Time Resolved X-Ray Absorption Spectroscopy

Time Resolved X-Ray Absorption Spectroscopy

2006. 10. 16Lee, Youhong

TR-XAS

LITR-XAS

• Laser-Initiated Time-Resolved X-ray Absorption Spectroscopy

TR-XAS

• Beer’s law

TR-XAS

I0 I

0

exp( )I xI

μ= −μ : absorption coefficient

X-Ray Absorption Spectroscopy

x

TR-XAS


TR-XAS


[ ]1/ 202 ( )m E E

k−

=

rkFT

0

0

( ) ( )( )( )

k kkk

μ μχμ−

=

TR-XAS


TR-XAS

K

L

M

hν

• K edge, LI edge, LII edge, …

The Nature of XAS

TR-XAS

XAFS Oscillations

2 22

2 ( )2

sin 2 ( )( ) ( )

j

j j

rj ijk k

j jj j

kr kk N F k e e

krσ λ δ

χ−

− ⎡ ⎤+⎣ ⎦∝∑

• (Extended) X-ray Absorption Fine Structure

XANES

• X-ray Absorption Near Edge Structure• Pre-edge : bound-bound transitions

Edge : ionization threshold• Multiple scattering effects

TR-XAS

Excited State Structures

TR-XAS

Far From Equilibrium

TR-XAS

( , ) ( , ) ( , ) ( , )( , )

( , )

i ii

d N k t R k R Rk t

d N k t

τ τ ζ τ χ ζ τ

τ τ

−=

−

∑ ∫∫

χ

At Transient Equilibrium

TR-XAS

( , ) ( ) ( )j jj

k t f t kχ χ=∑

Experiments in LITR-XAS


TR-XAS Experiments

Major Components

• X-ray source• Laser source• Detectors

TR-XAS Experiments

Experimental Set-Up

TR-XAS Experiments

X-Ray Sources

TR-XAS Experiments

• Continuum radiation• Ultrashort duration• High photon flux

X-Ray Sources

TR-XAS Experiments

Laser-Generated X-Ray Diodes

TR-XAS Experiments

High Harmonic Generation

TR-XAS Experiments

ω Nω

Laser Pump Sources

• The discrepancy in molar extinction coefficients for laser photons and for X-ray photons

TR-XAS Experiments

lCII ε=0

10log

32

ray-X

Laser 10or 10~⎟⎟⎠

⎞⎜⎜⎝

⎛Ο

εε

Laser Pump Sources

TR-XAS Experiments

)()()()](1[),( 010101 ktfktftk gs χχχ +−=

)101(00

lCa h

PNNN ε

ν−−==−

AlaCQPf

fCl

⋅⋅⋅⋅−⋅

=−⋅⋅−

ν

ε ]101[ )1(0

01

01

lCII ε=0

10log

absorbed photons of #consumed reactants of #

=Q

Detectors

• Avalanche PhotoDiode (APD)• Charge-Coupled Device (CCD)• Streak camera

TR-XAS Experiments

Example

TR-XAS Experiments

Taking Molecular Snapshots Using LITR-XAS


Taking Molecular Snapshots

NiTPP-L2


Nickel(II) tetraphenylporphyrin, L=piperidine

NiTPP vs. NiTPP-L2

NiTPP-L2Octahedral

Triplet

NiTPPSquare-planar

Singlet


Photodissociation of NiTPP-L2


XANES near the Ni K-edge


XAFS


CuOEP


Copper(II) octaethylporphyrin

Structures of Cu(II)OEP



XAFS of CuOEP


Transient XANES


Transient XAFS

[RuII(bpy)3]2+


Ruthenium(II) tris-2,2’-bipyridine

RuII vs. RuIII

t2g

eg

p3/2

t2g

eg

p3/2


Photochemistry of [RuII(bpy)3]2+


Transient XANES

( , ) ( , ) ( , )( ) ( , ) [1 ( )] ( ) ( )( )[ ( , ) ( )]

T E t A E t A Ef t P E t f t R E R Ef t P E t R E

= − −∞= + − −= −


Time-Resolved X-Ray Absorption Spectroscopy

Laser-Initiated Time-Resolved X-Ray Absorption Spectroscopy (LITR-XAS)

→ Laser pulse pump + X-ray pulse probe


→ The interference between the outgoing photoelectron wave from the X-ray

absorbing atom and the back-scattered photoelectron waves from the neighboring

atoms is presented.

→ The energy of the absorbed x-ray can be converted to the X-ray wave vector k

with respect to E0, the threshold energy for the ejecting core electron.

→ The absorption coefficient can be represented as the modulation term, χ(k), which

includes the structural parameters due to the oscillation from the outgoing

photoelectrons.

→ Finally, the momentum k space can be transformed to the distance R space.

EXAFS (Extended X-Ray Absorption Fine Structure) Oscillation

→ F(k), the backscattering amplitude; N, the coordination number; r, the average

distance; σ, the Debye-Waller factor due to thermal vibration and static disorder; λ,

the electron mean free path; and δ, the phase shift of the photoelectron wave by the

scattering atoms

→ The correlation between structural parameters and the modulation frequencies in the

EXAFS can be described by the above equation.

XANES (X-Ray Absorption Near Edge Structure)

→ XANES refers to the X-ray absorption spectrum near the transition edge, including

the pre-edge, the transition edge, as well as 30-50 eV above the edge.

→ Pre-edge : bound-bound transition / Edge : ionization threshold

→ The multiple scattering effect is dominant.

Following Atomic Movements Far From Equilibrium

→ Coherent atomic motions, namely vibrational motions arise immediately after

photoexcitation.

→ ςi(τ, R), the Born-Oppenheimer nuclear wavefunction for the occupied valence

electron configuration i; N(k, τ−t), the X-ray probe pulse profile.

→ The time dependence comes from the time evolution of R and the pulse shape of the

probe X-ray pulse.

Solving Molecular Structures at a Transient Equilibrium State

→ As excited molecules are transiently equilibrated, they reside there for a finite time

before leaving for the next excitation state potential surface or returning to the

ground state. Then,

where fj(t) and χj(k, t) are the fraction and absorption of jth species in the sample at

delay time t, respectively.

Experiments in LITR-XAS

2006. 10. 18

1. Pulsed X-ray Source

Time-resolved XAS demands three major components: the X-ray source, the laser

source, and the detectors.

The X-ray source not only needs to be short pulsed with high photon flux, but also to

have a sufficiently wide energy range.

Laser-generated X-ray diodes

High harmonic generation

2. Laser Pump Sources

The main concern in selecting a laser system as the pump source is the discrepancy

in molar extinction coefficients for laser photons and for X-ray photons.

3. Time-Resolved XAFS Detection

Most commonly used transmission detectors are avalanche photodiode(APD), X-ray

charge-coupled device(CCD), and X-ray streak camera.

Experimental Set-Up

Taking Molecular Snapshots Using LITR-XAS

1. NiTPP-L2 (Nickel(II) tetraphenylporphyrin, L=piperidine)

→ A laser pump pulse triggers the photodissociation of the axial ligands.

→ The transient species was captured by the x-ray probe pulse.

→ XANES spectra near the Ni-K region (E0=8.333 keV).

→ For the laser pumped spectra, NiTPP-L2 : NiTPP = 7 : 3.

→ NiTPP is attributed to the 1s-to-4pz transition; that would not be seen in an

octahedral or a square pyramid complex.

→ Fourier-transformed XAFS spectra weighed by k3.

→ The XANES agree so well with the two distance fitting from the XAFS spectra: the

transient structure has the square planar structure, NiTPP.

2. CuOEP (Copper(II) octaethylporphyrin)

→ CuOEP-THF complex: the transient species induced by coordinating solvent.

→ A new channel, charge transfer of the CuOEP-THF complex, makes the fast decay in

less than 100ps.

→ Calculated(by FEFF8) and experimental XAFS spectra of CuOEP.

→ Cu-N distances overlap well: almost identical Cu-N distances in both solid and

toluene solution.

→ XANES spectra of CuOEP without and with laser excitation(probed after 200ps).

→ A sharp peak at 8.985keV, which attributed to 1s-to-4pz transition in the square

planar structure, indicates the axial ligation in the triplet excited state of CuOEP in

presence of THF.

→ No peak in spectra in the toluene solution.

→ XAFS and FT-XAFS spectra of CuOEP in solutions.

→ No significant change in the toluene solution.

→ In the THF solution, the average Cu-N distance increases.

(ground state: 1.996Å, one-distance model: 2.01Å, two-distance model: 1.996, 2.03Å)

3. [RuII(bpy)3]2+ (Ruthenium(II) tris-2,2’-bipyridine)

→ The electronic structures of RuII and RuIII.

(a) Static x-ray absorption spectra at the Ru L3 edge of aqueous [RuII(bpy)3]2+, and of

[RuIII(NH3)6Cl2].

(b) Transient absorption spectrum of photoexcited aqueous [RuII(bpy)3]2+ recorded

300ps after the laser pump pulse. The curve is a fit accounting for the blueshift of the

static XAS due to the photoinduced increase of oxidation state, and the solid fit curve

includes the appearance of feature A’ for the reaction intermediate.

(c) The reactant state and the transient absorption fit.

1. 다음 중 XAS에 대한 설명으로 틀린 것은?

① XAS는 에너지에 따른 absorption coefficient를 측정하는 실험이다.

② XAS는 근사적으로 Beer의 법칙을 따른다.

③ 02 ( ) /k m E E= − 에서 는 photoelectron의 wavevector이다. k

④ 0( ) ( ( ) ) /k k 0χ μ μ= − μ 에서 0μ 는 sample을 넣지 않은 상태에서의 background

absorption을 가리킨다.

⑤ 근처를 XANES라 부른다. 0k =답) ④, 순수한 원자의 absorption이다.

2. 다음 중 XAFS oscillation에 대한 설명으로 틀린 것은?

① Debye-Waller factor는 thermal vibration과 static disorder를 반영한다.

② Oscillation의 phase는 shift가 없을 경우 의 형태를 가진다. kr③ Photoelectron의 mean-free-path가 커질수록 oscillation의 damping은 커진다.

④ 각각 원자의 특성은 scattering amplitude를 다르게 한다.

⑤ Multiple scattering까지 고려해 준 이론이 실험치와 잘 맞는다.

답) ③,

2

( )r

k e λχ−

∝ 이므로 damping은 작아진다.

3. 다음 중 XANES에 대한 설명으로 틀린 것은?

① XANES는 X-ray Absorption Near Edge Structure의 약자이다.

② Pre-edge의 영역에서는 bound-bound transition을 볼 수 있다.

③ Edge는 X-ray에 의해 photoelectron이 많이 나오기 시작하는 energy이다.

④ 모든 원자의 L-edge는 같다.

⑤ K-edge는 orbital의 관점에서 1s electron의 transition에 해당한다.

답) ④, 원자마다 다르다.

4. LITR-XAS 실험에 쓰이는 X-ray의 요건이 아닌 것은?

① High photon flux

② Short pulse duration

③ Broad energy range

④ Small beam size

⑤ Spatial overlap with pump beam

답) ④

5. 다음 중 XAS에 쓰이는 detector가 아닌 것은?

① Avalanche photodiode

② Charge-coupled device

③ Photomultiplier

④ Phosphor screen

⑤ Streak camera

답) ④

6, XAS 실험에서 Si(111)의 역할은?

① X-ray generator

② Monochromator

③ Fluorescence detector

④ Focusing optics

⑤ Synchronizer

답) ②

7. 다음 XAS spectrum에 나타난 NiTPP-L2의 photodissociation을 설명하시오.

답) 전체적인 photodissociation은 왼쪽 그림과 같이 일어나는 것으로 알려져 있는데 먼저

laser에 의해 octahedral T0 state가 T*로 된 후 ISC를 통해 S*, S0로 간 뒤 ground state로

돌아오게 된다. 마지막 ground state로 돌아오는 과정에서 ligand가 차례대로 붙는지 아니

면 동시에 붙는지 XAS spectrum을 통해 알 수 있는데 square pyramidal이나 octahedral

에서 나타나지 않는 1s→4pz peak이 나타나는 것으로 보아 후자가 맞음을 알 수 있다.

8. 아래 CuOEP의 XAS spectrum이 용매에 따라 다른 이유를 예측하시오.

답) Toluene의 경우 laser를 주었을 때와 그렇지 않았을 때의 spectrum 차이가 거의 없는

데 이는 둘의 구조적 차이도 거의 없음을 나타낸다. 반면에 THF를 넣어 주었을 때는 왼쪽

XANES에서 peak의 높이가 감소했고 오른쪽 XAFS에서 peak의 높이가 증가했음을 볼 수

있는데 이는 둘의 구조적 차이 즉 새로운 coordination이 생겼음을 짐작할 수 있다. 정리하

면 toluene은 non-coordinating solvent로서 반응에 참여하지 않으나 THF는 coordinating

solvent로서 새로운 반응 채널을 생성함을 알 수 있다.

9. 다음 그림은 [Ru(bpy)3]2+의 XAS spectrum이다. 각각 점선은 ground state, 실선은

transient state를 나타낸다. 차이를 설명하시오.

답) 차이는 두 가지로 나누어 볼 수 있는데 첫번째로 peak의 위치가 오른쪽으로 이동했음

을 알 수 있다. 그 이유는 Ru의 산화수가 2가에서 3가로 변함에 따라 core electron의

Coulomb interaction이 증가했고 그 결과 transition energy 또한 커졌기 때문이다. 두번째

로 새로운 peak(A’)이 생겼는데 이는 eg로의 transition뿐만 아니라 t2g로의 transition이 가

능해졌기 때문으로 해석할 수 있다.

10. 위 transient spectrum에서 A’과 B’은 peak function으로 fitting할 수 있는데 이를 이

용하면 FWHM(Full Width at Half Maximum)을 구할 수 있다. 이 FWHM은 peak이 얼마나

뾰족한가를 나타내는데(FWHM이 작을수록 뾰족) fitting 결과 B’보다 A’의 FWHM이 작은

것으로 밝혀졌다. 이유를 예측하시오.

답) 위 그림에서 보면 eg로 올라가는 경우는 여러 개 있을 수 있으나 t2g로 가는 경우는 하

나밖에 없으므로 A’의 peak이 뾰족하다.

Time Resolved X-Ray Absorption Spectroscopy

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