LETTER doi:10.1038/nature12044 Mapping molecular motions leading to charge delocalization with ultrabright electrons Meng Gao 1,2 *, Cheng Lu 1 , Hubert Jean-Ruel 1,2 , Lai Chung Liu 1,2 , Alexander Marx 2 , Ken Onda 3,4 , Shin-ya Koshihara 5,6 , Yoshiaki Nakano 7 , Xiangfeng Shao 7 {, Takaaki Hiramatsu 8 , Gunzi Saito 8 , Hideki Yamochi 7 , Ryan R. Cooney 1,2 , Gustavo Moriena 1,2 , Germa ´n Sciaini 1,2 * & R. J. Dwayne Miller 1,2 Ultrafast processes can now be studied with the combined atomic spatial resolution of diffraction methods and the temporal resolu- tion of femtosecond optical spectroscopy by using femtosecond pulses of electrons 1–14 or hard X-rays 15–19 as structural probes. However, it is challenging to apply these methods to organic mate- rials, which have weak scattering centres, thermal lability, and poor heat conduction. These characteristics mean that the source needs to be extremely bright to enable us to obtain high-quality diffraction data before cumulative heating effects from the laser excitation either degrade the sample or mask the structural dynamics 20 . Here we show that a recently developed, ultrabright femtosecond electron source 7–9 makes it possible to monitor the molecular motions in the organic salt (EDO-TTF) 2 PF 6 as it undergoes its photo-induced insulator-to-metal phase transition 21–24 . After the ultrafast laser excitation, we record time-delayed diffraction patterns that allow us to identify hundreds of Bragg reflections with which to map the structural evolution of the system. The data and supporting model calculations indicate the formation of a transient intermediate structure in the early stage of charge delocalization (less than five picoseconds), and reveal that the molecular motions driving its formation are distinct from those that, assisted by thermal relax- ation, convert the system into a metallic state on the hundred- picosecond timescale. These findings establish the potential of ultrabright femtosecond electron sources 7–14 for probing the pri- mary processes governing structural dynamics with atomic resolu- tion in labile systems relevant to chemistry and biology. (EDO-TTF) 2 PF 6 (where EDO-TTF is ethylenedioxytetrathiaful- valene) is a quasi-one-dimensional, 3/4-band-filled charge-transfer organic salt that undergoes a thermally induced insulator-to-metal phase transition at a critical temperature of T c < 280 K (ref. 21) and also a highly efficient and ultrafast photo-induced phase transition 22–24 . Its electronic structure resembles that of Bechgaard salts, which provided the first organic superconductor, (TMTSF) 2 PF 6 (see ref. 25). The origin of its insulator-to-metal phase transition involves a variety of collective phenomena 23 : Peierls-and-Holstein-type electron–phonon 26 and anti- ferromagnetic interactions 27 , that coexist with order–disorder 21,24 charge localization, and long-range Coulombic interactions have an important role in charge disproportionation 28 . In its metallic or high-temperature (HT) phase, electron donor EDO-TTF molecules (D) form columns of cations that are separated by sheets of acceptor PF 6 anions. The distri- bution of positive charges among EDO-TTF molecules along the stack- ing direction is represented by (D 10.5 D 10.5 D 10.5 D 10.5 ) as shown in Fig. 1 (top right) 21–24 . The high positive charge mobility along the cation stacks confers metallic properties at room temperature. The holes loca- lize below T c and endow the low-temperature (LT) phase with insulating properties. In the LT phase, the EDO-TTF molecules present a charge- ordered 21–24 state (D 11 D 10 D 10 D 11 ), in which a large bending of the neutral moieties promotes the doubling of the unit cell akin to a Peierls mechanism in a half-band-filled system, as shown in Fig. 1 (top left). Model calculations predict that vertical photo-excitation via the second charge-transfer band 26 leads to a localized {D 12 D 10 D 10 D 10 } excited state. Time-resolved optical reflectivity measurements in combination with theoretical modelling indicate 23 that this initial excited state evolves in less than 100 fs into a (D 11 D 10 D 11 D 10 ) charge-disproportionate state, which has a lifetime of about 4 ps. Time-resolved optical studies in the near- and mid-infrared region identify large charge fluctuations as the driving process that finally leads to the complete randomization and melting of the charge order after about 100 ps (ref. 24). To probe the structural evolution in (EDO-TTF) 2 PF 6 directly dur- ing its photo-induced insulator-to-metal phase transition, we per- formed femtosecond electron diffraction (FED) studies employing a recently developed ultrabright femtosecond electron source 7–9 that can *These authors contributed equally to this work. 1 Departments of Chemistry and Physics, University of Toronto, Toronto, Ontario M5S 3H6, Canada. 2 Max Planck Research Department for Structural Dynamics, Department of Physics, University of Hamburg, Hamburg Center for Ultrafast Imaging, Luruper Chaussee 149, 22761 Hamburg, Germany. 3 Interactive Research Center of Science, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 226-8502, Japan. 4 PRESTO, Japan Science and Technology Agency, Honcho, Kawaguchi 332-0012, Japan. 5 Department of Chemistry and Materials Science, Tokyo Institute of Technology, O ¯ okayama, Meguro-ku, Tokyo 152-8551, Japan. 6 CREST, Japan Science and Technology Agency (JST), 5-3, Yonbancho, Chiyoda-ku, Tokyo 102-8666, Japan. 7 Research Center for Low Temperature and Materials Sciences, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan. 8 Faculty of Agriculture, Meijo University, Shiogamaguchi 1-501 Tempaku-ku, Nagoya 468-8502, Japan. {Present address: State Key Laboratory of Applied Organic Chemistry, Lanzhou University, 222 Tianshui South Road, Lanzhou 730000, Gansu, China. b a c T c = 280 K Δ +1 +1 +0 +0 +0 +0 +1 +1 +1 +0 +0 +1 Low temperature High temperature +0.5 +0.5 +0.5 +0.5 +0.5 +0.5 +0.5 +0.5 +0.5 +0.5 +0.5 +0.5 Figure 1 | Insulator-to-metal first-order phase transition in (EDO- TTF) 2 PF 6 . Top panels, illustration of the molecular and electronic changes associated with the thermal insulator-to-metal phase transition. Bottom panels, diffraction patterns for the LT and HT phases, obtained at 230 K and 295K, respectively. The inset shows the assigned Miller indices (h, k, l ). The symmetry breaking (cell doubling) corresponds to peaks indexed with k 5 2n 1 1 in the LT phase. For details about the definition of unit cell axes, see Supplementary Information section 6.1. 18 APRIL 2013 | VOL 496 | NATURE | 343 Macmillan Publishers Limited. All rights reserved ©2013