We have studied the reaction dynamics for HgI2 in methanol by using time-resolved x-ray diffraction (TRXD). Although numerous time-resolved spectroscopic studies have provided ample information about the early dynamics of HgI 2, a comprehensive reaction mechanism in the solution phase spanning from picoseconds up to microseconds has been lacking. Here we show that TRXD can provide this information directly and quantitatively. Picosecond optical pulses triggered the dissociation of HgI 2, and 100-ps-long x-ray pulses from a synchrotron probed the evolving structures over a wide temporal range. To theoretically explain the diffracted intensities, the structural signal from the solute, the local structure around the solute, and the hydrodynamics of bulk solvents were considered in the analysis. The results in this work demonstrate that the determination of transient states in solution is strongly correlated with solvent energetics, and TRXD can be used as an ultrafast calorimeter. It also is shown that a manifold of structural channels can be resolved at the same time if the measurements are accurate enough and that global analysis is applied. The rate coefficients for the reactions were obtained by fitting our model against the experimental data in one global fit including all q-values and time delays. The comparison between all putative reaction channels confirms that two-body dissociation is the dominant dissociation pathway. After this primary bond breakage, two parallel channels proceed. Transient HgI associates nongeminately with an iodine atom to form HgI 2, and I2 is formed by nongeminate association of two iodine atoms.HgI2 ͉ hydrodynamics ͉ liquid phase ͉ molecular structural dynamics ͉ transient structure T he knowledge of temporally varying molecular structures during ultrafast processes is vital in understanding the mechanism and function of molecular reaction. During the last decades, the dynamics of molecular reactions have been investigated by numerous spectroscopic techniques with femtosecond time resolution. However, the direct determination, at atomic resolution, of the structural dynamics involved in such processes can only be obtained by time-resolved x-ray (1-5), electron diffraction (6-9) and x-ray absorption spectroscopy (10, 11) acting as atomic probes. The methodology of time-resolved x-ray͞electron diffraction is similar to ultrafast optical pumpprobe experiment: a femtosecond laser pulse triggers a reaction in the molecules, and a delayed electron or x-ray pulse, rather than an optical pulse as in optical spectroscopy, probes the structural evolution. The changes in the nuclear coordinates are directly recorded by varying the time delay between the laser and the x-ray͞electron pulse.Because the scattering cross section of hard x-rays is 6 orders of magnitude lower than for electrons (12, 13), time-resolved x-ray diffraction (TRXD) can penetrate condensed samples such as liquids. In the condensed phase, the dynamics are not only determined by the potential energy surfaces of the reactantp...

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Using fast imaging and atomic force microscopy, we demonstrate that the fraction of nanoparticles in ablation plumes produced by short pulse laser irradiation of metals is strongly altered when a second laser pulse of sufficiently large delay is applied. Comparing the results obtained for gold and copper, it is shown that a significant nanoparticle reduction is only observed if the delay between both laser pulses exceeds the characteristic time of electron-lattice thermalization. We propose the reduced electronic heat transport at large lattice temperature as the dominant mechanisms for the observed nanoparticle reduction.

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International audienceThe mechanisms of material ablation and nanoparticle generation from metal samples exposed to intense short laser pulses are experimentally investigated. We performed measurements of the ablated volume using optical microscopy and the analysis of the ablation plume by fast imaging. The results confirm the existence of two distinguished ablation regimes as a function of the laser fluence, and give a deeper insight in the involved physical mechanisms. Thus, both regimes are found to be related to the relative amount of atoms and nanoparticles within the plume. Comparing the results obtained for copper and gold, it is possible to determine the influence of electron-lattice coupling on the sample heat regime and the resulting plume properties

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