N. V. Tarasenko

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.

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.

Structure and Optical Properties of Carbon Nanoparticles Generated by Laser Treatment of Graphite in Liquids

In this paper, we report the one-step synthesis of luminescent carbon nanoparticles (NPs) via laser irradiation of a graphite target in a solvent [H2 O, ethanol, or a 0.008 m aqueous diethylenetriaminepentaacetic acid (DTPA) solution]. This is a simple approach for the fabrication of carbon dots with tunable photoluminescence (PL) that differs from other preparation methods, as no post-passivation step is required. The unfocused beam of the second harmonic (wavelength 532 nm) of the Nd:YAG laser was used in our experiments. The sizes of the prepared NPs were mainly distributed in the range of 1-8 nm with an average value of 3 nm. Carbon NPs of different inner structure were prepared: hexagonal diamond phase in aqueous DTPA solution, orthorhombic carbon phase in ethanol, and amorphous carbon in water. The synthesized carbon NPs have strong luminescence in the visible region, which makes them attractive for numerous biological applications. The photoluminescence of the synthesized NPs was investigated at different excitation wavelengths, from 260 to 450 nm. The highest intensities of the emission bands were detected for an excitation wavelength of 400 nm.

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.

Laser-induced size and shape transformation of silver colloidal nanoparticles

In this paper studies of the changes of the morphology of silver colloidal nanoparticles under laser irradiation at different fluences and wavelengths have been presented. Silver nanoparticles of size 10-30 nm were prepared in a gelatin stabilized AgNO3 solution under reduction with K-Na-tartrate. They were then exposed to pulsed laser radiation at 532 nm, 400 nm and 266 nm. In addition to the fragmented particles, relatively larger-size (1400-1500 nm) rightangled structures were found to be formed in the solution. The experimental conditions favouring laser-induced transformation of the morphology of nanoparticles have been determined.

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.

Laser ablation synthesis and optical characterization of zinc oxide nanostructures

Zinc oxide nanoparticles were synthesized by pulsed laser ablation technique in water with further deposition on the single-crystalline silicon substrate. The composition, morphology, absorbance and luminescent properties of the formed structures were studied. The study revealed that films of high structural and optical quality can be prepared by the developed method.