1. © Imperial College LondonPage 1 Probing molecular structure and dynamics using laser driven electron recollisions 30 th April 2009 Sarah Baker Quantum Optics and Laser Science group Blackett Laboratory

2. © Imperial College LondonPage 2 Introduction to our group We conduct a variety of experiments aiming to study molecular structure and dynamics, through both high-order harmonic generation, and electron rescattering. Femtolasers compactPRO; 1 kHz, 30 fs, 1 mJ Argon filled hollow core fibre HHG 7 fs 0.25 mJ Chirped mirrors for compression Coherent Legend-HE-USP 1 kHz, 50 fs, 2 mJ Electron rescattering in aligned molecules 14 fs 0.4 mJ 1.2 mJ 0.8 mJ KM Labs Red Dragon 1 kHz, 25 fs, 6 mJ Two-colour HHG <10 fs ~1 mJ 2 mJ 4 mJ

3. © Imperial College LondonPage 3 Introduction to our group Electron rescattering in aligned molecules HHG Two-colour HHG Underlying process is laser driven electron recollision… Returning electron wavepacket can have energy components up to ~ 150 eV, or wavelength 1 angstrom Recollision lasts ~ 1 fs We conduct a variety of experiments aiming to study molecular structure and dynamics, through both high-order harmonic generation, and electron rescattering.

4. © Imperial College LondonPage 4 Outline of talk Introduction 1 Laser driven electron recollisions 2. HHG and electron rescattering Experiments 3. Velocity map imaging of high energy rescattered electrons: towards harnessing these high energy electrons to obtain structural information 4. Probing attosecond dynamics by chirp encoded recollision (PACER): recent developments 5. HHG in larger molecules with 1300nm driving field: experiment conducted at Rutherford Appleton Laboratory, to search for signatures of the orbital structure in HHG signal Conclusions, outlook, acknowledgments, adverts…

5. © Imperial College LondonPage 5 Laser driven electron recollisions Returning electron wavepacket can have energy components up to ~ 150 eV, or wavelength 1 angstroms Recollision lasts ~ 1 fs Tunnel ionisation of atom picks up K.E. Acceleration of electron in laser field Recollision of electron with parent Ionisation can occur for a range of times around the peak of the electric field. Parts of the electron wavepacket born at different times follow different trajectories, and gain varying amounts of energy from the field.

6. © Imperial College LondonPage 6 Propagation in the laser field… o spreads the electron wavepacket in time and momentum o chirps the electron wavepacket Laser driven electron recollisions Continuous range of return energies 0 – 3.17U p At 800 nm, 5 x 10 14 Wcm -2 : U p = 30 eV Min. electron wavelength 1.2 angstroms Sign of chirp reverses during the recollision, at the time corresponding to recollision of maximum energy electrons

7. © Imperial College LondonPage 7 On recollision of the electron wavepacket with the parent ion… HHG and electron rescattering Recombination can occur, resulting in emission of high frequency photons High-order harmonic generation Scattering can occur, either elastically or inelastically. The scattered electron will then once again experience acceleration/deceleration in the laser field

8. © Imperial College LondonPage 8 HHG and electron rescattering Typical harmonic spectrum Typical electron spectrum Li et al, PRA 39, 5751 (1989)Grasbon et al, PRL 91, 173003 (2003). plateau rapid fall cut-off 3.17U p +I p 10U p

9. © Imperial College LondonPage 9 Electron rescattering Grasbon et al, PRL 91, 173003 (2003). 10U p Direct + rescattered electrons Rescattered electrons only ATI peaks

10. © Imperial College LondonPage 10 Structural information through electron rescattering The angular distribution of rescattered electrons may exhibit diffraction peaks/minima; a signature of the molecular internuclear separation [Lein et al, Phys. Rev. A 66, 051404 (2002)]. Spanner et al, J. Phys. B 37, L243 (2004). Best information found at high electron energies 5-6 a.u: 340-490 eV (Up ~ 42eV) Measuring angular distribution of such high energy electrons is experimentally difficult! TOF, scan angle Velocity map imaging COLTRIMS Okunishi et al [J. Phys. B 41, 201004 (2008)] measured ATI in O 2, N 2 up to electron energy 120 eV – but angle scans time consuming

11. © Imperial College LondonPage 11 Structural information through electron rescattering The angular distribution of rescattered electrons may exhibit diffraction peaks/minima; a signature of the molecular internuclear separation [Lein et al, Phys. Rev. A 66, 051404 (2002)]. Spanner et al, J. Phys. B 37, L243 (2004). Best information found at high electron energies Measuring angular distribution of such high energy electrons is experimentally difficult! 5-6 a.u: 340-490 eV (Up ~ 42eV) TOF, scan angle Velocity map imaging COLTRIMS AMOLF group recently investigated ionisation of O 2, N 2, CO 2 by XUV using VMI [JMO 55, 2693 (2008)] up to electron energy 60 eV.

12. © Imperial College LondonPage 12 Structural information through electron rescattering The angular distribution of rescattered electrons may exhibit diffraction peaks/minima; a signature of the molecular internuclear separation [Lein et al, Phys. Rev. A 66, 051404 (2002)]. Spanner et al, J. Phys. B 37, L243 (2004). Best information found at high electron energies Measuring angular distribution of such high energy electrons is experimentally difficult! 5-6 a.u: 340-490 eV (Up ~ 42eV) TOF, scan angle Velocity map imaging COLTRIMS Meckel et al [Science 320, 1478 (2008)] used COLTRIMS to detect diffraction signatures in O 2, N 2 at ~ 100 eV

13. © Imperial College LondonPage 13 A velocity map imaging spectrometer for high energy electrons We have been developing a VMI spectrometer capable of detecting few- hundred eV electrons. Basic design of grid assembly All electrodes electro- polished stainless steel - 15kV -11.7 kV High voltages required Large aperture MCP Flight tube with large diameter: length ratio Pulsed molecular beam in Occasional breakdown at > 13 kV Interaction region (laser into/out of page)

14. © Imperial College LondonPage 14 A velocity map imaging spectrometer for high energy electrons (Very) recently obtained electron images up to -10kV… Coherent Legend-HE-USP 1 kHz, 50 fs, 2 mJ 14 fs 0.1 mJ 0.8 mJ 1.2 mJ 45 deg OAP f=20 cm Skimmed molecular beam (axis out of page) delivered to interaction region Gas jet 1 kHz, backed with 250 mbar Xe CCD Vertical polarisation 1. Peaked structure confined to axis of polarisation 3. Continuous distribution extending to larger radii, emitted more isotropically 2. Weaker emission perpendicular to polarisation

15. © Imperial College LondonPage 15 A velocity map imaging spectrometer for high energy electrons Recently tested up to -10kV… Weak low order ATI peaks Separation 1.9 +/- 0.5 eV 14 fs, 2.2 x 10 14 Wcm -2 Peak of spectrum at 766 nm = 1.6 eV Spectrum at hollow fibre exit For short pulse, high intensity, do not expect clear ATI rings [Grasbon et al, PRL 91, 173003 (2003)]

16. © Imperial College LondonPage 16 A velocity map imaging spectrometer for high energy electrons Recently tested up to -10kV… To obtain angular distribution need to account for varying shift of rescattered electron momentum in different directions Energy distribution along polarisation direction U p computed from spot size (60 x 68 microns), energy (100 uJ at chamber entrance), and pulse duration (14 fs) measurements: U p = 13.1 eV

17. © Imperial College LondonPage 17 A velocity map imaging spectrometer for high energy electrons Recently tested up to -10kV… Energy distribution along polarisation direction Up computed from spot size (60 x 68 microns), energy (100 uJ at chamber entrance), and pulse duration (14 fs) measurements: Up = 13.1 eV Observe plateau and cut- off at 10Up as expected. ATI peaks just visible on log plot Maximum energy electron detected ~140 eV – but limited by laser intensity, not performance of VMI spectrometer. Promising progress towards measuring angular distribution of 200-300 eV electrons

18. © Imperial College LondonPage 18 High order harmonic generation Typical harmonic spectrum Li et al, PRA 39, 5751 (1989) rapid fall plateau cut-off 3.17U p +I p Spectral amplitude within strong field approximation described by Usually assumes single-active electron Assumes continuum states are approximated as plane waves. Ignores influence of laser field upon molecular bound state Within SFA there is a simple (Fourier) relationship between harmonic amplitude and orbital wavefunction  0

19. © Imperial College LondonPage 19 Effect of nuclear dynamics on HHG For a diatomic molecule, ionisation also launches a vibrational (expanding) nuclear wavepacket on the ionic PES. Time interval in which this wavepacket can evolve before electron returns 0-2.6 fs. For light nuclei significant motion can occur in this time window.

20. © Imperial College LondonPage 20 Vibrational wavefunctions (nuclear contribution) Electronic ground states Effect of nuclear dynamics on HHG Electron travel time For a diatomic molecule, ionisation also launches a vibrational (expanding) nuclear wavepacket on the ionic PES. Time interval in which this wavepacket can evolve before electron returns 0-2.6 fs. For light nuclei significant motion can occur in this time window. assuming superposition of plane waves for continuum electron wavefunction:. Harmonic signal proportional to. At the recollision, recombination amplitude given by

21. © Imperial College LondonPage 21 A single harmonic spectrum contains information about nuclear dynamics occurring on ionic PES during electron travel time, through amplitude vs frequency information. However, amplitude vs frequency also depends on a(k): Probing attosecond dynamics by chirp encoded recollision PACER: Compare harmonic yield in two isotopes and assume invariance of a(k) Ratio vs frequency information allows nuclear dynamics to be extracted. First proposed by M. Lein, Phys. Rev. Lett. 94, 053004 (2005). Harmonic emission is chirped: one-to-one mapping between electron travel time and harmonic frequency. Recombination amplitude decreases in time as nuclear wavepacket evolves.

22. © Imperial College LondonPage 22 Probing attosecond dynamics by chirp encoded recollision Laser electric field Time PACER: H2+H2+

23. © Imperial College LondonPage 23 Probing attosecond dynamics by chirp encoded recollision PACER: H2+H2+ Laser electric field Time D2+D2+ Detecting that motion occurs between emission of successive harmonic orders. Signal Ratio D 2 /H 2 Return time 1 Signature of the slower nuclear dynamics of D 2

24. © Imperial College LondonPage 24 PACER experimental set-up Gas jet rep rate 2 Hz (limited by pumping speed). Detect 17 th harmonic and beyond. Focus 9mm before jet to isolate short electron trajectories. CCD OAP f=40 cm Gas jet variable z Variable aperture 250 uJ 8 fs 800 nm 1kHz MCP Experimental set-up: gas jet position and confocal parameter/focal size can be continuously varied.

25. © Imperial College LondonPage 25 First PACER measurement Ratio H 2 /H 2 D2D2 H2H2 8fs, 2 x 10 14 Wcm -2, 800nm, pulsed gas jet source. Focus 9mm before jet to isolate short electron trajectories. Baker et al., Science 312 p 424 (2006). Ratio H 2 /H 2

26. © Imperial College LondonPage 26 First PACER measurement 8fs, 2 x 10 14 Wcm -2, 800nm, pulsed gas jet source. Focus 9mm before jet to isolate short electron trajectories. Baker et al., Science 312 p 424 (2006). Single molecule response, allowing for short trajectories only. Includes two-centre interference effects. Includes effect of reabsorption of generated XUV. Blue curve: averaged over randomly aligned angle distribution. Use BO potentials for H 2 + and D 2 +. Ratio H 2 /H 2

27. © Imperial College LondonPage 27 First PACER measurement Red: time evolution as reconstructed from harmonic spectra, different runs of genetic algorithm Blue: time evolution calculated from known BO potential The increasing ratio is a signature of the slower nuclear motion in D 2 +, and can be used to gain information about the nuclear motion: We have made a measurement with ~100 as time resolution, using 8 fs pulses: launch of electron and nuclear wavepackets is synchronous by nature of the process. Baker et al., Science 312 p 424 (2006). R (bohrs) Time (fs)

28. © Imperial College LondonPage 28 Dynamic interference through PACER Using longer, higher intensity pulses we were able to observe dynamic interference in the PACER signal, as the nuclear wavepacket expands. [Baker et al., PRL 101 053901 (2008)]. PACER can also be used to detect signatures of the structure of the orbital

29. © Imperial College LondonPage 29 PACER in methane Original experiment was also conducted in CH 4 and CD 4 : a harmonic ratio that strongly increased with order was detected. Baker et al., Science 312 p 424 (2006). We postulated that this was evidence of very fast bond angle changes following ionisation

30. © Imperial College LondonPage 30 PACER in methane Recent calculations support the very fast nature of conformational changes in CH 4 (Courtesy of Serguei Patchkovskii) Analytical calculation including only nuclear contribution, within BO approximation An electric field lifts the degeneracy of the 3 branches of the Jahn- Teller distortion allowing them to be treated independently. Very promising result regarding applicability of PACER to larger systems

31. © Imperial College LondonPage 31 HHG for structural information Can lead to complete reconstruction of  0 [Itatani et al. Nature 432, 867 (2004)]; so far in N 2 only. Within SFA there is a simple (Fourier) relationship between harmonic amplitude and orbital wavefunction  0 Larger molecules have low I p, so harmonic cut-off at relatively low orders Impulsively align molecules with pump pulse Time delayed probe pulse for HHG Measure HHG spectrum as function of angle between pump and probe polarisation We have been working towards retrieval of orbitals of more complecated organic molecules [Torres et al, PRL 98, 203007 (2007)].

32. © Imperial College LondonPage 32 HHG in larger molecules at 1300 nm Larger molecules have low I p, so harmonic cut-off at relatively low orders Recent experiment at ARTEMIS facility at Rutherford Appleton Laboratories Use longer wavelength driving field to extend the harmonic cut-off at a fixed intensity: sampling orbital with larger range of momentum components. KMLabs Red Dragon 1 kHz 10 mJ 80 fs 780 nm TOPAS Flat field spec CCD Imaging MCP 85:15 1300 nm 50 fs 1 mJ HR 1300 nm HT 780 nm f = 30 cm Gas jet (1 kHz) or continuous flow

33. © Imperial College LondonPage 33 HHG in larger molecules at 1300 nm We observed 27 th – 67 th harmonics in N 2 O, 27 th – 71 st in C 2 H 2 and good alignment revivals in both gases: N2ON2O C2H2C2H2 Half revival, parallel polarisation of pump and probe Angle scans should allow tomographic reconstruction within SFA to be tested for these polyatomic molecules.

34. © Imperial College LondonPage 34 Conclusions PACER is a promising new technique for probing fast nuclear motion in molecules Time resolution ~100 as May be applicable to larger molecules… We have developed a VMI spectrometer capable of measuring angular and energy distribution of few-hundred eV electrons: useful for obtaining structural information through electron rescattering. PACER with larger range of molecules: C 3 H 4 ? Outlook PACER with long trajectories: compare retrieved R(t) with that from short trajectories -> any detectable effect of the different field strengths at recombination? Full analysis of long wavelength HHG experiment to test tomographic reconstruction for polyatomic molecules Measurement of angular distribution of 200-300 eV electrons in diatomic molecule (N 2 …)

35. © Imperial College LondonPage 35 People Adverts! Delphine Darios; VMI of high energy electrons Imma Procino; Retrieval of molecular axis alignment from Coulomb explosion imaging experiments without cylindrical symmetry Two posters: PACER work: Joe Robinson Manfred Lein Ciprian Chirila Long wavelength HHG: Tom Siegel Ricardo Torres Leonardo Brugnera Imma Procino Jonathan Underwood Staff at ARTEMIS facility E. Springate, I.C. E. Turcu, C. Froud Jon Marangos John Tisch Electron VMI: Delphine Darios Marco Siano David Holland (STFC)