strong-field physics in the x-ray regime Louis DiMauro ITAMP FEL workshop June 21, 2006 fundamental studies of intense laser-atom interactions generation and application of attosecond pulses ultra-fast optical engineering

LCLS parameters XFELs are an x-ray source of unprecedented brightness only viable sources are accelerator-based

LCLS is great but not perfect  e - undulator B-field w ~ cm SASE FEL lasing builds up from noise

…so theorists beware what we think we are measuring may not be what we are measuring! …and welcome to the 70’s!

Linac Coherent Light Source at SLAC 15 GeV e-beam, 100 meter long undulator output: keV (1.5 A), 10 GW, photons/shot, 230 fs, 120 Hz five thrust areas slated for first operations

AMOP at the LCLS LCLS experimental halls AMOS team located in the near-hall initial experimental end-station on soft x-ray beamline (0.8-2 keV) scientific thrust: fundamental strong-field interactions at short wavelengths

AMOS team experimental menu short term strategy shine light on Roger’s wall (scientific impact) define a prominent role in XFEL development atomic ionization (nonlinear physics/metrology) x-ray scattering from aligned molecules (laser/x-ray experiments) cluster dynamics (non-periodic imaging)

contrast: long and short wavelength strong-field physics ponderomotive potential is everything at long wavelengths e - in Coulomb + laser fields electron ponderomotive energy (au): U p = I/4  2 displacement:  = E/  2 PW/cm 2 titanium sapphire laser: U p ~ 60 eV &  ~ 50 au for 10 2 PW/cm 2 U p &  are zero!

contrast: long and short wavelength strong-field physics laser multiphoton ionization x-ray multiphoton ionization n=2 neon photoabsorption n=1 laser multiphoton ionization

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

needle in the haystack: coincidence measurements reaction microscopy Schmidt-Böcking J. Ullrich et al., JPB 30, 2917 (1997) detect by correlating particle-particle events

coincidence measurement true-to-false ratio: T/F =  t C f  (  1 C + N 1 )(  2 C +N 2 )  t,1,2 ( , ,  )  detection coefficients N 1,2  noise counts C  average count rate f  repetition rate the 120 Hz operation of the LCLS makes coincidence experiments high risk, thus the initial AMOS end-station will not have this capability.

AMOS end-station: single-shot measurements courtesy of John Bozek

LCLS parameters for strong-field studies photon energy:800 eV number of photons:10 13 /shot pulse energy:1 mJ peak power:5 GW focused spot size:1  m flux:10 33 cm -2 s -1 intensity:10 17 W/cm 2 period:2 as number of cycles:40,000 ponderomotive energy:25 meV displacement:0.003 au

global survey of single atom response Keldysh picture optical frequency tunneling frequency    = (I p /2U p ) 1/2  -1 “photon description”      “dc-tunneling picture”      tunnel MPI Ti:sap & Nd excimer  CO 2 data compiled based on both electron and ion experiments mir  lcls

1-photon, 1-electron ionization consider a 1-photon K-shell transition:  K  cm 2 R K =  K F lcls  s -1 (saturated) t K = 1/R K = 1 fs photoionization L-shell neon photoabsorption K-shell rapid enough to ionize more than one electron! fast enough to compete with atomic relaxation? is the external field defining a new time-scale?

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

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

2-photon, 1-electron ionization consider a 2-photon K-shell transition: estimate 2-photon cross-section,  2 perturbative scaling laws 1 :  2  cm 4 s 2 nd -order perturbation theory 2 :  2  cm 4 s 1 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) transition probability: P 2 =  2 F 2  lcls  0.1-1

x-ray above-threshold ionization estimate (2+1)-photon cross-section,  2+1  2+1 =  2  cycle   cm 6 s 2 P 2+1 =  2+1 F 3  lcls  ATI should be observable bound-continuum ATI bound-c-c ATI Auger ATI distinguishable

can the LCLS get to the strong-field regime impose a Keldysh parameter of one    = (I p /2U p ) 1/2  U p  400 eV (8 800 eV, intensity needed is W/cm 2 (10 14 W/cm 2 )  = 0.3 au (19 au) number of photons is fixed, require tighter focus and shorter pulse  lcls ~ 10 fs (very possible) thus require a beam waist of 50 nm (in principle possible) tunnel MPI   lcls II

there are other sources on the horizon Cornell Energy Recovery Linac 10 7 photons/shot in 20 fs repetition rate: MHz

coincidence measurement true-to-false ratio: T/F =  t C f  (  1 C + N 1 )(  2 C +N 2 )  t,1,2 ( , ,  )  detection coefficients N 1,2  noise counts C  average count rate f  repetition rate the 120 Hz operation of the LCLS makes coincidence experiments high risk, thus the initial AMOS end-station will not have this capability. the ERL does have the advantage of high duty cycle! does it have high enough intensity for exploring multiphoton processes?

maybe? consider a 2-photon K-shell transition: estimate 2-photon cross-section,  2 perturbative scaling laws 1 :  2  cm 4 s 2 nd -order perturbation theory 2 :  2  cm 4 s 1 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) ERL parameters: 10 6 /shot, 100 fs, 1A 20 nm waist yields flux ~10 30 cm -2 s -1 (10 15 W/cm 2 ) transition probability: P 2  10 -(5  7)

two-photon ionization LCLS use near resonant enhancement of  2 increase number of photons/shot over average power open the possibility of temporal metrology! ERL

2-photon, 2-electron ionization 2-photon, 2-electron S.A. Novikov & A. N. Hopersky, JPB 35, L339 (2002) consider a 2-photon KK-shell transition:  KK  cm 4 s P KK  0.1 neon  KK  cm 4 s coincidence investigations: atomic aligned molecules cluster dynamics

x-ray-laser metrology photoionization incoherent x-rays ( eV) M L Auger M L Auger electron ~ 200 eV laser W/cm 2   W/cm 2 Auger