Chemical Physics 299 (2004) 163–170

www.elsevier.com/locate/chemphys

Probing photoinduced phase transition in a charge-transfermolecular crystal by 100 picosecond X-ray diffraction

Laurent Gu�erin a,*, Eric Collet a,*, Marie-H�el�ene Lem�ee-Cailleau a,Marylise Buron-Le Cointe a, Herv�e Cailleau a, Anton Plech b,c, Michael Wulff b,

Shin-Ya Koshihara d, Tadeusz Luty e

a Groupe Mati�ere Condens�ee et Mat�eriaux, UMR CNRS 6626, Universit�e Rennes1, 35042 Rennes Cedex, Franceb European Synchrotron Radiation Facility, B.P. 220, 38043 Grenoble, Francec Fachbereich Physik der Universit€at Konstanz, D-78457 Konstanz, Germany

d Department of Materials Science, Tokyo Institute of Technology, 2-12-1 Oh-okayama, Meguro-ku, Tokyo 152, Japane Institute of Physical and Theoretical Chemistry, Technical University of Wrocław, 50-370 Wrocław, Poland

Received 1 August 2003; accepted 7 October 2003

Abstract

Watching with an atomic resolution at structural changes as fast or ultra-fast photoinduced physical processes take place benefits

from recent progresses in time-resolved X-ray diffraction. Molecular materials where electronic and structural changes are strongly

coupled are model systems to perform such time-resolved crystallography studies. We report the structural investigation of pho-

toinduced phase transformations between ionic and neutral states in an organic charge-transfer molecular material, using 100 pi-

cosecond (ps) synchrotron pulses. This light-induced phenomenon, triggered by an ultra-short optical pulse from a femtosecond

laser, occurs by virtue of intrinsic cooperativity. Since electronic and structural changes are strongly coupled, it is of fundamental

interest to perform time-resolved X-ray diffraction to obtain information at the atomic scale. We also discuss the problem of co-

existence of phases and the interest of future investigations in faster timescale.

� 2003 Elsevier B.V. All rights reserved.

Keywords: Photoinduced phase transition; Time-resolved X-ray diffraction

1. Introduction

Fast and ultrafast X-ray science represents a new

emerging frontier. In particular, recent developments in

time-resolved X-ray diffraction promise direct access to

the dynamics of electronic, atomic and molecular mo-

tions in condensed matter, making possible to record

molecular movies during the transformation of matter

[1]. One major application is the direct investigation of

photoinduced structural phase transitions as they takeplace. Indeed, the ‘‘laser pump and X-ray probe’’ tech-

nique provides now an outstanding opportunity of the

* Corresponding authors. Tel.: +33-2-23-23-65-32; fax: +33-2-23-23-

67-17 (E. Collet); Tel.: +223236988; fax: +223236717 (L. Gu�erin).

E-mail addresses: laurent.guerin@univ-rennes1.fr (L. Gu�erin),

Eric.Collet@univ-rennes1.fr (E. Collet).

0301-0104/$ - see front matter � 2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.chemphys.2003.10.002

direct observation of symmetry and structural changes

triggered by a laser pulse irradiation. Short X-ray pulsesof about 100 picosecond (ps) around a third-generation

synchrotron source are used for structural investigations

of fast photoinduced processes with crystallographic

possibilities almost similar to that used at thermal

equilibrium [2,3]. Thus some structural investigations of

local chemical or biochemical photoinduced phenomena

occurring on this timescale have been recently reported

[4–7]. Other new X-ray sources, such as laser-producedplasma ones, provide ultrashort pulses down to 100

femtosecond (fs) [8,9]. This opens the way to femtosec-

ond crystallography and is at the origin of the obser-

vation of some fascinating non-thermal light-induced

phenomena, such as surface melting [10,11], coherent

optical phonon [12] or insulating-to-metal solid–solid

phase transition [13]. However the X-ray intensity

164 L. Gu�erin et al. / Chemical Physics 299 (2004) 163–170

provided by fs sources is rather low and these inves-

tigations have been still limited to simple systems where

the time evolution of only one or two Bragg peaks has

been followed. The study of photoinduced structural

phase transitions in more complex crystals such as mo-lecular ones requires more detailed diffraction mea-

surements. In this way we have recently reported the

direct observation of a laser-induced ferroelectric

structural order in an organic charge-transfer compound

by probing a single crystal with monochromatic 100 ps

synchrotron pulses [14], taking advantage of the X-ray

beam properties such as low divergence and high flux.

Photoinduced phase transitions constitute a new typeof fast or ultrafast manipulation of matter by light [15–

17]. In some photo-active materials, the structural re-

laxations of the electronic excited states following the

absorption of photons are not independent processes, as

in conventional excitonic or photo-chemical ones. In

fact these result in drastic electronic-structural changes

involving a large number of electrons and molecules.

Strong inter-molecular cooperative processes still fur-ther lead to a photoinduced phase transition towards

new lattice structure and electronic order [18]. There-

fore, possibilities appear to drive with light new self-

organized long-range ordering (structural, magnetic,

ferroelectric, etc.), opening the way to ultrafast photo-

switching of macroscopic physical properties of mate-

rials. In that frame, molecular charge-transfer (CT)

insulating materials are exemplary since they are readilytuned by a laser pulse between competing neutral (N)

and ionic (I) ground states on a ps timescale [19–21]. The

N–I transition occurs between two insulating phases.

Therefore, it was possible to photoinduce the ferro-

electric I phase from the N paraelectric phase, stable at

high temperature (Fig. 1). Time-resolved photo-crystal-

lography available with synchrotron sources makes it

Fig. 1. Schematic drawing of the photo-induced transformation path.

The stable neutral state is made of homogeneous chains (black) where

molecules are regularly stacked (1). Photons excite few DA pairs into

ionic (grey) (2) and chains becomes inhomogeneous by lattice relaxa-

tion of ionic strings growing along the stack (3). The coupling between

the strings make the system switch to a metastable macroscopic phase

with new electronic state and structural order (4) and a ferroelectric

dimerization reorganization takes place between the neutral stable

phase (left) and the photoinduced metastable ionic one (right).

possible to directly observe the photoinduced structural

changes, at the atomic scale, with the appropriate time-

resolution, giving also direct information on ordering or

disordering phenomena. Here we report new insights

around the structural investigation on the 100 ps time-scale of the photoinduced phase transformations, in

particular from the I ferroelectric low temperature phase

to the metastable N state. We also introduce the devel-

opment of adequate methodologies to discuss the nu-

cleation of the photoinduced phase, taking into account

the coexistence of phases, and open discussion on the

investigation of precursor phenomena in faster time re-

gion. Finally some general features on photoinducedphase transitions are discussed in relation with future

developments of photocrystallography.

2. N–I phase transition

The mixed-stack CT organic crystals which undergo

the N-I phase transition [22] belong to the class ofmultistable molecular materials. The switching between

degenerate or quasi degenerate states, driven by inter-

molecular cooperativity, may involve changes in the

molecular identity, such as charge [22] or spin [23–25].

In low-dimensional systems, interactions between elec-

tronic and lattice degrees of freedom are enhanced and

cause strong and ultrafast cooperative phenomena and

broken symmetries. This is the case of the N–I transitionwhich can be classified as a prototypical example of the

valence instability in CT solids. The mixed-stack se-

quence of alternating donor (D) and acceptor (A) mol-

ecules stimulates cooperative inter-molecular electron

transfer. This gives rise to a chain multistability [26,27]

between one regular N state . . . D0 A0 D0 A0 D0 A0. . .and two degenerate and polar dimerized I states

. . .(DþA�) (DþA�) (DþA�) (DþA�). . . and . . .(A�Dþ)(A�Dþ) (A�Dþ) (A�Dþ). . . The phase transition be-

tween macroscopic N (stable at high temperature) and I

(stable at low temperature) states, which are both insu-

lating, requires inter-chain cooperativity. Structural and

electronic changes are strongly coupled and the phase

transition mechanism is governed by the formation of

lattice-relaxed CT strings [27–31] along the stack, a se-

quence of I dimers within a N chain, . . .D0 A0 (DþA�)(DþA�) (DþA�) D0 A0. . . or vice versa, which interact

with each other (Fig. 1). The well-known TTF-CA

(tetrathiafulvalene-p-chloranil, C6H4S4–C6Cl4O2) CT

compound, located close to the N–I interface [22], un-

dergoes the first-order N–I phase transition at ambient

pressure, and structural changes are accompanying the

ionicity change of constituent molecules (Tc ¼ 81 K).

Thus, in relation with the dimerization process, a longrange ferroelectric ordering of dimers takes place in the I

phase (Iferro) [32]. In the high temperature N phase, the

monoclinic unit cell (Fig. 2) contains two symmetry re-

Fig. 2. Schematic drawing of the symmetry breaking associated with

the phase transition from the N (left) and I (right) states. In the N

states, donor (white) and acceptor (grey) molecules are located on

inversion symmetry site (�). In the I state, the dimerization process

gives rise to the loss of the inversion centre and the loss of the screw

axis corresponds to a ferroelectric ordering between the stacks.

L. Gu�erin et al. / Chemical Physics 299 (2004) 163–170 165

lated undimerized DA pairs, where both molecules are

located on inversion symmetry sites (space groupP21=n, Z ¼ 2 [33]). In the ionic low temperature phase,

inversion centres are lost in relation with the dimeriza-

tion process, whereas the simultaneous lost of the two-

fold screw axis, parallel to the b axis, is associated with

the ferroelectric ordering between the stacks (space

group Pn) as schematically shown on Fig. 2. Conse-

quently, the (0 k 0) Bragg reflections with k odd, for-

bidden by symmetry in the N phase, appear in the Iferroelectric phase. Such materials are highly photoac-

tive and they can switch under the effect of an ultra short

(fs) laser pulse in both directions from the I to the N

state and from the N to the I ones on the 100 ps time-

scale [19]. The photoinduced phenomena are highly

nonlinear because the efficiency is not simply propor-

tional to the total absorbed photon energy: it presents a

threshold behaviour and can be as high as few hundredsof DA pairs transformed per photon [19–21]. The

structural ordering phenomenon has been recently ob-

served by 100 ps X-ray diffraction in the photoinduced I

phase, triggered by laser irradiation of N phase [14].

New structural investigations are presented here.

3. Time-resolved crystallography experiment

In the present 100-ps time-resolved X-ray diffraction

study we have used monochromatic diffraction on a

single crystal, allowing an accurate measurement of

many Bragg reflections, well adapted to crystals com-

posed of small molecules. In addition, the use of a single

crystal makes it possible to control the laser polarization

effect. The experiments were performed on beamlineID09B at the European Synchrotron Radiation Facility

(ESRF), using the optical pump & X-ray probe method

[2,3]. The sample was cooled by a Helijet helium stream,

which allows to investigate not only the N-to-I photo-

induced transformation as previously [14], but also the

I-to-N from the low temperature phase.

A single crystal (1000� 150� 120 lm3) was strobo-

scopically pumped with a mode-locked Ti:sapphire laser

providing about 150 fs pulses at 1.55 eV (800 nm

wavelength), with a light polarization parallel to the

stacking axis a, the long axis of the crystal, which wasalso the oscillation axis for the X-ray data collections.

This allows to excite a large part of the crystal since the

excitation energy is located on the edge of the CT band,

centred on 0.65 eV (off-resonant excitation) [34]. Be-

cause the system relaxes on the ls–ms timescale, it is

possible to probe the sample by recording the diffraction

pattern stroboscopically using the ID09B set up. The

pulsed structure of the synchrotron radiation was usedto generate X-ray probing monochromatic pulses, in the

16 bunch ESRF mode. A rotating chopper (896 Hz)

synchronized with the laser was used to select the single

X-ray pulses of 100� 10 ps width and the flux on the

sample was 1.5� 108 photon/s. Monochromatic X-ray

(0.7701 �A) were obtained from a Si111 monochromator.

MAR-CCD camera (size ¼ 133� 133 mm2, pixel size ¼64� 64 lm2) was used to collected X-ray diffractionpatterns, with 2h Bragg angle limited to 49� in our ex-

perimental set-up. For each delay time 5 frames were

collected and about 210 reflections intensities were in-

tegrated, with 20 s of exposure and oscillation step of

the sample of 2�. Complete data collection (91 frames)

were also performed in the different phases at thermal

equilibrium.

4. Photoinduced structural changes from ionic to neutral

The N phase was photoinduced at 70 K from the

stable low-temperature I phase. The laser beam deliv-

ered about 2� 1016 photon/cm2/pulse on the sample.

Important changes on Bragg reflections intensities are

observed (Fig. 3), with some decreasing and some in-creasing excluding simple laser heating effects. Such

important intensity changes are a direct signature of a

strong three-dimensional (3D) structural reorganization

in the photoinduced state. In addition, the intensity of

many Bragg reflections is modified only after a delay of

about 500 ps. A similar behaviour was also observed in

an optical reflectivity study [27], where the photoinduced

modification becomes large not immediately after thelaser pulse excitation but after around 100 ps. At ther-

mal equilibrium an important characteristic of the

transition between the I low temperature and N high

temperature phases is the change of symmetry: the I

state is ferroelectric whereas the N one is paraelectric.

As discussed above, this is associated with a change of

space group from Pn for I to P21=n for N and then a

vanishing of the (0 k 0) Bragg reflections with k odd(systematic extinction in the N phase). During the

photoinduced N-to-I transition, it has been observed

that such reflections appear, directly indicating the

Fig. 4. Normalized intensity of the (0 6 7) Bragg reflection versus the

delay between the laser pump and the X-ray probe pulses. The im-

portant change in the intensity signs a structural reorganization as-

sociated with N–I transformation as previously reported. A significant

change occurs in the 500 ps range.

Fig. 3. Normalized intensity dependence of some Bragg reflections with

the delay between the laser pump and the X-ray probe pulses. This is

associated with the photoinduced transformation of the I phase to the

N one. General reflections (top, symbols include error bars) are

modified after an incubation time of about 500 ps, whereas the in-

tensity of the (0 3 0) reflection starts to decrease just after the laser

pulse excitation (bottom), indicating a two steps mechanism with an

intermediate Ipara disordered phase.

166 L. Gu�erin et al. / Chemical Physics 299 (2004) 163–170

ferroelectric nature of the photoinduced I phase [14]. In

this experiment on photoinduced I-to-N transition, we

notice the decrease of the (0 3 0) reflection intensity

(Fig. 3). However, it occurs on a shorter timescale since

its intensity starts to decrease just after the laser irradi-

ation and stay constant after about 500 ps. Such an

observation is very similar to the optical reflectivitystudy where the signal of second harmonic generation

decreases and disappears at the first stage what is

characteristic of the restoring of the centre of symmetry

[27]. This is discussed as resulting from a disordering

process of I strings (Ipara phase) after the photo-excita-

tion and it is responsible for the decrease of (0 3 0) re-

flection. The reason why the intensity of this reflection

does not reach zero is that the transformation of thesample is not complete. The penetration depth of

the laser in the 1–10 lm range is much smaller than the

sample size. As the laser light penetrates the sample, the

excitation density decreases and it may become lower

than the threshold value after a given depth, therefore

the transformation may not extend over all the volume.

In any case, this result is in qualitative agreement with

the physical features of the optical results: first the long-range polar order is destroyed by the laser excitation

and second the molecular state transforms from I to N

[27]. This qualitative picture corresponds to our obser-

vations (Fig. 3): in the case of order-disorder type

transition, only the peaks corresponding to the change

of symmetry are affected, such as the (0 3 0) between the

ordered Iferro and disordered Ipara states. The intensity of

general reflection is only affected when the structural

reorganization at the molecular level occurs, that is

when the structure factor of the molecules changes be-

tween the I and N states (changes of bond-length, angle,

etc.). The discussion of this point will be taken up again

in part 7.

For comparison, the I state was photoinduced at 90

K from the stable high-temperature N phase under thesame irradiation and probing conditions as mentioned

above. We show on Fig. 4 the changes of the intensity of

one Bragg reflection after the laser irradiation, with a

behaviour very similar to the previous results (Fig. 2 in

Ref. [14]). However, the present ones are obtained with

a better synchronization of the laser and synchrotron

sources (below 100 ps) and indicate a probable incuba-

tion time of about 300–400 ps.

5. Coexistence of photoinduced and stable phases

An important problem to debate is the coexistence of

stable and photoinduced phases since the transforma-

tion is not always complete. We have to stress out that

the transformation does not proceed via an homoge-neous random distribution of local photoinduced states,

as in independent chemical molecular processes [4,7]

where the crystal structure is described by an average

structure factor hF i. This factor is a weighted contri-

bution of the photoinduced molecular entity (Fphoto,concentration x) and stable one (Fstable, concentration1� x):

hF i ¼ xFphoto þ ð1� xÞFstable:

Fig. 6. Dependence of the variation of the intensity of more than 6000

Bragg reflections between the measurements in the N-to-I photoin-

duced experiment and the neutral state (Imeasured � Ineutral) with the

variation of the intensity between the neutral and ionic phases

(Iionic � Ineutral). The slope makes it possible to estimate the transfor-

mation rate around 35%.

L. Gu�erin et al. / Chemical Physics 299 (2004) 163–170 167

Here, a long range 3D transformation occurs, so that a

very large number of adjacent unit cells are modified in

the same way forming macroscopic domains. Therefore,

each photoinduced domain diffracts X-rays, with an

associated intensity. When the lattice parameters be-tween the stable and photoinduced phases are very

close, it is difficult to separate spatially the Bragg re-

flections coming from the two different coexisting pha-

ses. Therefore, for each Bragg spot the measured

intensity Imeasured is the weighted incoherent contribution

of stable (Istable) and photoinduced (Iphoto) domains, with

respective volume fraction (1� x) and x as schematically

shown on Fig. 5:

Imeasured ¼ xIphoto þ ð1� xÞIstable:It is very difficult to refine both the structure of the

photoinduced phase and the photoinduced fraction x

from the measured intensities, since they are highly

correlated. To illustrate this point, we discuss the N-to-I

photoinduced phase transition investigation. The spatial

resolution used, limited in particular by the pixel size of

the MAR CCD camera, did not allow to observe sig-

nificant shift of the lattice parameters or of the spot

position between the stable and photoinduced phases, as

it has been observed by neutron diffraction experimentin the hysteresis region, where both phases coexist,

during the temperature-induced transition [35]. Assum-

ing that the structure of the photoinduced ferroelectric

phase is the same as the one of the ferroelectric phase

stable at low temperature and therefore with identical

diffracted intensities, it is possible to estimate the

transformation rate from the variation of the intensities.

Fig. 5. (a) Schematic descriptions of the crystal in the stable state (left),

the fully transformed photoinduced one (right), and a partially

transformed state where the fraction volume of the photoinduced

domains is 50% (middle). (b) Schematic description by stack columns

of the intensity of two Bragg reflections, I1 and I2, in the stable (left)

and photoinduced (right) states. When the transformation is not

complete and occurs via a nucleation of domain process, each mea-

sured intensity I is the weighted incoherent contribution of stable and

photoinduced domains with respective volume fraction (1� x) and x:

I ¼ xIphoto þ ð1� xÞIstable. This is illustrated in a case of a 50% trans-

formation (middle).

In such a case there is a linear dependence of the vari-

ation of the intensities with x:

Imeasured � Ineutral ¼ xðIionic � IneutralÞ:

Such a comparison of more than 6000 reflections with

F 2=rðF 2Þ > 5 was performed for the N-to-I photoin-

duced transition. The intensities were measured for the

photoinduced state in the previous experiment [14] and

in this one for the stable I and N phases thanks to the

He cooling system. The result is reported on Fig. 6

giving an estimation of the transformation rate in the

35% range. This means that the crystal is transformedover about 20–40 lm, then over a few penetration

depths. One major problem originates from the depen-

dence of the penetration depth of the laser light with

regards to the orientation of the sample. Therefore,

during the data collection where the rotation of the

crystal extends over 180�, different transformation rates

should be generated for the 91 collected frames and our

result has to be considered as an average. The order ofmagnitude of the transformation rate is a few tenth of

percent, in agreement with the decrease of the (0 3 0)

reflection observed for the I–N transition (Fig. 3). As

this intensity is zero in the photoinduced N phase, a

decrease of 10% corresponds to 10% of transformation.

6. Towards the investigation of precursor phenomena

Understanding the mechanism at different timescales,

from the first step of the transformation to the long

living metastable photoinduced state is an important

aspect of photoinduced transformation. In the case of

TTF-CA, precursor phenomena were observed by op-

tical techniques on the ps timescale [17,20,21]. It is un-

derstood as the formation of one-dimensional (1D)

168 L. Gu�erin et al. / Chemical Physics 299 (2004) 163–170

strings, i.e. segments of several adjacent excited DA

pairs. In such a case, because it is associated with only

short-range order, it manifests by diffuse scattering. In-

deed, in addition to the Bragg reflections corresponding

to the average 3D structure, the local order gives rise todiffuse scattering more or less spread out within the

reciprocal space, depending on the nature of the corre-

lations. This diffuse scattering is governed by the Fou-

rier-transform of the correlation volume. In the present

case where 1D correlations occur along the stacking axis

Fig. 7. (a) Diffuse scattering signal at thermal equilibrium, associated

with 1D correlation observed with about 60,000 X-ray pulses of 100 ps

(1 min of exposure). (b) The projection of the intersection of the diffuse

planes with the sphere gives rise to curves on the 2D detector.

a, diffuse planes perpendicular to this stack are associ-

ated with such 1D fluctuations. Diffuse planes were al-

ready observed and analyzed in this type of CT

materials at thermal equilibrium [31]. Of course it is of

fundamental interest to make the dynamical analysis ofthis kind of observation around photoinduced phase

transitions but two main points have to be discussed.

First of all, the intrinsic dynamic of such non linear

excitation is quite fast, since the self-multiplication of

DA pairs propagates at typical speed around that of

sound. The increase of the size of the string will trigger

the width of the diffuse planes [31], therefore studying

the formation and proliferation of such 1D excitationsrequires at least 1 ps time resolution. The second

problem is that the diffuse scattering intensity is four or

five order of magnitude lower than that of Bragg re-

flections. In order to observe such a weak signal, a large

X-ray flux is required. We believe that in a near future,

the development of X-ray sources will combine both

ultra short time resolution and high flux. Just to give an

idea about the intensity of this signal, we show onFig. 7(a) the diffuse scattering signal observed at thermal

equilibrium using 100 ps X-ray pulses (static measure-

ment). As shown schematically on Fig. 7(b), the stacking

axis is horizontal so that the planes cross the Ewald�ssphere vertically and circles corresponding to the inter-

section of the planes with the sphere are projected on the

flat 2DMAR CCD detector. This shows how the flux on

ID09B should allow to investigate long living disorder-ing phenomena, such as the disappearance of long range

polar order around 500 ps. Going beyond the average

structure dynamics is a new target for future.

7. Discussions and perspectives

The results presented here illustrate how 100 ps X-ray

diffraction may contribute to describe the physical pic-

ture for a photoinduced structural phase transition in a

molecular crystal, which is a process basically different

from independent photoinduced local structural chan-ges. It is essential to compare the X-ray results with

those obtained from time-resolved optical measure-

ments and to stress the differences between the two

techniques. Since the TTF-CA crystal is composed of

relatively light elements X-ray probe the bulk, i.e. X-ray

probe the global development of the photoinduced

transformation inside the crystal. In contrast, optical

reflectivity measurements probe a region close to thesurface. Consequently the probed dynamics may be

different because nucleation processes may be strongly

influenced by the vicinity of the surface. Since the pen-

etration depth of laser light is much smaller than the

crystal thickness, an observed photoinduced transfor-

mation within about 500 ps extending over 30 lm (i.e. a

few penetration depth of the laser) does not proceed by

L. Gu�erin et al. / Chemical Physics 299 (2004) 163–170 169

a simple propagation of front phase from the surface

because the propagation speed of the front phase could

not be one or two orders of magnitude larger than the

speed of sound. The development of such a large mac-

roscopic transformed part is due to the fact that thepump laser fluence is a few times larger than the

threshold one, and so the 3D transformation can be

induced on a few penetration depths. The difference

between the two probes is also probably at the origin

that the timescale observed here for destroying the long

range polar order (Fig. 3), is longer than in optical ex-

periments [27] (Fig. 2): a phase may be more stabilized

in the bulk than close to the surface. In addition thetime resolutions of the two experiments are very differ-

ent. Indeed the X-ray diffraction technique presented

here has a time resolution of 100 ps whereas optical

measurements are performed with about 200 fs time

resolution.

Photoinduced phase transitions provide a richness

of coherent atomic motions triggered by ultra-short

laser pulses. However these take place on different in-trinsic spatial scales and associated multiple timescales.

It has been shown that the N–I phase transition at

thermal equilibrium proceeds via a cascade of coop-

erative phenomena: in a first step the formation of

lattice-relaxed CT 1D strings and in a second one their

3D condensation and ordering [30]. A similar physical

picture has been used to interpret optical reflectivity

behaviour. Some precursor phenomena take place onthe subps–ps timescale and may be associated with the

formation of 1D nano-scale photoinduced structures

through coherent molecular motions (oscillating and/or

not) which immediately follow the laser pulse

[17,20,21]. Femtosecond X-ray diffraction is particu-

larly well adapted to directly observe such coherent

motions as it was recently demonstrated [12]. Thus

coherent optical phonon excitation manifests by anoscillation of the intensity of Bragg peaks, which al-

lows to determine not only its frequency but also its

polarization (by comparison between the relative am-

plitudes of oscillation). As far as the number of pho-

toinduced strings is small, the photoinduced process is

linear, i.e. one string per photon forms. However when

a sufficient number of strings are simultaneously cre-

ated the inter-chain interactions lead to the formationof 3D domains of a photoinduced phase. It is another

type of coherent process but at another spatial and

temporal scales. It is mainly governed by the motion of

phase fronts (often called domain walls) and then takes

place at the acoustic phonon timescale. Some oscilla-

tory behaviours of optical reflectivity have been ob-

served and ascribed to generated acoustic phonons and

also to domain wall oscillations [17,20,21]. One excit-ing objective is the direct observation of the transfor-

mation of characteristic diffuse scattering planes

characteristic of 1D precursor phenomena into Bragg

peaks characteristic of 3D order. Our observation of

such diffuse scattering at thermal equilibrium, but with

the experimental conditions of 100 ps measurements,

shows that the goal is not so far. Notice that probably

many photoinduced phase transitions in different ma-terials proceed in a similar way, with in a first step

short-range precursor phenomena, often close to the

surface consequently to small laser light penetration,

and in a second step the formation of macroscopic

photoinduced phase. A deep understanding of such

phenomena at different scales requires the combined

use of X-ray diffraction over different timescales (100 fs

and 100 ps) and temporal optics techniques. This is thekey for controlling ultrafast macroscopic switching of

materials.

Acknowledgements

Authors are grateful to Simone Techert, and Mathias

Meyer for their contributions during the advancement

of this work.

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