Olivier Bräm

Ultrafast fluorescence studies of dye sensitized solar cells

Time-resolved fluorescence spectra from the RuN719 dye exhibit very short lifetimes (<30 fs) in solutions, on non-injecting substrates and on injecting ones. This reveals <10 fs intramolecular energy redistribution competing with the injection. We conclude that injection proceeds on a sub-10 fs time scale from non-thermalized levels of the dye.

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.

Real-time observation of the charge transfer to solvent dynamics

Intermolecular electron-transfer reactions have a crucial role in biology, solution chemistry and electrochemistry. The first step of such reactions is the expulsion of the electron to the solvent, whose mechanism is determined by the structure and dynamical response of the latter. Here we visualize the electron transfer to water using ultrafast fluorescence spectroscopy with polychromatic detection from the ultraviolet to the visible region, upon photo-excitation of the so-called charge transfer to solvent states of aqueous iodide. The initial emission is short lived (~60 fs) and it relaxes to a broad distribution of lower-energy charge transfer to solvent states upon rearrangement of the solvent cage. This distribution reflects the inhomogeneous character of the solvent cage around iodide. Electron ejection occurs from the relaxed charge transfer to solvent states with lifetimes of 100-400 fs that increase with decreasing emission energy.

Femtosecond UV Studies of the Electronic Relaxation Processes in Cytochrome c

We report on an experimental study with UV and visible ultrafast time-gated emission and transient absorption of the early photodynamics of horse heart Cytochrome c in both ferric and ferrous redox states. A clear separation in time and energy of tryptophan and haem emission is observed. Excitation of the haem via resonant energy transfer from the tryptophan residue is observed in the subsequent haem electronic relaxation. Different Trp-haem energy transfer time constants of the ferrous and ferric forms are obtained. An almost instantaneous relaxation to the lowest singlet excited state (corresponding to the so-called Q band) characterizes the earliest electronic dynamics of the haem, independent of excitation energy, while dark intermediate states govern the ground-state recovery. The information gathered in these two experiments and in the literature allows us to propose a simple scheme for the electronic relaxation leading to ligand dissociation.

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.

A cascade through spin states in the ultrafast haem relaxation of met-myoglobin

We report on a study of the early relaxation processes of met-Myoglobin in aqueous solution, using a combination of ultrafast broadband fluorescence detection and transient absorption with a broad UV-visible continuum probe at different pump energies. Reconstruction of the spectra of the transient species unravels the details of the haem photocycle in the absence of photolysis. Besides identifying a branching in the ultrafast relaxation of the haem, we show clear evidence for an electronic character of the intermediates, contrary to the commonly accepted idea that the early time relaxation of the haem is only due to cooling. The decay back to the ground state proceeds partially as a cascade through iron spin states, which seems to be a general characteristic of haem systems.