Capturing molecular structural dynamics by 100 ps time-resolved X-ray absorption spectroscopy.

Abstract

An experimental set-up for time-resolved X-ray absorption spectroscopy with 100 ps time resolution at beamline NW14A at the Photon Factory Advanced Ring is presented. The X-ray positional active feedback to crystals in a monochromator combined with a figure-of-merit scan of the laser beam position has been utilized as an essential tool to stabilize the spatial overlap of the X-ray and laser beams at the sample position. As a typical example, a time-resolved XAFS measurement of a photo-induced spin crossover reaction of the tris(1,10-phenanthrorine)iron(II) complex in water is presented.

Full text

Page 1

research papers
110
doi:10.1107/S0909049508034596
J. Synchrotron Rad. (2009). 16, 110–115
Journal of
Synchrotron
Radiation
ISSN 0909-0495
Received 4 September 2008
Accepted 23 October 2008
Capturing molecular structural dynamics by 100 ps
time-resolved X-ray absorption spectroscopy
Tokushi Sato,a,bShunsuke Nozawa,bKohei Ichiyanagi,bAyana Tomita,a,b
Matthieu Chollet,aHirohiko Ichikawa,bHiroshi Fujii,cShin-ichi Adachib,dand
Shin-ya Koshiharaa,b,d,e*
aDepartment of Materials Science, Tokyo Institute of Technology, 2-12-1-H61 Ohokayama,
Meguro-ku, Tokyo 152-8551, Japan,bNon-Equilibrium Dynamics Project, ERATO, Japan Science
and Technology Agency, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan,cInstitute for Molecular
Science and Okazaki Institute for Integrative Bioscience, Myodaiji, Okazaki 444-8787, Japan,dHigh
Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan, and
eFrontier Research Center, Tokyo Institute of Technology, 2-12-1 Ohokayama, Meguro-ku, Tokyo
152-8551, Japan. E-mail: skoshi@cms.titech.ac.jp
An experimental set-up for time-resolved X-ray absorption spectroscopy with
100 ps time resolution at beamline NW14A at the Photon Factory Advanced
Ring is presented. The X-ray positional active feedback to crystals in a
monochromator combined with a figure-of-merit scan of the laser beam position
has been utilized as an essential tool to stabilize the spatial overlap of the X-ray
and laser beams at the sample position. As a typical example, a time-resolved
XAFS measurement of a photo-induced spin crossover reaction of the tris(1,10-
phenanthrorine)iron(II) complex in water is presented.
Keywords: time-resolved X-ray absorption; spin crossover; tris(1,10-phenanthrorine)iron(II).
1. Introduction
Ultrafast time-resolved X-ray measurements such as diffrac-
tion, scattering, absorption and imaging using synchrotron
radiation sources are becoming general and powerful tools for
exploring structural dynamics in the fields of materials and
biological sciences (Sˇrajer et al., 1996; Collet et al., 2003;
Schotte et al., 2003; Cavalleri et al., 2005; Ihee et al., 2005;
Jeong et al., 2005; Gawelda et al., 2007; Ichiyanagi et al., 2007;
Kong et al., 2008; Wang et al., 2008). Among these time-
resolved X-ray techniques, time-resolved X-ray absorption
spectroscopy (TR-XAS) provides dynamic information on the
electronic state and molecular structure of non-crystalline
samples with temporal resolution of the pulse width of X-rays
(Saes et al., 2003, 2004; Cavalleri et al., 2005; Khalil et al., 2006;
Gawelda et al., 2007). TR-XAS can reveal the dynamics of the
oxidation and valence states via XANES and the local mole-
cular structure via EXAFS with sub-angstrom spatial resolu-
tion, which compliments diffraction and scattering techniques
(Ihee et al., 2005; Kong et al., 2008). Several groups have been
developing TR-XAS using synchrotron facilities, and this
method has been successfully applied to probe the dynamics
of the spin crossover system (Khalil et al., 2006; Gawelda et al.,
2007) and the photodissociation of ligands in NiTPP (Chen et
al., 2001).
For successful TR-XAS measurement in the pump–probe
mode, precise spatial and temporal overlap between the X-ray
and laser beam is essential. However, maximizing the incident
X-ray beam intensity causes difficulties in keeping the spatial
overlap during the experiment. The intensity of the incident
X-ray beam is usually maximized to obtain as strong a time-
dependent signal as possible, whereas most of the incident
photons are wasted by X-ray chopper or electronic gating in
order to reduce the repetition rate of the X-ray. The maxi-
mized X-ray beam gives a large heat load to X-ray optics,
mainly to the monochromator or mirror, which destabilizes
the X-ray beam position. In order to maintain the spatial
overlap during data collection, the X-ray beam position must
be stabilized by active positional feedback. At the same time,
the spatial overlap must be reproducibly tuned during the long
data collection period to avoid the mismatch of the X-ray and
laser positions owing to positional drift of the two beams. The
most effective check is the laser positional scan using the
photo-induced X-ray signal itself as a figure of merit of the
spatial overlap. Here we report the development of a TR-XAS
system equipped with stable alignment of the X-ray and laser
beam positions at the beamline NW14A, Photon Factory
Advanced Ring, and present an example of TR-XAS
measurements using these systems.
2. Experimental details
2.1. X-ray and laser specifications
The 6.5 GeV Photon Factory Advanced Ring (PF-AR) is a
full-time single-bunch synchrotron radiation source especially

Page 2

suited for time-resolved X-ray studies using pulsed X-rays
because time-resolved experiments primarily require a rela-
tively sparse bunch-filling mode. Electrons with a ring current
of 60 mA (75.5 nC per bunch) are stored in a single bucket
with a lifetime of ?20 h. The frequency of the RF cavities and
harmonic number of the PF-AR are 508.58 MHz and 640,
respectively, corresponding to a revolution frequency of
794 kHz. As the PF-AR is operated in the single-bunch mode,
the X-ray pulses are delivered at a frequency of 794 kHz with
a pulse duration of 60 ps (r.m.s.). The PF-ARis operationalfor
?5000 h annually, which is a major advantage in terms of
time-resolved beam time over other synchrotron facilities.
In this TR-XAS study, all measurements were performed
using the fluorescence XAFS method on the undulator
beamline NW14A at the PF-AR, as illustrated in Fig. 1.
Details of the beamline have been reported previously
(Nozawa et al., 2007). The X-ray pulses at 794 kHz were
monochromized by a Si(111) monochromator and used for the
probe. The energy resolution was ?E/E = 1.6 ? 10?4at 7 keV,
which is estimated from the rocking curve of the Si(111)
reflection. The higher harmonics of the reflection were
rejected by the double flat mirrors coated with rhodium with a
thickness of 100 nm. The X-rays were focused to 250 (H) mm
? 150 (V) mm at the sample position by the Rh-coated bent-
cylindrical mirror. The intensity of the incident X-ray was
detected by ionization chambers.
A 945 Hz regenerative amplified Ti:sapphire laser was used
for the pump source. The timing between the X-ray and laser
is defined on the basis of the RF master clock that drives the
electron bunch in the storage ring. In order to achieve precise
synchronization between the laser and X-ray, we used the
CANDOX delay-timing system (Ohshima et al., 2007), which
provides an external trigger signal within a 3 ps jitter. This
system is composed of frequencydividers, IQ modulator phase
shifters and digital counters. The external trigger of the sine
wave (84.76 MHz = 508.58 MHz/6) for the mode-locked laser
and pulse signal (945 Hz = 508.58 MHz/537600) was provided
by the frequency divider. The timing of the laser pulse can be
changed by shifting the amount of the IQ modulator phase up
to 2 ns (1/508.58 MHz). Delay timing of over 2 ns can be set by
the digital counter combined with the IQ modulator up to
about 1 ms (1/945 Hz). The laser was converted to 400 nm by a
BBO crystal and stretched to a duration of up to 2 ps and
focused using a lens with ’ = 300 mm at the sample position.
The angle between the X-ray and the laser was almost parallel
(?10?).
2.2. Sample environments, detector and data acquisition
We used an open jet and circulation pump system because
liquid samples can be damaged by laser excitation (Saes et al.,
2004). The liquid sample was circulated by the magnetic gear
pump at a flow rate of 180 ml min?1. The stability of the
fluorescence signal and overlap of the X-ray and laser depend
on the positional fluctuation of the liquid surface. We used a
sapphire nozzle head to provide a stable liquid flow with a size
of 7 mm (width) ? 300 mm (thickness). The liquid surface was
set at 45?against the X-ray.
A fast scintillation detector was located at 90?to the inci-
dent X-ray beam, coplanar with the X-ray polarization vector
to avoid elastic scattering, and a Soller slit and a metal filter
were set in front of the detector. The 794 kHz X-ray fluores-
cence signal was measured using a fast scintillation counter,
which consists of a plastic scintillator (Saint-Gobain Crystals
BC-420) with a nanosecond-order decay time, a 29% light
output of NaI(Tl), and a photomultiplier tube (Hamamatsu
H7195) with ’ = 46 mm acceptance, 10 nA dark current and
3 ? 106gain. The response time of the detector was fast
enough to separate each X-ray pulse at 794 kHz. The pump
and probe measurement was performed by detecting the
fluorescence X-ray signals just after and just before the laser
pulse using gated integrators (Stanford Instruments SR250)
synchronized with the laser pulse (945 Hz).The output voltage
from the gated integrators was converted to a frequency signal
using a voltage-to-frequency (V–F) converter (Ohyo Koken
Kogyo 733-1). The output pulses from the V–F converter were
counted using a 100 MHz counter (Ortec 974) and further
used to obtain the X-ray absorption spectrum.
research papers
J. Synchrotron Rad. (2009). 16, 110–115Tokushi Sato et al.
? Time-resolved X-ray absorption spectroscopy
111
Figure 1
Layout of the time-resolved XAFS experiment at the NW14A beamline at the PF-AR.

Page 3

Fig. 2(a) shows a schematic diagram of the measurement.
The fluorescence X-ray signal from the scintillation counter
was fed into the two inputs of the boxcar integrator as the
signal and reference. The laser timing was sampled by a fast
metal–semiconductor–metal photodiode (Hamamatsu G4176-
03). All signals were monitored by a 2.5 GHz digital oscillo-
scope (Tektronix DPO 71254). Fig. 2(b) shows the signals
measured by the oscilloscope during measurement.
2.3. X-ray positional feedback from the fixed-exit
double-crystal monochromator and the figure-of-merit
scan for the spatial overlap between the X-ray and laser
During the TR-XAS measurement it is important to
produce a high-quality X-ray beam in order to measure a small
differential signal between the photo-excited and ground
states. This requisite involves accurate X-ray energy tuning
(0.1 eV), a fixed beam position, and stable beam intensity over
a wide range of energy scanning up to 1.5 keV. For this
purpose the X-ray beam is generally monochromized by a
fixed-exit double-crystal monochromator (DCM). The first
crystal monochromatizes the incident white X-ray and the
second crystal defines the X-ray energy. Because the incident
white X-ray heats up the first crystal and causes thermal
expansion of the crystal, the heat load causes instabilities in
the energy, intensity and position of the exit beam from the
DCM. We implemented a positional feedback control of the
X-ray beam with the piezo actuator of ??1to stabilize the
beam position and intensity (Kudo & Tanida, 2007). A posi-
tion-sensitive ion chamber (PSIC; Ohyo Koken Kogyo
S-2403B) was set downstream of the DCM, and the output was
converted to the positional signal. The beam position was
stabilized by a proportional-integral-derivative (PID) control
by monitoring the difference of the positional signal from the
standard beam position. The feedback signal was converted to
the voltage and led to the piezo actuator of the first crystal of
the DCM. Fig. 3 shows the beam intensity measured by the I0
ionization chamber and the position measured using the PSIC
at 7 keV with and without the feedback loop used for tuning
??1. Without the feedback loop the beam intensity and
position are not stabilized, since the two crystals are not
maintained in parallel. In contrast, a smooth and stable beam
position and intensity are obtained with the feedback loop.
The spatial overlap of the X-ray and laser beam is typically
achieved by positional scans using a slit or a pinhole. This
method is not straightforward because two independent scans
for laser and X-ray are required and the slit or pinhole must
be aligned properly for each positional check. The most
straightforward way to achieve this is by a laser positional scan
using the X-ray signal itself as the figure of merit of the spatial
overlap. This allows a reproducible alignment of the laser and
X-ray during the experiment. This method can record the
actual laser profile using a pump–probe signal at the actual
laser intensity used in the experiment and makes it possible to
estimate the laser shape and focal size without attenuating the
laser power. In order to perform the figure-of-merit scan
properly, the difference spectrum before and after photo-
excitation must be known in advance. Thus, we align the X-ray
and laser at the sample position by the pinhole scan first, and
utilize the figure-of-merit scan as soon as the difference
spectrum is successfully obtained.
As shown in Fig. 4(a), a mirror just before the liquid jet is
mounted on motorized rotational and swivel stages. First, the
research papers
112
Tokushi Sato et al.
? Time-resolved X-ray absorption spectroscopy
J. Synchrotron Rad. (2009). 16, 110–115
Figure 3
Stabilization of the beam position by the feedback system of the
monochromator.
Figure 2
Schematic drawing of the detection system and the signal output
observed on the oscilloscope during the time-resolved measurement.

Page 4

X-ray energy and laser delay timing are fixed at the maximum
pump–probe signal, and the laser beam position is scanned
using these two stages. The distance between the sample and
the mirror was 300 mm, and a mirror rotational angle of 0.02?
corresponds to 100 mm at the sample position. Fig. 4(b) shows
the result of the figure-of-merit scan at 7125 eV; the overlap of
both lights was achieved at 100 mm precision.
3. Pump–probe TR-XAS experiment of a spin crossover
system
3.1. Spin crossover transition of [FeII(phen)3]2+
Here we report a real example for the measurement of TR-
XAS with 100 ps time resolution utilizing our experimental
set-up. We have observed the photo-induced spin crossover
(SCO) transition of [FeII(phen)3]2+in solution. It is known
that this system undergoes structural change and electronic
spin transition with a ?680 ps decay time (McCusker et al.,
1993). This time scale of transition is suitable for our time
resolution of 100 ps.
The structure of FeIIcomplexes used in this study is shown
in Fig. 5(a). [FeII(phen)3]2+is the six-coordinated low-spin
complex in its ground state. The spectrum shown by the solid
line is the absorption spectrum of [FeII(phen)3]2+. The
absorption peaks at 438, 477 and 510 nm correspond to the
transition from
1968; Sullivan et al., 1978). In addition to the [FeII(phen)3]2+
complex, we prepared the high-spin analogue [FeII(2-CH3-
phen)3]2+as a reference compound of the photo-excited state
of [FeII(phen)3]2+. The absorption spectrum of [FeII(2-CH3-
phen)3]2+, shown by the dotted line, is much weaker than that
1A1 to
1MLCT (Jørgensen, 1957; Bosnich,
of the low-spin [FeII(phen)3]2+. Fig. 5(b) shows an energy
diagram of photo-induced SCO of [FeII(phen)3]2+suggested
by the ultrafast transient absorption spectroscopy (McCusker
et al., 1993). The photo-excitation of the low-spin complex at
the MLCT band induces a high-spin state5T2, and the excited
state decays from a high-spin to a low-spin state at a time
constant of ?680 ps. We observed this spin transition and
structural change by TR-XAS at a 100 ps time resolution.
[FeII(phen)3]2+and [FeII(2-CH3-phen)3]2+were prepared as
reported previously (Pfeiffer & Christeleit, 1938; Irving et al.,
1953). All the reagents were reagent-grade and used without
further purification.
research papers
J. Synchrotron Rad. (2009). 16, 110–115Tokushi Sato et al.
? Time-resolved X-ray absorption spectroscopy
113
Figure 5
(a) Structure of [FeII(phen)3]2+and [FeII(2-CH3-phen)3]2+and their
absorption spectra in the low-spin (solid line) and high-spin (dotted line)
states. (b) Simplified energy diagram of the photo-induced spin crossover
transition of [FeII(phen)3]2+.
Figure 4
Figure-of-merit scan of the laser position by the motorized stage.

Page 5

3.2. Time-resolved XAS of [FeII(phen)3]2+: results and
discussion
The upper part of Fig. 6 shows the steady-state XAS spectra
of [FeII(phen)3]2+in the low-spin state and [FeII(2-CH3-
phen)3]2+in the high-spin state in aqueous solution at the
Fe K-edge. The difference spectrum of these steady-state
spectra is shown in the lower part of Fig. 6 (solid line). The
open circles show the transient difference spectrum of
[FeII(phen)3]2+observed at 50 ps after laser excitation. The
steady-state difference spectrum is scaled by 0.06, which
accounts for the 6% photoexcitation yield of the ground-state
species in solution by the laser irradiation. The spectral
features labeled as A and B arise from the transition to the
unoccupied Fe state. These unoccupied orbitals are caused by
hybridization between the Fe (4s, 4p) and the N (2p) states. As
the Fe—N bond length increases upon photo-excitation, going
from low-spin to high-spin states, the overlap between the Fe
and N orbitals diminishes. As a result, the ligand field strength
decreases and a hole on the Fe 4p orbital is generated, leading
to the enhancement of the dipole transition from 1s to 4p, as
shown by spectral feature A. Since this transition involves a
contribution of the orbital that is formally anti-bonding, the
stretching of the bond length lowers the energy of the un-
occupied Fe—N state, accounting for the red shift of feature
B. Features C and D are attributed to multiple-scattering
processes of the photoelectron (Hannay et al., 1997; Briois et
al., 2001; Boillot et al., 2002).
As discussed previously, spectral feature A directly reflects
the change in the bond length between Fe and N atoms.
Therefore the observed intensity enhancement at this photon
energy can be utilized as a proper probe of the photo-induced
structural change with the spin state transition. In order to
study the time evolution of the spectral features, we measured
the time course of feature A at 7125 eV, as shown in Fig. 7. The
temporal evolution of the difference X-ray signal is modeled
as a single exponential function convoluted with the instru-
ment response function. The instrument function is estimated
by the pulse duration of the X-ray and modeled by a Gaussian
error function (thick solid curve) with ? ’ 60 ps. This result
indicates that the structural evolution after the1A1to1MLCT
excitation of the low-spin compound is complete within our
time resolution. The decay time of 700 ps agrees well with the
life-time estimated by ultra-fast spectroscopy (McCusker et al.,
1993). The structural changes will be obtained by further
analysis of the TR-EXAFS, which is directly linked to the
transiently generated high-spin species of the [FeII(phen)3]2+
compound in solution. A detailed structural analysis of the
TR-XAS is now underway, and the results of the analysis will
be presented elsewhere.
4. Concluding remarks
We have presented an experimental set-up for time-resolved
X-ray absorption spectroscopy with 100 ps time resolution at
beamline NW14A at the PF-AR. We have demonstrated that
the X-ray positional feedback of the monochromator and the
figure-of-merit scan of the laser beam position are powerful
tools for achieving the efficient observation of precise and
detailed photo-induced change in TR-XAS. By utilizing this
feedback and scanning system, a photo-induced transient
signal of a spin crossover reaction of the [FeII(phen)3]2+
complex in water was successfully detected. Further analysis
of the pre-edge region and EXAFS structure will reveal the
details of the photo-induced spin state and structure.
We thank Michael Wulff (ESRF) for discussion of the
figure-of-merit scan and Hyotcherl Ihee (KAIST) for the use
of the liquid jet system. This work was performed under the
research papers
114
Tokushi Sato et al.
? Time-resolved X-ray absorption spectroscopy
J. Synchrotron Rad. (2009). 16, 110–115
Figure 7
Time course of the difference signal at 7125 eV. Fit of the data using a
single exponential function convoluted with the instrument function (? =
60 ps) (solid line).
Figure 6
(a) Fe K-edge XANES spectra of a 50 mM aqueous solution of
[FeII(phen)3]2+(solid line) and [FeII(2-CH3-phen)3]2+(dashed line). (b)
Difference spectrum between [FeII(phen)3]2+and [FeII(2-CH3-phen)3]2+
(solid line). Difference ?1.3 ms before and 50 ps after excitation of
[FeII(phen)3]2+(circles).