Scientific opportunities with ultrafast electron diffraction & microscopy

Probe dynamics on the atomic and molecular time and length scale, with sub-Angstrom spatial and sub-ps temporal resolutions

Frontier of ultrafast science

Transition pathways Rate and time scaleElementary steps

MeV UEDJim Cao

Correlation of microscopic structure-properties of matter

Properties of matter are largely determined by the arrangement of constituent atoms and molecules

softconductorblack

hardinsulatortransparent

Dynamics of transformation

Phase transition

The atomic-level knowledge of structural changes is essential to the thorough understanding the dynamics of physical processes

ABC AB + C A + B + C

IR P

+III

FF

I

F F

Chemical reaction:Pathway, elementary steps, immediate

Ultimate goal: to reveal and control the materials processes at the level of atomic motions

Physical and chemical changes involve changes of structure of matter

W. E. King, et al., J. Appl. Phys. 97, 111101 (2005)

Require atomic level spatial and temporal resolutions:~ 100 fs and sub-Angstrom

All these dynamical

processes are driven

by the atomic

motion on the

timescale of single

vibrational period

First ps pump-probe electron diffractionMelting 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/cm2, Al film melt within 20 ps.

Ultrafast electron probes: diffraction

DC UED( 30- 100 keV)

RF MeV UED( a few MeV)

Electrons/Pulse 103 – 105 106 - 108

Pulses/Image few - 104 1 – few-10

Time Resolution ~300 fs <100 fsBond Change < 1Å < 1ÅSpectroscopy Not yet May be difficultPulse compression yes yes

Timing Jitter Not an issue Could be largeEnvironmental cell Possible

Main Function fs diffraction Single-shot diffraction

Limitation Reversible processes Imaging is difficult

Ultrafast electron probes: imaging

UEM(200 keV)

DTEM(200 keV)

Electrons/Pulse ~10- 103 ~107- 109

Pulses/Image 105+ 1

Time Resolution ~100 fs ~ 10 nsImage Resolution ~2Å a few nmSpectroscopy yes Not yetPulse compression No need yes

Timing Jitter Not an issue Not an issueEnvironmental cell Under development Under development

Main Function Imaging and spectroscopy

Single-shot imaging

Limitation Reversible processes Resolution

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

High energy density physics (~1talk)Dynamics of warm dense matter

Nano-science (~5 talks)Plasmonics, single-particle imaging, nanomechanical motions

Biology (~3 talks)macromolecular structure, functioning processes (near future)

Chemistry (~5 talks)reaction dynamics in gas and solids, solvation dynamics

1 ns after time zero

010

020

030

011

021

031

2101

2102

2103

before time zero

T=273K,  1.5 mJ/cm2 pump

010

001

J. Li, et al., Ultrafast Phenomena XVII, postdeadline sesscion (2010)

Without pump in FED

Photo induced phase transition in Colossal Magnetoresistance (CMR) materials

La1-xSrxMnO3

Photo Induced Structure Phase Transition (PIPT)

1. New nonequilibrium phase by photo excitation2. Different structure, electronic and magnetic orders3. 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?

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

open new horizon for materials science Ultrafast structural probe: indispensable

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

PIPT in Colossal Magnetoresistance (CMR) materials

Colossal Magnetoresistance (CMR)and phase diagram

R.M. Kusters, et al., Physica B 155, 362-365 (1989)

Resistivity of Nd0.5Pb0.5MnO3

A. Urushibara et al., PRB 51, 14103 (1995)

La1-xSrxMnO3(LSMO)

Large resistivity drop in B field Rich phase diagram for PIPT study

Perovskite structure and strongly correlated systemMn 3d level

Mn 3d level determine the magnetic and electronic properties

Hund coupling: magnetic moment

crystal field split: eg and t2glevels

Jahn-Teller distortion: further split eg and t2g levels

Mechanism of CMR effect: double exchange, JT distortion, phase separation, still not resolved

Structure O-R phase transition transitions

OrthorhombicJT distortion

bicOrthorPbmnbaa hom

RhombohedralNo JT distortion

bohedralRcRaaa hom3

2

10

3

xz

y

At x = 0.16, O phase at T<310KR phase at T > 318 K, and bi-stable in between.This O-R phase transition can also be triggered by applying B field.

Static electron diffraction studies

2100

T=80 K: Orthorhombic

M. Arao et al., Phys. Rev. B. 62, 5399 (2000)

T=330 K: RhombohedralLa0.825Sr0.175MnO3

orthorhombic symmetry has a superlatticestructure with a lattice constant twice that of the parent perovskiteunit cell along the [001] direction

Study the O-R phase transition dynamics by tracing the fractional point

Melting of orthorhombic structure

1 ns after time zero

010

020

030

011

021

031

2101

2102

2103

before time zero

T=273K,  1.5 mJ/cm2 pump

010

001

Confirmed Ts ~328K without pump in FED system

J. Li, et al., Ultrafast Phenomena XVII, postdeadline sesscion (2010)

Without pump in FED

Temporal evolution of diffraction intensity

1=1.56    0.17 ps1.55    0.28 ps1.48    0.19 ps

Fast component: Photo‐induced structure change

No Base temperature dependenceChange amplitude is linear on pump

bi-exponential relaxation : involves two distinct mechanisms with different time scales

Fast process: photo-induced quench of Jahn-Teller distortion

K. Takenaka, et al., Phys. Rev. B. 62, 13864 (2000)

1. weak pump fluence dependence of 12. 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

Temporal evolution of diffraction intensity

2=960      140  ps102     32   ps30     6 ps

0 500 1000 1500-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Nor

mal

ized

inte

nsity

Time delay (ps)

0.75 mJ/cm2

1.12 mJ/cm2

1.50 mJ/cm2

Slow component: growing of new structure domain

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

Monitoring the dynamics of chemical reactions using MeV UED

ABC AB + C A + B + C

IR P

Study the reaction kinetics and dynamics by following the temporal evolution of molecular structures

+III

FF

I

F F

Dynamics of chemical reactions

ABC AB + C A + B + C

IR 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

Photo-dissociation dynamics in gas phase

- ideal for studying reaction dynamics - isolated, collision-free environment- steering the reaction by changing pump pulse

M. Dantus, M. J. Rosker, and A. H. Zewail, J. Chem. Phys. 87(4), 2395-2397 (1987).

Ultrafast and require ~100 fsresolution

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

Study of dissociation dynamics with UED

EEEEEEDDDDDD

ProductDistribution rij; nij

VibrationEnergy lij

Reaction Rate

Transient Structures

EnergyRedistribution

UUUUUUEEEEEEDDDDDD

t

Reaction Dynamics

0 1 2 3 4 5 6 7 8 9

s (Å-1)

sM(s

)

0 1 2 3 4 5 6 7

r (Å)

f(r) C-F

C--IF--I

I--I

C-I F--I

Photodissociation of C2F4I2

Ground state: the isomer ratio: 75% : 25%

Anti Gauche

2I FCI IFCIFC 424224221

Two-step reaction pumped at 307 nm

J. Cao, H. Ihee, & A. H. Zewail. (1999) PNAS, 96, 338-342.

0

2

4

6

8

10

-30 30 90 150 210 270 330

dealy time (ps)

+

Clocking

-20 -10 0 10 20delay time (ps)

Zero of timeC2F4I

Temporal evolution of radicals:C2F4I and C2F4

0

2

4

6

8

10

-30 30 90 150 210 270 330

delay time (ps)

2 = 17 ± 2 ps

C2F4

C2F4I2

2nd I dissociation time ~17 ps < 25 ps FTS results at 277 nm2nd iodine dissociation yield ~82% > 30% FTS results at 277 nmthe 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, 5978-5979 (1997)

C2F4 I Radical structure: classical

f(r) for bridged radical model

0 1 2 3 4 5 6 7

r (Å)

f(r) for non-bridged radical model

0 1 2 3 4 5 6 7

r (Å)

+

H. Ihee et al, Science 291, 458 (2001)

Bridged radical structure: stereo-selectivity observed in many reactions involving the haloethyl radicals

Skell P. S. & Traynham, J. G. (1984) Acc. Chem. Res. 17, 160-166

C2F4 I radical structure is classical

A separate experiment with higher SNR to measure the C2F4I structure

Time resolution is bond by the velocity mismatch between the pump optical and probe electron pulse to 1 ps

1(x)1

x

y

uP

d 1

M(x ,y)

d 2

2(u)2

t(P) = d1v1

- d2v2

Cross beam geometry velocity mismatch between pump and probe beam reduce time resolution,

t(P) > 1 ps

Velocity mismatch

∆tVM = 0

∆tVM > 0

Making molecular movie

Dr. Peter Hamm

Reveal the dynamics of chemical reaction in large molecules by monitoring temporal evolution of molecular structures

MeV UED: overcome velocity limit together with fssynchronization, improve the overall time resolution to sub 100 fs

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.