 10 x20 W/cm 2 Γ U Γ e A+ P > nγ e A W/cm 2 > ω hω ATI, HHG laser laser Γ n+ 1 γ e A+ Γ U Γ e A+ P e A W/cm 2 << < ω hω Γ laser laser 2 e A+ +? sequential vs. non - sequential Γ U Γ e A+ P e A+ << << hω Γ ω laser laser Auger,Γ Hollow atoms? 2 e A+ +

11 11 Physics comes from inner shells laser multiphoton ionization neon photoabsorption n=2 n=1 x-ray multiphoton ionization

12 12 x-ray strong field experiment x-ray multiphoton ionization photoionization correlated ionization Auger sequential 2-photon, 2-electron

13 13 Baseline properties of LCLS radiation Photon Energy 825 ev 8250 ev Repetition rate 120 Hz 120 Hz Photons per pulse Pulse duration (rms) 137 fs 73 fs Beam divergence 5.7 μrad 0.8 μrad 1 st harmonic bandwidth 0.07% 0.03% Peak power in 1 st harmonic 4 GW 8 GW

14 AMOS end-station: single-shot measurements 6/20/2007 PULSE Ultrafast X-ray Summer 14

15 15 AMO Instrument Preliminary design of AMO instrument with Five TOF electron spectrometers One of three ion spectrometers Pulsed supersonic gas jet (from below) X-ray fluorescence spectrometer(s)

16 16 Diagnostics Chamber Would like to measure properties of LCLS beam on shot-byshot basis pulse power total beam energy bolometer temporal properties (duration, temporal overlap with laser ) magnetic bottle etof photon wavelength & bandwidth magnetic bottle etof beam position and trajectory CCD imaged screens beam size & divergence wavefront sensor Transparent AMO sample allows diagnostics to be after the experiment

17 AMO in Hutch 2 Layout 6/20/2007 PULSE Ultrafast X-ray Summer 17

18 Hutch Assignments 6/20/2007 PULSE Ultrafast X-ray Summer 18

19 19 AMO end-station refocus optics Need adaptive optics to change spot size (control power), move focus ~1m between end-station and diagnostics Ideally two sets of optics for ~1μm focus and <100nm focus

20 20 AMO end-station Field Strengths LCLS Pulse: ev 5 B 4 C mirrors with ~92% reflectivity = ph in 137fs (rms) pulse Intensity will depend on focus size in end-station Focus W/cm 2 1mm μm μm μm nm

21 21 Scientific Goals of the AMO Instrument Investigate multiphoton and high-field x-ray processes in atoms, molecules and clusters Multi-photon ionization/excitation in atoms/molecules/clusters are well known in optical and recently EUV regime little known about multi-photon inner-shell processes Accessible intensity on verge of high-field regime where field of light interacts with electrons in the sample Study time-resolved phenomena in atoms, molecules and clusters using ultrafast x-rays Inner-shell side band experiments using LCLS and laser photons Photoionization of aligned molecules Temporal evolution of state-prepared systems

22 22 1-photon, 1-electron ionization consider a 1-photon K-shell transition: σ K cm 2 Γ K = σ K F lcls s -1 t K = 1/ Γ K = 1 fs (saturated) photoionization rapid enough to ionize more than one electron! fast enough to compete with atomic relaxation? neon photoabsorption L-shell K-shell

23 23 Coherent effects? 3d generation x-ray sources have << 1 photon per mode; LCLS will have coherence spikes containing a billion photons or more, in ~femtoseconds! What about coherent excitation? Rabi rate Ω μ = f ex i F 0 μf E 1 binding 1then implies Ω for 1keV fsec

24 24 Two-photon ionization Γ (2) = σ (2) F 2 σ (2) ~ cm 2 s What s that?

25 25 Two-photon cross section ΔE σ (2) σ ΔE h σ (2) σ h ΔE cm 54 2 ω 10 cm σ = 1Mb 18 s 4 1 s

26 26 2-photon, 1-electron ionization consider a 2-photon K-shell transition: estimate 2-photon cross-section, σ 2 perturbative scaling laws 1 : σ cm 4 s 2 nd -order perturbation theory 2 : σ cm 4 s transition probability: P 2 = σ 2 F 2 τ lcls P. Lambropoulos and X. Tang, J. Opt. Soc. Am. B 4, 821 (1987) 2 S. A. Novikov and A. N. Hopersky, JPB 33, 2287 (2000)

27 27 two-photon ionization 2-photon probability σ 2 10 (52 54 ) cm 4 s 20 nm waist 100 fs, 1A ERL LCLS photons/shot use near resonant enhancement of σ 2 increase number of photons/shot over average power open the possibility of temporal metrology!

28 28 2-photon, 2-electron ionization consider a 2-photon KK-shell transition: σ KK cm 4 s Γ KK = σ KK F 2 lcls s -1 (saturated) t KK 10 as 2-photon, 2-electron so Γ KK >>Γ K σ KK cm 4 s neon S.A. Novikov & A. N. Hopersky, JPB 35, L339 (2002)

29 29 2-photon, 2-electron ionization 2-photon, 2-electron are the electrons correlated? in a strong optical field single electron dynamics dominate. i.e. is it sequential ionization? photoionization Auger photoionization

30 30 Scientific Goals temporal resolution Two photons of different energy present at same time can result in multiphoton ionization (FLASH FEL & laser) Phenomenon provides a means to measure temporal overlap of two pulses i.e. providing measure of temporal overlap between LCLS & laser i.e. cross correlation of 12ps frequency doubled Nd:YLF laser pulse with 35.5eV FLASH pulse in photoionization of He M. Meyer et al, PRA, 74, (R) 2006.

31 31 x-ray-laser metrology Weingartshofer Phys. Rev. Lett. 39, 269 (1977) e- scattering from argon in the presence of a laser.

32 x-ray-laser metrology Ι W/cm 2 Auger Schins, Agostini, et al., 1994: M incoherent x-rays ( ev) Auger electron ~ 200 ev M laser W/cm 2 L photoionization L Auger 6/20/2007 PULSE Ultrafast X-ray Summer 32

33 33 Scientific Goals temporal resolution Improved FLASH using shorter laser pulse Ti:sapphire laser Measured relative jitter between two beams of 250fs using Xe 5p photoionization P. Radcliffe et al, APL, 90, , 2007.

34 34 Sidebands for higher energy x-rays Classical considerations: Number of sideband should INCREASE E E 200eV U obs P = p 1eV pe + p 2m Pond sidebands go out to 40eV 2 ~ E pe + 8E pe U P

35 35 Single-shot x-ray diagnostics? R. Kienberger, MPQ

36 36 More background on the science: The LCLS Science Thrust Areas Coherent scattering at the nanoscale (XPCS) Atomic, Molecular, and Optical Science Pump/probe diffraction dynamics Pump/probe high-energy-density (HED) science Nano-particle and single-molecule (non-periodic) imaging