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Originally published in Science Express on 11 December 2008
Science 23 January 2009:
Vol. 323. no. 5913, pp. 489 - 492
DOI: 10.1126/science.1165733
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Reports

Femtosecond XANES Study of the Light-Induced Spin Crossover Dynamics in an Iron(II) Complex

Ch. Bressler,1 C. Milne,1 V.-T. Pham,1 A. ElNahhas,1 R. M. van der Veen,1,2 W. Gawelda,1,2* S. Johnson,2 P. Beaud,2 D. Grolimund,2 M. Kaiser,1,2 C. N. Borca,2 G. Ingold,2 R. Abela,2 M. Chergui1{dagger}

X-ray absorption spectroscopy is a powerful probe of molecular structure, but it has previously been too slow to track the earliest dynamics after photoexcitation. We investigated the ultrafast formation of the lowest quintet state of aqueous iron(II) tris(bipyridine) upon excitation of the singlet metal-to-ligand-charge-transfer (1MLCT) state by femtosecond optical pump/x-ray probe techniques based on x-ray absorption near-edge structure (XANES). By recording the intensity of a characteristic XANES feature as a function of laser pump/x-ray probe time delay, we find that the quintet state is populated in about 150 femtoseconds. The quintet state is further evidenced by its full XANES spectrum recorded at a 300-femtosecond time delay. These results resolve a long-standing issue about the population mechanism of quintet states in iron(II)-based complexes, which we identify as a simple 1MLCT->3MLCT->5T cascade from the initially excited state. The time scale of the 3MLCT->5T relaxation corresponds to the period of the iron-nitrogen stretch vibration.

1 Ecole Polytechnique Fédérale de Lausanne, Laboratoire de Spectroscopie Ultrarapide, ISIC, FSB-BSP, CH-1015 Lausanne, Switzerland.
2 Swiss Light Source, Paul-Scherrer Institut, CH-5232 Villigen PSI, Switzerland.

* Present address: Laser Processing Group, Instituto de Optica, Consejo Superior de Investigaciones Científicas, Serrano 121, E-28006 Madrid, Spain.

{dagger} To whom correspondence should be addressed. E-mail: Majed.chergui{at}epfl.ch

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THIS ARTICLE HAS BEEN CITED BY OTHER ARTICLES:
Dynamics of Chemical Bonding Mapped by Energy-Resolved 4D Electron Microscopy.
F. Carbone, O.-H. Kwon, and A. H. Zewail (2009)
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