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Les agrégats métalliques présentent des propriétés extraordinaires, en particulier chimiques et catalytiques, qui dépendent fortement de leur taille. Ce comportement en fait des candidats idéaux pour l'analyse en temps réel de processus photo-induits ultrarapides, le but ultime étant la conduite de scénarios de contrôle cohérent. Nous avons réalisé des expériences d'ionisation multiphotonique non stationnaire sur de petits agrégats alcalins de différentes tailles et dans différents états électroniques rovibrationnels, y compris leur état fondamental, sondant ainsi leur dynamique ondulatoire, leur orientation structurelle, leur transfert de charge et leur dissociation. Les processus observés dépendent grandement des paramètres du faisceau laser d'excitation, tels que sa phase, son amplitude et sa durée ; cette sensibilité plaide pour l'emploi d'un système de contrôle rétroactif permettant de générer les formes d'impulsion optimales. Les caractéristiques spectrales et temporelles de ces dernières reflètent les propriétés du système étudié ainsi que les processus photochimiques que l'irradiation y induit. Nous présentons d'abord la dynamique vibrationelle d'états électroniques excités, liés, dissociatifs et prédissociatifs, des dimères et trimères alcalins. Suit une description du principe d'observation par pompage optique par effet Raman stimulé de la dynamique d'un paquet d'onde dans l'état électronique fondamental. Puisque les paramètres de l'impulsion d'excitation influencent de façon significative le poids des différentes trajectoires dans l'espace des phases, nous avons mené des expériences sur les canaux de fragmentation concurrents d'une réaction photo-induite, en étudiant les différents embranchements des voies d'ionisation et de fragmentation de Na2K excité électroniquement. L'utilisation d'un algorithme évolutif, pour l'optimisation de la phase et de l'amplitude de l'onde électromagnétique appliquée, permit d'influencer significativement la quantité résultante d'ions parents ou fragments. Des propriétés intéressantes sont déduites de la forme des impulsions laser optimales obtenues, qui révèlent la période de vibration moléculaire et qui, combinées aux courbes de potentiel, permettent de proposer les trajectoires que l'optimisation impose aux paquets d'onde. Pour finir, nous avons étudié comment une variété plus grande d'agrégats, en contribuant au canal de fragmentation NaK, influe sur la forme optimale d'impulsion. Là encore, la structure de cette dernière apporte des éclaircissements sur les canaux de fragmentation empruntés durant le processus de contrôle.

The ionization potentials of Linand Nanclusters are obtained by photoionization. Shell effects are observed at n = 8 and n = 20, as well as strong odd/even alternations. The results are compared with the spherical shell model and the ab initio calculations.

Closed loop optimization of fs pulse shapes is investigated on the model systems consisting of small mixed alkali clusters. By controlling the photo-excited wave packet dynamics the multi-photon fragmentation and ionization of the Na_mK_n clusters are optimized. The resultant optimized pulse shapes carry fingerprints of the chosen dynamical pathways of the involved molecules. The dependence of fragmentation with cluster size is additionally investigated by gradually shifting the cluster size distribution towards larger masses. The resulting modification of the optimized pulse forms offers new insights into competing fragmentation channels during the control process. Characteristic motions of the involved wave packets are proposed to explain the optimized dynamic pathways. In particular, the steering of the wave packet in the trimer Na₂K by the optimized pulse can be described in terms of avoiding the predissociative curve crossing and thereby achieving an effective and selective fragmentation.

Using a container-integrated mobile femtosecond terawatt laser system with integrated detection unit Teramobile, we have demonstrated remote laser-induced breakdown spectroscopy R-LIBS on copper and aluminium samples with targets located at 25 m away from the container. The ability of our laser system to generate pulses in the femtosecond, picosecond and nanosecond regimes allowed us to perform direct comparisons between these three pulse durations. The dependence of the fluorescence signal on laser pulse energy showed a nonlinear behavior with a threshold, which is consistent with the previous observations for laser ablation. Such nonlinear behavior leads to a dependence of the LIBS signal on the temporal-spectral shape of the laser pulse. We showed especially that the transform-limited pulse does not optimize the fluorescence. A properly applied chirp allows an increase of the LIBS signal. Understanding and optimization of the chirp effect would improve the detection limit of the LIBS using a femtosecond laser Femto-LIBS and lead to a larger detection distance. Furthermore the use of pulse shaping should enhance the detection specificity for the cases of spectral overlapping between several elements to be identified.

Using a container-integrated mobile femtosecond terawatt laser system with integrated detection unit (Teramobile), we have demonstrated remote laser-induced breakdown spectroscopy (R-LIBS) on copper and aluminium samples with targets located at 25 m away from the container. The ability of our laser system to generate pulses in the femtosecond, picosecond and nanosecond regimes allowed us to perform direct comparisons between these three pulse durations. The dependence of the fluorescence signal on laser pulse energy showed a nonlinear behavior with a threshold, which is consistent with the previous observations for laser ablation. Such nonlinear behavior leads to a dependence of the LIBS signal on the temporal-spectral shape of the laser pulse. We showed especially that the transform-limited pulse does not optimize the fluorescence. A properly applied chirp allows an increase of the LIBS signal. Understanding and optimization of the chirp effect would improve the detection limit of the LIBS using a femtosecond laser (Femto-LIBS) and lead to a larger detection distance. Furthermore the use of pulse shaping should enhance the detection specificity for the cases of spectral overlapping between several elements to be identified.

Closed loop optimization of fs pulse shapes is investigated on the model systems consisting of small mixed alkali clusters. By controlling the photo-excited wave packet dynamics the multi-photon fragmentation and ionization of the NamKnclusters are optimized. The resultant optimized pulse shapes carry fingerprints of the chosen dynamical pathways of the involved molecules. The dependence of fragmentation with cluster size is additionally investigated by gradually shifting the cluster size distribution towards larger masses. The resulting modification of the optimized pulse forms offers new insights into competing fragmentation channels during the control process. Characteristic motions of the involved wave packets are proposed to explain the optimized dynamic pathways. In particular, the steering of the wave packet in the trimer Na2K by the optimized pulse can be described in terms of avoiding the predissociative curve crossing and thereby achieving an effective and selective fragmentation.

One parameter control experiments are carried out on two simple model systems with different complexities, the dimer NaK and the trimer Na₂K. The influence of the most important pulse features in formally found optimal control results1 are investigated independently for both molecules: the chirp and the separation of subpulses in pulse trains. For both molecules, the photoinduced processes under control are the transient three photon ionizations. Studies are presented that reveal the dependence of the measured ion yield on the applied chirps and the pulse separations. The pulse trains consisting of several subpulses are created by applying a sinusoidal spectral phase modulation. These studies may as well aid the interpretation of generated complex pulse shapes obtained in optimal control experiments.

One parameter control experiments are carried out on two simple model systems with different complexities, the dimer NaK and the trimer Na2K. The influence of the most important pulse features in formally found optimal control results1are investigated independently for both molecules: the chirp and the separation of subpulses in pulse trains. For both molecules, the photoinduced processes under control are the transient three photon ionizations. Studies are presented that reveal the dependence of the measured ion yield on the applied chirps and the pulse separations. The pulse trains consisting of several subpulses are created by applying a sinusoidal spectral phase modulation. These studies may as well aid the interpretation of generated complex pulse shapes obtained in optimal control experiments.

We describe the Teramobile system, a new mobile femtosecond multi-terawatt laser and detection system based on a state-of-the art CPA laser system embedded in a standard freight container, as well as a mobile detection unit allowing a characterization of the nonlinear propagation of high power laser pulses over long horizontal distances. The unique mobility feature of the whole system opens the way to previously unreachable applications for high-power laser pulses in the field of atmospheric research (lidar, laser-triggered lightning), which are also briefly reviewed.

We describe the Teramobile system, a new mobile femtosecond multi-terawatt laser and detection system based on a state-of-the art CPA laser system embedded in a standard freight container, as well as a mobile detection unit allowing a characterization of the nonlinear propagation of high power laser pulses over long horizontal distances. The unique mobility feature of the whole system opens the way to previously unreachable applications for high-power laser pulses in the field of atmospheric research (lidar, laser-triggered lightning), which are also briefly reviewed.

The gas-phase infrared spectroscopy of V₄O₁₀+ produced by laser vaporization has been studied in the spectral region from 7 to 16 µm. Mass-selected V₄O₁₀+ cations were stored in a helium filled radio frequency hexadecapole ion trap and excited using tunable infrared radiation from a free electron laser. The photodissociation spectrum was recorded by monitoring the V₄O₈+ yield (O₂ loss) as a function of the excitation wavelength. Two absorption bands at 842 and 1032 cm−1 are observed, which are assigned to resonant excitation of the antisymmetric V–O–V stretching and V&z.dbd6;O stretching vibrations, respectively. Comparison to recent theoretical and experimental studies indicate that the absorbing species consists of a V₄O₈+ ionic core weakly bound to an oxygen molecule.

Abstract:: We have performed adaptive feedback optimization of phase-shaped femtosecond laser pulses to control the wavepacket dynamics of small mixed alkali-metal clusters. An optimization algorithm based on Evolutionary Strategies was used to maximize the ion intensities. The optimized pulses for NaK and Na2K converged to pulse trains consisting of numerous peaks. The timing of the elements of the pulse trains corresponds to integer and half integer numbers of the vibrational periods of the molecules, reflecting the wavepacket dynamics in their excited states.

Abstract:: We present results on the ultrafast dynamics of mass-selected neutral Ag4 clusters using NeNePo (negative ion - neutral - positive ion) femtosecond pump-probe spectroscopy. One-color pump-probe spectra of the Ag4-/Ag4/Ag4+ system measured at 385 nm and an internal cluster temperature of 20 K display a complex beat structure over more than 60 ps. The oscillatory structure is attributed to vibrational wave packet dynamics in an excited “dark" state of neutral Ag4. A dominant 740 fs wave packet period as well as wave packet dephasing and rephasing are observed in the spectra. Fourier analysis of the spectra yields a group of frequencies centered around 45 cm-1 and an anharmonicity χe2νχeχe of 2.65 cm-1 for the active vibrational mode.

Abstract:: The diffusion and aggregation of preformed Agn-clusters ( n = 2-9) deposited onto a highly oriented pyrolytic graphite (HOPG) substrate is studied by two-photon-photoemission (2PPE). The sample is irradiated with ultrashort laser pulse pairs and the kinetic energy of the emitted photoelectrons is analyzed in a magnetic bottle type time-of-flight spectrometer. During annealing of the sample from 100 K up to room temperature, nanoparticles are formed on the surface by diffusion and aggregation of the silver clusters. A steep increase of the total photoelectron yield at a sample temperature of about 150 K is explained by the excitation of plasmons in the silver nanoparticles. From the kinetic energy distribution of the photoelectrons we deduce a strong variation of the work function of the sample during the formation of the nanoparticles, which is attributed to a quantum size effect.

Abstract:: In this paper we characterize the ultrafast fragmentation in electronically excited Fe(CO)2(NO)2 and CpMn(CO)3 by means of femtosecond time-resolved spectroscopy combined with mass spectrometry. From the transient two-color multi-photon ionization data, it was possible to record the transients of the parent molecule ions and their photofragment ions. The experimentally observed decay times indicated an ultrafast loss of the first ligands (sub-100 fs decay times). Further we performed a feedback control experiment on the photofragmenting CpMn(CO)3 molecular system in order to maximize the yield of desired ionic products through pulse modulation. The shape of the pulses obtained from optimization reflect well the intrinsic molecular dynamics during photofragmentation and the change of the CpMn(CO)+/CpMn(CO)3+ ratio shows a clear evidence for the capability of the optimization method to find tailor-made system-specific pulses.

We report a new differential absorption lidar technique for measuring simultaneously the concentrations of NO and NO_2 in the atmosphere. The technique is based on the correlation of the 450-nm absorption band of NO_2 and 227-nm absorption band of NO by frequency doubling. This performance has been allowed by the advent of a new and highly efficient frequency doubling crystal: the β-BaB_2O_4. A test experiment on a NO/NO_2 emitter has been performed, demonstrating the efficiency of the technique. The detection limit is estimated to be ~1 ppm · m for NO_2 and 100 pp · m for NO. The range of measurement is limited to 1 km, due to the strong UV Rayleigh scattering and O_2 absorption.

The method of 2-photon-ionisation has successfully been applied to alcali aggregates for a mass selective spectroscopy. The detection efficiency obtained is by far superior to conventional ionisation methods. The spectra allow conclusions about the geometry and temperature of the investigated particles. Therefore the method seems to be well suited for analyzing sputtered neutral particles.

A supersonic molecular beam, pulsed laser and time-of-flight mass spectrometer are arranged together in order to study photo- and auto-ionization, two-photon-ionization and fragmentation processes of alkali clusters as a function of the laser wavelength. In spite of an unfavourable duty cycle, however, the apparatus reaches a considerably higher sensitivity than cw experiments. Alkali clustersMn (n?21) have been observed and investigated. The well resolved TOF mass spectra are commonly accompanied by significant and broad signals of metastable ions, a phenomenon which cannot be observed by quadrupole mass spectrometry. ParticlesMn (n?4) have been investigated by twophoton ionization using two different laser wavelengths. Several new electronic transitions for Na3 are found; commonly, however, the excitation and ionization channels are accompanied by strong fragmentation processes. The fragment patterns are very sensitively dependent upon the laser wavelength even when working near the ionization threshold. The results are a strong indication, that the peak intensities of cluster mass spectra cannot easily be related to the intensity distribution of the neutral cluster beam.

Abstract: Positively and negatively charged metal clusters ions generated by sputtering are investigated by a variety of experimental techniques. Gas-phase experiments include photofragmentation measurements, which reveal information about the ionization energies of silver and aluminum clusters, and gas-phase nickel carbonyl formation, from which the structures of nickel clusters are determined. In addition, the size dependence of the catalytic activity of deposited silver clusters in the development of photographic films are presented.

In this paper, we present an overview of the capabilities of linear and non-linear spectroscopy for the 3D-analysis of gaseous pollutants and aerosols in the atmosphere. The basic technique is Lidar (Light Detection and Ranging). Results obtained in Sevilla and Lyon about the ozone smog formation and the dynamics of the transport of dust particles are presented. Finally, non-linear Mie scattering on atmospheric aerosols has been demonstrated in the laboratory, opening new perspectives for remote sensing.

A spectroscopic dye laser system-pumped by a Copper Vapor Laser (CVL)-has been built and optimized for different dye solutions operating in the spectral range from 530 to 890 nm. Conversion efficiencies up to 40% were reached in broad band operation, 24% with a prism expander grating cavity, and 20% in grazing incidence configuration, which operated at typical band widths of 3 GHz. Second harmonic generation (SHG) of the dye laser output produced tunable uv-radiation between 260 and 408 nm at conversion efficiencies ?5%. Fundamental and SHG output were used for resonant ionization experiments in molecular beams.

Abstract:: The interaction of a sequence of two identical ultrashort laser pulses with an atomic system results in quantum interferences as in Ramsey fringes experiments. These interferences allow achievement of temporal coherent control of the excitation probability. We present the results of a temporal coherent control experiment on two different atomic systems: one-photon absorption in K (4s-4p) and two-photon absorption in Cs (6s-7d). In K, the quantum interferences between the two excitation paths associated with the laser pulses are revealed through rapid oscillations of the excitation probability as a function of the time delay between the two pulses. These oscillations take place at the transition frequency (period T = 2.56 fs). The interferences are modulated by beats (at about 580 fs) resulting from the doublet structure of the excited state (4p (2 P 1/2 , 2 P 3/2 )). Three complementary interpretations of this experiment are presented: in terms of beats of quantum interferences, of variation in the spectrum intensity, and of wave packet interferences. Whenever the two laser pulses are temporally overlapped, optical interferences are superimposed on to the quantum interferences. The distinction between these two types of interference is clearly revealed in the two-photon excitation scheme performed on Cs (6s-7d (2 D 3/2 , 2 D 5/2 )) because quantum interferences occur at twice the frequency of the optical interferences.

Generation of continuous-wave, tunable UV radiation (250–260 nm) by intracavity doubling a coumarin-515 ring dye laser is described. A cooled (200–280 K) ADP crystal with end faces cut at Brester's angle is placed inside the laser ring cavity which has been compensated for astigmatism and coma. UV output powers at 254 nm of 120 μW and 60 μW are achieved with the laser operating multimode (bandwidth ≅ 20 GHz) and single-mode (bandwidth ⪅ 50 MHz), respectively. Continuous single-mode scans over the 253.7 nm mercury profile demonstrate sub-Doppler resolution of the Hg 6s6p 3PO1- 6s21S0transition.

We report a new differential absorption lidar technique for measuring simultaneously the concentrations of NO and NO_2 in the atmosphere. The technique is based on the correlation of the 450-nm absorption band of NO_2 and 227-nm absorption band of NO by frequency doubling. This performance has been allowed by the advent of a new and highly efficient frequency doubling crystal: the β-BaB_2O_4. A test experiment on a NO/NO_2 emitter has been performed, demonstrating the efficiency of the technique. The detection limit is estimated to be ~1 ppm .m for NO_2 and 100 ppb.m for NO. The range of measurement is limited to 1 km, due to the strong UV Rayleigh scattering and O_2 absorption.

The immission load of nitric oxide due to traffic is investigated in the cities of Lyon, Geneva, and Stuttgart using a mobile differential absorption LIDAR system (DIAL). Horizontal and vertical maps of the NO mixing ratio, as well as 24 hour records are presented. It is shown that street canyons favor high immission values, whereas broad main axes are efficiently ventilated (Lyon); inversion conditions rise the average immission load within a city mainly independent of traffic (Geneva and Stuttgart), whereas highly frequented intersections favor high immission values regardless of weather conditions (Geneva); the observed maximum immission loads also depend on the traffic density during rush hours (Stuttgart).

We have demonstrated the ability to trigger and guide high-voltage discharges with ionized filaments generated by femtosecond terawatt laser pulses. The plasma filaments extended over the whole gap, providing a direct ohmic connection between the electrodes. Laser-guided straight discharges have been observed for gaps of as much as 3.8 m at a high voltage reduced to 68% of the natural breakdown voltage. The triggering efficiency was found to depend critically on the spatial connection of the laser filaments to the electrode as well as on the temporal coincidence of the laser with the peak of the high voltage.

We investigated the spectral behavior of a white-light continuum generated in air by 2-TW femtosecond laser pulses at 800 nm. The spectrum extends at least from 300 nm to 4.5 μm. From 1 to 1.6 μm the continuum's intensity increases strongly with the laser energy and depends on the initial chirp.

We investigated the spectral behavior of a white-light continuum generated in air by 2-TW femtosecond laser pulses at 800 nm. The spectrum extends at least from 300 nm to 4.5 µm. From 1 to 1.6 µm the continuum’s intensity increases strongly with the laser energy and depends on the initial chirp.