Pump-Probe Experiments in the Gas Phase using the TESLA-FEL

Transcription

1 Pump-Probe Experiments in the Gas Phase using the TESLA-FEL Michael Meyer, L.U.R.E., Orsay, France Introduction Pump-Probe Experiments SR + opt. Laser <--> FEL + opt. Laser FEL characterization Cross correlation experiments on atomic Ar Electronic correlations in excited atomic states Coupling of autoionization states Dynamics of molecular fragmentation Conclusion Time-resolved analysis Quantum beat spectroscopy

2 Pump - Probe - Experiments photoexcited states (atomic case) E "A" e - l = s, d electron interaction Preparation of initial state - manipulation of electron cloud - controlled change of interaction Probe by photoionization hν 2 hν 1 l = p excited state l = s ground state Selection of final state - two-photon excitation - partial cross sections comparison with theoretical models

3 Pump - Probe - Experiments photoexcited states (molecular case) A + + e - 1 photodissociation E AB* B + + e photoinduced processes - characterization of products - dynamics of excited states A + B ---> pulsed sources AB r A-B time-evolution of the process

4 Relevant time scales SR ps HHG FEL fs ms 10-3 s µs 10-6 s ns 10-9 s rotation s Auger autoionisation vibration dissociation intra-molecular processes s radiative relaxation

5 Characteristics of laser and synchrotron sources laser SR FEL E/ E ph/s ph/pulse hν IR - UV (1µm - 300nm) IR - X-Ray ( 1eV - kev) VUV -XUV eV tuneability limited easy limited rep. rate c.w. or pulsed 8.32 MHz pulsed 8.32 MHz pulsed 10 Hz pulse width Ar+ : 250 ps dye : <10 ps 600ps ---> 50ps fs

6 Synchronization FEL - optical Laser bending magnet FEL to user linac electrons Ti:Sa laser 790 nm 830 nm 150 fs FEL electrons visible synchrotron radiation opt. laser to user Cross correlation master clock from injector rack! 300 m long cables HASYLAB/DESY (Coordinator) Research Centre Jülich / BESSY Max-Born-Institute, Berlin Dublin City University synchroscan streak camera! slow feedback Goal: synchonization < 200 fs CNRS / LURE Lund Laser Centre / MAX-Lab Funded by the 5th Framework Programme of the European Commission

7 Characterization of Pump-Probe time-structure and stability using cross correlation experiments Photoionization of Ar presence of IR: e - VUV IR Ar + 3p 5 -shift of IP - broadening of PES peaks - sidebands ----> t(max) = 3fs Ar 3p 6 Ti:Saph: 800nm, 40fs, 1 khz dressing beam: up to 50µJ --> 13 TW/cm 2 E.S. Toma, H.G. Muller, P.M. Paul, P. Berger, M. Cheret, P. Agostini, C. LeBlanc, G. Mullot, G. Cheriaux Phys. Rev. A 62, (2000)

8 Two-photon (femtosecond)photoionization of Ar XUV-IR Cross-Correlation e - 20 H21 H19 H17 (26eV) 15,76 ev IR VUV Ar + 3p Delay / fs electron kinetic energy /ev T (laser) = 30fs Ar 3p 6 Low field regime: Int(IR) W/cm 2 Γ(sideband) 2 = Γ(IR) 2 + Γ(VUV) 2 P.O Keeffe, M. Meyer (LURE), V. Veniard, R. Taïeb, A. Maquet (LCP-MR), R. Lopez-Martens, J. Mauritsson, A. Johannson, R. Lopez-Martens, A. L Huillier (LLC, Lund), PRA69, (R) (2004)

9 Cross correlation experiments using high order harmonics E( T) I( T) I eff ~ T > E ( T) + I SB ( T) ~ f [ I IR ( t - T)] I VUV (t) dt -

10 High Intensity Regime: Effect of the ponderomotive shift on the harmonic peak Photoelectron IR and XUV pulses I IR (max) = 10 TW/cm 2 τ IR = 200 fs τ XUV = 100 fs Resolution E kin = 125 mev Ar +2 P 1/2 Ar +2 P 3/2 LLC - experiment (HHG): I (VUV) ph. / pulse Single pulse : ~ 3 e - / pulse ---> Σ shots I (FEL) ph. / pulse Single shot experiments

11 Application: Electronic Correlation in Excited Atomic States

12 Two-photon double-resonant ionization ( SR + Laser ) 13.4 ev nf' mp' Xe + 5p 5 ( 2 P 1/2 ) R6G 5f' 8p' 12.1 ev 4f [5/2] 2 R6G 7p' Xe + 5p 5 ( 2 P 3/2 ) laser pulse 11.2 ev 5p 5 ( 2 P 3/2 ) 6d [3/2] ev 5p 5 ( 2 P 3/2 ) 5d [3/2] 1 τ 5d = 600ps rad S.R. pulse τ 6d rad = 1.2ns Xe 5s 2 5p 6 ( 1 S 0 )

13 Two-photon excitation of even-parity states (SR + Laser) Xe* 5d --> 4f Xe* 6d --> nf, mp f' [5/2] 2 8f' [5/2] 2 14 Xe* 4f' [5/2] p' [3/2] 1 12p' [3/2] 1 intensity (cts/s laser wavelength (cm-1) 16.8x10 3 9f' 10f' 11f' 12f' 13f' 13p' 14p' 15p' 16p' laser wavelength [cm -1 ] x τ(5d) = 600ps I pulsed / I cw = x10 3 laser wavelength (cm-1) τ(6d) = 1.4 ns

14 Photoexcitation in the valence shell Core-excitation intensity (arb.units) 6s 5d 6d Xe + 6d 8s E = 100meV E < 10 µev photon energy (ev) E = 0.1 mev 5p 5 ns, md autoionisation 4d 9 5p 6 np 5p 5 2 P 3/2 e - 10ps e - Auger decay 5p 5 ns, md 10fs radiative relaxation Xe + Fluo 100ps - 10ns Xe 5p 6 Xe 4d 10 5p 6

15 Two-photon (femtosecond) double-resonant excitation of autoionization states 69.5 ev 67.5 ev 7d laser 7p Xe* 4d 9 5p 6 6p (Γ=0.11eV, τ = 6 fs) ----> Xe* 4d 9 5p 6 7d 2 D 3/2 7p' Xe + 4d 92 D 5/2 alternatives: Li* 1s(2s2p 3 P) 2 P (2.6meV, 250fs) ----> Li* 1s(2p 2 ) 2 D Ar* 2p 5 4s, 3d (hν (exc.) = 245 ev) ----> Ar* 2p 5 np, mf H. Aksela et al., Phys.Rev.A51,1291(1995) 6p Xe + 5p 4 nd??? 5s 65.1eV XUV Auger Xe 5s 2 5p 6 Xe + 5p 4 6p, 7p --> resonant Auger decay of even-parity AIS

16 Dynamics of autoionization states coupling of two AIS by a (strong) laser field He 2s np resonances 2s3d photoionization yield 2s2p 2 1 VUV laser Γ = 20 fs = 34 mev He 1s 2 photon energy (ev) M. Domke et al., PRL 69, 1171 (1992) A.I. Magunov, I. Rotter, S.I. Strakhova, J. Phys. B32, 1489 (1999) H. Bachau, Lambropoulos, Shakeshaft, PRA 34, 4785 (1986)

17 Dynamics of autoionization states Effect of intermediate state and its lifetime Xe Xe Xe E laser 9 4d 6d 9 4d 6p E 9 4d 6d 9 4d 6p E 9 4d 6d 9 4d 6p VUV VUV VUV g.s. VUV g.s. laser 5 5p 6d g.s. two-photon double-resonant VUV + laser direct two-photon VUV + VUV two-photon double-resonant laser (3hν) + VUV

18 Application: Dynamics of Molecular Fragmentation

19 Laser Induced Fluorescence spectroscopy on N 2 + (SR + Laser) N 2 + hν(s.r.) ---> N 2* (Rydberg) ---> N 2+ (X 2 Σ, v = 0, J ) + e A + ndσ g 1 Π u ion signal (arb.units) X 2 Σ g 4 A + ndδ g 1 Π u A 2 Π u N v= photon energy of S.R. (ev) ion yield spectrum 17.5 excitation v = 0 --> v = 4 λ (laser) = 610nm relaxation v = 4 --> v = 1 λ (fluo) = 710nm

20 Analysis of rotational structure of N 2+ (SR + Laser) R12&Q11 R11 R21 P22 R22&Q21 selection rules: Experiment J = -1, 0, +1 P, Q, R branch relative intensity: simulation (T = 80 K) exp (-B J(J+1) / kt) laser wavenumber (cm -1 x10 3 ) c.w. laser high resolution laser spectroscopy on species prepared by synchrotron radiation

21 Resonant Core Excitations in molecules N 2 O** ---> N 2+ + O intensity (arb. units) intensity (10 3 cts/s) sN T π* 1s N T _1 Π * 1sN C π* 1s N C _1 Π * 1s N _1 σ * N T N C N 2 O 1sN σ* photon energy (ev) photon energy (ev) hν (401eV) <10fs laser NNO absorption N*NO Auger NNO + dissociation NN + + O coupling electron <-> ion

22 Dissociation dynamics of core excited molecules Time-resolved studies Fe (CO) 2 (NO) 2 + hν (S.R.) > Fe (CO) 2 (NO) eV (Fe3p) Fe (CO) 2 (NO) 2+ + NO Fe (CO) (NO) CO τ 1 τ 1 Fe (CO) (NO) + + CO + Fe (NO) CO + τ 2 τ 2 Fe + + CO + NO Fe NO τ i = 1ps --> 1ns

23 Dynamics of molecular fragmentation H 2 + hν (FEL) ---> H + H* (2s, 2p, 3s, 3p ) Fluorescence decay Ionization by laser Lα theory laser 2p 2 P 3/2 2p 2 P 1/2 Exp.: H* + hν --> H + + e - IP(2p ) = 3.4 ev λ(las) < 365 nm E E = 10 GHz 0.04 mev Lα Y. S. Kimura Lauer et et al.,j.phys.b al., Phys.Rev.A56, 31, ( 98), ( 97), Fluorescence calculation, beamfoil-experiment Hα, hν (exc) = 16.6 ev 1s 2 S 1/2 quantum beats: Ω 100 ps

24 Interference effects in molecular fragmentation Example: H 2 H 2 + hν (FEL) ---> H + + H* (2l, 3l ) ---> H + + H + + H H + H + H* + H + H + Experiment: H*(3l) + hν (laser) --> H + + e - Doppler-Interference λ(las) 800 nm --> E kin (e - ) = 0.1 ev hν(fel) = 16.8 ev --> E kin (H*) = 0.3 ev 2π / Ω = 2 ps Ca 2 --> Ca* + Ca fluorescence Ω = 2 ω 0 (v / c) 2 v = 1100 m/s 2π / Ω = 385 ps P. Grangier, A. Aspect, J. Vigue, PRL 54, 418 (1985)

25 Conclusion Pump-Probe experiments using femtosecond-x-ray sources Ultrafast dynamics of Photoionisation and Photodissociation - Characterization of FEL pulses Cross correlation experiments - Electronic Relaxation Photoexcitation of Autoionization States Coupling of Autoionization States Dynamics of Rydberg Wavepackets - Molecular Fragmentation Time-Resolved Spectroscopy of Dissoziation Interference Effects in Dissoziation

26 Collaboration SR + Laser Experiment L.U.R.E. M. Gisselbrecht P. O Keeffe A. Marquette S. Aloise Moscow State University (Russia) A. N. Grum-Grzhimailo University of Rostov (Russia) V. Sukhorukhov I. Petrov HHG + Laser LLC, Lund (Sweden) A. L Huillier A. Johansson C.G. Wahlström R. Lopez-Martins J. Mauritsson Laboratoire de Chimie Physique (France) A. Maquet V. Veniard R. Taïeb Proposal: TESLA-FEL 2005 Participating groups: L.U.R.E., Orsay, France M. Meyer, L. Nahon, P. O'Keeffe HASYLAB, Hamburg, Germany J. Feldhaus, E. Plönjes, S. Düsterer Lund Laser Center / MAX-Lab, Lund, Sweden J. Larsson, A. L'Huillier, S. Sorenson Dublin City University, Dublin, Ireland E. Kennedy, J. Costello, P. v. Kampen, J.P. Mosnier Max-Born Institut, Berlin, Germany I. Will, H. Redlin