Scientific opportunities with ultrafast electron diffraction & microscopy

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1 Scientific opportunities with ultrafast electron diffraction & microscopy Jim Cao Frontier of ultrafast science MeV UED Transition pathways Rate and time scale Elementary steps Probe dynamics on the atomic and molecular time and length scale, with sub-angstrom spatial and sub-ps temporal resolutions

2 Correlation of microscopic structureproperties of matter Properties of matter are largely determined by the arrangement of constituent atoms and molecules soft conductor black hard insulator transparent

3 Dynamics of transformation The atomic-level knowledge of structural changes is essential to the thorough understanding the dynamics of physical processes ABC AB + C A + B + C I I I + I R I P F F F F Chemical reaction: Pathway, elementary steps, immediate Phase transition Ultimate goal: to reveal and control the materials processes at the level of atomic motions

4 Physical and chemical changes involve changes of structure of matter All these dynamical processes are driven by the atomic motion on the timescale of single vibrational period W. E. King, et al., J. Appl. Phys. 97, (2005) Require atomic level spatial and temporal resolutions: ~ 100 fs and sub-angstrom

5 First ps pump-probe electron diffraction Melting of Thin-Film Al (20 ps) Mourou et al., Appl. Phys. Lett. 41, 44 (1982) Williamson et al., Phys. Rev. Lett. 52, 2364 (1984) Diffraction to reach sub-angstrom spatial resolution. It was modified from a steak camera. At pump fluence 13 mj/cm 2, Al film melt within 20 ps.

6 Ultrafast electron probes: diffraction DC UED ( kev) RF MeV UED ( a few MeV) Electrons/Pulse Pulses/Image few few-10 Time Resolution ~300 fs <100 fs Bond Change < 1Å < 1Å Spectroscopy Not yet May be difficult Pulse compression yes yes Timing Jitter Not an issue Could be large Environmental cell Possible Main Function fs diffraction Single-shot diffraction Limitation Reversible processes Imaging is difficult

7 Ultrafast electron probes: imaging UEM (200 kev) DTEM (200 kev) Electrons/Pulse ~ ~ Pulses/Image Time Resolution ~100 fs ~ 10 ns Image Resolution ~2Å a few nm Spectroscopy yes Not yet Pulse compression No need yes Timing Jitter Not an issue Not an issue Environmental cell Under development Under development Main Function Imaging and Single-shot imaging spectroscopy Limitation Reversible processes Resolution

8 Reveal dynamics at atomic level time and length scales Physics and materials science (~10 talks) melting, diffusionless transformation, photo-induced phase transition, dynamics in correlated system, nucleation and crystal growth, surface charge dynamics, thermal transport Chemistry (~5 talks) reaction dynamics in gas and solids, solvation dynamics Nano-science (~5 talks) Plasmonics, single-particle imaging, nanomechanical motions Biology (~3 talks) macromolecular structure, functioning processes (near future) High energy density physics (~1talk) Dynamics of warm dense matter

9 Photo induced phase transition in Colossal 010 Magnetoresistance (CMR) materials La 1-x Sr x MnO Without pump in FED before time zero 1 ns after time zero T=273K, 1.5 mj/cm 2 pump J. Li, et al., Ultrafast Phenomena XVII, postdeadline sesscion (2010)

10 Photo Induced Structure Phase Transition (PIPT) 1. New nonequilibrium phase by photo excitation 2. Different structure, electronic and magnetic orders 3. Not accessed by changing T K. Nasu, Photo-induced phase transition, 2004 why and how photoexcited few electrons can finally result in an excited domain with a macroscopic size?

11 PIPT generates new and novel state Conventional chemical design and synthesis: changing chemical composition PIPT: realized new states without changing chemical composition, not realized by heating PIPT: selective excitation of specific subsystem by changing momentum, phase, helicity and energy; study the interaction in time domain to gain understanding of transition mechanism open new horizon for materials science Ultrafast structural probe: indispensable PIPT in Colossal Magnetoresistance (CMR) materials

12 Colossal Magnetoresistance (CMR) and phase diagram Large resistivity drop in B field Rich phase diagram for PIPT study Resistivity of Nd 0.5 Pb 0.5 MnO 3 R.M. Kusters, et al., Physica B 155, (1989) La 1-x Sr x MnO 3 (LSMO) A. Urushibara et al., PRB 51, (1995)

13 Perovskite structure and strongly correlated system Mn 3d level determine the magnetic and electronic properties Hund coupling: magnetic moment crystal field split: e g and t 2g levels Jahn-Teller distortion: further split e g and t 2g levels Mn 3d level Mechanism of CMR effect: double exchange, JT distortion, phase separation, still not resolved

14 Structure O-R phase transition transitions y z x At x = 0.16, O phase at T<310K R phase at T > 318 K, and bistable in between. This O-R phase transition can also be triggered by applying B field. Orthorhombic JT distortion a a b Pbmn Orthorhombic a a a R3c Rhombohedral Rhombohedral No JT distortion

15 Static electron diffraction studies T=80 K: Orthorhombic orthorhombic symmetry has a superlattice structure with a lattice constant twice that of the parent perovskite unit cell along the [001] direction M. Arao et al., Phys. Rev. B. 62, 5399 (2000) La Sr MnO 3 T=330 K: Rhombohedral Study the O-R phase transition dynamics by tracing the fractional point

16 Melting of orthorhombic structure Confirmed Ts ~328K without pump in FED system Without pump in FED before time zero 1 ns after time zero T=273K, 1.5 mj/cm 2 pump J. Li, et al., Ultrafast Phenomena XVII, postdeadline sesscion (2010)

17 Temporal evolution of diffraction intensity 1 = ps ps ps No Base temperature dependence Change amplitude is linear on pump bi-exponential relaxation : involves two distinct mechanisms with different time scales Fast component: Photo induced structure change

18 Fast process: photo-induced quench of Jahn-Teller distortion 1. weak pump fluence dependence of 1 2. no sample temperature dependence Local effect: Mn O bond change due to intra and inter site electronic transition destroy the JT, which in turn results in the meting of O phase K. Takenaka, et al., Phys. Rev. B. 62, (2000)

19 Temporal evolution of diffraction intensity 1.2 Normalized intensity mj/cm mj/cm mj/cm 2 2 = ps ps 30 6 ps Time delay (ps) Slow component: growing of new structure domain

20 Slow process: growing of new structure domain Pump fluence dependent relaxation time 2 Time scale can be up to ~1 ns K. Nasu, Photo induced phase transition, 2004

21 Monitoring the dynamics of chemical reactions using MeV UED ABC AB + C A + B + C R I P I I I + I F F F F Study the reaction kinetics and dynamics by following the temporal evolution of molecular structures

22 Dynamics of chemical reactions ABC AB + C A + B + C R I P Kinetics & Dynamics - Reaction rate: K = 1/ - Product distribution - Transient structures - Energy redistribution & elementary steps UED: study the reaction kinetics and dynamics by following the temporal evolution of molecular structures

23 Photo-dissociation dynamics in gas phase - ideal for studying reaction dynamics - isolated, collision-free environment - steering the reaction by changing pump pulse Ultrafast and require ~100 fs resolution M. Dantus, M. J. Rosker, and A. H. Zewail, J. Chem. Phys. 87(4), (1987).

24 FTS and UED Optical probe Indirect monitoring r(t) via electronic energy - Rely on PES, not direct structural probe - Not one-to-one for poly-atomic molecules - Limited knowledge of excited PES - Single channel UED Direct monitoring r(t) via diffraction - Direct and not rely on PES - Multi-channel (each atomic pair diffracts) - Absolute concentration

25 Study of dissociation dynamics with UED ED Product Distribution r ij; n ij t UED Vibration Energy l ij Transient Structures Reaction Rate Energy Redistribution Reaction Dynamics

26 Photodissociation of C 2 F 4 I 2 Two-step reaction pumped at 307 nm C 2F4 I2 C 2F4 I I 2 C 2F4 1 2I Anti sm(s) Gauche f(r) C-I C-F C--I F--I F--I I--I s (Å -1 r (Å) ) Ground state: the isomer ratio: 75% : 25% J. Cao, H. Ihee, & A. H. Zewail. (1999) PNAS, 96,

27 Temporal evolution of radicals: C 2 F 4 I and C 2 F C 2 F 4 I Zero of time Clocking 10 8 C 2 F 4 I delay time (ps) 6 C 2 F = 17 ± 2 ps dealy time (ps) delay time (ps) 2 nd I dissociation time ~17 ps < 25 ps FTS results at 277 nm 2 nd iodine dissociation yield ~82% > 30% FTS results at 277 nm the reaction in our experiment at 307 nm is more likely to be initiated by two-photon absorption. Zhong, D., Ahmad, S. & Zewail, A. H. J. Am. Chem. Soc. 119, (1997)

28 C 2 F 4 I Radical structure: classical Bridged radical structure: stereoselectivity observed in many reactions involving the haloethyl radicals A separate experiment with higher SNR to measure the C 2 F 4 I structure f(r) for non-bridged radical model + f(r) for bridged radical model Skell P. S. & Traynham, J. G. (1984) Acc. Chem. Res. 17, r (Å) r (Å) H. Ihee et al, Science 291, 458 (2001) C 2 F 4 I radical structure is classical Time resolution is bond by the velocity mismatch between the pump optical and probe electron pulse to 1 ps

29 Velocity mismatch t VM = 0 t VM > 0 2 (u) 2 y 1 P d 1 1 (x) d 2 u Cross beam geometry velocity mismatch between pump and probe beam reduce time resolution, M (x,y) x t(p) > 1 ps t(p) = d 1 v 1 - d 2 v 2

30 Making molecular movie Reveal the dynamics of chemical reaction in large molecules by monitoring temporal evolution of molecular structures MeV UED: overcome velocity limit together with fs synchronization, improve the overall time resolution to sub 100 fs Dr. Peter Hamm

31 Summary Rapid advancement of ultrafast electron diffraction and microscopy in the past ten years have provided the unprecedented capability of directly probing the dynamical processes at the relevant atomic time and length scales, opening new scientific opportunities in the fields of biology, chemistry, physics and materials science.