A novel experimental method for the measurement of the caloric curves of clusters

A novel experimental scheme has been developed in order to measure the heat capacity of mass selected clusters. It is based on controlled sticking of atoms on clusters. This allows one to construct the caloric curve, thus determining the melting temperature and the latent heat of fusion in the case of first-order phase transitions. This method is model-free. It is transferable to many systems since the energy is brought to clusters through sticking collisions. As an example, it has been applied to Na\_90\^+ and Na\_140\^+. Our results are in good agreement with previous measurements

Experimental Determination of Nucleation Scaling Law for Small Charged Particles

We investigated the nucleation process at the molecular level. Controlled sticking of individual atoms onto mass selected clusters over a wide mass range has been carried out for the first time. We measured the absolute unimolecular nucleation cross sections of cationic sodium clusters Na_{n}^{+} in the range n=25-200 at several collision energies. The widely used hard sphere approximation clearly fails for small sizes: not only should vapor-to-liquid nucleation theories be modified, but also, through the microreversibility principle, cluster decay rate statistical models

Observation of coherent transients in ultrashort chirped excitation of an undamped two-level system

The effects of Coherent excitation of a two level system with a linearly chirped pulse are studied theoretically and experimentally (in Rb (5s - 5p)) in the low field regime. The Coherent Transients are measured directly on the excited state population on an ultrashort time scale. A sharp step corresponds to the passage through resonance. It is followed by oscillations resulting from interferences between off-resonant and resonant contributions. We finally show the equivalence between this experiment and Fresnel diffraction by a sharp edge.Comment: 4 pages, 4 figures, to appear in PR

Dynamique femtoseconde dans des atomes et molécules - précession de spin et dynamique de photoélectrons - transitoires cohérents - dynamique des états excités de l'acétylène

This thesis is about the study of dynamics of atoms and molecules on the femtosecond time scale by the pump-probe technique. In a first part the oscillation of a wave packet created by a superposition of fine structure states in atomic potassium has been observed. This oscillation is interpreted as the precession of the spin momentum around the total angular momentum. This is followed by a theoretical study showing that this oscillation can be used to produce spin polarised electrons. Another experiment performed has permitted the observation of interferences of free electron wave packets created by ionising potassium atoms with a sequence of two time delayed ultrashort pulses. The pump-probe technique has been applied to the study of the population transfer in a two level system interacting with a chirp pulse. This experiment has led to the observation of coherent transients due to interferences between the amplitude of probability transferred at and after resonance. In a second part, I have studied the predissociation of highly excited states of acetylene, which leads to very short lifetimes of these states. A first step consists in a nanosecond REMPI (3+1) experiment. The photoelectron spectra of the different states were collected. They show a strong Rydberg-Valence mixing. The dynamics of these states has been then studied in a pump-probe experiment. The pump step consists in the excitation by a VUV (132 nm) photon while the probe step is done by ionising the system with UV (396 nm) pulse. We show that the determination of the lifetimes must be done carefully by taking into account the pulses characteristics.Cette thèse présente l'étude de dynamique à l'échelle femtoseconde de systèmes atomiques et moléculaires. Dans une première partie nous décrivons l'oscillation d'un paquet d'onde dans un doublet de structure fine de l'atome de potassium, observée expérimentalement. Cette oscillation est reliée à la précession du spin autour du moment angulaire total. Nous montrons théoriquement comment mettre à profit cette oscillation pour produire des électrons polarisés en spin. En ionisant ce même atome par deux impulsions ultracourtes séparées en temps, deux paquets d'électrons libres sont émis. Nous montrons qu'il est possible de les faire interférer. L'étude expérimentale, par la technique pompe-sonde, de l'excitation d'un système à deux niveaux dans l'atome de rubidium par une impulsion à dérive de fréquence nous a permis d'observer en temps réel le transfert de population de l'état fondamental vers l'état excité. Ce transfert s'accompagne de transitoires cohérents résultant d'interférences entre les amplitudes de probabilité transférées au passage et après le passage par la résonance. Dans une seconde partie, nous nous intéressons à la prédissociation d'états très excités de l'acétylène, de durées de vie très courtes (~100 fs). Nous avons dans un premier temps réalisé des expériences en régime nanoseconde REMPI (3+1). Les signatures des spectres de photoélectrons correspondant aux différents états permettent de mettre en évidence le mélange des caractères Rydberg-Valence dans cette gamme d'énergie. La dynamique de ces états est observée en fonction du délai entre des impulsions ultracourtes pompe VUV (132 nm) et sonde UV (396 nm) en collectant le signal d'ion et/ou le spectre de photoélectrons. Nous montrons par ces résultats que la détermination des durées de vie doit passer par une analyse des signaux pompe-sonde tenant compte des caractéristiques des impulsions laser

Dynamique femtoseconde dans des atomes et molécules (précession de spin et dynamique de photoélectrons transitoires cohérents , dynamique des états excités de l'acétylène)

TOULOUSE3-BU Sciences (315552104) / SudocSudocFranceF

Dynamical simulation of collision-induced dissociation of pyrene dimer cation

International audienceWe report a theoretical investigation of the collision induced dissociation of pyrene dimer cation, as recently investigated in the experimental work by Zamith et al. (J. Chem. Phys. 153, 054311 (2020)). Molecular dynamics simulations using potential energies and forces computed at the self-consistent charge density functional based tight binding level were conducted for different collision energies between 2.5 and 30 eV. It appears that most of the dissociation occurs on a short timescale (less than 3 ps). The dynamical simulations allow to visualise the dissociation processes. At low collision energies, the dissociation cross section increases with collision energies whereas it remains almost constant for collision energies greater than 10-15 eV. The analysis of the kinetic energy partition is used to get insights into the collision/dissociation processes at the atomic scale. The simulated time of flight mass spectra of parent and dissociation products are obtained from the combination of molecular dynamics simulations and phase space theory to address the short and long timescales dissociation, respectively. The agreement between the simulated and measured mass spectra suggests that the main processes are captured by this approach

A two-state model analysis of the melting of sodium clusters: Insights in the enthalpy-entropy compensation

International audienceExperimental melting temperatures and associated latent heats of size-selected sodium clusters are analyzed in the frame of a two-state model of melting. The strong variations of these quantities as a function of size make clusters an excellent benchmark for testing speculative ideas about melting. The relation between cohesive energy of clusters and their latent heat of melting is demonstrated, and a correlation between cohesive energy and vibrational frequency in the solid state is found. It is shown that this relation may throw light on the enthalpy-entropy compensation, which is observed in a variety of systems undergoing a first-order phase transition. A relation is established between the Lindemann melting criterion, the variation of the vibrational frequency with the cohesive energy, and the enthalpy-entropy compensation