Sébastien Zamith

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.

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.

TIME-DEPENDENT WAVE PACKET STUDY OF THE ONE ATOM CAGE EFFECT IN I2-AR VAN DER WAALS COMPLEXES

We performed a time-dependent wave packet study to investigate the fragmentation and recombination of the I2–Ar Van der Waals complex following excitation above the B-state dissociation limit. Based on a recently published ab initio potential energy surface of the ground state [C. F. Kunz, I. Burghardt, and B. Hess, J. Chem. Phys. 109, 359 (1998)], we studied the possible kinematic origin of the “one-atom cage effect” by three-dimensional wave packet propagation within the rotational infinite order sudden approximation. We found that final vibrational distributions depend strongly on the ground and excited state equilibrium geometries. Taking uncertainties in the excited state potential into account, we confirm a possible kinematic origin of the one-atom cage effect from a collinear isomer of the I2–Ar complex, initially proposed by Valentini and Cross [J. J. Valentini and J. B. Cross, J. Chem. Phys. 77, 572 (1982)].

Two-step melting of Na41+

The heat capacity of the mass selected Na(41) (+) cluster has been measured using a differential nanocalorimetry method. A two-peak structure appears in the heat capacity curve of Na(41) (+), whereas Schmidt and co-workers [M. Schmidt, J. Donges, Th. Hippler, and H. Haberland, Phys. Rev. Lett. 90, 103401 (2003)] observed, within their experimental accuracy, a smooth caloric curve. They concluded from the absence of any structure that there is a second order melting transition in Na(41) (+) with no particular feature such as premelting. The observed difference with the latter results is attributed to the better accuracy of our method owing to its differential character. The two structures in the heat capacity are ascribed to melting and premelting of Na(41) (+). The peak at lower temperature is likely due to an anti-Mackay to Mackay solid-solid transition.

Attachment cross-sections of protonated and deprotonated water clusters

Attachment cross-sections of water molecules onto size selected protonated (H(2)O)(n)H(+) and deprotonated (H(2)O)(n - 1)OH(-) water clusters have been measured in the size range n = 30-140 for 10 eV kinetic energy of the clusters in the laboratory frame. Within our experimental accuracy, the attachment cross-sections are found to have the same magnitude and size dependence for both species. It is shown that electrostatic interactions are likely to play a role even for the largest sizes investigated.

Heat capacities of mass selected deprotonated water clusters

Heat capacities of mass selected deprotonated water clusters (H(2)O)(n-1)OH(-) have been measured in the size range n = 48-118, as a function of temperature. We have found that they undergo a melting-like transition in the range 110-130 K. The transition temperature is size dependent with a strong correlation with the dissociation energy around the shell closure at n = 55.

New device to study unimolecular cluster nucleation

We have developed an apparatus which allows measuring the sticking cross section of a neutral atom onto a mass selected charged cluster of known temperature. The main point is to reduce the kinetic energy dispersion in the mass selected cluster beam in order to work with ions of very low (near thermal) kinetic energy. A novel device is presented which focuses in energy with only a small loss in the beam intensity. An application is shown to the sticking of sodium atoms onto sodium clusters at an energy of a few tenths of an eV in the center-of-mass frame.

Polarization-dependent pump-probe studies in atomic fine-structure levels: towards the production of spin-polarized electrons

The precession of orbital and spin angular momentum vectors has been observed in a pump-probe study of the 4P fine-structure states of atomic potassium. A femtosecond pump pulse prepared a coherent superposition of the two fine-structure components. A time-delayed probe pulse then ionized the system after it had been allowed to evolve freely. Oscillations recorded in the ion signal reflect the evolution of the orientation of the orbital and spin angular momentum due to spin-orbit coupling. This interpretation gives physical insight into the cause of the half-period phase shift observed when the relative polarizations of the laser pulses were changed from parallel to perpendicular. Finally, it is shown that these changes in the orientation of the spin momentum vector of the system can be utilized to produce highly spin-polarized free electrons on the femtosecond scale

Experimental nanocalorimetry of protonated and deprotonated water clusters

An experimental nanocalorimetric study of mass selected protonated (H2O)nH(+) and deprotonated (H2O)n-1OH(-) water clusters is reported in the size range n = 20-118. Water clusters heat capacities exhibit a change of slope at size dependent temperatures varying from 90 to 140 K, which is ascribed to phase or structural transition. For both anionic and cationic species, these transition temperatures strongly vary at small sizes, with higher amplitude for protonated than for deprotonated clusters, and change more smoothly above roughly n ≈ 35. There is a correlation between bonding energies and transition temperatures, which is split in two components for protonated clusters while only one component is observed for deprotonated clusters. These features are tentatively interpreted in terms of structural properties of water clusters.

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.