Ultrafast X-Ray Absorption Spectroscopy at an ERL X-ray

Source

Christian Bressler

ETH Lausanne

Why ultrafast X-ray Spectroscopy ?• Element specific• Applicable to all media (gas, liquids, solids,

biological samples)• Probes the electronic structure via XANES

(in particular valence orbitals)• Probes the local geometric structure

Short time scales ↔ short distance scales• Precision! (< 10 % of typical bond lengths)

2+

ħω

1. IntramolecularCharge Transfer

2. Light-Induced Spin Crossover

3. Solvation Dynamicsin Liquids

Aqueous [Ru(bpy)3]2+

I. Electron Transfer Reactions

• H-atom of coordination chemistry• Photosensitizer• Solar Cells• Catalyst in Redox-Reactions• Model for metalloproteins• Marker in Biology,…

-0.02

0.00

0.02

2850 2900 2950 30000.00

0.25

0.50

0.00

0.25

0.50

x 10L2 edgeL3 edge

Tra

nsie

nt A

bsor

ptio

n / a

.u.

Δt = 50 ps

C D C

BB

C'

B'

A'D'C'A'

Abs

orpt

ion

/ a.u

.

X-Ray Probe Energy /eV

B'c)

b)

Abs

orpt

ion

/ a.u

.a) Δt < 0

Ground, Transient and Excited State XAFS of Ru(bpy)3 (aq)

IIIII

2 830 2 840 2 850 2 860 2 870 2 880 2 890 29 00 29 10 29 20

D

Abso

rptio

n / a

.u.

X -R a y P ro b e E n e rg y / e V

+ 0 .95 eV

I.P .CBA2 p 3 /2

+ 2 .0 eV + 3 eV

+ 2 eV L3-edge spectrum

Saes et al, Phys. Rev. Lett. 90 (2003) 47403

-0.02

0.00

0.02

2850 2900 2950 30000.00

0.25

0.50

0.00

0.25

0.50

x 10L2 edgeL3 edge

Tra

nsie

nt A

bsor

ptio

n / a

.u.

Δt = 50 ps

C D C

BB

C'

B'

A'D'C'A'

Abs

orpt

ion

/ a.u

.

X-Ray Probe Energy /eV

B'c)

b)

Abs

orpt

ion

/ a.u

.a) Δt < 0

Ground, Transient and Excited State XAFS of Ru(bpy)3 (aq)

ElectronicStructure

L2 edgeL3 edge

2835 2840 28450.0

0.1

0.2

0.3

0.4

0.5

Abso

rptio

n / a

.u.

X-Ray Probe Energy / eV2965 2970 2975

0.0

0.1

0.2

Abs

orpt

ion

/ a.u

.

X-Ray Probe Energy / eV

(eg) (eg)(t2g) (t2g)(A’) (B’)

4d 6

W. Gawelda et al., submitted to JACS (2005)

(A’) (B’)

4d 6

Comparison of experimental and simulated line shapes

∆E (10Dq) (ground - excited) = -0.15 eV ∆R (Ru-N)= -0.02 Ǻ

L3 (2p1/2) L2 (2p1/2)

II. Light-induced spin cross-over compounds

[FeII(bpy)3]2+

Photoinduced Low Spin (S=0) → High Spin (S=2) transition

Aqueous [Ru(bpy)3]2+ and [Fe(bpy)3]2+2+

3MLCTMC

3MLCT

LF

LF

300 ns0.6 ns

1GS 1GS

5HS

Ru(bpy)3]2+: Fe(bpy)3]2+:

Photocycle

~0.6 ns

Fe(bpy)3]2+(aq):

(LS) (HS) (<1ps)

(HS) (LS) (700 ps)

0.0

0.5

1.0

1.57000 7200 7400 7600 7800

7000 7200 7400 7600 7800

-0.1

0.0

0.1

0.2 Δt = 50 ps

Tran

sien

t Abs

orpt

ion

/ Δχ

X-Ray probe Energy / eV

[Fe(bpy)3]2+ (aq)

static absorption

Nom

aliz

ed A

bsor

ptio

n

SLS (Nov. 2005)

c = 1-100 mMol/l

Fluorescence Detection

100 ps

700 ps

Fe K-edge XAS

-400 -200 0 200 400 600 800 1000 1200

0.0

0.5

1.0 XAFS optical XAFS fit

Nor

mal

ized

Tra

nsie

nt A

bsor

ptio

n / a

.u.

Time Delay / ps

0 2 4 6 8 10-1

0

1

-1

0

1

2 4 6 8 10

Δt = 50 ps

Δχ(

k) x

k2

Photoelectron Wavevector k / Å-1

Transient FEFF

Δt < 0

Photoelectron Wavevector k / Å-1

χ(k

) x k

2

FEFF Ground State

Transient spectrum

Fe-N Bond increase:

ΔR = 0.2 ± 0.015 Å

Ground state spectrum

III. Solvation Dynamics

5160 5180 5200 5220 5240 5260

NaI / H2O Solid I2

2s 5p

X-Ray Probe Energy / eV

Abs

orpt

ion

/ a.u

.

I- (5p6) + (H2O)n + hν → I0 (5p5) + (H2O)m + e- (aq)

5.18 5.20 5.22 5.24

-0.2

0.0

0.2

0.4

2

0

-2

-4

-6

-8Δt = 50 ps

Sta

tic S

pect

rum

/ a.

u.

X-Ray probe Energy / eV

Iodine L1 edge

Tra

nsie

nt S

igna

l / a

.u.

Time-Resolved XANES of aqueous Iodide

I. Tavernelli, unpublished results (2006)

5180 5200 5220 5240

0.00

0.05

0.10

Iodine L1 edge

5p

Abs

orpt

ion

/ a.u

.

X-Ray Probe Probe Energy / eV

I- / H2O

I0 / H2O

2s

• 2s 5p intensity: occupancy of 5p orbitals• EXAFS: dramatic structural changes

Using an ERL Hard X-Ray Source

• Flux (Photons/pulse)

• Pulse Width

1000 100 10 1 0.1 0.01100

103

106

109

1012

Time-Sliced

SR

Synchrotrons

X-RayPlasma

ERL

X-FEL

Puls

e In

tens

ity /

phot

ons/

(0.1

% b

.w.)

Pulse width / ps

1000 100 10 1 0.1 0.0100

103

106

109

1012

Time-Sliced

SR

SynchrotronRadiation

X-RayPlasma

ERL

X-FEL

Puls

e In

tens

ity /

phot

ons/

(0.1

% b

.w.)

Pulse width / ps

Pulsed X-Ray Sources

Sc Ti V Cr Mn Fe Co Ni Cu Zn Y Zr Nb Mo Tc Ru Rh Pd Ag106

107

108

109

1010

Ultrafast EXAFS on

L3

K

(a)

Num

ber o

f inc

iden

t X-r

ay p

hoto

ns fo

r S/N

= 1

Element

2-20 keV

Transition Metal Compounds

Bressler and Chergui, Chem. Rev. (2004)

f = 10 %

χ = 1 %

S/N = 1

Thiswork

0 10 20 30 40 50 60 70 80 90108

1010

1012

1014

1016

K (EXAFS) L3 (XANES) K (XANES) L3 (XANES)

Num

ber o

f Pho

tons

for S

/N =

1

Atomic Number Z

Liquid Phase XAFS (1 mM)

0 10 20 30 40 50 60 70 80 90108

1010

1012

1014

1016

1018

K edge L3 edge

Gas Phase XAFS (1011 cm-3)

Num

ber o

f Pho

tons

for S

/N =

1

Atomic Number Z

Feasibility Range of Possible Experiments

One Possible ERL Experiment

50-300 fs50 ps

e-

0 2 4 6 8 10-1

0

1

Δt = 50 ps

Δχ(

k) x

k2

Photoelectron Wavevector k / Å-1

Transient FEFF

2 4 6 8 10

-1

0

1

Δt < 0

χ(k

) x k

2

Photoelectron Wavevector k / Å-1

FEFF Ground State ΔR = 0.2 Ǻ

Molecular PhysicsNon-adiabatic dynamics in Molecules

ApplicationsAtomic and Plasma Physics

Molecular PhysicsCondensed Phase Chemical Dynamics

Coordination ChemistryBiological systemsSolid state physics

NanosystemsMagnetic systems

Material science,…

Possible experimental lay-outs

UNDULATOR SAMPLE

DETECTOR

QEXAFSMONOLASER

SASE

SAMPLE

DETECTORQEXAFSMONOLASER

0.0

4.55 4.60 4.65

0

1

S

tatic

Abs

orpt

ion

/ a.u

.

X-Ray probe Energy / keV

Iodine L3 edge

Δt = 50 ps

Tra

nsie

nt S

igna

l / a

.u.

Time-Resolved XANES of aqueous Iodide

0.0

4500 4550 4600 4650 4700 4750 4800

0

1

Sta

tic A

bsor

ptio

n / a

.u.

X-Rray probe Energy / eV

Δt = 50 ps

Tra

nsie

nt S

igna

l / a

.u.

Time-Resolved EXAFS of aqueous Iodide

ABSO

RP

TIO

N

λ X-RAY

Structural Information via X-Ray Absorption (EXAFS)

PHOTON ENERGY

R1

t1

t=0

t2

Pump

R2

• Single scattering events due to higher energy photoelectrons

• Bond distances and coordination numbers from simple FT of energyspectrum

LaserLaser--Pump XPump X--rayray--Probe SetProbe Set--upup

Bend Magnet Beamline 5.3.1 Advanced Light Source, Berkeley

X-RayDetector

UHV

Sample(Liquid Jet)

From Storage Ring

X-Ray ProbeGe(111)Mono

Data Acquisition Strategy

ALS “Camshaft” Mode

T = 656 nsec

2 nsec100 psec

t

2.4 x 10-6

Usable X-Ray Flux =

Laser (1 kHz)

BoxCar

Trigger(2 kHz)

PC

Saes et al, Rev. Sci. Inst. 75 (2004) 24

2825 2850 2875 2900 2925 2950 2975

3x106

4x106

5x106

2825 2850 2875 2900 2925 2950 2975

-4x104

-2x104

02x104

4x104

X-Ray Energy / eV

Tran

smitt

ed P

hoto

ns

ΔI /

Phot

ons

= 3000 Photons-+

Δt = 60 psec

L3

L2

0.0 0.5 1.0 1.5 2.00

100

200

300

400

-1 0 1 2 3 4 5 6 7 8 9 100.0

0.1

0.2

0.3

0.4

0.5

0.6

Readout Signal / V

Even

ts /

Bin

Poisson Fit:μ = 2.818 (0.001)

Pro

babi

lity

Number of Photons