INDUCED BREAKDOWN SPECTROSCOPY; ABLATION; EMISSION; SPARK

The influence of distance between sample surface and focal point on optical emission spectroscopy of laser-induced silicon plasma by a nanosecond Nd:YAG laser operating at the wavelength of 1064 nm was investigated in air. Our results show that the emission intensity of Si (I) 390.6 nm line and N (II) 399.5 nm line depends strongly on the distance between sample surface and focal point. When the surface of ablated sample is away from the focal point of focusing lens, the neutral atomic line (Si(I) signal to be measured) is much higher than the ionic line (interference signal N (II)). Therefore, we can improve the intensity of Si (I) signal to be measured, and reduce the intensity of interference signal N (II). The presented result is mainly based on the reduction of interaction between the plasma plume and the ambient air, leading to much weaker collisions. (C) 2017 Author(s).

Silicon; Laser-induced breakdown spectroscopy; Emission spectroscopy; Surface morphology; Periodic surface structures;PLASMA PARAMETERS; SURFACE-MORPHOLOGY; SILICON; ZINC; ALLOY; TEMPERATURE; DIAGNOSTICS; SPECTRA; DENSITY; GAS

The effect of nature and pressure of ambient environment on laser-induced breakdown spectroscopy (LIBS) and ablation mechanisms of silicon (Si) have been investigated. A Q-switched Nd-YAG laser with the wavelength of 1064 nm, pulse duration of 10 ns, and pulsed energy of 50 mJ was employed. Si targets were exposed under ambient environments of inert gases of argon, neon, and helium for different pressures ranging from 5 to 760 torr. The influence of nature and pressure of ambient gases on the emission intensity of Si plasma have been explored by using the LIBS spectrometer system. The plasma parameters such as electron temperature and number density were determined by applying Boltzmann plot and Stark broadening method, respectively. Our experimental results suggest that the nature and pressure of ambient environment play a significant role for generation, recombination, and expansion of plasma and consequently affect the excitation temperature as well as electron density of plasma. The surface morphological analysis of laser-irradiated Si was performed by using scanning electron microscope (SEM). Various kinds of structures, for example laser-induced periodic surface structures or ripples, cones, droplets, and craters have been generated and their density and size are found to be strongly dependent upon the ambient environment. A quantitative analysis of particulate size and crater depth measured from SEM images showed a strong correlation between plasma parameters and the growth of micro/nanostructures on the modified Si surface.

Laser induced plasma spectroscopy; Spectroscopic plasma diagnostics; Stark broadening; Electron number density; Spectral line shapes and intensities;SINGLY IONIZED SILICON; VISIBLE SIII LINES; II SPECTRAL-LINES; EMISSION-SPECTROSCOPY; OPTICAL-EMISSION; THERMODYNAMIC-EQUILIBRIUM; BROADENING PARAMETERS; SELF-ABSORPTION; HYDROGEN-ATOMS; NEUTRAL ATOM

In this work we summarize, analyze and critically evaluate experimental procedures and results of LIBS electron number density plasma characterization using as examples Stark broadened Si I and Si II line profiles. Selected publications are covering the time period from very beginning of silicon LIBS studies until the end of the year 2015. To perform the analysis of experimental LIBS data, the testing of available semiclassical theoretical Stark broadening parameters for Si I and Si II lines was accomplished first. This is followed by the description of experimental setups, results and details of experimental procedure relevant for the line shape analysis of spectral lines used for plasma characterization. Although most of results and conclusions of this analysis are related to the application of silicon lines for LIBS characterization they are of general importance and may be applied to other elements and different low-temperature plasma sources. The analysis of experimental procedures used for LIBS diagnostics from emission profiles of non-hydrogenic spectral lines is carried out in the following order: the influence of laser ablation and crater formation, spatial and temporal plasma observation, line self-absorption and experimental profile deconvolution, the contribution of ion broadening in comparison with electron impacts contributions to the line width in case of neutral atom line and some other aspects of line shape analysis are considered. The application of Stark shift for LIBS diagnostics is demonstrated and discussed. Finally, the recommendations for an improvement of experimental procedures for LIBS electron number density plasma characterization are offered. (C) 2017 Elsevier B.V. All rights reserved.

INDUCED BREAKDOWN SPECTROSCOPY; SATURATION VAPOR-PRESSURE; INDUCED INCANDESCENCE; SI-I; NANOSECOND; RADIATION; SURFACE

The temporal luminescence behavior of silicon atoms during and after laser-heating of gas-borne silicon nanoparticles was investigated. Silicon nanoparticles were formed in the exhaust stream of a microwave plasma reactor at 100 mbar. The observed prompt atomic line intensities correspond with thermal excitation of the evaporated species. A prompt signal at 251.61 and 288.15 nm originating from the 3s(2)3p(2) -> 3s(2)3p4s transitions showed a lifetime of 16 ns that matches the documented excited-state lifetime for the respective transitions. A secondary delayed signal contribution with similar peak intensities was observed commencing approximately 100-300 ns after the laser pulse and persisting for hundreds of nanoseconds. This signal contribution is attributed to electron impact excitation or recombination after electron impact ionization of the silicon evaporated as a consequence of the laser heating of the plasma leading to non-thermal population of electronically excited silicon. The observations support a nanoparticle evaporation model that can be used to recover nanoparticle sizes from timeresolved LII data. (C) 2017 Optical Society of America

LIBS; spatial confinement; cavity height; signal enhancement;INDUCED ALUMINUM PLASMA; TIME-RESOLVED EMISSION; OPTICAL-EMISSION; MAGNETIC-FIELD; PULSE; ENHANCEMENT; ABLATION; SILICON; LIBS; OPTIMIZATION

In this paper, we present a study on the spatial confinement effect of laser-induced plasma with a cylindrical cavity in laser-induced breakdown spectroscopy(LIBS). The emission intensity with the spatial confinement is dependent on the height of the confinement cavity. It is found that, by selecting the appropriate height of cylindrical cavity, the signal enhancement can be significantly increased. At the cylindrical cavity (diameter = 2mm) with a height of 6 mm, the enhancement ratio has the maximum value (approximately 8.3), and the value of the relative standard deviation (RSD) (7.6%) is at a minimum, the repeatability of LIBS signal is best. The results indicate that the height of confinement cavity is very important for LIBS technique to reduce the limit of detection and improve the precision.

FEMTOSECOND DOUBLE-PULSE; OPTICAL-EMISSION; PLASMA; ENHANCEMENT; SILICON

The spatial confinement effect in laser-induced plasma with different distances between the target surface and focal point is investigated by optical emission spectroscopy. A Nd: YAG laser is used to produce plasma from a silicon sample in air atmosphere. When the appropriate distance is selected, the duration of spectral emission enhancement is much longer, and the enhancement effect is much stronger. The phenomenon is attributed to the aspect ratio of the lateral to axial direction of the plasma plume. The plasma plume of a large aspect ratio will interact with the reflected shockwave in a long range of delay time, leading to high particle density. This provided a better understanding about the effect of the distance between the target surface and focal point, leading to better conditions for spatially confined laser-induced breakdown spectroscopy.

Laser-induced breakdown spectroscopy; femtosecond laser; double-pulse; signal enhancement;INDUCED BREAKDOWN SPECTROSCOPY; AIR; IRRADIATION; ABLATION; DAMAGE; FILMS

In this paper, we present a study on the influence of interpulse delay in laser-induced silicon plasma with femtosecond double-pulse, and two subpulses have different laser energies. The optical emission line collected by a lens is the Si (I) at 390.55 nm. The range of double-pulse interpulse delay is from -150 ps to 150 ps. Unlike the femtosecond double pulses with two same energies, the combination of low + high energies can enhance the spectral emission intensity, while the combination of high + low energies probably reduces the spectral line intensity compared with single-pulse femtosecond laser. The results indicate that the interpulse delay is very important for laser-induced breakdown spectroscopy with femtosecond double-pulse to improve the optical emission intensity.

Laser-induced breakdown spectroscopy; spatial confinement; signal enhancement;INDUCED BREAKDOWN SPECTROSCOPY; PULSE

In laser-induced breakdown spectroscopy (LIBS), a serious of cylindrical cavities are used to spatially confine the plasmas produced from silicon (Si) target by using a Nd: YAG laser in air. Time-resolved spectrum of Si plasma with different diameters of cylindrical spatial cavities is measured. Compared with the plasmas generated without the confined cavity, the spectral emission intensity of plasma generated with cylindrical cavity is enhanced at a certain delay time. As the diameter of cylindrical cavity increases, the spectral emission is enhanced later in time and the enhancement becomes weaker. The result shows that the presence of the confined cavity leads to an increase in the spectral emission of plasma which is attributed to compressed plasma by the reflected shockwave.

In this study, a new approach for Laser-induced Breakdown Spectroscopy (LIBS) with gold nanofilm (NF) deposited on sample surface was carried out by using femtosecond laser. Different materials including metallic sample, semiconductor sample and dielectric sample were tested. Strong emission enhancement was obtained on SiO2, ceramic and Si sample. It was found that different lines in ceramic sample had different enhancement effect, which was probably related to concentration of different substances in ceramic. The enhancement effect was found to weaken with the increase of laser fluence. Enhancement factors of about 14 for SiO2, 5 for ceramic and 3 for Si sample were achieved at laser fluence of 3.3 J/cm(2), which decreased to 9 for SiO2, 3 for ceramic and 1.5 for Si sample at laser fluence of 8.8 J/cm(2). The mechanism of nanofilm-enhanced LIBS (NFELIBS) was localized surface plasmon resonance, which could change the dominant photon ionization process of sample.