Matthieu Nicoul

Picosecond acoustic response of a laser-heated gold-film studied with time-resolved x-ray diffraction

We apply time-resolved x-ray diffraction using ultrashort x-ray pulses from a laser-produced plasma to probe the picosecond acoustic response of a thin laser-heated gold film. Measurements of the temporal changes in the angular distribution of diffracted x-rays provide direct quantitative information on the transient evolution of lattice strain. This allows to disentangle electronic and thermal pressure contributions driving lattice expansion after impulsive laser excitation. The electron-lattice energy equilibration time τE=(5±0.3) ps as well as the electronic Gruneisen parameter γe=(1.48±0.3) have been determined.

Ultrafast reversible ligand isomerisation in Na2[Fe(CN)5NO]·2H2O single crystals

We present a time-resolved absorption study on the light-induced generation of reversible linkage NO isomers in single crystals of Na(2)[Fe(CN)(5)NO] x 2 H(2)O using laser pulses of 160 fs width. Using the pump wavelength lambda = 500 nm the singlet-singlet (1)A(1)-->(1)E excitation induces the NO rotation by about 90 degrees from the linear Fe-N-O configuration to a side-on configuration [structure: see text]. The formation of the isomer is monoexponential with a characteristic time of tau = 300(20) fs and proceeds along a diabatic potential surface without occupation of further intermediate states. The side-on structure has a lifetime of 270(30) ns at T = 23 degrees C.

Bent crystal x-ray mirrors for time-resolved experiments with femtosecond laser-produced x-ray pulses

In the last few years, bent crystal X-ray mirrors have played an important role in time-resolved X-ray diffraction experiments when X-ray pulses from femtosecond laser- produced plasmas were used (1-4). Improvements in manufacturing techniques have significantly increased the quality of this type of mirror.

On the Photophysical Properties of New Luminol Derivatives and their Synthetic Phthalimide Precursors

The photophysical properties of a series of structurally related 4-aminophthalimides and the corresponding 5-aminophthalic hydrazides (luminols) are reported. Absorption, steady-state, and time-resolved fluorescence spectra of luminols exhibited substitution, solvent, and pH dependence. Singlet lifetimes have been determined by time-resolved laser flash spectroscopy. UV spectra in gas phase and DMSO solution were calculated by TD-DFT which revealed the existence of two low-energy excited singlet states with strong pH-sensitivity.

Toward ultrafast time-resolved Debye-Scherrer x-ray diffraction using a laser-plasma source

An elliptical glass capillary has been used to focus ultrashort Cu K alpha x-ray pulses emitted from a femtosecond laser-produced plasma. Due to its high magnification (7x), the optic transforms the divergent x-ray emission of the plasma into a quasicollimated x-ray beam with a divergence of only 0.18 degrees. As an application we demonstrate the possibility to perform Debye-Scherrer diffraction experiments with the simultaneous detection of several diffraction orders. This will allow one to extend time-resolved x-ray diffraction with femtosecond laser-plasma x-ray sources to a much wider range of materials, which are not easily available as single crystals.

Extreme Phonon Softening in Laser-excited Bismuth – Towards an Inverse Peierls-transition

Large amplitude coherent optical phonons have been investigated in laser-excited Bismuth by means of femtosecond time-resolved X-ray diffraction. For absorbed laser fluences above 2 mJ/cm 2 , the experimental data reveal an extreme softening of the excited A 1g -mode down to frequencies of about 1 THz, only 1/3 of the unperturbed A 1g -frequency. At even stronger excitation the measured diffraction signals no longer exhibit an oscillatory behavior presenting strong indication that upon intense laser-excitation the Peierls-distortion, which defines the equilibrium structure of Bismuth, can be transiently reversed.

Transient Reversal of a Peierls-Transition: Extreme Phonon Softening in Laser-Excited Bismuth

Laser-excited coherent optical phonons in Bismuth were investigated using time-resolved X-ray diffraction. The observed extreme softening of the excited A1g-mode presents strong indication that the Peierls-distortion defining the equilibrium structure of Bismuth is transiently reversed.

Ultrafast Lattice Dynamics in FeRh during a Laser-Induced Magnetic Phase Transition

Time-resolved X-ray diffraction is used to study the lattice response of FeRh during a laser-driven anti-ferromagnetic to ferromagnetic phase transition. The experiments reveal a fast and a slow component in the induced expansion dynamics.

Coherent acoustic and optical phonons in laser‐excited solids studied by ultrafast time‐resolved X‐ray diffraction

We apply ultrafast time‐resolved X‐ray diffraction to directly study coherent acoustic and optical phonons in laser‐excited materials. In Au the ps acoustic response has been investigated with the particular goal to clarify the interplay of electronic and thermal pressure contributions. In Bi the extreme softening of the coherently excited A1g optical phonon presents strong indication that upon intense laser‐excitation the Peierls‐transition which determines the equilibrium structure of Bi can be reversed. In FeRh we studied the lattice response after a fast laser‐induced anti‐ferromagnetic to ferromagnetic phase transition.

The Role of Thermal and Electronic Pressure in the Picosecond Acoustic Response of Femtosecond Laser-excited Solids

Ultrafast time-resolved X-ray diffraction has been used to study the dynamics of coherent acoustic phonons in fs laser-excited Ge and Au, with the particular goal to clarify the interplay of the electronic and thermal pressure contributions. For semiconductors it is usually assumed that the electronic pressure is the dominant driving force. Our measurements reveal that in Ge the relative strength of the electronic pressure decreases with increasing laser fluence. Only for low fluences the electronic pressure dominates, while at high fluences the thermal pressure exceeds the electronic pressure. For the case of Au the data are well described within the established theoretical framework using the known values for those material parameters which determine the laser-induced pressure, namely the energy relaxation time and the electronic and lattice Gruneisen parameters.