Spatio-temporal evaluation of Zr plasma parameters in a single-beam-splitting double-pulse laser-induced plasma

The plasma shielding effect is one of the major weaknesses of laser-induced breakdown spectroscopy(LIBS) as it causes non-linearity in signal strength. Although LIBS is typically carried out in constant laser energy, this non-linearity causes a reduction in sensitivity. In this work, we systematically examine laser-induced plasma, formed by two different excitation source modes, i.e. single pulse(SP)-excitation and single-beam-splitting double-pulse(SBSDP)-excitation over Zr-2.5% Nb alloy. The two most important plasma parameters influencing the emission line intensity, plasma temperature(T_e) and electron density(N_e) were studied and compared for both modes of laser excitation. Comparison of the results conclusively demonstrates that due to the splitting of the laser energy in the SBS-DP mode, the plasma shielding effect is significantly reduced. The reduced plasma shielding translates to an increased laser–sample coupling under SBS-DP mode. Temporal imaging of the total intensity of the laserinduced plasma in both excitation modes was also studied. The study shows how the plasma shielding effect can be reduced to improve the analytical quality of the LIBS methodology.     

Influence of target temperature on femtosecond laser-ablated brass plasma spectroscopy

Spectral intensity,electron temperature and density of laser-induced plasma(LIP) are important parameters for affecting sensitivity of laser-induced breakdown spectroscopy(LIBS).Increasing target temperature is an easy and feasible method to improve the sensitivity.In this paper,a brass target in a temperature range from 25℃ to 200℃ was ablated to generate the LIP using femtosecond pulse.Time-resolved spectral emission of the femtosecond LIBS was measured under different target temperatures.The results showed that,compared with the experimental condition of 25℃,the spectral intensity of the femtosecond LIP was enhanced with more temperature target.In addition,the electron temperature and density were calculated by Boltzmann equation and Stark broadening,indicating that the changes in the electron temperature and density of femtosecond LIP with the increase of the target temperature were different from each other.By increasing the target temperature,the electron temperature increased while the electron density decreased.Therefore,in femtosecond LIBS,a hightemperature and low-density plasma with high emission can be generated by increasing the target temperature.The increase in the target temperature can improve the resolution and sensitivity of femtosecond LIBS.     

Influence of target temperature on AlO emission of femtosecond laser-induced Al plasmas

The influence of the target temperature on the molecular emission of femtosecond laser-induced breakdown spectroscopy(LIBS) was investigated experimentally. An Al target was ablated to produce laser-induced plasma. The Al target was uniformly heated to a maximum of 250℃. The measured molecular emission was AlO(△ν=0) from the femtosecond LIBS of the Al target.The measurements indicated that the molecular emission of AlO increased as the temperature of the Al target increased. In addition, a two-temperature model was used to simulate the evolution of the electron and lattice temperature of the Al target with different initial temperatures. The simulated results showed that the electron and lattice temperatures of Al irradiated by the femtosecond laser increased as the initial temperature of the Al target increased; also, the simulated ablated depth increased. Therefore, an increase in the initial Al target temperature resulted in an enhancement in the spectral signal of AlO from the femtosecond LIBS of Al,which was directly related to the increase in the size of the ablated crater. The study suggested that increasing the temperature of the target improves the intensity of molecular emission in femtosecond LIBS.     

Optical Emission Spectroscopy Analysis of the Early Phase During Femtosecond Laser-Induced Air Breakdown

Single-pulse and double-pulse optical emission spectroscopy （OES） analyses were carried out in air by using ultrashort laser pulses at atmospheric pressure. The aim of this work is to use spectroscopic methods to analyze the early phase of laser-induced plasma after the femtosecond laser pulse. The temporal behavior of emission spectra of air plasma has been characterized. In comparison with the single-pulse scheme, the plasma emission obtained in the double-pulse scheme presents a more intense continuum along with several additional ionic lines. As only one line is available in the single-pulse scheme, the plasma temperature measurements were performed using only the relative line-to-continuum intensity ratio method, whereas the relative line-to-line intensity ratio method and the relative line-to-continuum intensity ratio method were used simultaneously to estimate the electron temperature in the double-pulse scheme. The results reveal that the temperature values obtained by the two methods in the double-pulse scheme agree. Moreover, this shows that the relative line-to-continuum intensity ratio method is suitable for early phase of laser-induced plasma diagnostics. The electron number density was estimated using the Stark broadening method. In the early phase of laser-induced plasma, the temporal evolution of the electron number density exhibits a power law decrease with delay time.     

Effect of distances between lens and sample surface on laser-induced breakdown spectroscopy with spatial confinement

Spatial confinement can significantly enhance the spectral intensity of laser-induced plasma in air.It is attributed to the compression of plasma plume by the reflected shockwave.In addition,optical emission spectroscopy of laser-induced plasma can also be affected by the distance between lens and sample surface.In order to obtain the optimized spectral intensity,the distance must be considered.In this work,spatially confined laser-induced silicon plasma by using a Nd:YAG nanosecond laser at different distances between lens and sample surface was investigated.The laser energies were 12 mJ,16 mJ,20 mJ,and 24 mJ.All experiments were carried out in an atmospheric environment.The results indicated that the intensity of Si (I) 390.55 nm line firstly rose and then dropped with the increase of lens-to-sample distance.Moreover,the spectral peak intensity with spatial confinement was higher than that without spatial confinement.The enhancement ratio was approximately 2 when laser energy was 24 mJ.     

Time-resolved spectroscopy of femtosecond laser-induced Cu plasma with spark discharge

The combination of spark discharge and laser-induced breakdown spectroscopy (LIBS) is called spark discharge assisted LIBS. It works under laser-plasma triggered spark discharge mode, and shows its ability to enhance spectral emission intensity. This work uses a femtosecond laser as the light source, since femtosecond laser has many advantages in laser-induced plasma compared with nanosecond laser, meanwhile, the study on femtosecond LIBS with spark discharge is rare. Time-resolved spectroscopy of spark discharge assisted femtosecond LIBS was investigated under different discharge voltages and laser energies. The results showed that the spectral intensity was significantly enhanced by using spark discharge compared with LIBS alone. And, the spectral emission intensity using spark discharge assisted LIBS increased with the increase in the laser energy. In addition, at low laser energy, there was an obvious delay on the discharge time compared with high laser energy, and the discharge time with positive voltage was different from that with negative voltage.     

Laser-induced plasma electron number density: Stark broadening method versus the Saha-Boltzmann equation

We report spectroscopic studies on plasma electron number density of laser-induced plasma produced by ns-Nd:YAG laser light pulses on an aluminum sample in air at atmospheric pressure.The effect of different laser energy and the effect of different laser wavelengths were compared.The experimentally observed line profiles of neutral aluminum have been used to extract the excitation temperature using the Boltzmann plot method,whereas the electron number density has been determined from the Stark broadened as well as using the Saha-Boltzmann equation(SBE).Each approach was also carried out by using the Al emission line and Mg emission lines.It was observed that the SBE method generated a little higher electron number density value than the Stark broadening method,but within the experimental uncertainty range.Comparisons of N_e determined by the two methods show the presence of a linear relation which is independent of laser energy or laser wavelength.These results show the applicability of the SBE method for N_e determination,especially when the system does not have any pure emission lines whose electron impact factor is known.Also use of Mg lines gives superior results than Al lines.     

Comparison of emission signals for different polarizations in femtosecond laser-induced breakdown spectroscopy

In this study, a femtosecond laser was focused to ablate brass target and generate plasma emission in air. The influence of lens to sample distance(LTSD) on spectral emission of brass plasma under linearly and circularly polarized pulses with different pulse energies was investigated. The results indicated that the position with the strongest spectral emission moved toward focusing lens with increasing the energy. At the same laser energy, the line emission under circularly polarized pulse was stronger compared with linearly polarized pulse for different LTSDs. Next, electron temperature and density of the plasma were obtained with Cu(Ⅰ) lines,indicating that the electron temperature and density under circularly polarized pulse were higher compared to that under linearly polarized pulse. Therefore, changing the laser polarization is a simple and effective way to improve the spectral emission intensity of femtosecond laserinduced plasma.     

Comparison of sample temperature effect on femtosecond and nanosecond laser-induced breakdown spectroscopy

In this paper, we investigated the emission spectra of plasmas produced from femtosecond and nanosecond laser ablations at different target temperatures in air. A brass was selected as ablated target of the experiment. The results indicated that spectral emission intensity and plasma temperature showed similar trend for femtosecond and nanosecond lasers, and the two parameters were improved by increasing the sample temperature in both cases. Moreover, the temperature of nanosecond laser-excited plasma was higher compared with that of femtosecond laser-excited plasma, and the increase of the plasma temperature in the case of nanosecond laser was more evident. In addition, there was a significant difference in electron density between femtosecond and nanosecond laser-induced plasmas. The electron density for femtosecond laser decreased with increasing the target temperature, while for nanosecond laser, the electron density was almost unchanged at different sample temperatures.     

Image-based plasma morphology determination and LIBS spectra correction in combustion environments

Spectra correction is essential for the quantification of laser-induced breakdown spectroscopy (LIBS) due to the uncertainties in plasma morphology. In this work, we determined the plasma morphology using a charge-coupled device camera and introduced the spectral correction method based on plasma images to a combustion environment. The plasma length, width, volume, and location were extracted from the plasma images. Using a back-scattering setup, the contribution of plasma location fluctuation to the total spectral fluctuation was mitigated. The integral intensity of the plasma image was used as a proxy of the total number density to correct the spectra. Linear relationships were established between the integral intensities of the plasma images and the spectral intensities, under different laser energy levels and gas temperatures. The image-based correction method could significantly reduce the fluctuation of raw spectral intensities when the laser energy was below 240 mJ. Compared with the correction method based on total spectral areas, the proposed method offered significant improvements in the low energy region, which promises to reduce the signal fluctuations in combustion environments while preserving the spatial resolution and mitigating the flow disturbance.     

Influence of target temperature on AIO emission of femtosecond laser-induced AI plasmas

The influence of the target temperature on the molecular emission of femtosecond laser-induced breakdown spectroscopy (LIBS) was investigated experimentally.An Al target was ablated to produce laser-induced plasma.The Al target was uniformly heated to a maximum of 250 ℃.The measured molecular emission was AlO (△υ =0) from the femtosecond LIBS of the Al target.The measurements indicated that the molecular emission of AIO increased as the temperature of the A1 target increased.In addition,a two-temperature model was used to simulate the evolution of the electron and lattice temperature of the Al target with different initial temperatures.The simulated results showed that the electron and lattice temperatures of Al irradiated by the femtosecond laser increased as the initial temperature of the A1 target increased;also,the simulated ablated depth increased.Therefore,an increase in the initial A1 target temperature resulted in an enhancement in the spectral signal of AlO from the femtosecond LIBS of Al,which was directly related to the increase in the size of the ablated crater.The study suggested that increasing the temperature of the target improves the intensity of molecular emission in femtosecond LIBS.     

Simultaneous Determination of Trace Lead and Chromium in Water Using Laser-Induced Breakdown Spectroscopy and Paper Substrate

Toxic metals such as lead and chromium in aqueous solutions have been analyzed simultaneously by laser-induced breakdown spectroscopy(LIBS), in which the ordinary printing paper is used as a liquid absorber which was immerged into Pb(NO3)2and Cr(NO3)3aqueous solution to enrich the heavy metals. This method overcomes the drawbacks of splashing and low sensitivity in ordinary LIBS analysis of water, in which a laser beam is directly focused on a liquid surface. A good signal intensity and reproducibility has been demonstrated. The Pb 405.78 nm and Cr 427.48 nm spectral lines are used as the analytical lines. The variation of line intensity with immersion time was investigated. The calibration curve for quantitative measurement of Pb and Cr in water was established, and the detection limits are 0.033 mg/L and 0.026 mg/L respectively,which is about 2-3 orders of magnitude better than that in the ordinary LIBS analysis of heavy metal in solution.     

Spectral Characteristics of Laser-Induced Graphite Plasma in Ambient Air

An experimental setup of laser-induced graphite plasma was built and the spectral characteristics and properties of graphite plasma were studied. From the temporal behavior of graphite plasma, the duration of CN partials(B~2∑~+→X~2∑~+) emission was two times longer than that of atomic carbon, and all intensities reached the maximum during the early stage from0.2 μs to 0.8 μs. The electron temperature decreased from 11807 K to 8755 K, the vibration temperature decreased from 8973 K to 6472 K, and the rotational temperature decreased from7288 K to 4491 K with the delay time, respectively. The effect of the laser energy was also studied, and it was found that the thresholds and spectral characteristics of CN molecular and C atomic spectroscopy presented great differences. At lower laser energies, the electron excited temperature, the electron density, the vibrational temperature and rotational temperature of CN partials increased rapidly. At higher laser energies, the increasing of electron excited temperature and electron density slow down, and the vibrational temperature and rotational temperature even trend to saturation due to plasma shielding and dissociation of CN molecules. The relationship among the three kinds of temperatures was T_(elec)T_(vib)T_(rot) at the same time. The electron density of the graphite plasma was in the order of 10~(17)cm~(-3) and 10~(18)cm~(-3).     

Effect of lens focusing distance on laser-induced silicon plasmas at different sample temperatures

We investigated the dependence of laser-induced breakdown spectral intensity on the focusing position of a lens at different sample temperatures(room temperature to 300 ℃) in atmosphere.A Q-switched Nd:YAG nanosecond pulsed laser with 1064 nm wavelength and 10 ns pulse width was used to ablate silicon to produce plasma. It was confirmed that the increase in the sample's initial temperature could improve spectral line intensity. In addition, when the distance from the target surface to the focal point increased, the intensity firstly rose, and then dropped.The trend of change with distance was more obvious at higher sample temperatures. By observing the distribution of the normalized ratio of Si atomic spectral line intensity and Si ionic spectral line intensity as functions of distance and temperature, the maximum value of normalized ratio appeared at the longer distance as the initial temperature was higher, while the maximum ratio appeared at the shorter distance as the sample temperature was lower.     

Time-resolved spectroscopy of collinear femtosecond and nanosecond dual-pulse laser-induced Cu plasmas

In this paper,we investigate the time-resolved spectroscopy of collinear femtosecond(fs)and nanosecond(ns)dual-pulse(DP)laser-induced plasmas.A copper target was used as an experimental sample,and the fs laser was considered as the time zero reference point.The inter-pulse delay between fs and ns laser beams was 3 μs.First,we compared the time-resolved peak intensities of Cu(Ⅰ)lines from Cu plasmas induced by fs+ns and ns+fs DP lasers with collinear configuration.The results showed that compared with the ns+fs DP,the fs+ns DP laser-induced Cu plasmas had stronger peak intensities and longer lifetimes.Second,we calculated time-resolved plasma temperatures using the Boltzmann plot with three spectral lines at Cu(Ⅰ)510.55,515.32 and 521.82 nm.In addition,time-resolved electron densities were calculated based on Stark broadening with Cu(Ⅰ)line at 521.82 nm.It was found that compared with ns+fs DP,the plasma temperatures and electron densities of the Cu plasmas induced by fs+ns DP laser were higher.Finally,we observed images of ablation craters under the two experimental conditions and found that the fs+ns DP laser-produced stronger ablation,which corresponded to stronger plasma emission.     

Synchrotron micro-XRF study of metal inclusions distribution and variation in fused silica induced by ultraviolet laser pulses

Metal inclusions play critical roles in laser-induced damage for large fused silica optics. Here, the spatial distribution of sodium, aluminum, iron and copper in as-prepared samples is analyzed by synchrotron based X-ray fluorescence spectrometry microprobe system at the BL15U1 beam line at the Shanghai Synchrotron Radiation Facility. The as-prepared fused silica samples are induced by 355 nm laser pulses with no, or low, or high fluences. The spatial resolution of the obtained elemental maps is up to 50 μm. Analysis of the elemental maps indicates that the distribution of metals has a close association with the laser fluence and pulses. The normalized fluorescence signal attenuation for metal inclusions corresponds to the laser fluence. The decrement of metals depends chiefly upon the fluence other than pulses of the incidence laser, which is most pronounced for iron and least for copper. The decrement is evident for high fluence laser irradiation, while the amount is negligible for low fluence laser irradiation. Among the four metals, iron concentration is suggested as the most destructive factor for optics lifetime, especially under high fluence irradiation. The quasi-periodic feature of elemental distribution is partly ascribed to laser intensity modulation induced by Fresnel diffraction.     

ULTRASOUND INDUCED AND LASER ENHANCED COLD FUSION CHEMISTRY

The standard model of sonoluminescence suggests that the coulomb barrer to deuterium fusion may be overcome by high bubble gas temperatures caused by compression heating if the bubble diameter remains spherical during bubble collapse.However,in the more likely collapse geometry of a pancake shape,the temperature rise in the bubbles is negligible.But the collapsing pancake bubble is fund to significantly increase the frequency of the infrared energy available in the vibrational state of the water molecules at ambient temperature.For a collapse to liquied density,ultraviolet radiation at about 10eV is fund.Although the ultraviolet radiation is of a low intensity,higher intensities may be possible if the bubble collapse is enhanced by visible and infrared lases.Neither hot nor cold fusion is predicted in bubble collapse but the ultraviolet energy at about 10eV developed in the bubble is sufficient to provide the basis for a new field of chemistry called ultrasound induced and laser enhanced cold fusion chemistry.     

Evaluation of gases'molecular abundance ratio by fiber-optic laser-induced breakdown spectroscopy with a metal-assisted method

A metal-assisted method is proposed for the evaluation of gases'molecular abundance ratio in fiber-optic laser-induced breakdown spectroscopy(FO-LIBS).This method can reduce the laser ablation energy and make gas composition identification possible.The principle comes from the collision between the detected gases and the plasma produced by the laser ablation of the metal substrate.The interparticle collision in the plasma plume leads to gas molecules dissociating and sparking,which can be used to determine the gas composition.The quantitative relationship between spectral line intensity and molecular abundance ratio was developed over a large molecular abundance ratio range.The influence of laser ablation energy and substrate material on gas quantitative calibration measurement is also analyzed.The proposed metal-assisted method makes the measurement of gases'molecular abundance ratios possible with an FO-LIBS system.     

Enhancement of optical emission generated from femtosecond double-pulse laser-induced glass plasma at different sample temperatures in air

In double-pulse laser-induced breakdown spectroscopy(DP-LIBS), the collinear femtosecond double-pulse laser configuration is experimentally investigated with different initial sample temperatures using a Ti:sapphire laser. The glass sample is ablated to produce the plasma spectroscopy. During the experiment, the detected spectral lines include two Na(I) lines(589.0 nm and 589.6 nm) and one Ca(I) line at the wavelength of 585.7 nm. The emission lines are measured at room temperature(22 ℃) and three higher initial sample temperatures(T_s?=?100 ℃, 200 ℃, and 250 ℃). The inter-pulse delay time ranges from-250 ps to 250 ps.The inter-pulse delay time and the sample temperature strongly influence the spectral intensity,and the spectral intensity can be significantly enhanced by increasing the sample temperature and selecting the optimized inter-pulse time. For the same inter-pulse time of 0 ps(single-pulse LIBS), the enhancement ratio is approximately 2.5 at T_s?=?200 ℃ compared with that obtained at T_s?=?22 ℃. For the same inter-pulse time of 150 ps, the enhancement ratio can be up to 4 at T_s?=?200 ℃ compared with that obtained at T_s?=?22 ℃. The combined enhancement effects of the different initial sample temperatures and the double-pulse configuration in femtosecond LIBS are much stronger than that of the different initial sample temperatures or the double-pulse configuration only.     

Analysis of Pulverized Coal by Laser-Induced Breakdown Spectroscopy

Laser-induced breakdown spectroscopy （LIBS） has been used to detect atomic species in various enviromnents. The quantitative analysis （C, H, O, N and S） of representative coal samples are being carried out with LIBS, and the effects of particle size are analyzed. A powerful pulse Nd：YAG laser is focused on the coal sample at atmosphere pressure, and the emission spectra from laser-induced plasmas are measured by time-resolved spectroscopy, and the intensity of analyzed spectral lines is obtained through observing the laser plasma with a delay time of 0.4 #s. The experimental results show that the slope of calibration curve is nearly 1 when the concentration of the analyzed element is relatively low, and the slope of curve is nearly 0.5 when the concentration of C is higher than other elements. In addition, using the calibration-free model without self-absorption effect, the results show that the decreasing of particle size leads to an increase of the plasma temperature.