Three pulse UV photon echo studies of molecules in solution: effect of the chirp.

We report on the electronic dephasing times of the nonpolar chromophore diphenylacetylene (DPA) in ethanol and in cyclohexane (polar and nonpolar solvents respectively) by photon echo measurements in the ultraviolet. Contrary to previous reports, we observed sub-100-fs electronic dephasing times for DPA in both solvents. We identify fast dynamics of tau=40+/-10 fs on the photon echo peak shift (PEPS) traces of DPA in ethanol. In addition, we observed a dependence of the PEPS asymptotic value on the temporal chirp of the pulses. We propose a model to describe it in terms of phase-matching condition and beam geometry.

Relaxation dynamics of tryptophan in water: A UV fluorescence up-conversion and molecular dynamics study.

We report on an ultrafast experimental and simulations study of the early relaxation events of photoexcited tryptophan in water. Experimentally, we used fluorescence up-conversion in both polychromatic and single wavelength detection modes in the 300-480 nm range with polarization dependence. We report on the time evolution of the Stokes shift, bandwidth, and anisotropy from tens of femtoseconds to picoseconds. These observables contain signatures of the simultaneous occurrence of intramolecular and solvent-molecule interactions, which we disentangle with the help of nonequilibrium molecular dynamics simulations. We also observe a breakdown of the linear response approximation to describe our results.

Ultrafast nonresonant response of TiO2 nanostructured films.

We present a detailed study of the nonresonant third-order ultrafast response of TiO(2) nanostructured films, combining a classical heterodyned optical Kerr effect experiment, with two polarization selective Kerr techniques, based on transient lensing and phase modulation effects. The complementarity of these techniques is highlighted and demonstrated with calculations. Different aspects of the experimental results are addressed in detail and, finally, the possibilities of performing experiments on liquid dynamics in the pores of TiO(2) films, are discussed in the light of the present results.

Femtosecond fluorescence upconversion setup with broadband detection in the ultraviolet.

We show a femtosecond fluorescence upconversion setup with broadband detection to measure time-resolved emission spectra in the 300-550 nm range, upon excitation between 250 and 300 nm, with a time resolution of 100 fs. We present time-resolved fluorescence emission spectra of 2,5-diphenyloxazole in solution, which demonstrate the capabilities of the setup.

Correlation of femtosecond wave packets and fluorescence interference in a conjugated polymer: Towards the measurement of site homogeneous dephasing.

Probing electronic femtosecond (fs) coherence among segmental sites that are congested by static and dynamic site disorder and subject to structural relaxation is a big, experimental challenge in the study of photophysics of poly(p-phenylenevinylene). In this work, fs-wave-packet fluorescence interferometry experiments are presented that measure macroscopic coherent kernels and their phase-relaxation in the low-temperature, bottom-state regime of the density-of-states below the migrational threshold energy where downhill site-to-site transfer is marginal. By using freely propagating and tunable 70 fs excitation/probing pulses and employing narrow-band spectral filtering of wave packets, fluorescence interferograms with strongly damped beatings can be observed. The coherences formally follow the in-phase superpositions of two site-optical free-induction-decays and originate from distinct pairs of coherent doorway-states, different in energy and space, each of them being targeted, by two discrete quantum-arrival-states 1(alpha) and 1(beta), via independent, isoenergetic 0-->1 fluorescence transitions. The coherent transients are explained as site-to-site polarization beatings, caused by the interference of two fluorescence correlation signals. The numerical analysis of the damping regime, based upon second-order perturbational solutions, reveals the lower limit value of homogeneous dephasing in the range from T(2) approximately 100 fs to T(2) approximately 200 fs depending on the site-excitation energy of the bottom-states. The experiments enable to look into the formation of the relaxed state as a special molecular process of electron-phonon coupling and hence open-up a quite new perspective in the puzzle of multichromophore optical dynamics and structural relaxation in conjugated polymers.

Sensitivity of flavin fluorescence dynamics in neuronal nitric oxide synthase to cofactor-induced conformational changes and dimerization.

The fluorescence intensity of the two flavin prosthetic groups, FMN and FAD, in neuronal nitric oxide synthase (nNOS) was found to decay highly nonexponentially, being best described by four fluorescence lifetimes. This excited state heterogeneity is the result of multiple flavin quenching sites which are due to several flavin microenvironments created mainly by stacking with aromatic amino acids. Investigating nNOS in the absence of one or more of Ca2+/calmodulin, tetrahydrobiopterin, and heme revealed an influence of these cofactors on the microenvironments of the flavin prosthetic groups. Similar effects on the flavin rotational dynamics were found by analyzing the fluorescence anisotropy decay of the holo and of the different apo forms of nNOS. Since the tetrahydrobiopterin and the heme are located in the N-terminal oxygenase domain of nNOS, their effect on the flavins in the C-terminal reductase domain is explained by a folding back of the reductase domain onto the oxygenase domain. Thereby a domain-domain interface is created containing the FAD, FMN, heme, and tetrahydrobiopterin prosthetic groups which allows for efficient electron transfer during catalysis. The heme group, which is known to be essential for homodimerization of nNOS, was also found to be essential for the formation of the domain-domain interface.