Space-resolved Emission Spectroscopy Research Articles

Influence of a Static Magnetic Field on Laser Induced Tungsten Plasma in Air

In this work, laser induced tungsten plasma has been investigated in the absence and presence of 0.6 T static transverse magnetic field at atmospheric pressure in air. The spectroscopic characterization of laser induced tungsten plasma was experimentally studied using space-resolved emission spectroscopy. The atomic emission lines of tungsten showed a significant enhancement in the presence of a magnetic field, while the ionic emission lines of tungsten presented little change. Temporal variation of the optical emission lines of tungsten indicated that the atomic emission time in the presence of a magnetic field was longer than that in the absence of a magnetic field, while no significant changes occurred for the ionic emission time. The spatial resolution of optical emission lines of tungsten demonstrated that the spatial distribution of atoms and ions were separated. The influence of a magnetic field on the spatial distribution of atoms was remarkable, whereas the spatial distribution of ions was little influenced by the magnetic field. The different behaviors between ions and atoms with and without magnetic field in air were related to the various atomic processes especially the electrons and ions recombination process during the plasma expansion and cooling process.

Fs Laser‐Induced Plasmas from Energetic Polymers: Towards Micro‐Laser Plasma Thruster Application

The micro‐laser plasma thruster (μ‐LPT) is a device aimed at orienting and repositioning small satellites above the atmosphere. These devices are based on IR diode lasers utilized for the ablation of polymers and production of thrust for the control of the satellite motion. In this work, different polymer systems, i.e., poly(vinyl chloride) (PVC), glycidyl azide polymer (GAP), and poly(vinyl nitrate) (PVN) with two absorbers, i.e., carbon nanoparticles (C) and an IR‐dye (IR), have been investigated for an application in μ‐LPT. The properties of a plasma produced by the irradiation of GAP + C, GAP + IR, PVC + C, and PVN + C with 100 fs laser pulses has been investigated by time‐resolved plasma imaging and time and space resolved plasma emission spectroscopy.

Characteristics of the ablation plume induced on glasses for analysis purposes with laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) has been demonstrated as an efficient tool for elemental analyses of transparent dielectric materials such as glasses or crystals for more than ten years. The induced plasma is however much less studied compared to that induced on the surface of a metal. The purpose of this work is therefore to characterize the plasma induced on the surface of a glass sample for analytical purpose as a function of the ablation laser wavelength, infrared (IR) or ultraviolet (UV), and the ambient gas, air or argon. The surface damage of the samples was also observed for ablation with IR or UV laser pulse when the sample was a float glass or a frosted one. Optimized ablation fluence was then determined. The morphology of the plasma was observed with time-resolved spectroscopic imaging, while the profiles of the electron density and temperature were extracted from time- and space-resolved emission spectroscopy. The analytical performance of the plasmas was then studied in terms of the signal-to-noise ratio for several emission lines from some minor elements, Al, Fe, contained in glasses, and of the behavior of self-absorption for another minor element, Ca, in the different ablation conditions.

Vacuum ultraviolet assessment of ionization temperature by space-resolved emission spectroscopy

In this article, the electron ionization temperature in plasmas generated by 1064 nm laser pulses in the vacuum ultraviolet spectral range is evaluated as a function of the axial distance from a steel target surface using emission spectroscopy. The temperature was determined using the relative line intensities ratio of C II 90.41 nm and C III 97.7 nm spectral lines, applied to the Saha–Boltzmann equation. Ionization temperatures determined in this way changed from 33 900 K at the target surface to 26 800 K (2.92–2.31 eV) at 4.0 mm away from it. Large differences between the measured excitation and ionization temperatures suggest nonthermal equilibrium conditions between electrons and heavier ionic species. Based upon the results obtained from this and a previous study under the same operating conditions, the validity of the local thermal equilibrium condition in the plasmas investigated is presented and discussed.

Two-dimensional space-resolved emission spectroscopy of laser ablation plasma in water

We developed a method for two-dimensional space-resolved emission spectroscopy of laser-induced plasma in water to investigate the spatial distribution of atomic species involved in the plasma. Using this method, the laser ablation plasma produced on a Cu target in 5 mM NaCl aqueous solution was examined. The emission spectrum varied considerably depending on the detecting position. The temperature and the atomic density ratio NNa/NCu at various detecting positions were evaluated by fitting emission spectra to a theoretical model based on the Boltzmann distribution. We are successful in observing even a small difference between the distributions of the plasma parameters along the directions vertical and horizontal to the surface. The present approach gives direct information for sound understanding of the behavior of laser ablation plasma produced on a solid surface in water.

Diagnostics of nonuniform plasmas for elemental analysis via laser-induced breakdown spectroscopy: demonstration on carbon-based materials

We investigate the plasma produced by the interaction of ultraviolet nanosecond laser pulses with a carbon fiber composite tile from the inner wall of a fusion reactor. The experiments are carried out in argon at a pressure of 5 × 104 Pa. Fast imaging is used to characterize the plume expansion dynamics and excited plasma species are followed by time- and space-resolved emission spectroscopy. The measurements show that ionized and highly excited plasma species are located in the front of the expanding plume, whereas neutral atoms and lower excited species dominate in the plasma core. By measuring the excitation temperature of metallic ions and neutral atoms, we evidence the existence of a temperature gradient that appears during the early expansion stage and remains up to delays typically used in material analysis via laser-induced breakdown spectroscopy. The knowledge of the spatial distribution of the plasma properties is then used to model the plasma emission spectrum. Assuming two plasma zones in local thermodynamic equilibrium of different temperatures and electron densities, we calculate the spectral radiance and compare it to the spatially integrated spectrum recorded with an Echelle spectrometer. From the best agreement between measured and computed spectra we deduce the elemental concentrations of carbon, hydrogen and metal impurities. The validity of the model is critically discussed and the measurement uncertainties are evaluated. The present approach is foreseen to improve the accuracy of analysis via laser-induced breakdown spectroscopy in many applications, in particular when materials of elements with significantly different ionization potentials are investigated.

Early stage optical emission in nanosecond laser ablation

This paper describes the results of an experimental and theoretical investigation of the physical origin of the visible continuum emission usually observed in the early stages of nanosecond laser ablation of solid materials. It has been suggested, but not confirmed that the continuum is due to radiative recombination and bremsstrahlung emission. Time and space-resolved emission spectroscopy with an absolutely calibrated spectrometer was used to study the spectral emission in laser ablation of zinc in vacuum at 4.1 J cm−2 using a 8 ns, 1064 nm laser pulse. By modelling the spectral emission with a spectral synthesis code, it has been shown that the continuum emission is primarily due to bound-bound transitions between strongly Stark broadened energy levels. Similarly, it can be concluded that the optical absorption is primarily due to bound-bound transitions.

Effect of ablation photon energy on the distribution of molecular species in laser-induced plasma from polymer in air

Distribution of molecular species, C2 and CN, in laser-induced plasma from a polymer target (polyvinyl chloride: PVC) was observed for ablation with 266nm and 355nm pulses. The influence of ablation photon energy on the distribution of molecular species in the plasma has been thus studied. Time- and space-resolved emission spectroscopy was used for the observation which led to the determination of emission intensity profiles of C2 molecule and CN radical for different delays after the impact of the laser pulse on the target. The profiles of related elements, C, N, and excitation temperature in the plasma were further determined to correlate with those of molecular emission intensity. Different behaviors were clearly observed between plasmas induced by pulses with the two different wavelengths chosen to be close each other in the near ultraviolet (UV). A closer analysis shows the photon energy corresponding to 266nm pulse of 4.66eV is larger than bond energies of all the chemical bonds in the studied polymer, while that of 355nm radiation of 3.49eV is smaller than or in the same range of the involved bond energies. Observed different behaviors suggest therefore different ablation mechanisms of polymer by laser radiation, and consequently different channels of molecule formation in the plasma. Observation of the morphology of the craters on the target surface left by laser ablation confirmed further different ablation mechanisms with the two used wavelengths.

Composition and species evolution in a laser-induced LuMnO3 plasma

Pulsed laser deposition is often used to grow multi-elemental thin films from stoichiometric targets. The growth process is influenced by a wide variety of parameters like the target composition, background gases, laser wavelength, laser fluence, or spot size. The changes these parameters induce in the film growth also affect the plasma plume and species formed during laser ablation. For oxide growth O2, and sometimes N2O, is utilized as background gas to achieve the required oxygen composition for the as-grown film. Mass spectrometry combined with time- and space resolved emission spectroscopy is used to investigate the behavior and evolution of plasma species in the plasma plume during the ablation process of LuMnO3 dependent on the background gas.

Ultraviolet versus infrared: Effects of ablation laser wavelength on the expansion of laser-induced plasma into one-atmosphere argon gas

Laser-induced plasma from an aluminum target in one-atmosphere argon background has been investigated with ablation using nanosecond ultraviolet (UV: 355 nm) or infrared (IR: 1064 nm) laser pulses. Time- and space-resolved emission spectroscopy was used as a diagnostics tool to have access to the plasma parameters during its propagation into the background, such as optical emission intensity, electron density, and temperature. The specific feature of nanosecond laser ablation is that the pulse duration is significantly longer than the initiation time of the plasma. Laser-supported absorption wave due to post-ablation absorption of the laser radiation by the vapor plume and the shocked background gas plays a dominant role in the propagation and subsequently the behavior of the plasma. We demonstrate that the difference in absorption rate between UV and IR radiations leads to different propagation behaviors of the plasma produced with these radiations. The consequence is that higher electron density and temperature are observed for UV ablation. While for IR ablation, the plasma is found with lower electron density and temperature in a larger and more homogenous axial profile. The difference is also that for UV ablation, the background gas is principally evacuated by the expansion of the vapor plume as predicted by the standard piston model. While for IR ablation, the background gas is effectively mixed to the ejected vapor at least hundreds of nanoseconds after the initiation of the plasma. Our observations suggest a description by laser-supported combustion wave for the propagation of the plasma produced by UV laser, while that by laser-supported detonation wave for the propagation of the plasma produced by IR laser. Finally, practical consequences of specific expansion behavior for UV or IR ablation are discussed in terms of analytical performance promised by corresponding plasmas for application with laser-induced breakdown spectroscopy.

Temporal and spatial dynamics of laser-induced aluminum plasma in argon background at atmospheric pressure: Interplay with the ambient gas

Laser ablation in background gas implies supplementary complexities with respect to what happens in the vacuum. It is however essential to understand in detail the involved mechanisms for a number of applications requiring the ablation to be performed in an ambient gas at relative high pressure, such as pulsed-laser deposition, or laser-induced breakdown spectroscopy. In this paper, the expansion of a vapor plume ablated from an aluminum target into an argon gas at atmospheric pressure is experimentally investigated using time- and space-resolved emission spectroscopy. The obtained results provide a detailed description of the interplay between the vapor and the gas. The electron density, the temperature and the number densities (and therefore the partial pressures) of aluminum vapor and argon gas have been measured in and surrounding the vapor plume. Our observations show a confinement of the vapor plume by the gas, which is expected as predicted by the usual hydrodynamics models. The result is a plasma core with quite uniform distributions in electron density, temperature and number densities. Such plasma core presents an ideal emission source for spectroscopic applications. It is however evidenced by our observations that a large amount of argon is mixed into the aluminum plume in the plasma core, which invalidates in the experimental conditions that we used, the hydrodynamic “piston” model where the background gas is pushed out by the shock wave surrounding the vapor plume. Instead, other mechanisms such as laser-supported detonation wave should play important roles in the early stage of the expansion of the plasma for the determination of its morphology at longer delays.

Influence of pressure on the Pt nanoparticle growth modes during pulsed laser ablation

Pulsed laser deposition of a platinum target was performed in solution and in a He background gas atmosphere at pressures ranging from 10−5 to 11 Torr. The influence of the plasma dynamics on the structural properties of the nanostructured Pt films was investigated by time-of-flight and space-resolved emission spectroscopy (velocity measurements). It is shown that two different growth modes exist. In the first, formation of nanoparticle is occurring in the surrounding media (gas or solution), while in the second one, diffusion and reorganization of atomic species at the substrate surface is favored. In a gaseous environment, the transition between both modes is occurring at He pressure of ∼0.5 Torr, which corresponds to a velocity of ∼5.8×103 m s−1.

Ultra-fast laser ablation and deposition of TiO2

In this work we report on the properties of the ablation plume and the characteristics of the films produced by ultra-fast pulsed laser deposition (PLD) of TiO2 in vacuum. Ablation was induced by using pulses with a duration of ≈300 fs at 527 nm. We discuss both the composition and the expansion dynamics of the TiO2 plasma plume, measured by exploiting time- and space-resolved emission spectroscopy and gated imaging. The properties of the TiO2 nanoparticles and nanoparticle-assembled films were characterized using different techniques, i.e. environmental scanning electron microscopy (ESEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). It is suggested that most of the material decomposes in the form of nanoparticles.

Space- and time-resolved optical diagnosis for the study of laser ablation plasma dynamics

The dynamics of transient plasmas generated by high-fluence nanosecond laser ablation has been investigated by means of optical methods (time- and space-resolved emission spectroscopy and fast ICCD imaging). Systematic measurements have been carried out on plasma produced in vacuum (10−8Torr residual pressure) by Nd:YAG laser (10ns, 532nm) irradiation of Aluminum targets. Al neutral atoms and different charge state ions have been monitored through the evolution of corresponding spectral lines. The study evidenced the presence of two different groups of particles, tentatively related to two distinct ejection mechanisms. This behavior has been confirmed by the fast ICCD (20ns gate) recording of the total optical emission of the plume. The application of the relative line intensity method to the study of the excitation temperature axial profile is equally discussed.

Space-resolved soft X-ray emission from laser produced lithium plasma

Space-resolved emission spectroscopy was used to study the evolution of 13.5 nm line intensity and electron temperature of the plasma generated by laser ablation of lithium target. Two emitting regions were observed, their intensities depending on laser fluency. Plasma image is discussed in the frame of a Gaussian model of particle expansion.

Polymers as fuel for laser-based microthrusters: An investigation of thrust, material, plasma and shockwave properties

The micro-laser plasma thruster (μ-LPT) is a micropropulsion device, designed for steering and propelling of small satellites (1–10 kg). A laser is focused onto a polymer layer on a substrate to form a plasma, which produces the thrust that is used to control the satellite motion. Three different polymers were tested to understand the influence of their specific properties on the thrust performance: poly(vinyl chloride) (PVC) as a low-energetic material, a glycidyl azide polymer (GAP), and poly(vinyl nitrate) (PVN) as high-energetic polymers. Different absorbers (carbon nanoparticles or an IR dye) were added to the polymer to achieve absorption at the irradiation wavelength (1064 nm). The influence of the material and dopant properties on the decomposition characteristics and the energy release were investigated by thrust measurements and ns-shadowgraphy. Mass spectrometry and time- and space-resolved plasma emission spectroscopy in air and vacuum were used to analyze the degree of fragmentation as function of the material properties. The kinetic energies of selected fragments were calculated from the spectra. GAP + C showed the best performance in all measurements at high fluences, while at low fluences PVN + C revealed the best performance.

Electrode microwave discharge in nitrogen: Structure and gas temperature

Electrode microwave discharges in nitrogen at pressures of 1–16 Torr and input microwave powers of 30–180 W have been studied by space-resolved emission spectroscopy. It is shown that the discharge is highly nonuniform. The relative intensities of the first and second positive nitrogen bands, as well as of the first negative band of nitrogen ions, are found to vary significantly throughout a discharge because, in different discharge regions, emitting particles are excited by different mechanisms. The gas temperature was determined by the method of the unresolved rotational structure of different sequences of the emission spectra of the second positive system of nitrogen.

Spatial characterization of laser-induced plasmas: distributions of neutral atom and ion densities

Spatial distributions of temperature and relative number densities of Fe neutral atoms and ions in a laser-induced plasma have been measured by time- and space-resolved emission spectroscopy. The deconvolution of the intensity spectra has been carried out to obtain the local emissivity of neutral atom and ion lines as a function of the axial (z) and radial (r) coordinates. The plasma was generated with a Fe-Ni-Al alloy in air at atmospheric pressure using a Nd:YAG laser. The distributions present a dark region with negligible neutral atom and ion emissivities and densities, situated close to the sample surface at axial distances z≤1 mm. In the emitting region, the temperature has its maximum at the plasma axis (16000 K) and shows a monotonous decrease towards the border (6000 K). The neutral atoms and ions occupy regions corresponding to a great extent to different radial positions. At an axial distance z=1.8 mm, at which the maximum emissivities are produced, the maximum of the ion density is at r≅0.6 mm, whereas the neutral atom density maximum is at r≅1.3 mm.

Spatial characterization of laser induced plasmas obtained in air and argon with different laser focusing distances

Spatial distributions of spectral line emissivities and plasma characteristics (temperature, electron density and neutral atom relative number density) in laser-induced plasmas have been measured for varying laser focusing distances. Time- and space-resolved emission spectroscopy, combined with a deconvolution procedure of the spectra, has been used to obtain the radial profiles of emissivity starting from the lateral profiles of intensity. The distributions have been obtained for the time window 5–6 μs after the laser pulse. The plasmas have been generated in air and argon at atmospheric pressure by focusing a Nd:YAG laser onto an iron sample. The distributions obtained in both ambient gases have similar features, showing a notable variation in shape and magnitude values with the focusing distance. In air, the Fe emissivity and atom density show maximum values when the focal plane of the lens (128-mm focal length) is placed 10 mm and 12 mm, respectively, below the sample surface, whereas the higher temperature is obtained when the focus is 5 mm below the surface and decreases for deeper focusing positions. This behavior is explained by the plasma shielding effect produced at high laser irradiances. For the plasmas generated in argon, the emissivities and atom densities of Fe, coming from the sample and Ar coming from the ambient gas, are obtained. The results show that, at moderate and high irradiances, the distributions of Fe and Ar atom densities are separated to a large extent due to the existence of a region of minimum Ar atom density, which is interpreted to result from the displacement of the ambient gas by the shockwave propagating away from the sample surface.

Spectroscopic measurement of plume emission from femtosecond laser ablation

We present a time and space resolved emission spectroscopy technique that enables a full characterisation in three dimensions density of emitting species generated by femtosecond pulsed laser ablation. Applied to Al neutral atoms, it reveals a population that, at the lowest fluences used, can be roughly described by a half range Maxwell–Boltzmann function. This simple configuration evolves towards a more complex population as the energy density increases. Specificities connected to the measured emitted spectral rays are evidenced and taken into account.