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Nous avons utilise des cristaux de KTP et de KNbO 3 comme oscillateurs parametriques optique (OPO) pompes par un laser Titane:Saphir accordable. Pour l'OPO base sur un KTP «monolithique» en configuration d'accord de phase non critique de type II (θ=90° et Φ=0°), on obtient un seuil de 0,2 MW/cm 2 et une efficacite de conversion totale de 37%. L'OPO base sur le KNbO3 permet d'obtenir un signal et une complementaire variant de 0,95 a 3,8 μm lorsque l'on balaie la pompe entre 740 et 860 nm. Implemente dans notre systeme Lidar, ce cristal nous a permis de mesurer a 3,5 μm la taille et la concentration des gouttelettes d'eau lors d'un episode de brouillard, sur une distance de 1500 m. Le rayon moyen des gouttelettes d'eau est de 8,7 μm et la concentration est de 10 mg/m 3 .

La detection optique a distance de type Lidar (Light Detection and Ranging) pourrait etre appliquee pertinemment pour diagnostiquer la presence dans l'atmosphere d'agents biologiques. R. Chang et al [1] ont recemment demontre qu'il est possible de detecter des bacteries par fluorescence induite par laser (LIF). Toutefois, en milieu reel, il faut eviter toute interference avec d'autres sources d'emission: les hydrocarbures et leurs residus de combustion. Nous presentons ici les spectres LIF comparatifs de plusieurs aerosols candidats a la detection.

We present the results of an extensive study on urban aerosols performed in the summer of 1996 in Lyon. Non-volatile urban particles have been sampled using filters. Size distribution and composition, determined by scanning electronic microscopy (SEM) and X-Ray microanalysis, reveal two main modes at 0.1 and 0.9 μm, the composition of which is soot for the first and 60% in number soot — 40% silica for the second. Mie calculations have been performed using the actual size distribution and composition to determine the optical backscattering and extinction coefficients. The optical characteristics were then used for Lidar measurements. The combined SEM and Lidar techniques allowed to obtain the first quantitative 3D-profiles of urban aerosols concentrations. Potential and limitations of the method are critically discussed.

The unimolecular decomposition of metal rich LinHm+ clusters (1 ≤ m ≤ 6, n ≤ 22 and (n − m) > 3) is studied. The evaporative rates of the mixed clusters display features characteristic of metallic clusters. This confirms and extends to a larger size range the previous results obtained by photoionization and absorption cross section measurements. The evaporative rates are simulated by considering that there is a segregation between a metallic Lin-m+ part and an insulating (LiH)m part in the mixed cluster.

Narrow bandwidth operation within 250 MHz of a flashlamp-pumped Ti:Al 2 O 3 laser is successfully demonstrated with injection seeding from a CW laser diode. Seeding is shown sensitive even to very small injected power. Typically, 70% energy of a 100 mJ and 50 ns pulse is locked into the narrow bandwidth. This seeding efficiency is essentially limited in our experiments by the spatial hole burning in a laser rod with linearly polarized laser beam. It is also limited by mode mismatching between slave laser and seeding laser due to thermal effect on the laser rod induced by short and intense pumping flash.

The ionization-potential measurements of Li n H m clusters with 3≤n≤11 and 0≤m≤6 are presented. Their evolution is dominated by the number of electrons which are delocalized in the cluster. The analysis is in favor of a segregation between a metallic part and an insulating part in the mixed cluster. By comparison with alkali-halide clusters results, the existence of possible cuboid structures is discussed.

We have studied the unimolecular evaporation of ionized lithium clusters for different conditions of ionization. In this article we present the total evaporative rates and the monomer–dimer branching ratios for [Formula: see text]. We observe nonstatistical effects, associated with the dissociation along a less energetically favorable channel. Even after a few microseconds, the internal energy is probably not statistically distributed in all the vibrational modes.

Optical absorption spectra of Li4H and Li9H clusters have been recorded by depletion spectroscopy in the visible range. From comparison with ab initio calculations, geometries of both clusters are identified. The hydrogen atom assumes a peripheral position bridging two and three Li atoms in the planar and three‐dimensional structures of Li4H and Li9H, respectively. Na4F and Na9F clusters are also theoretically studied and it is shown how the strong electronegativity of the F atom leads to different geometries than in lithium hydrids. Finally, the metallic character of these clusters is discussed and in both cases, the hydrogen or fluorine atom localizes one valence electron. However, the optical absorption spectra are much broader than in pure Lin and Nan clusters due to the lower symmetry.

Ionization Potentials of LinHm clusters have been measured by photoionization. As in Lin, odd-even alternations and shell closing effects are observed. In a first approximation, we find that LinH clusters behave like Lin−1 and LinH2 like Lin−2. The results may be interpreted by assuming that the bonding of one hydrogen localizes one electron and that the other electrons remain delocalized.

We report the first experimental optical spectra of Li 2 H. Vibrational structure of the Li 2 H B state has been obtained by two-photon ionization spectroscopy, as well as high-resolution rotational spectra. Simulations of the rotational structure in the first vibrational band allowed us to determine the geometry of Li 2 H in the ground and excited B states: LiLi = 2.52 A, LiH = 1.715 A in the ground A 1 state and LiLi = 3.24 A, LiH = 2.29 A in the B 1 state. The geometry found for the ground state is in excellent agreement with several ab initio calculations. The ionic character of both states is discussed.

We present a novel method of characterizing urban aerosols that combines scanning-electron microscopy, x-ray microanalysis, and lidar measurements. Inversion algorithms, based on fractal aerosol models, allowed us to compute the scattering coefficients of the measured size distribution. The α and β coefficients were used to invert lidar data, yielding what to our knowledge are the first quantitative three-dimensional measurements of the aerosol mass concentrations in urban conditions. The combined method was used during an extensive experiment in Lyon in the summer of 1996. Size distributions exhibit two main modes, at 0.1 and 0.9 μm, the composition of which was determined by x-ray microanalysis. The first mode is soot, and the second is composed of 60% coarse soot particles and 40% silica particles. Lidar measurements showed a homogeneous aerosol concentration within the mixing layer and a steep gradient above. Measurements made over 24 h also showed loads that were due to traffic rush hours and the dynamics of the height of the planetary boundary layer.

We present the first broadly tunable KNbO 3 OPO in No-Tracking Configuration (NTC), pumped by a flashlamp-pumped Ti:sapphire laser. Tuning the pump laser from 733 to 841 nm yielded a tuning range from 908 nm to 1402 nm (for the signal), and 2103 to 3803 nm (for the idler). This range was limited by the mirror coatings, and continuous tuning should be achievable up to and beyond 4000 nm. Threshold was as low as 15 NW/cm 2 and efficiencies up to 10% have been observed without AR-coatings on the crystal.

We report the performance of a type-II KTP optical parametric oscillator-amplifier tandem pumped in a non-critical phase-matching configuration by a flashlamp-pumped Ti:Sapphire laser, scanned from 730 to 900 nm. Tunable signal and idler wavelengths are generated from 1.065 to 1.290 µm and from 2.320 to 2.970 µm, respectively, without crystal tracking. Slope efficiency is as high as 32% for the signal conversion only. A KNbO3 crystal has also been tested as a parametric amplifier in a no-tracking configuration and yields comparable gain.

Photoabsorption spectra of free LinH clusters have been recorded for n=2 to 9. Clusters are produced by coexpansion of 10–100 mbars of lithium vapor and 2–4 bars of a 98 % / 2 % mixture of argon with hydrogen. Two photon ionization was used to study the X ← B system of Li2H while beam depletion spectroscopy was used to measure the absorption cross sections of larger clusters. Most observations may be interpreted by the localization of one electron on the hydrogen bonding in agreement with ionization potential measurements.

Ionization Potentials of LinHm clusters have been measured by photoionization. As in Lin, odd-even alternations and shell closing effects are observed. In a first approximation, we find that LinH clusters behave like Lin−1 and LinH2 like Lin−2. The results may be interpreted by assuming that the bonding of one hydrogen localizes one electron and that the other electrons remain delocalized.

The LinHm clusters are produced in a seeded molecular beam. The lithium vapor is expanded in the vacuum with a mixture of 99% of argon and 1% of H2. The formed clusters are analyzed by a time of flight mass spectrometer and the clusters electronic structure is probed through ionization potential measurements. The electronic structure of LinH and LinH2 may be interpreted by assuming that the bonding of one hydrogen atom localizes one electron of the lithium atoms and that the other electrons remain delocalized.

We present the results of a detailed Lidar study on urban aerosols performed in summer 1996 in Lyon. Mie calculations havebeen performed to determine the optical backscattering and extinction coefficients of the real size distribution obtained bysampling, and then used for Lidar measurements. The solid particles have been sampled using cut-off filters. Size distributionand composition, determined by scanning electronic microscopy (SEM) and X-Ray microanalysis, reveal two main modes at0. 1 and 0.9 .tm, the composition of which is soot for the first one and 60 % soot - 40 % silica for the second. The combinedSEM and lidar techniques allowed to obtain the first quantitative Lidar profiles of urban aerosols. Potential and limitations ofthe method are critically discussed.Keywords : lidar, aerosols, microscopy, urban 1. INTRODUCTION Atmospheric aerosols are today a key issue in air pollution. Their impact on public health and activity in heterogeneouschemistry are well recognised although few comprehensive quantitative studies have been reported so far. This is especiallytrue for urban aerosols, which variety in size and composition make a complete characterisation difficult. For instance, due tothe lack of analysing techniques, today's legislation is limited to specify a certain mass of particles per cubic centimetre',whatever the size and the composition. This is particularly unsatisfactory since obviously particles smaller than 1 im aremuch more relevant for public health than bigger ones .

Narrow bandwidth operation within 250 MHz of a flashlamp-pumped Ti: Al2O3 laser is successfully demonstrated with injection seeding from a CW laser diode. Typically, 70% energy of a 100 mJ and 50 ns pulse is injection-locked into the narrow bandwidth.

An all-solid-state infrared lidar system has been developped mainly to detect hydrocarbons in the 3300nm stetching band. As an infrared laser source, an optical parametric oscillator (OPO) has been built. It is pumped by a tunable flashlamps pumped Ti:Sapphire which allows the very convenient “no-tracking” configuraion for the OPO. It has been implemented in a mobile, very compact lidar system.

We developed a new concept for lidar measurements: multispectral differential absorption. Instead of comparing the backscattered light at one absorbed and one reference wavelengths (DIAL), a broadband laser is sent into the atmosphere and the returned signal is spectrally resolved on the receiver side (MULTIDIAL). Potential advantages are thoroughly reviewed. First, experiments combining lidar with a Fourier transform spectrometer are presented. Broadband nontracking OPOs are being built for this application in the infrared to detect VOCs. This idea was also applied to the UV range. A tunable (280-300 nm) broadband Ce:LiSAF laser has been developed for this purpose. The detection is performed by a gated amplified CCD-OMA.

We present results of experiments to determine the spectral and angular dependence of nonlinear scattering by water microdroplets of radii ranging from 1 to 32 μm illuminated by ultrashort laser pulses. The main process observed is third-harmonic generation (THG). Moreover, in contrast with linear Mie scattering, the angular pattern of the THG is remarkably stable over the size range 8–32 μm.

Static electric dipole polarizabilities of lithium clusters made of n $(n=2--22)$ atoms have been measured. The experiment consists of deflecting a collimated cluster beam through a static inhomogeneous electric field. The strong decrease per atom from Li to ${\mathrm{Li}}_{3}\ensuremath{-}{\mathrm{Li}}_{4}$ shows that electronic delocalization is reached for very small sizes. Moreover, directly measured polarizabilities are consistent with photoabsorption data. They thus confirm unambiguously the ``missing'' optical strength in lithium clusters.