時間分解XAFS - 立命館大学...Step Scan XAFS モノクロメーターの角度を止めて、吸光度データを溜め込む。1スペクトルを得るのに最低でも数分かかる。時間分解能は分オーダー

時間分解XAFS上村洋平

(分子科学研究所)

時間分解XAFS測定とは？

○短時間でXAFSを測定したい。

○時間と共に変化する様を観測したい（定性的）。

○反応速度や活性化エネルギーを求めたい（定量的）。

○励起状態、準安定状態に関する情報を得たい。

原子分子レベルでの”速い”変化

Nat. Commun. 2017, 8 , 15342

Fe[bpy]3の光励起過程: fs ~ ps Co触媒での電子移動: µs

Rh3+

TiO2

Rh3+

Rh3+ Rh3+

Rh3+ Rh3+

光触媒TiO2上での光電析: min～h

hνTiO2

J. Phys. Chem. C 2013, 117, 17367−17375

Au3+

Au3+Au3+Au3+Au3+Au3+

Au3+Au3+ Au3+

J. Phys. Chem. C. 2008, 112, 8495–8498

Auナノ粒子の核形成・粒子成長: µs - s

NaBH4

ChemPhysChem 2011, 12, 127 – 131

Step Scan XAFS

モノクロメーターの角度を止めて、吸光度データを溜め込む。1スペクトルを得るのに最低でも数分かかる。

時間分解能は分オーダー

モノクロ掃引

データ蓄積

もっと早く測定することはできないものか…

I0 It

もっと”短時間”のXAFSスペクトルを測定するには？

1. 出来るだけ”素早く”モノクロメータを動かす！⇒Quick XAFS

2. 一度にXAFSスペクトルを取得する！⇒DXAFS

3. 時間情報を空間情報に置き換える⇒Pump-flow-probe XAFS

4. X線のパルス性を利用する⇒ポンプ-プローブXAFS

1,2 ⇒ なんとか短時間でXAFSスペクトルを得る 3,4(,2) ⇒ 時間0からある揃った時刻のスペクトルを得る













1. Quick XAFS

Prof. Ronald Frahmhttp://www.qexafs.comより転載

モノクロメーターを止めることなく連続で角度を掃引。1スペクトルを得るのに数秒～分かかる。時間分解能は秒~分オーダー

QXAFSでどこまで早く測定出来る？

QXAFSの高速化　　高速クイックXAFS

分光器を高速に角度スキャンしながら、分光器角度と検出器出力を連続計測・保存

ガルバノスキャナー分光器

X線

チャネルカット分光結晶

(27×14 mm)

X線電離箱

高速角度掃引分光器試料

蛍光X線検出器

電流アンプローパスフィルタ

メモリトリガ

PC

ファンクションジェネレータ

デジタイザー

ローパスフィルタ

計測系角度エンコーダ

T. Uruga et al., AIP Conf. Proc., 882, 914 (2007).

QXAFSの高速化　　高速クイックXAFS

分光器を高速に角度スキャンしながら、分光器角度と検出器出力を連続計測・保存

ガルバノスキャナー分光器

X線

チャネルカット分光結晶

(27×14 mm)

X線電離箱

高速角度掃引分光器試料

蛍光X線検出器

電流アンプローパスフィルタ

メモリトリガ

PC

ファンクションジェネレータ

デジタイザー

ローパスフィルタ

計測系角度エンコーダ

T. Uruga et al., AIP Conf. Proc., 882, 914 (2007).

SPring-8で開発されたガルバノQXAFS: 最速でxx msでの測定が可能 T. Uruga et al., AIP Conf. Proc., 882, 914 (2007).

QXAFSの特徴

! 通常のXAFS実験と基本的に同じ構成 ⇒　既存のビームラインに組み込める

! 種々の検出法（蛍光法、転換電子収量法等）の適応が可能

☹ 機械的な再現性が求められる⇒ XANESの微小な変化を議論する時は要注意

# スペクトルの各点で観測している時刻が異なる⇒ スペクトル測定時間より十分に遅い変化測定用

Auナノ粒子の核形成・粒子成長

of the white line decreases with an increase in DT/Au from 0to 2, and further addition of DT does not change the spectralfeatures. The Au atoms in the solution of DT/Au = 16 exist asAu+ ions, whose 5d states are fully occupied by electrons.[19, 20]

The spectra were fitted with the linear combination of thoseobtained for DT/Au = 0 and 16, that is, for Au3+ and Au+ ions,

to evaluate the fractions of Au3 + and Au+ in the solution. Thefractions listed in Table 1 are well consistent with those calcu-lated as per the stoichiometric reaction (Supporting Informa-

tion, Table S1), which indicates the reduction of Au3 + ions bytwo molecules of DT to form Au+ ions. Recently, Lennox et al.reported that an Au3+ ion is reduced by two molecules of DTto form [TOA][Au+X2] (X = Cl! or Br!) complex, and further ad-dition of DT did not lead the formation of Au+ thiolate speciesin two-phase Shiffrin–Brust method.[21] Herein, XANES spectralanalysis showed that [TOA][Au3+X4] is reduced to [TOA][Au+Cl2]by 2 eq. molar of dodecanethiol. The XANES spectra changelittle by the further addition of dodecanethiol, which indicatesno formation of Au!S bonds. These results are consistent withthe Lennox’s work. Accordingly, the equimolar mixture of[TOA][Au+X2] and [TOA][Au3+X4] is a precursor in the toluenesolution of DT/Au = 1.

The UV/Vis spectra of the solutions for various DT/Au ratioswere also measured (Supporting Information, Figure S1). Theintensity and shape of the LMCT band varied as per the con-version of Au3+ to Au+ when DT/Au was between 0 and 2.The intensity of the LMCT band changed with almost no varia-tion in the peak wavelengths (316 nm and 368 nm) whenDT/Au was between 2 and 16. Au3 + ions may be reduced toAud+ via Au+ in the solution of DT/Au = 4 and 16. By observ-ing the XANES spectra carefully, we can see that the shoulderpeaks at the edge become slightly smaller with an increase inDT/Au from 2 to 16. The Au+ could be further reduced by DTto Aud+ . It is also noted that the precipitation due to the for-mation of Au+ thoilate species was not observed in the pres-ence of more than 2 equiv molar of DT.

The DMF solution of NaBH4 was added to the toluene solu-tion of DT/Au = 1 containing an equal quantity of Au3 + andAu+ ions. The small peak that appears at the edge after thedisappearance of the white line in the early stage of the seriesof the XANES spectra is shown in Figure 1. The XANES spec-trum exhibits the most specific small peak at 4.6 s. A similarsmall peak is also observed in the XANES spectrum ofAu11(PPh3)8Cl3,[22] but it is not observed in that of the Au+ ions.This result suggests that small AuNPs, that is, Au nuclei, areformed at 4.6 s. The XANES spectral feature at 4.6 s is broaderthan that of Au11(PPh3)8Cl3, which is represented as the broadersmall peak at the edge. This might imply that the Au nucleivary in size. It should also be noted that the XANES spectra ob-tained before the formation of the Au nuclei were fitted bythe spectra of the Au nuclei and those of the solution of

Figure 1. Series of Au L3-edge XANES spectra during AuNP formation to-gether with those of HAuCl4, Au foil, and Au11(PPh3)8Cl3 as references. TheXANES spectra during the formation of AuNPs in the presence of the equi-molar of DT to HAuCl4 from 0 s to 178.5 s (c). The spectra vary from thetop to the bottom region with an increase in time. References: HAuCl4

(g), Au foil (a), and Au11(PPh3)8Cl3 (d).

Figure 2. XANES spectra of toluene solution for a series of DT/Au ratios.DT/Au ratios are a) 0, b) 0.1, c) 0.4, d) 1, e) 2, f) 4, and g) 16.

Table 1. The fractions of Au3 + and Au+ in the solution with variousDT/Au ratios before NaBH4 reduction. The fractions were evaluated by fit-ting of the XANES spectra of various DT/Au with linear combination ofthose of DT/Au = 0 and 16.

DT/Au 0.1 0.4 1 2 4

Au3 + [%] 90.3 79.0 50.3 0 0Au+ [%] 9.7 21.0 49.7 100 100R factor 0.006 0.004 0.005 0.009 0.007

128 www.chemphyschem.org ! 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPhysChem 2011, 12, 127 – 131

DT/Au = 16 (Supporting Information, Figures S2 and S3). Au+

ions are the intermediate in the formation process of the Aunuclei. Furthermore, the appearance of the small peak aroundthe edge immediately after the addition of NaBH4 indicatesthat DT passivates Au just after the reduction of Au ions toAu0, because the small peak is derived from the formation ofAu!S bonds[4] and Au ions do not have Au!S bonds.

The extended XAFS (EXAFS) spectrum at 4.6 s was analyzedto confirm the formation of the Au nuclei and to investigatetheir structure. Figure 3 a shows the EXAFS spectrum at 4.6 s.

The EXAFS spectrum at 4.6 s shows oscillations in the 7.5–11and 13–15 !!1 k ranges (gray regions in Figure 3) and almostno oscillations in the other k ranges. The oscillation phase ofthe Au foil is in accordance with that of the solution ofDT/Au = 1 before its reduction in the 7.5–11 and 13–15 !!1 krange, although these oscillations are in antiphase in the otherk ranges. These results suggest that the EXAFS spectrum at4.6 s consists of the oscillations due to Au!Au scatterings re-sulting from a face-centered cubic (fcc) crystal structure andthose due to Au!X scatterings. A similar EXAFS spectrum asthat obtained at 4.6 s is represented by the linear combination

of 12.5 % of the EXAFS spectrum of the DT/Au = 1 solution(Figure 3 c) and 25 % of that of Au foil (Figure 3 d), as shown inFigure 3 b. The coordination number of the Au!Au pair of Aunuclei is evaluated to be 3. This is based on the assumptionthat the coordination number of the Au!Au pair is proportion-al to the amplitude of the EXAFS spectrum of the Au foil, thecoordination number of which is 12. Therefore, the structure ofthe Au nuclei is estimated to be an Au4 cluster with an fcccrystal system, as shown in the inset of Figure 3. It should alsobe noted that almost all the Au atoms in the cell belong tothe Au nuclei at 4.6 s, because an XAFS spectrum provides in-formation about the average state of an absorption atom. Asfar as we know, this is the first report of an observation of Aunuclei in the formation process of AuNPs.

After the formation of Au nuclei, the XANES spectra ap-proach those of Au foil. Each XANES spectrum after 4.6 s wasfitted with the linear combination of the spectrum of the Aunuclei observed at 4.6 s and that of the Au foil (Figure S4).Figure 4 shows the change in the fraction of XANES spectrum

at 4.6 s and that of the Au foil. The fraction of the Au nucleidecreases with time and approaches ca. 30 %, and that of theAu foil increases and approaches ca. 70 % for ca. 180 s. Thisresult indicates that the AuNPs possess less than 100 % of thefraction of the Au foil and more than 0 % of that of the Aunuclei. In fact, the XANES spectrum of purified AuNPs coveredwith DT having 3.3"0.5 nm diameter, estimated from TEM ob-servation (Supporting Information, Figure S5), was representedby the linear combination of 67.5 % of the XANES spectrum ofthe Au foil and 32.5 % of that of the Au nuclei (Supporting In-formation, Figure S5). AuNPs have a larger fraction of surfaceatoms, that is, coordinatively unsaturated atoms, than the Aufoil, and the Au nuclei almost exclusively consist of coordina-tively unsaturated atoms. Therefore, we inferred that the frac-tion of the XANES spectrum of the Au nuclei was that of the

Figure 3. EXAFS spectrum of Au nuclei observed during AuNPs formation.a) EXAFS spectrum of Au nuclei observed at 4.6 s after the addition ofNaBH4 to the HAuCl4 solution. b) The simulated spectrum obtained by linearcombination of the reference spectra of c) 12.5 % of the solution ofDT/Au = 1 before reduction and d) 25 % of the Au foil. The 7.5–11 and 13–15 !!1 k ranges where the oscillations of (c) and (d) are in phase are in gray.The inset is the estimated structure of Au nuclei from the linear combina-tion, an Au4 cluster with fcc crystal system.

Figure 4. Time course of the fractions of the surface and inner atoms ofAuNPs. The fraction of the XANES spectrum of Au nuclei observed at 4.6 sduring the formation of AuNPs, representing that of the surface atoms ofAuNPs (*). The fraction of the XANES spectrum of the Au foil representingthat of the inner atoms (&). The fractions were evaluated by least-squares fit-ting of the series of XANES spectra with a linear combination of the spectraof the Au nuclei and Au foil, resulting in a R factor (~).

ChemPhysChem 2011, 12, 127 – 131 " 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemphyschem.org 129

ChemPhysChem 2011, 12, 127 – 131

時間分解能: 100 ms

2. DXAFS: スペクトルを一括で測定する

松下正 KEK名誉教授

DXAFSの計測システム

PF-AR集光ミラー

必要なエネルギー領域を機械的動作なく一度に測定

湾曲結晶

試料

検出器Elow

Ehigh

DXAFSの分光原理

湾曲結晶

湾曲結晶

Bragg配置（反射分光）

Laue配置（透過分光）

試料

試料

一次元検出器

一次元検出器

低エネルギー領域 Si(111), Si(311) etc

高エネルギー領域 Si(311), Si(511) etc

湾曲結晶（Polychromator）分散型XAFS（Dispersive XAFS : DXAFS)

白色X線

白色X線

DXAFSの分光原理(続き)

θ

θ

sin2sinRp

Rpq−

=

結晶から集光点までの距離qは

R : 結晶の湾曲半径p : 光源から結晶までの距離

※Laue配置の場合はθ = θB – π/2

DXAFSの検出器

蛍光体（Gd2O2S(Tb),CsI(Tl)など）

Fiber Optical PlatePDA

〇各素子を切り替えながら1個のADC（14 bit, 525 kHz）で読み出し。〇検出器の最小露光時間：2 ms（1024素子PDAの場合）〇蛍光体を用いてX線を可視光へ変換して検出するため結果的にスペクトルのエネルギー分解能が劣化。

蛍光体と組み合わせたPDA一次元検出器(Photodiode Array (PDA)

PFでのDXAFSシステム

DXAFSの特徴

! 機械的に動く部分が無く、高速化が可能 ⇒検出器次第では、サブナノ秒の実験も可能! 全領域同時測定なので単発現象を追える

☹ 基本的に、透過法で測定する

　⇒試料の均一性を要求#

☹ 特殊な光学系が必要

Pump-flow-probe XAFS空間情報を時間情報に置き換えて測定を行う

変化の開始: CWレーザー時刻tの見積: t = d/v [ms]

繰り返し測定が可能な試料に適応

experiment and continuously purged and kept under N2 duringthe experiment.Pump-Flow-Probe Experiment. The conceptual idea of

this experimental setup is to perform time-resolved measure-ments by focusing on a fast flowing jet a continuous wave(CW) laser beam for sample excitation, spatially separated at avariable distance from an X-ray probe beam. Figure 2 shows in

schematic form the experimental setup, with the samplecirculating in the flow system through a capillary where theinteraction with the pump and probe beams take place. TheCW laser is focused on the capillary (1 mm diameter, 10 μmwall thickness) and excites the sample, and downstream the X-ray beam probes it. The distance between the two beams d andthe flow speed v of the liquid together determine the timedelay, while the focusing of the laser and the X-ray beams andthe velocity profile of the sample solution define the effectivetime resolution. Therefore, we refer to this experiment aspump-flow-probe.In order to achieve efficient excitation of [Ru(bpy)3]

2+, weselected a laser with a 447 nm wavelength and 1 W power(CNI lasers MDL-III-447). The laser beam was focused withcylindrical lenses to a spot size of 0.1 × 1 mm2 (vertical ×horizontal) at the sample position and the X-ray beam wasfocused to a 0.1 × 0.1 mm2 spot. The use of a capillary andclosed flow system allowed the sample to be kept underoxygen-free conditions. For the experiments presented here, theaverage flow speed in the probed region in the capillary was 3m/s corresponding to a 333 μs delay between pump and probeper millimeter of spatial separation between laser and X-raybeams. XAS measurements were performed in differential modewhere the X-ray energy is first changed, and then the XASsignal is measured with laser on for 1−30 s, following which theXAS signal without laser is measured for the same time. The X-ray energy is then stepped to the next energy point, and thesame measurement sequence is repeated.In order to control the spatial separation between laser and

X-ray beams, one has to first determine the position where theyoverlap. A tungsten pinhole with 100 μm diameter and 50 μmthickness was used to find the X-ray beam position and then to

achieve spatial overlap of the beams the transmitted laserintensity through the pinhole was maximized by adjusting thealignment mirror (3). As an alternative, optical fluorescenceinduced by both the pump and probe beams was found to besufficiently intense to allow accurate control of beam positionsby webcam-imaging of the sample. A neutral density filter withan optical density of ∼7 was used during the alignment in orderto reduce the fluorescence induced by the laser and registeredby the camera.The pump-flow-probe measurements were performed at the

SuperXAS beamline of the SLS. The storage ring was operatedin the top-up mode (400 mA). XAS spectra were detected inthe fluorescence mode (Co Kα line) using an energy-resolving13-element germanium detector (Canberra) with an additionalZ-1 (Fe) filter. A double-crystal Si[111] monochromator wasused to define and scan the X-ray energy, while a Rh-coatedtoroidal mirror provided 0.1 × 0.1 mm2 focusing of the X-raybeam on the sample position.

Pump-Sequential-Probes Experiment. Pump-sequential-probes measurements were performed at the 11ID-D beamlineof the APS using a highly optimized setup for time-resolvedXAS which has been described in detail elsewhere.18 In contrastto experiments aiming at picosecond time resolution and thususing only one X-ray bunch from the storage ring to probe theexited state after each laser pulse, the present experimentutilized a train of X-ray pulses with a separation of 153 ns fromthe APS storage ring operating in 24-bunch mode to record aseries of spectra. Thus, the time evolution of the transient XASsignal was probed at successive time delays of n × 153 ns afterthe excitation event. Therefore, we refer to this type ofexperiment as pump-sequential-probes. The pump pulses wereprovided by the Nd:YLF regenerative amplifier laser installed atthe beamline, and the second harmonic at 527 nm (pulseenergy of 0.6 mJ at 1 kHz repetition rate, ∼5 ps (fwhm) pulseduration, and 0.8−1 mm spot size) was used in the presentexperiments. The shortest time delay between laser and X-raypulses investigated here was 100 ps. The X-ray beam had ∼106photons/pulse, focused to a spot smaller than the sample jetdiameter, and XAS data sets were acquired in fluorescencemode using the dedicated pulse-to-pulse digitization setup ofthe beamline. The sample jet diameter was 0.6 mm.

■ RESULTS AND DISCUSSIONExcitation Conditions and Time Resolution: Simula-

tions. On the basis of the reaction scheme and the rateequations given in the Supporting Information, we performedsimulations of the time evolution of intermediate concen-trations to guide the choice of experimental conditions towardmaximizing the expected transient signal. In the pump-flow-probe experiment the sample (or, more precisely, eachelementary volume flowing through the capillary) is excitedby the CW laser during the time window that is defined by theflow speed and laser spot size. The typical excitation time is onthe order of 20 μs, which is quite different from the conditionsof pump−probe experiments with pulsed lasers that usuallyhave pulse durations <100 ps. The excitation time of 20 μscorresponds to 100 μm focusing of the laser beam and flowspeed of 5 m/s; it can be used as lower limit for the excitationtime that gives also lower estimate for the concentration ofreaction intermediates. The irradiated volume (1 mm × 0.1 mmspot size and 1 mm thickness) receives 4.4 × 1013 photons fromthe 1 W laser at 447 nm for 20 μs. The number ofchromophores (0.4 mM concentration) in this volume is 1.9

Figure 2. Scheme of the setup for the pump-flow-probe experiment:(1) CW laser, (2) cylindrical lenses for focusing, (3) mirror, (4)capillary with continuously flowing sample, (5) X-ray beam, (6)reservoir with the sample, (7) gear pump. The vertical size of the laserbeam is S1 and of the X-ray beam is S2. The distance between laserand X-ray beams is d, and average flow speed of the sample in thecapillary is v.

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp4010554 | J. Phys. Chem. C 2013, 117, 17367−1737517369

J. Phys. Chem. C 2013, 117, 17367-17375

X-ray

Laser

!比較的簡便なセットアップ(通常のステップスキャンXAFS)で行える $繰り返し可能な試料にのみ適用可 %時間分解能はµsが限界(レーザーの集光による)

Pump-flow-probe XAFS

(formation of aggregates or supramolecular complexes) thatcan lead to fast electron transfer. Each individual transientspectrum is quite noisy, but there is high temporal over-sampling over the lifetime of the species. As a firstapproximation one can average a series of spectra correspond-ing to a rather wide time window, but shorter than the lifetimeof the transient species. There are data analysis methods thatallows to extract the structure of intermediates from time-resolved XAFS data sets without binning of the spectra andtherefore without losing the time resolution.47 In many casesstructural parameters correlate with the percentage ofintermediate, and both these parameters cannot be determinedsimultaneously. Therefore, it is important to have goodestimate of the percentage of the intermediate fromindependent experiments in order to obtain the high accuracyof structure determination.The pump-flow-probe experiment allows information about

the reaction kinetics to be obtained by varying the spatialseparation between the laser and the X-ray beams, whilekeeping the X-ray energy fixed at the spectral feature ofparticular interest, e.g., at the position where the differencesignal exhibits the largest magnitude. Figure 4 shows the datafrom such a measurement series, and from the discussion aboveand in the Supporting Information (section: time resolution),we conclude that there is significant contribution of thetemporal response function to the rise time of the signal shownin Figure 4. Note that if the reaction kinetics in the consideredtime range are simple, for example, if there is only oneintermediate state present, then the extraction of the structureof the intermediate is possible by using either a single spectrum

or a longer series of spectra. In this case the two time-resolvedXAS methods can be considered equivalent.In the pump-sequential-probes experiment no significant

temporal evolution of the difference spectrum was observed inthe range 150 ns−3.7 μs, justifying the averaging of the timeseries to improve S/N. Pump-flow-probe measurements wereperformed for 100 μs delay between pump and probe. For boththe pump-sequential-probes and pump-flow-probe experimentswe have calculated the averaged spectrum and the error barsanalyzing the series of spectra using standard statisticalmethods. Figure 5 shows that both methods produce similar

Figure 3. Co K-edge X-ray transient absorption spectra of themulticomponent Ru/MV/Co photocatalytic system corresponding toa series of different time delays between laser and X-ray pulses andacquired using the pump-sequential-probes method. Dots correspondto the measured data points while lines are the smoothed spectra(adjacent averaging with window of five points). Error bars have beenomitted for clarity, but the measurement uncertainty is well-represented by the scatter around the averaged curve.

Figure 4. Transient X-ray absorption signal as a function of delay timebetween laser and X-ray beams in the pump-flow-probe experiment,measured at the energy of the first maximum of the transient XASspectrum (7720 eV) associated with Co reduction. The delay betweenpump and probe is defined by the relative position of the beams in thepump-flow-probe configuration.

Figure 5. Top: Co K-edge XANES of the multicomponent Ru/MV/Co photocatalytic system in the ground state. Bottom: transient X-rayabsorption spectra of the Ru/MV/Co measured at the Co K-edgeusing the pump-flow-probe and pump-sequential-probes methods.



(formation of aggregates or supramolecular complexes) thatcan lead to fast electron transfer. Each individual transientspectrum is quite noisy, but there is high temporal over-sampling over the lifetime of the species. As a firstapproximation one can average a series of spectra correspond-ing to a rather wide time window, but shorter than the lifetimeof the transient species. There are data analysis methods thatallows to extract the structure of intermediates from time-resolved XAFS data sets without binning of the spectra andtherefore without losing the time resolution.47 In many casesstructural parameters correlate with the percentage ofintermediate, and both these parameters cannot be determinedsimultaneously. Therefore, it is important to have goodestimate of the percentage of the intermediate fromindependent experiments in order to obtain the high accuracyof structure determination.The pump-flow-probe experiment allows information about

the reaction kinetics to be obtained by varying the spatialseparation between the laser and the X-ray beams, whilekeeping the X-ray energy fixed at the spectral feature ofparticular interest, e.g., at the position where the differencesignal exhibits the largest magnitude. Figure 4 shows the datafrom such a measurement series, and from the discussion aboveand in the Supporting Information (section: time resolution),we conclude that there is significant contribution of thetemporal response function to the rise time of the signal shownin Figure 4. Note that if the reaction kinetics in the consideredtime range are simple, for example, if there is only oneintermediate state present, then the extraction of the structureof the intermediate is possible by using either a single spectrum

or a longer series of spectra. In this case the two time-resolvedXAS methods can be considered equivalent.In the pump-sequential-probes experiment no significant

temporal evolution of the difference spectrum was observed inthe range 150 ns−3.7 μs, justifying the averaging of the timeseries to improve S/N. Pump-flow-probe measurements wereperformed for 100 μs delay between pump and probe. For boththe pump-sequential-probes and pump-flow-probe experimentswe have calculated the averaged spectrum and the error barsanalyzing the series of spectra using standard statisticalmethods. Figure 5 shows that both methods produce similar

Figure 3. Co K-edge X-ray transient absorption spectra of themulticomponent Ru/MV/Co photocatalytic system corresponding toa series of different time delays between laser and X-ray pulses andacquired using the pump-sequential-probes method. Dots correspondto the measured data points while lines are the smoothed spectra(adjacent averaging with window of five points). Error bars have beenomitted for clarity, but the measurement uncertainty is well-represented by the scatter around the averaged curve.

Figure 4. Transient X-ray absorption signal as a function of delay timebetween laser and X-ray beams in the pump-flow-probe experiment,measured at the energy of the first maximum of the transient XASspectrum (7720 eV) associated with Co reduction. The delay betweenpump and probe is defined by the relative position of the beams in thepump-flow-probe configuration.

Figure 5. Top: Co K-edge XANES of the multicomponent Ru/MV/Co photocatalytic system in the ground state. Bottom: transient X-rayabsorption spectra of the Ru/MV/Co measured at the Co K-edgeusing the pump-flow-probe and pump-sequential-probes methods.



possible, and there will be a slow recovery to the initial state byback electron transfer. Since all main components are insolution and do not have specific interactions with each other,charge transfer and all subsequent reaction steps occur ondiffusion-limited time scales. Therefore, the relevant inter-mediate states of the catalytic compound(s) often havelifetimes in the microsecond to millisecond range, dependingon reactivity and concentrations.Time-resolved X-ray absorption spectroscopy with pico-

second time resolution is currently well developed. Pump−probe setups with kilohertz (kHz) repetition rate exist.17−23

They use synchrotron sources (that have X-ray pulses withmegahertz (MHz) repetition rates) for probing and kHz lasersfor sample excitation. Powerful optical laser pulses allowefficient excitation of the studied molecules (reported values arebetween 7 and 50% for kHz pump−probe XAS). Achieving alarge fraction of excited molecules in the probed volume hasbeen one main requirement for the feasibility of suchexperiments, since only a small fraction (∼10−3) of the X-rayflux is used for probing due to the mismatch in laser vs X-raypulse repetition rates. Only one X-ray pulse out of a thousandin the MHz pulse train delivered by the synchrotron ring mightdetect laser-induced changes in the sample that occur with kHzfrequency. Another factor that affects the feasibility of such anexperiment is the difference in optical and X-ray absorptioncross sections. In order to improve the detection efficiency,setups with high laser pulse repetition rate (∼1 MHz) havebeen developed recently.24,25 At such high frequencies thefilling pattern of the storage ring becomes important, and themuch larger bunch separation at large storage rings allows toobtain higher efficiency for the given laser repetition rate. Sinceit is extremely difficult to refresh the sample with such highrepetition rate, the application of this pump−probe scheme iscurrently limited to compounds with short-lived intermediates(<1 μs) to avoid complications arising from the buildup oflong-lived transients. On the other hand, long-lived transientswith a lifetime in the microsecond range can be very efficientlymeasured with kHz laser excitation and using X-ray probepulses that follow with MHz frequency as described in theExperimental Method section. For similar applications the useof a laser with a repetition rate of 10−200 kHz allows to getmaximal efficiency and such setups are under development atthe SuperXAS beamline of the Swiss Light Source (SLS) and ata few beamlines of the Advanced Photon Source (APS).There are several challenges related to the application of

time-resolved XAS to study the structure of intermediate statesof multicomponent photocatalysts. First, the (relative) amountsof intermediates formed are usually low, and even if thechromophore is efficiently excited by the laser, the diffusion-limited character of the electron transfer reduces the percentageof intermediates in a given time window. Second, under realistic

operating conditions, the concentration of the photocatalyst isusually low (∼0.1 mM). A significant increase of thisconcentration can lead to undesired effects, such as aggregationof complexes and formation of bimetallic species.26

Examples of X-ray absorption experiments with microsecondtime resolution are rare in the scientific literature. After a fewpioneering works,27−30 this field appears to have been almostabandoned. In ref 27 the combination of rapid-flow andexcitation with a CW laser was used, and a time resolution of∼1 μs was achieved.In this article we present the first application of time-resolved

X-ray absorption spectroscopy with microsecond time reso-lution to a multicomponent photocatalytic system. A pump-flow-probe setup with CW laser excitation having highdetection efficiency and thus the ability to measure time-resolved spectra at sub-millimolar concentrations has beendeveloped and applied to the cobaloxime-based photocatalyticsystem. The results obtained with this pump-flow-probe XASsetup are compared with those obtained using a time-resolvedmeasurement technique that utilizes a pump-sequential-probesscheme with kHz pulsed picosecond sample excitation.

■ EXPERIMENTAL METHODSamples. The catalyst [Co(dpgBF2)2(OH2)2] (dpgH2 =

diphenylglyoxime) was prepared as previously described31 andis shown in Figure 1. [Ru(bpy)3]Cl2, methyl viologendichloride (MVCl2), NH4PF6, and NBu4PF6 were purchasedfrom Sigma-Aldrich and used without further purification. Asphotosensitizer and electron relay, [Ru(bpy)3](PF6)2 andMV(PF6)2 were prepared from NH4PF6 and [Ru(bpy)3]Cl2or MVCl2 respectively, following a standard anion-exchangeprocedure. The solid-state metal complexes were handled in air.Anhydrous acetonitrile was purchased from Fisher Scientific.For the pump-flow-probe experiments the sample consisted

of an acetonitrile solution of [Ru(bpy)3](PF6)2 (0.4 mM),MV(PF6)2 (8 mM), tetrabutylammonium hexafluorophosphateNBu4PF6 (0.1 M), and [Co(dpgBF2)2(OH2)2] (0.3−0.7 mM).The same sample composition was used for the pump-sequential-probes experiment but with significantly higherconcentrations: [Ru(bpy)3](PF6)2 3 mM, MV(PF6)2 8 mM,NBu4PF6 0.1 M, and [Co(dpgBF2)2(OH2)2] 1−2 mM.The optical density of the samples was 0.5 for pump-flow-

probe conditions (at the 447 nm excitation wavelength with 1mm sample thickness) and 0.2 for pump-sequential-probesconditions (525 nm excitation and 0.6 mm sample thickness).The Co compound used for the measurements reported in thisstudy was not very soluble in CH3CN, and the solution had tobe filtered before use, leading to an uncertainty in final sampleconcentrations stated above. After the time-resolved XASmeasurements, the identity of the unused sample was checkedwith cyclic voltammetry and electron spray ionization massspectroscopy, and the presence of [Co(dpg)3(BF)2]

+ wasdetected.32 This contamination does not significantly influencethe main aim of this work, which is to compare and evaluate thetwo different excitation and detection schemes for microsecondtime-resolved XAS. However, the presence of this species in thereaction mixture may contribute to the measured XAS spectraand kinetics, and we will therefore refrain from a detailedanalysis of the transient XAS spectra. Such an analysis aiming atmore detailed mechanistic information has to await furtheroptimization of the conditions of the pump-flow-probeexperiment. For both setups the freshly prepared samplesolution was degassed with N2 at least 30 min before the

Figure 1. Scheme of hydrogen evolution by multicomponentphotocatalytic system. The structure of cobaloxime [Co(dpgBF2)2L2]is also shown, with L representing the solvent molecule.



J. Phys. Chem. C 2013, 117, 17367-17375

© 1993 Nature Publishing Group



Nature 1993, 362, 40-43

Pump probe XAFS

Fe[bpy]3

励起状態にある物質の電子状態・構造はどうなっているの？

パルスレーザーが物質に当たる時刻(t0)から、X線が試料に照射されるまでの時間差(⊿t)で、それぞれの状態をスペクトルを測定する。

Paul Sherrer Instituteホームページから

Pump probe XAFS

刺激

X線パルス

Δt1

t

Δt2

Δt3

X線パルスを用いた時間分解測定

Sample : 0.6 mM WO3 suspension pump laser : 400 nm 270 mJ/cm2@945 Hz X-ray pulse width : 100 ps(FWHM)

J. Synchrotron Rad., 14, 313 (2007). J. Synchrotron Rad., 16, 110 (2009).

Photoexcited State of WO3 : LIII XANES@NW14A, PF-AR

Uemura et al., Chem. Lett., 43, 977 (2014).


k = 0.5(1) ns -1

The excited state decayed in 10 ns. →It was successfully fitted with a single exponential function.

Photoexcited State of WO3 : LIII XANES


SPring-8

SACLA

~10 fs

~50 fs

Sample: 4 mM WO3 suspension pump laser: 520 mJ/cm2@15 Hz X-ray pulse width: 30 fs(FWHM) Time resolution: 700 fs

Uemura, et al., Angew. Chem. Int. Ed., 55, 1364 (2016).

Photoexcited State of WO3 : LIII XANES@SACLA


peak A : Edge shift due to formation of W5+ < 1 ps peak C : decrease of absorption from eg orbitals ~ 200 ps

Three distinct peaks were found in the differential spectra.

peak B which was not found in the previous experiments was observed.

Uemura, et al., Angew. Chem. Int. Ed., 55, 1364 (2016).

Photoexcited State of WO3 : LIII XANES@SACLA


XSTRIP

ポリクロメーター

Nd:YAGレーザー

Single Shot DXAFS


検出部

約25 mm

シリコンマイクロストリップ検出器センサーチップシリコンダイオードピクセル数 1024個ピクセル幅 15 µm

ピクセル間隔 25 µm

ピクセル厚 500 µm

電荷容量 10 pC/pixel

読み出し回路32個のADC（14 bit, 5 MHz）最小繰り返し測定間隔 22 µs

最小露光時間 0.5 µs

X線を直接入射可能→高エネルギー分解能

エネルギー領域：～15 keV

Daresbury Lab.で開発岩澤康裕教授（電通大）と共同で導入

J. Headspith et al., Nucl. Instrum. Methods. Phys. Res., A512, 239 (2003).

一次元検出器: XSTRIP

XSTRIPを使った1パルスXAFS測定

最小露光時間：0.5 µs

XSTRIPReset

1 µs

PF-ARX-ray pulse

Scan

0.5 µs

Frame

1.26 µs1.26 µs 1.26 µs1.26 µs

Reset

約100ps同期

バンチ周回周期：794 kHz

PF-AR1.26 µs 1.26 µs

約100 ps

Single Shot DXAFSX線パルスを用いた時間分解測定

Time-resolved observation of structural change of copperinduced by laser shock using synchrotron radiation withdispersive XAFSYasuhiro Niwaa, Tokushi Satob, Kohei Ichiyanagia, Kei Takahashia and Masao Kimuraa,c

aPhoton Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization,Tsukuba, Japan; bCenter for Free-Electron Laser Science, Deutsches Elektronen-Synchrotron, Hamburg,Germany; cSOKENDAI, Tsukuba, Japan

ABSTRACTWe report a single-shot nanosecond time-resolved dispersive XAFSmeasurement of copper foil under laser-induced shock compression.Obtained EXAFS results indicated clearly the structural evolution,though XANES spectra showed that the laser irradiation does notaffect the copper electronic state. The nearest (ca. 0.22 nm) andlonger (ca. 0.44 nm) Cu–Cu distances are changed at 4 and 200ns, respectively, in the radial structure functions (RSFs). It wasrevealed that an average compression of 2% occurs at 4 ns and isreleased within 200 ns. It was found that the fragmentation of thecopper begins at around 200 ns according to the increase indistance of the RSF.

ARTICLE HISTORYReceived 31 May 2016Accepted 7 July 2016

KEYWORDSDispersive XAFS;compression; laser shock;copper

1. Introduction

Properties of materials depend on environments, such as temperature and pressure. Asupercritical fluid is a good example of it. It does not exist at ordinary temperature andpressure, and appears at a temperature and pressure above its critical point. Thus amaterial which has new properties could be obtained by controlling its pressure and temp-erature. There are many reports about extreme conditions. X-ray absorption fine structure(XAFS) is one of the most powerful tools to determine the local structure in a material.Since XAFS can be applied not only to crystals but also to materials that do not havelong-range order such as amorphous systems, it is particularly useful for high pressureapplications. Many high pressure studies of metals such as copper and iron usingdiamond-anvil cell (DAC) by means of XAFS have been reported in the 1970–1980s.[1–4]Iron and copper have been studied from the viewpoint of understanding phase transitionsand for use as EXAFS pressure calibrants.[2,4] Recently, Ishimatsu et al. have studied the α–εtransition of iron using XAFS under quasi-hydrostatic pressure conditions to gain a betterunderstanding of the mechanism of martensitic transformation.[5]

Recently, a high-power laser has been used, instead of DAC, as a high pressure source ina time-resolved study under extreme conditions. The merit of a high-power pulse laser is

© 2016 Informa UK Limited, trading as Taylor & Francis Group

CONTACT Yasuhiro Niwa [email protected] Photon Factory, Institute of Materials Structure Science, HighEnergy Accelerator Research Organization, 1-1 Oho, Tsukuba 305-0801, Japan

HIGH PRESSURE RESEARCH, 2016VOL. 36, NO. 3, 471–478http://dx.doi.org/10.1080/08957959.2016.1211647

summarized in Table 1. The local structures at 4 and 200 ns can be assumed to be similar tothose in the initial state because the shapes of the XANES spectra are almost the same, andthe coordination number N was fixed at 12. Table 1 shows that the nearest-neighborCu–Cu distance of R = 0.256 (2) nm before the laser shock becomes shorter (R = 0.254(1) nm) at 4 ns corresponds to ca. 2% compression in volume, and almost returns to theinitial value of R = 0.257 (1) at 200 ns. In other words, the 2% compression in volumeon average is expected at 4 ns and is released at 200 ns from the change of thenearest-neighbor distance.

However, the peaks around R = 0.43 nm, corresponding to the third-nearest-neighbordistances in the <1, 0.5, 0.5 > directions, showed a different behavior. The peak remainednear that of the initial state (R = 0.43 nm) at 4 ns, and shifted to a larger value (R = 0.44 nm)at 200 ns. These results suggest that the change of atomic-scale structure induced by thelaser shock is not isotropic, in line with the expectation that laser shock produces uniaxialcompression. Unfortunately, quantitative distances R for the second and higher neighbor-ing interaction could not be obtained because of insufficient energy range and low S/Nratio of the EXAFS oscillations.

The atomic-scale change of structure found in this study is compared with the result ofthe laser shock-induced lattice deformation of CdS single crystal examined by time-resolved Laue diffraction.[13] The laser conditions used in that study were almost thesame as those in this study except for the laser energy, which was 0.86 J instead of1.3 J. They reported that the whole sample was shock-compressed after 15 ns.

Table 1. Structure parameters for the nearest Cu–Cu interaction for various delay times.Delay time/ns S0

2 N R/10–1nm σ2/10–2 nm2 ΔE0/eV R-factor

Laser off 1.1 ± 0.1 12* 2.56 ± 0.02 0.011 ± 0.003 6 ± 2 0.0154 ns 1.1* 12* 2.54 ± 0.01 0.010 ± 0.001 6* 0.024200 ns 1.1* 12* 2.57 ± 0.01 0.012 ± 0.001 6* 0.016

*The values were kept constant during the analysis.

Figure 3. The radial structure function observed (solid line) and calculated for the first shell (dashedline) at 4 ns (blue line), 200 ns (red line) after laser irradiation and without laser irradiation (black line).

HIGH PRESSURE RESEARCH 475

summarized in Table 1. The local structures at 4 and 200 ns can be assumed to be similar tothose in the initial state because the shapes of the XANES spectra are almost the same, andthe coordination number N was fixed at 12. Table 1 shows that the nearest-neighborCu–Cu distance of R = 0.256 (2) nm before the laser shock becomes shorter (R = 0.254(1) nm) at 4 ns corresponds to ca. 2% compression in volume, and almost returns to theinitial value of R = 0.257 (1) at 200 ns. In other words, the 2% compression in volumeon average is expected at 4 ns and is released at 200 ns from the change of thenearest-neighbor distance.

However, the peaks around R = 0.43 nm, corresponding to the third-nearest-neighbordistances in the <1, 0.5, 0.5 > directions, showed a different behavior. The peak remainednear that of the initial state (R = 0.43 nm) at 4 ns, and shifted to a larger value (R = 0.44 nm)at 200 ns. These results suggest that the change of atomic-scale structure induced by thelaser shock is not isotropic, in line with the expectation that laser shock produces uniaxialcompression. Unfortunately, quantitative distances R for the second and higher neighbor-ing interaction could not be obtained because of insufficient energy range and low S/Nratio of the EXAFS oscillations.

The atomic-scale change of structure found in this study is compared with the result ofthe laser shock-induced lattice deformation of CdS single crystal examined by time-resolved Laue diffraction.[13] The laser conditions used in that study were almost thesame as those in this study except for the laser energy, which was 0.86 J instead of1.3 J. They reported that the whole sample was shock-compressed after 15 ns.

Table 1. Structure parameters for the nearest Cu–Cu interaction for various delay times.Delay time/ns S0

2 N R/10–1nm σ2/10–2 nm2 ΔE0/eV R-factor

Laser off 1.1 ± 0.1 12* 2.56 ± 0.02 0.011 ± 0.003 6 ± 2 0.0154 ns 1.1* 12* 2.54 ± 0.01 0.010 ± 0.001 6* 0.024200 ns 1.1* 12* 2.57 ± 0.01 0.012 ± 0.001 6* 0.016

*The values were kept constant during the analysis.

Figure 3. The radial structure function observed (solid line) and calculated for the first shell (dashedline) at 4 ns (blue line), 200 ns (red line) after laser irradiation and without laser irradiation (black line).

HIGH PRESSURE RESEARCH 475

• 短時間でXAFSを測定したい。 QXAFS

• 時間と共に変化する様を観測したい（定性的）。 QXAFS, DXAFS

• 反応速度、活性化エネルギーを求めたい（定量的）。 DXAFS, QXAFS

• 励起状態、準安定状態に関する情報を得たい。 ultrafast (t<ps) pump-probe実験繰り返し測定が必要

どの方法をとるべきか？

Documents

時間分解XAFS - 立命館大学...Step Scan XAFS モノクロメーターの角度を止めて、吸光度データを溜め込む。1スペクトルを得るのに最低でも数分かかる。時間分解能は分オーダー