Time Resolved X-Ray Absorption Spectroscopy 2006. 10. 16 Lee, Youhong TR-XAS
LITR-XAS Laser-Initiated Time-Resolved X-ray Absorption Spectroscopy TR-XAS
X-Ray Absorption Spectroscopy I 0 I Beer s law x I I 0 = μ exp( x) μ : absorption coefficient TR-XAS
X-Ray Absorption Spectroscopy TR-XAS
X-Ray Absorption Spectroscopy k = [ 2 me ( E) ] 1/2 0 χ ( k) = μ ( k) μ ( k) μ 0 0 ( k) k FT r TR-XAS
X-Ray Absorption Spectroscopy TR-XAS
The Nature of XAS K edge, L I edge, L II edge, M L hν K TR-XAS
XAFS Oscillations (Extended) X-ray Absorption Fine Structure 2rj kr 2 σ λ ( k) j + δij 2 2 sin 2 ( k) jk j χ( k) N jfj( k) e e kr j 2 j TR-XAS
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 χ ( k, t) = i dτ N ( k, τ t) ζ ( τ, R) χ( k, R) ζ ( τ, R) i dτ N ( k, τ t) i TR-XAS
At Transient Equilibrium χ ( kt, ) = f( t) χ ( k) j j j TR-XAS
Experiments in LITR-XAS 2006. 10. 18 Lee, Youhong TR-XAS Experiments
Major Components X-ray source Laser source Detectors TR-XAS Experiments
Experimental Set-Up TR-XAS Experiments
X-Ray Sources Continuum radiation Ultrashort duration High photon flux TR-XAS Experiments
X-Ray Sources TR-XAS Experiments
Laser-Generated X-Ray Diodes TR-XAS Experiments
High Harmonic Generation ω Nω TR-XAS Experiments
Laser Pump Sources The discrepancy in molar extinction coefficients for laser photons and for X-ray photons log 10 I 0 = ε lc I Ο ε ε Laser ~ 10 2 or 10 X -ray 3 TR-XAS Experiments
Laser Pump Sources log 10 χ ( k, t) = [1 f01( t)] χ gs ( k) + f01( t) χ01( k) I I 0 = εlc N 0 N = N a = P h 0 ν (1 10 εlc ) Q = # of reactantsconsumed # of photons absorbed f 01 = P 0 Q[1 10 ν ε l C C a l (1 A f 01 ) ] TR-XAS Experiments
Detectors Avalanche PhotoDiode (APD) Charge-Coupled Device (CCD) Streak camera TR-XAS Experiments
Example TR-XAS Experiments
Taking Molecular Snapshots Using LITR-XAS 2006. 10. 30 Lee, Youhong Taking Molecular Snapshots
NiTPP-L 2 Nickel(II) tetraphenylporphyrin, L=piperidine Taking Molecular Snapshots
NiTPP vs. NiTPP-L 2 NiTPP Square-planar Singlet NiTPP-L 2 Octahedral Triplet Taking Molecular Snapshots
Photodissociation of NiTPP-L 2 Taking Molecular Snapshots
XANES near the Ni K-edge Taking Molecular Snapshots
XAFS Taking Molecular Snapshots
CuOEP Copper(II) octaethylporphyrin Taking Molecular Snapshots
Structures of Cu(II)OEP Taking Molecular Snapshots
XAFS of CuOEP Taking Molecular Snapshots
Transient XANES Taking Molecular Snapshots
Transient XAFS Taking Molecular Snapshots
[Ru II (bpy) 3 ] 2+ Ruthenium(II) tris-2,2 -bipyridine Taking Molecular Snapshots
Ru II vs. Ru III e g e g t 2g t 2g p 3/2 p 3/2 Taking Molecular Snapshots
Photochemistry of [Ru II (bpy) 3 ] 2+ Taking Molecular Snapshots
Transient XANES T( E, t) = A( E, t) A( E, ) = f ( tpet ) (, ) + [1 f( t)] RE ( ) RE ( ) = f()[ t P( E,) t R( E)] Taking Molecular Snapshots
Time-Resolved X-Ray Absorption Spectroscopy Laser-Initiated Time-Resolved X-Ray Absorption Spectroscopy (LITR-XAS) Laser pulse pump + X-ray pulse probe X-Ray Absorption Spectroscopy 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 E 0, 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. ς(τ, i 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 f j (t) and χ j (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 (E 0 =8.333 kev). For the laser pumped spectra, NiTPP-L 2 : 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 k 3. 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-4p z 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. [Ru II (bpy) 3 ] 2+ (Ruthenium(II) tris-2,2 -bipyridine) The electronic structures of Ru II and Ru III.
(a) Static x-ray absorption spectra at the Ru L 3 edge of aqueous [Ru II (bpy) 3 ] 2+, and of [Ru III (NH 3 ) 6 Cl 2 ]. (b) Transient absorption spectrum of photoexcited aqueous [Ru II (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. 다음중 XAS 에대한설명으로틀린것은? 1 XAS 는에너지에따른 absorption coefficient 를측정하는실험이다. 2 XAS 는근사적으로 Beer 의법칙을따른다. k = 2 m( E E )/ 에서 k 는 photoelectron의 wavevector이다. 3 0 χ( k) = ( μ( k) μ )/ μ0 에서 μ0 는 sample을넣지않은상태에서의 background 4 0 absorption 을가리킨다. 5 k = 0 근처를 XANES라부른다. 답 ) 4, 순수한원자의 absorption 이다. 2. 다음중 XAFS oscillation 에대한설명으로틀린것은? 1 Debye-Waller factor 는 thermal vibration 과 static disorder 를반영한다. 2 Oscillation의 phase는 shift가없을경우 kr 의형태를가진다. 3 Photoelectron 의 mean-free-path 가커질수록 oscillation 의 damping 은커진다. 4 각각원자의특성은 scattering amplitude 를다르게한다. 5 Multiple scattering 까지고려해준이론이실험치와잘맞는다. 답 ) 3, χ( k) 2r e λ 이므로 damping 은작아진다. 3. 다음중 XANES에대한설명으로틀린것은? 1 XANES는 X-ray Absorption Near Edge Structure의약자이다. 2 Pre-edge의영역에서는 bound-bound transition을볼수있다. 3 Edge는 X-ray에의해 photoelectron이많이나오기시작하는 energy이다. 4 모든원자의 L-edge는같다. 5 K-edge는 orbital의관점에서 1s electron의 transition에해당한다. 답 ) 4, 원자마다다르다. 4. LITR-XAS 실험에쓰이는 X-ray의요건이아닌것은? 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. 다음중 XAS에쓰이는 detector가아닌것은? 1 Avalanche photodiode 2 Charge-coupled device 3 Photomultiplier 4 Phosphor screen 5 Streak camera 답 ) 4 6, XAS 실험에서 Si(111) 의역할은? 1 X-ray generator 2 Monochromator 3 Fluorescence detector 4 Focusing optics 5 Synchronizer 답 ) 2 7. 다음 XAS spectrum 에나타난 NiTPP-L 2 의 photodissociation 을설명하시오. 답 ) 전체적인 photodissociation은왼쪽그림과같이일어나는것으로알려져있는데먼저 laser에의해 octahedral T 0 state가 T * 로된후 ISC를통해 S *, S 0 로간뒤 ground state로돌아오게된다. 마지막 ground state로돌아오는과정에서 ligand가차례대로붙는지아니면동시에붙는지 XAS spectrum을통해알수있는데 square pyramidal이나 octahedral 에서나타나지않는 1s 4p z peak이나타나는것으로보아후자가맞음을알수있다.
8. 아래 CuOEP 의 XAS spectrum 이용매에따라다른이유를예측하시오. 답 ) Toluene의경우 laser를주었을때와그렇지않았을때의 spectrum 차이가거의없는데이는둘의구조적차이도거의없음을나타낸다. 반면에 THF를넣어주었을때는왼쪽 XANES에서 peak의높이가감소했고오른쪽 XAFS에서 peak의높이가증가했음을볼수있는데이는둘의구조적차이즉새로운 coordination이생겼음을짐작할수있다. 정리하면 toluene은 non-coordinating solvent로서반응에참여하지않으나 THF는 coordinating solvent로서새로운반응채널을생성함을알수있다. 9. 다음그림은 [Ru(bpy) 3 ] 2+ 의 XAS spectrum이다. 각각점선은 ground state, 실선은 transient state를나타낸다. 차이를설명하시오.
답 ) 차이는두가지로나누어볼수있는데첫번째로 peak의위치가오른쪽으로이동했음을알수있다. 그이유는 Ru의산화수가 2가에서 3가로변함에따라 core electron의 Coulomb interaction이증가했고그결과 transition energy 또한커졌기때문이다. 두번째로새로운 peak(a ) 이생겼는데이는 e g 로의 transition뿐만아니라 t 2g 로의 transition이가능해졌기때문으로해석할수있다. 10. 위 transient spectrum에서 A 과 B 은 peak function으로 fitting할수있는데이를이용하면 FWHM(Full Width at Half Maximum) 을구할수있다. 이 FWHM은 peak이얼마나뾰족한가를나타내는데 (FWHM이작을수록뾰족 ) fitting 결과 B 보다 A 의 FWHM이작은것으로밝혀졌다. 이유를예측하시오. 답 ) 위그림에서보면 e g 로올라가는경우는여러개있을수있으나 t 2g 로가는경우는하나밖에없으므로 A 의 peak이뾰족하다.