Laser vapor cloud

When the light from a pulsed laser is focused onto a solid surface, electrons 292 a) jgns, and neutral particles are evaporated 293-295) when the laser intensity exceeds a threshold value. Kinetic energy, temperature, degree of ionization and total mass of the expanding vapor cloud depend on incident laser power and pulse duration 296.297)... 

The laser irradiance (laser power/cm2) is an important parameter Different irradiance values lead to a vapor cloud of different density, and consequently the ion-molecule reactions can take place with highly different yields. 

Di.screle solution samples are intriHluced by transferring an aliquot of (he sample to the aioini/er. The vapor cloud produced with electrothermal atomizers is tran.sient bccnu.se of Ihe limited amount of sample available. Solid samples can be introduced inlo plasmas by vaporizing them wilh an eleciric spark or with a laser beam.. Solutions are generally introduced into the atomizer by one of the lirsi three methods listed in Table 8-2. 

Laser Microprobe A plasma produced by the absorption of laser radiation, forming an emitting vapor cloud. Sometimes supplemented by auxiliary excitation by spark. conducting and nonconducting solids 19,000... 

A method which combines laser vaporization of metal targets with controlled condensation in a diffusion cloud chamber is used to synthesize nanoscale metal oxide and metal carbide particles (10-20 nm). The silica nanoparticles aggregate into a novel web-like microstructure. These aggregates are very porous and have a very large surface area (460 m /g). Bright blue photoluminescence from the nanoparticle silica has been observed upon irradiation with UV light. The photoluminescence is... 

The chapter consists of three major sections. The first is a brief review of the processes of nucleation and growth in supersaturated vapors for the formation of clusters and nanoparticles. The second section deals with the application of laser vaporization for the synthesis of nanoparticles in a diffusion cloud chamber. In the third section, we present some examples of nanoparticles synthesized using this approach and discuss some selected properties. 

There has been renewed interest in the method, mainly due to the availability of improved Nd YAG laser systems. In addition, different types of detectors, such as microchannel plates coupled to photodiodes and CCDs, in combination with multi-channel analyzers make it possible for an analytical line and an internal standard line to be recorded simultaneously, by which the analytical precision can be considerably improved. By optimizing the ablation conditions and the spectral observation, detection limits obtained using the laser plume as a source for atomic emission spectrometry are in the 50-100 pg/g range and RSDs are in the region of 1% as shown by the determination of Si and Mg in low-alloyed steels [255, 259]. This necessitates the use of slightly reduced pressure, so that the atom vapor cloud is no longer optically very dense and the background emission intensities become lower. In the case of laser ablation of brass samples at normal pressure and direct... 

Plume (laser) The cloud of vapor that rises from the heated spot during laser vaporization. The cloud adsorbs some of the laser radiation to produce ions and electrons. 

This detector was first described by Charlesworth (8). Nebulization of the column effluent in a stream of tepid (30-35°C) gas, followed by total vaporization of the solvent in a warm (40-45°C) drift tube, leaves a cloud of particles made of the nonvolatile material contained in the eluent. These particles are carried by the gas stream across a laser beam and then vented. 

Atomization should completely convert the elements in the sample into an atomic vapor of high density. To meet these requirements a large amount of energy is injected rapidly into the sample hence, arcs, sparks, high temperature flames and lasers are used for this purpose. The shape of the atomic cloud generated is determined by thermal expansion of the vapor and the flow of inert or flame gases. This system forms a dynamic atom cell or reservoir. 

An ensemble of cold Rydberg atoms is easily obtained after laser excitation of a cold atomic cloud, as those performed in a Gs or lib vapor-cell magnetooptical trap, at a temperature of 135 /jK or 300 fiK respectively. In the case of cesium (for the experiments performed at Laboratoire Aime Cotton) or rubidium (for the experiment performed at the University of Virginia), the atoms p-excited by the cooling lasers are Rydberg-excited by using a laser pulse provided by a dye laser pumped by the third harmonic of a Nd YAG laser... 

Elements with low intensity fluorescence lines (e.g. Eu, Tm and Y) have been determined in aqueous solutions by depositing and drying nanoliter amounts of sample on the Ni cathode of a miniature GD source used as the atom reservoir [665], The atomic cloud thus formed was extited by a Cu-vapor laser-pumped dye laser to detect fluorescence directly. Absolute detection limits of 2 fg for Eu, 0.08 fg for Tm and 1.2 pg for Y were achieved and the total time for analysis from sample probing to data acquisition did not exceed 5 min. 

This goal is most clearly approached by methods based on vaporization of a metal by laser pulses, leading to a cloud of clean clusters of bare metal containing as few as one atom. Catalytic activity of clusters as a function of number of atoms can be obtained by time-of-flight mass spectrometry (16-18). 

In a cloud chamber detector, ions at atmospheric pressure are electrically focused into a cloud chamber filled with cold water or octane vapors. The presence of the ion serves as the nucleus for the formation of small droplets that can scatter light from a laser beam passing through the cloud chamber. When mobility-separated ions entered the cloud chamber, perturbation in the laser light due to the formation of ion-nucleated particles was detected by a PMT. When the chamber was supersaturated with water, the scattered light intensity increased in the presence of ions, but when the chamber was supersaturated with octane, the intensity decreased in the presence of ions. Mobility spectra of difluorodibromomethane have been reported using cloud chamber detection." ... 

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