The ultrafast (<100 fs) conversion of delocalized exciton into charge-separated state between the primary donor P700 (bleaching at 705 nm) and the primary acceptor A0 (bleaching at 690 nm) in photosystem I (PS I) complexes from Synechocystis sp. PCC 6803 was observed. The data were obtained by application of pump-probe technique with 20-fs low-energy pump pulses centered at 720 nm. The earliest absorbance changes (close to zero delay) with a bleaching at 690 nm are similar to the product of the absorption spectrum of PS I complex and the laser pulse spectrum, which represents the efficiency spectrum of the light absorption by PS I upon femtosecond excitation centered at 720 nm. During the first approximately 60 fs the energy transfer from the chlorophyll (Chl) species bleaching at 690 nm to the Chl bleaching at 705 nm occurs, resulting in almost equal bleaching of the two forms with the formation of delocalized exciton between 690-nm and 705-nm Chls. Within the next approximately 40 fs the formation of a new broad band centered at approximately 660 nm (attributed to the appearance of Chl anion radical) is observed. This band decays with time constant simultaneously with an electron transfer to A1 (phylloquinone). The subtraction of kinetic difference absorption spectra of the closed (state P700+A0A1) PS I reaction center (RC) from that of the open (state P700A0A1) RC reveals the pure spectrum of the P700+A0- ion-radical pair. The experimental data were analyzed using a simple kinetic scheme: An*-->k1[(PA0)*A1--><100 fs P+A0-A1]-->k2P+A0A1-, and a global fitting procedure based on the singular value decomposition analysis. The calculated kinetics of transitions between intermediate states and their spectra were similar to the kinetics recorded at 694 and 705 nm and the experimental spectra obtained by subtraction of the spectra of closed RCs from the spectra of open RCs. As a result, we found that the main events in RCs of PS I under our experimental conditions include very fast (<100 fs) charge separation with the formation of the P700+A0-A1 state in approximately one half of the RCs, the approximately 5-ps energy transfer from antenna Chl* to P700A0A1 in the remaining RCs, and approximately 25-ps formation of the secondary radical pair P700+A0A1-.

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This study presents the detailed nature of iron clusters formed on Fe 3+ -Nafion membranes. The catalytic nature of these clusters during immobilized Fenton processes was observed to be a function of the deposition method of Fe ions on the Nafion. The nonbiodegradable azo-dye Orange II and 2-propanol were utilized as convenient organic model compounds in photoassisted Fenton degradation processes. The highest photocatalytic activity was observed when samples were prepared by ion exchange between iron(III) aquacomplexes and H + or Na + as counterions of the Nafion SO3group. Spectroscopic techniques show that iron(III) in the membrane was present mainly as a mononuclear complex of [Fe(H2O)6] 3+ and binuclear complexes [Fe(H3O2)Fe] 5+ and [Fe-O-Fe] 4+ . If NaOH or ammonia was added to the former samples prepared by ion exchange, Nafion-Fe membranes with low photocatalytic activity were obtained showing R-Fe2O3 and [Fe-O-Fe] 4+ . Detailed high-resolution transmission electron microscopy was carried out for the Nafion-Fe ion-exchanged and also base-treated membranes showing R-Fe2O3 nanocrystallites of 3.5-5 nm. Spectral bands were found for iron oxides in the Fe 3+ -Nafion by femtosecond laser spectroscopy. The R-Fe2O3 nanocrystallites in the Nafion exchanged base-treated membranes presented a relaxation dynamics for the excited states close to that observed with R-Fe2O3 nanocrystallite colloids taken as reference compounds. Multiexponential transient absorption decay of R-Fe2O3 in SO3 --water clusters was observed with time constants close to 320 fs, 1.5 ps, and 31 ps after the excitation pulse. Samples of Fe 3+ -Nafion membranes with high activity show different transient dynamics relative to the Fe 3+ -Nafion with low activity. Correlation of the photocatalytic activity of Fe 3+ -Nafion with UV-vis, Fourier transform infrared, Mo ¨ssbauer, and X-ray photoelectron spectroscopic results suggests that the photocatalytic activity correlates with the amount of mononuclear [Fe(H2O)6 ] 3+ , binuclear complexes [Fe(H3O2)Fe] 5+ and oxo-bridged [Fe-O-Fe] 4+ found in the membranes.