Photooxidation and photoaquation of iron hexacyanide in aqueous solution: A picosecond X-ray absorption study

We present a picosecond Fe K-edge absorption study of photoexcited ferrous and ferrichexacyanide in water under 355 and 266 nm excitation. Following 355 nm excitation, thetransient spectra for the ferrous and ferric complexes exhibit a red shift of the edgereflecting an increased electron density at the Fe atom. For the former, an enhancedpre-edge transition is also observed. These observations are attributed to the aquated[Fe(CN)5OH2]3− species, based on quantum chemicalcalculations which also provide structural parameters. Upon 266 nm excitation of theferric complex, a transient reminiscent of the aquated species is observed (appearance ofa pre-edge feature and redshift of the edge) but it is different from that obtained under 355 nmexcitation. This points to a new reaction channel occurring through an intermediatestate lying between these two excitation energies. Finally, 266 nm excitation of theferrous species is dominated by the photooxidation channel with formation of the ferriccomplex as main photoproduct. However, we observe an additional minor photoproduct, whichis identical to the 266 nm generated photoproduct of the ferric species, suggesting thatunder our experimental conditions, the pump pulse photooxidises the ferrous complex andre-excites the primary ferric photoproduct