Ultrafast reversible ligand isomerisation in Na2[Fe(CN)5NO] x 2 H2O single crystals.

We present a time-resolved absorption study on the light-induced generation of reversible linkage NO isomers in single crystals of Na(2)[Fe(CN)(5)NO] x 2 H(2)O using laser pulses of 160 fs width. Using the pump wavelength lambda = 500 nm the singlet-singlet (1)A(1)-->(1)E excitation induces the NO rotation by about 90 degrees from the linear Fe-N-O configuration to a side-on configuration [structure: see text]. The formation of the isomer is monoexponential with a characteristic time of tau = 300(20) fs and proceeds along a diabatic potential surface without occupation of further intermediate states. The side-on structure has a lifetime of 270(30) ns at T = 23 degrees C.