A femtosecond visible pump-infrared probe time resolved absorption experiment makes it possible to reveal the ultrafast vibrationnal kinetic associated to formation of light-induced linkage isomers in Na 2 [Fe(CN) 5 NO]2H 2 O (SNP) single crystals. Time-resolved spectroscopy on a femtosecond scale makes it possible to observe and to record photochemical processes [1-2]. The ultrafast study of electronic, vibrational and structural changes during light-induced isomerization reveals the correlation between the changes of the electron density and the structural response of matter. Consider an electronic transition that excites a molecule from a (bonding) ground state to an (reactive) excited state that is the starting point for, e.g., the rotation of a ligand in a molecule. According to the Born-Oppenheimer approximation the direct electronic excitation in the sub-femtosecond range is followed by a slower nuclear response in the fs-ps range. The nuclear motion (e.g. rotation) of the ligand starts in a highly excited state and in the absence of luminescence will end in highly excited vibrational-rotational states of the novel geometry. The excess energy will be dissipated during the thermalization of this highly excited vibrational-rotational state towards its ground state. A typical example for such ultrafast photochemical processes is the photo-induced linkage isomerism of the nitrosyl ligand in coordination complexes [3]. Here we study the prototypic case of [Fe(CN) 5 NO] 2-anion. As shown in Figure 1 the ground state (GS) is characterized by a linear Fe-NO coordination. The irradiation with light in the blue-green spectral range (e.g. λ ~ 500 nm) induces a charge-transfer transition. Thereby the system changes symmetry from a 1 A 1 state to a 1 E doubly degenerate state. As a consequence the doubly degenerate deformational mode δ(Fe-NO) can induce a rotation of the NO ligand. The rotation of about 90° yields the side-on configuration of Fe< N O (metastable state MS2) while a rotation of 180° results in the isonitrosyl configuration Fe-ON (metastable state MS1). The transition from the excited ground state 1 E towards MS2 occurs radiationless in about 300±30 fs [3]. In the case of the NO ligand the structure of GS and the metastable states MS1 and MS2 is known from X-ray and neutron diffraction measurements at low temperatures in the static regime [4,5,6]. Moreover, the GS, MS1, and MS2 have clearly distinguished (NO) vibration frequency centered at 1961 cm-1 (5100 nm), 1831 cm-1 (5460 nm) and 1631 cm-1 (6130 nm) respectively [7]. Hence optical This is an Open Access article distributed under the terms of the Creative Commons Attribution License 2.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.< Réduire