Femtosecond Time-resolved Technique Research Articles

Optical nonlinearity of Cu1.81S nanodisks decorated with silver nanoparticles

The nonlinear absorption (NLA) and ultrafast dynamics process of Cu1.81S nanodisks (NDs) and Ag-Cu1.81S nanocomposites are investigated by using femtosecond Z-scan measurements and femtosecond time-resolved transient absorption spectrum, respectively. After decoration with Ag particles, Ag-Cu1.81S nanocomposites exhibit stronger saturable absorption and smaller saturable intensity at 600 and 800 nm. The carrier relaxation time is measured using femtosecond time-resolved techniques, and the results showed that the prolongation of the carrier relaxation time of the composite material can lead to a stronger saturation absorption effect.

The influence of interactions between isotopoloques on coherent, ultrafast vibrational dynamics of liquid C2Cl4.

The dynamics of intramolecular and intermolecular vibrations in liquid tetrachloroethylene are studied for the first time by use of femtosecond time-resolved techniques, such as transient transmission spectroscopy and optical Kerr effect spectroscopy. Fourier transforms of time signals are compared with spontaneous Raman spectra for both isotropic and anisotropic components. The isotopic effect resulting from natural abundance of chlorine isotopes manifests itself as splitting of the isotropic spectra of intramolecular symmetric vibrations. Application of windowed Fourier transform enables us to study the dynamics of both spectral responses in real time and to analyze the role of intermolecular interactions on the coherence in the system. In order to describe the dynamics of molecules in a liquid and to explain the experimental results, we use a simple theoretical model taking into account intermolecular interactions, which allowed us to find vibrational and rotational life times.

Tryptophan Fluorescence from G. Weber to the Present

Gregorio Weber (Symposium on Light and Life, W.D. McElroy and Glass, B. Eds., 1961) described the fluorescence of tryptophan in proteins. Since that time more than 12,000 papers have appeared related to tryptophan fluorescence. It is used to measure the folding of proteins and the interaction of proteins with each other, with nucleic acids, membranes and small molecules. Time-resolved fluorescence studies have revealed details regarding the excited-state of tryptophan in proteins. Ground-state and excited-state heterogeneity influence the fluorescence and its relation to protein conformation. Excited-state electron transfer, proton transfer, energy transfer, solvent and protein relaxation are among the processes that have been implicated in tryptophan fluorescence in proteins The availability of new experimental, computational and theoretical methods suggest that there are now opportunities for using tryptophan fluorescence for probing protein structure and dynamics. Femtosecond time-resolved techniques and molecular dynamics computations have been of particular value and should provide new information about folded and unfolded structural regions in proteins.

Photophysical Properties of Quinolizinium Salts and Their Interactions with DNA in Aqueous Solution

The photophysical properties of three quinolizinium salts (naphto[2,1-b]quinolizinium bromide (Q2), naphto[1,2-b]quinolizinium bromide (Q3), and indolo[2,3-b]quinolizinium tetrafluoroborate (HI)) in fluid media and their interactions with DNA were investigated by steady-state and by nanosecond and femtosecond time-resolved techniques. The main decay pathways of the excited singlet state S(1), fluorescence, intersystem crossing, and internal conversion, were characterized in terms of quantum yields and rate constants. The lowest triplet state of the quinolizinium salts is able to sensitize singlet oxygen in rather high efficiency (phi(Delta) = 0.4 to 0.5 in MeCN). The complexes between quinolizinium salts and DNA formed in the ground state were characterized in terms of lifetimes and decay channels to give more details of the mechanism of photoinduced DNA strand break.

Synthesis and Characterization of Dinuclear Ruthenium Complexes Covalently Linked to RuII Tris‐bipyridine: An Approach to Mimics of the Donor Side of Photosystem II

To mimic the electron-donor side of photosystem II (PSII), three trinuclear ruthenium complexes (2, 2a, 2b) were synthesized. In these complexes, a mixed-valent dinuclear Ru2(II,III) moiety with one phenoxy and two acetato bridges is covalently linked to a Ru(II) tris-bipyridine photosensitizer. The properties and photoinduced electron/energy transfer of these complexes were studied. The results show that the Ru2(II,III) moieties in the complexes readily undergo reversible one-electron reduction and one-electron oxidation to give the Ru2(II,III) and Ru2(III,III) states, respectively. This could allow for photooxidation of the sensitizer part with an external acceptor and subsequent electron transfer from the dinuclear ruthenium moiety to regenerate the sensitizer. However, all trinuclear ruthenium complexes have a very short excited-state lifetime, in the range of a few nanoseconds to less than 100 ps. Studies by femtosecond time-resolved techniques suggest that a mixture of intramolecular energy and electron transfer between the dinuclear ruthenium moiety and the excited [Ru(bpy)3]2+ photosensitizer is responsible for the short lifetimes. This problem is overcome by anchoring the complexes with ester- or carboxyl-substituted bipyridine ligands (2a, 2b) to nanocrystalline TiO2, and the desired electron transfer from the excited state of the [Ru(bpy)3]2+ moiety to the conduction band of TiO2 followed by intramolecular electron transfer from the dinuclear Ru2(II,III) moiety to photogenerated Ru(III) was observed. The resulting long-lived Ru2(III,III) state decays on the millisecond timescale.

Kinetics of charge separation and energy transfer in photosystem II reaction center

In the present study we investigated the kinetics of the initial charge separation and energy transfer in the PS II reaction center using pico-second and femto-second time-resolved techniques. We conclude that there are energy transfer processes between s-Car or Pheo and P680 in the PSII reaction center. The time constant of energy transfer from Pheo to P680 is less than 100 ps, while that from (s-Car to P680 is about 350 ps. For the initial charge separation in the PS II reaction center, the preliminary finding supported the about 3-ps time constant of charge separation. The possible kinetic scheme in PS II reaction center was proposed. Further experiments are in progress.