Single-wall carbon nanotubes are formed by Nd:YAG laser vaporization of a graphite/(1 at.% Ni, 1 at.% Co) target into flowing argon (500 Torr) within a quartz tube furnace (1000 degrees Celsius). Here, this process is investigated for the first time with time-resolved laser-induced luminescence imaging and spectroscopy of Co atoms, C₂ and C₃ molecules, and clusters. These measurements under actual synthesis conditions show that the plume of vaporized material is segregated and confined within a vortex ring which maintains an approximately 1 cm³ volume for several seconds. Using time-resolved spectroscopy and spectroscopic imaging, the time for conversion of atomic and molecular species to clusters was measured for both carbon (200 microseconds) and cobalt (2 ms). This rapid conversion of carbon to nanoparticles, combined with transmission electron microscopy analysis of the collected deposits, indicate that nanotube growth occurs during several seconds of time from a feedstock of mixed nanoparticles in the gas-suspended plume. By adjusting the time spent by the plume within the high- temperature zone using these in situ diagnostics, single- walled nanotubes of controlled length were grown at an estimated rate of 0.2 micrometers /s.

Time- and spatially-resolved spectroscopy in conjunction with detailed modeling constitutes a powerful technique for the understanding of plasma plume dynamics. To this end, in a series of experiments performed at Sandia National Laboratories, laser-generated LiAg plasma plumes were produced by irradiation of solid targets using a Nd laser. Time- and spatially-resolved (along a direction normal to the target's surface) optical spectra were recorded with a framing spectrograph. In order to limit gradients along a direction perpendicular to the target normal, targets with strips of LiAg coated on top of Pt were used. The PT plume collisionally confines the LiAg, thus reducing the LiAg lateral expansion. This technique allows a better characterization of the LiAg plasma. The spectra displays line transitions in Li and Ag atoms, and evidence of ions in the plume is suggested by the presence of forbidden lines and Stark-broadened line shapes. A spectroscopic model based on collisional-radiative atomic kinetics, detailed line shapes, and radiation transport calculations is used to interpret the data. From this analysis temperature, density and ionization in the plume as a function of time and position along the normal to the target surface can be extracted.

This paper describes diagnostics experiments of pulsed excimer laser induced plasma. During excimer laser heating, the intense radiation flux from the laser is transformed to the target material and raises the temperature of the target surface rapidly. Melting and evaporation can occur, and the evaporated vapor can be ionized to form a plasma plume. Under rapid heating by a short laser pulse, the melted material can be superheated, and undergoes a phase explosion that turns the melt into a mixture of liquid and vapor. A number of experiments are carried out to investigate the laser-ablated plume. The velocity of the laser-evaporated plasma plume, absorption and scattering of the laser energy by the plasma, the threshold laser fluence for phase explosion, the evaporation pressure at the target surface, and the ablation rate are determined. Experiments are performed in a laser fluence range between 2.5 J/cm² and 10 J/cm² (between 100 MW/cm² and 400 MW/cm²) on nickel specimens. Results of these studies reveal phase change mechanisms and the kinetics at the evaporating surface during excimer laser interaction with the Ni target.

Characterization of a plasma produced from a gas puff target irradiated with a high-power nanosecond laser pulse from a Nd:glass laser is presented. The gas puff target was formed by puffing gas into the laser focus using a fast, high-pressure electromagnetic valve. The parameters of the plasma have been determined using soft x-ray diagnostics, including crystal spectrographs, pinhole cameras, and semiconductor detectors. The results of measurements are compared with the results of the numerical simulations performed using the 2-D magnetohydrodynamic code with the gasdynamical description of the plasma motion. Strong self-focusing of the laser beam in a high-density gas puff target and formation of a plasma channel observed in earlier experiments has been confirmed.

When a wide bandgap insulator is exposed to UV laser radiation at fluences close to the damage threshold, many laser pulses may be required before the onset of damage. Typically, damage is accompanied by the formation of a highly localized, bright, fluorescent plume. In this work, we explore the onset of damage and plume formation by imaging the laser-induced fluorescence from cleaved, single crystal NaCl exposed to pulsed 248-nm laser radiation (KrF excimer). Subsequent observations of the resulting surface topography are made by scanning electron microscopy. The onset of plume fluorescence is accompanied by localized fracture, usually associated with cleavage steps but originating some microns beneath the surface. Strong laser interactions are typically confined to the fractured region, which grows from pulse to pulse. We interpret these results in terms of absorption by deformation- induced defects along cleavage steps. Deformed material, produced by pushing a small metal sphere against the surface, damages at especially low fluences, consist with this interpretation. Indirect evidence suggests that thermal effects are localized along dislocation cores, which serve as recombination centers for laser-induced excitations.

Laser ablation is one of the most effective ways of making single-wall carbon nanotubes. Although the process is poorly understood, the importance of nanoparticle formation to initiate tube growth is evident. While some groups have concluded that nanoparticles can form in vacuum, we have argued that this is unlikely because the expansion of the plume is so rapid that the 'freezing limit' is reached too rapidly for nucleation and growth to the observed size. A background gas changes the dynamics completely. Calculations show that in a few microseconds the ablated plume is dramatically slowed by the 'snowplowing' of the background gas into a peak whose density is much greater than its initial density. The ablated material is trapped within this peak. The question then arises as to how this peak dissipates by diffusion. A simple calculation shows that it is at this point that a drastic change in the timescale of the process occurs so that there is ample time (milliseconds) for nanoparticles to nucleate and grow.

Molecular-dynamics simulations are used to investigate single- shot pulsed laser ablation and desorption of crystalline silicon. The motion of approximately 32000 atoms, contained in a 5 X 5 X 27 nm³ surface rectangular box irradiated by a single 308 nm, 10 ps, Gaussian laser pulse is followed on the picosecond time scale. Because melting and, possibly, ablation or desorption of the target following absorption of the laser pulse are described within the thermal annealing model, care is taken not to exceed carrier densities of approximately 10²² cm^-3. More precisely, the interaction of photons with the target is thought to cause the generation of a dense gas of hot electrons and holes which thermalizes, at first, on a time scale of a few tens of femtoseconds through carrier-carrier scattering. These hot photocarriers then transfer their kinetic energy to the lattice by means of carrier-phonon interactions characterized by a very fast initial cooling rate. The result is the creation, above a characteristic threshold energy, of a plume containing single atoms and clusters leaving the target with high axial velocities. Preliminary results about the melting fluence threshold and mechanisms underlying ablation are presented. Carrier diffusion is found to be an essential mechanism for relaxation and is presented as a possible cause of subsurface boiling.

Molecular dynamics calculations were carried out to follow the first 100 ps of the plume formation in matrix-assisted laser desorption ionization (MALDI). Two specific issues were addressed: the desorption of guest species in the form of matrix adducts and some distinctive features of infrared (IR) MALDI. Co-desorption of guest and matrix ions in the form of an adduct was studied using positive substance P ions surrounded by sinapinic acid anions. Upon laser heating the surface layers of the matrix underwent phase transition and the adduct was transferred into the gas phase without decomposition. IR-MALDI was studied using succinic acid as a matrix in combination with triglycine guest molecules. The IR laser induced desorption process was modeled by coupling the succinic acid O-H bond to an external 900 K heat bath. The energy redistribution within the matrix molecules themselves and the transfer between the matrix and guest molecules resulted in an increase of the system temperature. The kinetic temperature of the matrix reached plateau at approximately 670 K within a few ps. The temperature of the guest molecule increased on a slower time scale and during the calculation the values remained several hundred K below the O-H vibrational temperature. The liftoff velocity of the guest species (approximately 300 m/s) was similar to the values obtained in our previous calculations for different matrices and excitation methods.

It is now well established that thin films of a wide variety of materials can be deposited by ablation of a target material by a laser. Here, expansion of the laser-ablated plume in vacuum and in a background gas is comparatively studied using continuum hydrodynamics models, molecular gas dynamics models, and a recently developed multiple scattering model which combines continuum hydrodynamics and inter-species collisions. Continuum hydrodynamics models and molecular gas dynamics models predict, for the most part, that background plasma would reach the deposition substrate (or an ion probe placed at the same distance away from the target) first. On the other hand, the multiple scattering model shows that a component of the plume can indeed reach the substrate at vacuum speed, followed by a second plume component which is more or less slowed down by the presence of the background gas depending on its ambient pressure. Quantitative fits to the experimental data have been obtained with this multiple scattering model for expansion of Silicon in Helium and in Argon. The successful application of the multiple scattering model serves to explain the phenomenon of 'plume splitting' which is frequently observed in laser ablation processes for thin film deposition.

Pulsed-laser deposition has proved to be a promising method for producing complex inorganic thin films. One of its major advantages, relative to other methods, is the capability of controlling many process parameters, such as laser pulse width, energy, and wavelength along with background reactive gas pressure and substrate bias. Adjusting these parameters provides a pre-tuning of the laser plasma thereby allowing for optimum process conditions in a particular thin film deposition. Understanding and fully characterizing such highly-dynamic and rapidly-streaming plasmas requires multiple techniques for monitoring the plasmas at different stages. By combining different diagnostic methods, it is possible to analyze the broad time window over which these ablation plasmas develop and to understand the related processes that occur. We present in this work new results involving correlation of time-resolved Langmuir probe data, optical emission spectroscopy, and electrostatic energy analysis to characterize the laser-induced plasmas generated from targets of titanium, tin-dioxide and aluminum. Two laser sources, an 80 fs Ti:Sapphire laser (780 nm) and a 6 ns Nd:YAG laser (1.06 micrometer), were used in this work. Examples of very high quality, epitaxial tin-dioxide films grown on sapphire by femtosecond-laser MBE are presented. These films are evaluated by high-resolution, cross-sectional TEM and x-ray diffraction. Film quality is considered in relation to the ablation plasma parameters, wherein femtosecond and nanosecond plasmas are compared.

Expansion of ablation plumes created by intense ultrashort lasers is determined by various complicated physical processes which have very different spatial and time scales. Since complete simulation by one model is practically impossible, we suggest using two models describing initial and final stages that can be matched at an intermediate time. The proposed modeling procedure connects laser parameters to plume properties far away from the ablation spot. Laser material interaction and beginning of the expansion are simulated with a one-dimensional hydrodynamics code and the final stage is modeled using an analytical solution for an expanding three- dimensional ellipsoidal gas cloud.

In the ultraviolet, visible and near-infrared, single and multiphoton electronic transitions can explain the production and emission of charged atoms, molecules and photoelectrons during laser ablation and desorption. However, the process of charge transfer and ionization during ablation of dielectrics in the mid-infrared is not well understood. Even though significant electronic excitation is unlikely, copious emission of charged particles, e.g. atoms, molecules and electrons, is observed. No evidence of laser plume interactions is observed and inverse Bremsstrahlung (IB) is ruled out as a primary ionization/charge transfer mechanism. By irradiating with an ultrashort pulse-width mid-infrared laser tuned to a vibrational resonance it is possible to generate a high vibrational excitation density in dielectric materials. This high excitation density creates a non- equilibrium state of matter that exists until the deposited energy fully thermalizes. In this paper we report measurements of the kinetic energy of ions and electrons from CaCO₃, NaNO₃ and dihydroxybenzoic (DHB) acid that are highly non- thermal. This non-thermal energy distribution is evidence that the primary production of charged species occurs while the material is in a non-equilibrium state. The fact that it occurs in quite different materials, and without some of the characteristic signatures of electronically induced desorption and ionization, points toward a new mechanism.

We present experimental observations of plasma formation during single picosecond laser pulse (35 ps, 1064 nm) ablation of copper in air. We studied an early-stage air plasma which forms before the emergence of a material vapor plume. Using picosecond time-resolved shadowgraphs and interferograms, we measured the evolution of this air plasma that has an electron density on the order of 10²⁰ cm^-3. At post-pulse times, the longitudinal expansion of the plasma was found to be suppressed, while a power-law relation, r approximately t^1/2, was obtained for the temporal lateral expansion of the plasma radius. The results indicate that a large amount of the overall energy from the picosecond laser pulse is absorbed by the early-stage air plasma.

Laser induced breakdown and laser-excited atomic fluorescence spectroscopy were used to analyze trace metal atoms in water. Laser breakdown plasma was generated by a Q-switched Nd:YAG laser and an optical parametric oscillator (OPO) laser was used to excite target atoms in water. The wavelength of second laser was tuned to the absorption line of the target atom, and its time delay from the first one was optimized in order to maximize the fluorescence signal under low background light. Furthermore, the optimum transition lines were selected in terms of oscillator strength, branching ratio, and level population of target atom. In the case of Fe doped solution, the Fe concentration of less than a hundred ppb was detected. With this method, a lower detection limit of Fe was achieved than that in the case of the method using only laser-induced breakdown spectroscopy. The developed system is applicable for quick and supersensitive detection of trace metal atoms in water.

Process monitoring is a vital part of industrial laser applications that enables intelligent control of processes by observing acoustic, optical, thermal and other emissions. By monitoring these emission during laser processing, it is possible to ascertain characteristics that help diagnose features of the laser processed material and hence to optimize the technique. An experimental set up of observing plasmas during laser spot welding is described here. A pulsed Nd:YAG laser was used to spot-weld a variety of materials of different thickness, the plasmas generated during welding were monitored by a number of techniques, and the data obtained was used to characterize the welds. In the study photodiodes were set at different angles and observed the intensity and generation of the plasmas during the laser spot-welding process thereby giving a weld 'signature.' A portable spectrometer was used off-axis to obtain spectra of the emissions from the plasmas. Post process analysis was performed on the materials by mechanical polishing and chemical etching and observations of weld penetration depth and weld quality were correlated with the data collected on the plasmas. Different cover gases were also used during laser welding and the results of the effects of the various gases on the plasma are shown. The results indicate the relationship between laser weld generated plasma characteristics and weld features such as penetration depth. A direct correlation between the intensities of the photodiode and portable spectrometer signals was observed with weld penetration depth.

The problem of small particle size detection is important for industrial and environmental applications. Previous investigations have shown the possibility of using the laser breakdown method to achieve this goal; the sensitivity of this method is a thousand times higher than that of conventional methods. However, for small particle sizes, the damage threshold of the solid target in this case is very close to the breakdown point of pure air. After breakdown, there is a small volume of dense hot plasma that emits radiation. We analyzed this radiation using an analytical model and simulation code as well as by experiment, and found that the emission intensity varied depending on the laser type and plasma parameters including initial particle size.

Short pulsed X-rays have been experimentally generated by 90 degree Thomson scattering of 2 TW, 90 fs laser pulses by 17 MeV electron beams. A few 100 fs X-ray pulses have been generated via backward Thomson scattering from a few 100 fs electron bunches made by a bunch compression chicane. 100 TW laser and microtron as a 150 MeV electron beam source will be prepared, and the laser and the electron beam will be interacted as a hard X-ray source. Soft X-ray may be generated via laser-plasma nonlinear Thomson scatterings as a source of X-ray microscope.