V. S. Burakov

Plasma chemistry in laser ablation processes

Abstract Based on the results of quantitative spectroscopic diagnostics (LIF in combination with time resolved emission spectroscopy) chemical dynamics in laser-produced plasmas of metallic (Ti, Al,), and graphite samples have been examined. The Nd-YAG (1064 nm, 10 ns, 100 mJ) and excimer XeCl (308 nm, 10 ns, 10 mJ) lasers were employed for ablation. The main attention was focused on the elucidation of a role of oxide and dimer formation in controlling spatio-temporal distributions of different species in the ablation plume. The results of the spatial and temporal analysis of a laser-produced plasma in air indicates the existence of diatomic oxides in the ablation plume both in the ground and excited states, which are formed from reactions between ablated metal atoms and oxygen. The efficiency of the oxidation reaction depends on the intensity and spot diameter of the ablation laser beam. The maximal concentration of TiO molecules are estimated to be of 1×10 14 cm −3 at the time of 10 μs after the start of the ablation pulse. A comparison of spatial–temporal distributions of Ti atoms and excited TiO molecules allow us to find a correlation in their change, which proves that electronically excited Ti oxides are most probably formed from oxidation of atoms in the ground and low lying metastable states. The spectroscopic characterization of pulsed laser ablation carbon plasma has also been performed. The time–space distributions as well as the high vibrational temperature of C 2 molecules indicate that the dominant mechanism for production of C 2 is the atomic carbon recombination.

An absolute measurement of the neutral density profile in the tokamak plasma by resonance fluorescence on the Hα line

The authors discuss the use of the resonance fluorescence method with laser pumping of optical transitions of excited hydrogen atoms for high-temperature plasma diagnostics. The calculations were performed for the Balmer Hα line, λ = 6563 A. The conclusions reached may also be generalized to other hydrogen lines. Experimental results obtained on the FT-1 tokamak are presented and discussed.

Intracavity laser spectroscopic method for determining trace amounts of iodine and barium in water and biological samples

An intracavity laser spectroscopic method and the necessary instrumentation are described for the direct determination of trace amounts of iodine in water and in biological media. Minimal sample preparation is required for the laser probing of vapours over the surface of the heated liquid in a closed cell. This laser method is also applied to measurements of ultra-low barium contents in water by sample evaporation in a standard graphite furnace electrothermal atomizer. Detection limits of 0.015 mg l–1 and 0.2 ng l–1 were obtained for iodine and barium, respectively, which are 2–3 orders of magnitude lower than those obtained using a pulsed dye laser. The precision of the method was 10% for the lowest concentrations measured. The procedure is comparatively simple, quick and inexpensive.

Near-threshold laser-induced sputtering of aluminum surface by UV and IR irradiation

Abstract Laser-induced particle emission (Al, Al * , Al + ) from Al and Al 2 O 3 surfaces irradiated by laser beams at different wavelengths has been examined to investigate primary processes of laser-initiated sputtering. The Nd-YAG (1064 nm, 10 ns, 1–4 J/cm 2 ), excimer XeCl (308 nm, 10 ns, 0.1–0.5 J/cm 2 ) lasers or Stokes components of stimulated Raman scattering of XeCl laser radiation in pressed hydrogen (353 nm, 414 nm) were employed for irradiation of the surfaces of the samples in the helium (air) atmosphere at pressures (10 −3 –400 Torr). The near-threshold energy fluences on the sample surface did not result in detectable surface damages but changed the surface structure and composition. LIF spectroscopy was used to detect nonemitting ground state species, while excited species were observed by time-resolved optical emission spectroscopy. The role of photochemical (electronic) and thermal effects in the initiation of particle emission have been identified in dependence on the laser wavelength. The predominance of the photochemical mechanism over the thermal one has been established in the initiation of laser-induced sputtering by radiation of excimer XeCl laser. Additionally, the resonant aspects of laser–surface–plume interaction as determined from the resonant and nonresonant irradiation of aluminum at 308.2 nm and 307.8 nm have been discussed.

Dynamics of the Emission of Light by C2 and C3 Molecules in a Laser Plasma Produced by Two-Pulse Irradiation of the Target

The laser plasma produced by irradiation of a graphite target simultaneously by the first and second harmonics of two Nd3+:YAG lasers has been studied by emission spectroscopy methods. The delay between radiation pulses (τ) varied from 0 to 700 μsec. It is established that in the absence of a delay between pulses (τ = 0) the increase in the intensity of plasma emission at the wavelengths corresponding to the radiative transitions of the C2 and C3 molecules is of nonradiative character. The plasma produced by laser radiation at the wavelength λ = 1064 nm exerts its influence on the radiation spectrum of the plasma produced by laser radiation at the wavelength λ = 532 nm, if the magnitude of the delay between laser pulses τ does not exceed 30 μsec. The most probable reason for this character of influence of the prepulse on the laser plasma radiation spectrum is sublimation of soot particles caused by laser radiation at λ = 532 nm.

Intracavity Laser Spectroscopy: Plasma Diagnostics and Spectral Analysis (Review)

Based on the analysis of the results obtained for the most part in the past ten years, the present state of the art and the potential for the development of two current trends in the application of selective intracavity laser spectroscopy are considered, that is, the dynamic diagnostics of plasma and the highly sensitive spectral analysis. The practically useful dependences and relationships reflecting the quantitative trends in the intracavity absorption in pulsed lasers have been systematized.

Formation of chemical compounds in a laser plasma

Using the methods of laser-induced fluorescence and emissive spectroscopy, we carried out investigations of the formation of TiO molecules in a laser plasma produced by focusing the radiation of an AYG:Nd3+ laser on the surface of a titanium target in air. The radiation flux density varied within the range 108–1010 W/cm2. We investigated the distribution of molecules over internal states and the space-time distributions of Ti atoms in the ground, metastable, and excited states, as well as of TiO molecules in the ground and excited states. We found that gas-phase reactions with participation of Ti atoms in the ground state provide the most probable channel for the formation of TiO molecules; the role of reagents in ionized, excited, and metastable states is of secondary importance.

Laser absorption and fluorescence diagnostics of a plasma

Diagnostics of a near-surface laser plasma, the plasma of a strong-current pulse gaseous discharge in inert gases, nitrogen, and carbon dioxide under conditions of intense evaporation of the wall of the discharge chamber, the plasma of a discharge with a hollow cathode, and the active medium of an excimer laser was conducted by methods of intracavity laser spectroscopy and laser-induced fluorescence. The dynamic fields and absolute concentrations of atoms, ions, molecules and electrons, the plasma temperature, and the velocities of flows of particles were measured. The quantitative determination of the density of particles in the erosion laser flame prior to breakdown and the phenomena associated with the formation of molecules in the laser plasma received primary consideration.

Study of the profiles of atomic absorption lines in an electrothermal atomizer

Narrow atomic absorption lines of Ba, Cs, Mg, and Tl are recorded in an electrothermal atomizer with a graphite tube furnace by the intracavity laser spectroscopy method. The conditions of applicability of this laser absorption method for reducing the observed profiles of spectral lines to the true ones are found. Measurement of the integral absorption coefficient has made it possible to determine the absolute number of free atoms of each element in the analyte volume at the maximum of the absorption peak under optimum conditions of atomizer heating. The Voigt parameters are determined from the profiles of absorption lines.

Spectroscopic characterization of plasmas produced by dual-laser ablation

Based on the results of spectroscopic diagnostics a role of laser-surface and laser-plasma interactions in enhancement of line emission from plasma produced by the sequence of two laser pulses at different wavelengths is discussed.