imre femtochemistry final 100300

7/28/2019 Imre Femtochemistry Final 100300

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LCLS

Dan Imre, Brookhaven National Laboratory

Philip Anfinrud, National Institutes of Health

John Arthur, Stanford Synchrotron Radiation LaboratoryJerry Hastings, Brookhaven National Laboratory

Chi-Chang Kao, Brookhaven National Laboratory

Richard Neutze, Uppsala University, Sweden

Mark Renner, Brookhaven National LaboratoryWilson-Squire Group, University of California at San Diego

Ahmed Zewail, California Institute of Technology

Femtochemistry


2/26

LCLSA Chemists View of Nature

Description of static molecular properties in terms of bondlengths and angles has served us well.

Virtually every new discovery in biology and chemistry can betraced to a structure being solved.


3/26

LCLSChemistry is about Motion

Chemical transformations are about dynamics, i.e. rapid

changes in bond lengths and bond angles.What is needed is a tool that will make possible a simpleconnection between the static picture and its time evolution.


4/26

LCLSChemistry is about Motion

The ultimate goal of any molecular dynamics study is to produce amotion picture of the nuclear motions as a function of time.


5/26

LCLSSpectroscopy of the Transition State

Femtosecond lasers are fast enough

BUT

Their greater than 200-nm wavelength does not allow for anyspatial information

Capturing molecules in the processof reacting has been along-time dream


6/26

LCLSSpectroscopy of the Transition State

Spectroscopy of the transition state is an attempt to compensatefor the inability of lasers to provide the spatially neededresolution

Ultrafast Electron Diffraction (UED) is the only experimentalsystem that attempts to break that limit


7/26

LCLSTemporal and Spatial Scales

Putting things in perspective

What are the time-scales?

What are the length-scales?


8/26

LCLSTemporal and Spatial Scales

Time in femtoseconds, distance in

H2OOH + H

CH2I2CH2I + I


9/26

LCLSTemporal and Spatial Resolution

TIME

The very light systems require a time resolution of a fewfemtoseconds, while heavier ones can be studied with pulses a

few hundred femtosecond long.

BOND LENGTH

The LCLS will make it possible to map out the nuclear motionswith a resolution of 0.1 , which is clearly sufficient.


10/26

LCLSUED Experimental Set-up

Ultrafast Electron Diffraction

H. Zewail


11/26

LCLSUED CH

2

I-CH2

I Photodissociation

femtoseconds tens picoseconds

UED will never break thepsec time limit because of

the fundamentalrelationship between thenumber of electrons in thebunch and pulse length.

The LCLSis the only tool with the required temporal andspatial resolution


12/26

LCLSComparison between UED and LCLS

Comparison between Ultrafast Electron Diffraction (UED)and the LCLS

1 time resolution; 2 relative crossection; 3 relative signals

The predicted signals are comparable but the LCLStimeresolution is at least 50 times better.

W )OX[ &URVVVHFWLRQ

5DWH

+]

6LJQDO

8(' SV

/&/6 IV


13/26

LCLSProposed Experiments

Exp 1. Gas phase photochemistry

Exp 2. Condensed phase photochemistry

Exp 3. Dynamics in nanoparticles


14/26

LCLSPump-Probe Experiments

ProbeLCLS

Pumplaser

Samp

le

CCD

The femtochemistry experiments use an ultrafast laserto initiate the process and the LCLSbeam as a probe


15/26

LCLSExperimental Approaches

Time resolved diffraction

Time resolved Mie scattering (small angle scattering)


16/26

LCLS

Photodissociation of an isolateddiatomic molecule is the simplest ofchemical reactions.

t=0 is easily defined

The initial wave-function is well

defined

The wave-function remains localized

throughout the reaction

The LCLSis ideally suited toinvestigate these reactions

Experiment 1. Gas phase photodissociation reactions

Absorption

ResonanceRama

n

t0

t1

t2

t3

t4 t5


17/26

LCLS

The coupling between nuclear andelectronic motion is a universalphenomenon that dominatesalmost all photochemistry.

It is essential that we develop anintuitive picture of this behavior

LCLSwill make it possible todirectly observe this complexmotion.

Nuclear and Electronic Coupling is Universal Phenomenon


18/26

LCLS

The solvent cage changes thedynamics and provides a meansto study recombination reactions.

Experiment 2. Condensed phase photochemistry


19/26

LCLS

I2 in dichloromethane (Neutze et al.)

Third Generation Sources Have a Limited Time Resolution

2.0 3.0 4.0 5.0

0

10

20

30

A/A1u

X

B

Energy(103c

m-1)

Internuclear separation (A)

0

4

11 2

2 3

3 4

4 0

23

300fsec

100psec

200psec

200psec

Diffuse X-ray scattering with80 psec time

resolution fromEuropean Synchrotron

Radiation Facility


20/26

LCLSAn Example from E S R F

I2 in dichloromethane (Neutze et al.)

2.0 3.0 4.0 5.0

0

10

20

30

A/A1u

X

B

Energy

(103

cm-1)

Internuclear separation (A)

0

4

1

23

0.0 0.2 0.4 0.6

1

0

1

Time nsec

Relativesignal


21/26

LCLSExperiment 3. Dynamics in nanoparticles

Nanoparticles

Semiconductors and metal nanocrystals also known asquantum dots possess unique size-dependent electronicand optical properties that result from quantum size

confinement of charge carriers and very large surface tovolume ratios.

These properties hold great promise for applications inareas such as microelectronics, electro-optics,

photocatalysis, and photoelectrochemistry. They are alsoparticularly attractive, because of their large surface areaand fast charge transport properties, for photovoltaicsand photo-degradation of chemical wastes and pollutants.


22/26

LCLSExperiment 3. Dynamics in nanoparticles

The size distribution problem

Under most experimental conditions size dependent propertiestend to be masked by the presence of a wide size distribution.The high intensity of the LCLSwill make it possible to conductexperiments on single particles.

The solvent effect

Under most experimental conditions the high surface to

volume ratio results in extreme sensitivity to solvent. Toprovide for a controlled, reproducible, well defined, inertenvironment, with low scattering background particles will beisolated in Ne crystals for study.


23/26

LCLS

Melting a single nanoparticle

Laser photon

LCLS LCLS

Experiment3. Melting single nanoparticles


24/26

LCLSExperiment 3. Vibrations in nanoparticles

The time evolution of the Mie scattering spectrum at 1.5 willmake it possible to map out internal particle vibrational modesas well as surface capillary modes of a single nanoparticle.

LCLS

LCLS

LCLS

Laserp

hoton

t=0 t=1 t=2 t=3Pump Probe Probe Probe

Signal Signal Signal


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LCLS

550

500

450

400

350

300

250

200

150

100

50

0

Intensity

3.02.82.62.42.22.01.81.61.41.21.0

Particle Diameter (nm)

Mie scattering spectrum at 1.5 A

Simulation of MieScattering at 1.5

Simulated scattering intensity at a single angle as a function ofparticle size. A similarly rich spectrum is obtained for a fixed

particle size as a function of scattering angle.

Mie spectra are extremely sensitive to changes in particle size and

shape.


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LCLSFemtochemistry at the LCLS : Conclusion

The LCLSis the only tool that will, in the foreseeable future,make it possible to observe nuclear motion during areaction in real time.

The LCLScan be applied to a wide range of problems in thefield of chemistry, some of which were touched upon here,from the most fundamental photodissociation reaction, tothe more applied problem of characterizing the propertiesof nanoparticles.

imre femtochemistry final 100300

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