Chemometrics and Numerical Methods in LIBS E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 679

Veröffentlichungsjahr: 2022

Beschreibung

Chemometrics and Numerical Methods in LIBS A practical guide to the application of chemometric methods to solve qualitative and quantitative problems in LIBS analyses Chemometrics and Numerical Methods in LIBS, delivers an authoritative and practical exploration of the use of advanced chemometric methods to laser-induced breakdown spectroscopy (LIBS) cases. The book discusses the fundamentals of chemometrics before moving on to solutions that can be applied to data analysis methods. It is a concise guide designed to help readers at all levels of knowledge solve commonly encountered problems in the field. The book includes three sections: LIBS information simplification, LIBS classification, and quantitative analysis by LIBS. Each section of the book is divided into a description of relevant techniques and practical examples of its applications. Contributors to this edited volume are the most recognized international experts on the chemometric techniques relevant to LIBS analysis. Chemometrics and Numerical Methods in LIBS also includes: * A thorough introduction to the simplification of LIBS information, including principal component analysis, independent component analysis, and parallel factor analysis * Comprehensive explorations of classification by LIBS, including spectral angle mapping, linear discriminant analysis, graph clustering, self-organizing maps, and artifical neural networks * Practical discussions of linear methods for quantitative analysis by LIBS, including calibration curves, partial least squares regression, and limit of detection * In-depth examinations of multivariate analysis and non-linear methods, including calibration-free LIBS, the non-linear Kalman filter, artificial and convolutional neural networks for quantification Relevant for researchers and PhD students seeking practical information on the application of advanced statistical methods to the analysis of LIBS spectra, Chemometrics and Numerical Methods in LIBS will also earn a place in the libraries of students taking courses involving LIBS spectro-analytical techniques

Leseprobe

Chemometrics and Numerical Methods in LIBS

Edited by

Vincenzo Palleschi

Applied and Laser Spectroscopy Laboratory, Institute of Chemistry of Organometallic Compounds, Research Area of National Research Council Pisa, Italy

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Library of Congress Cataloging‐in‐Publication Data

Names: Palleschi, V., editor.Title: Chemometrics and numerical methods in LIBS / edited by Vincenzo Palleschi.Description: Hoboken, NJ : Wiley, 2023. | Includes bibliographical references and index.Identifiers: LCCN 2022030870 (print) | LCCN 2022030871 (ebook) | ISBN 9781119759584 (cloth) | ISBN 9781119759560 (adobe pdf) | ISBN 9781119759577 (epub)Subjects: LCSH: Laser-induced breakdown spectroscopy. | Chemometrics.Classification: LCC QD96.A8 C44 2022 (print) | LCC QD96.A8 (ebook) | DDC 543/.52--dc23/eng20220919LC record available at https://lccn.loc.gov/2022030870LC ebook record available at https://lccn.loc.gov/2022030871

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List of Contributors

Mohamed Abdel‐HarithNational Institute of Laser‐Enhanced ScienceCairo UniversityCairo, Egypt

Zienab Abdel‐SalamNational Institute of Laser‐Enhanced ScienceCairo UniversityCairo, Egypt

Sabrina Messaoud AberkaneIonized Media and Lasers DivisionCenter for Development of Advanced TechnologiesAlgiers, Algeria

Asia BottoApplied and Laser Spectroscopy LaboratoryInstitute of Chemistry of Organometallic CompoundsResearch Area of the National Research CouncilPisa, Italy

Fausto BrediceAtomic Spectroscopy Laboratory Centro de Investigaciones Ópticas (CIOp)La Plata, Argentina

Beatrice CampanellaApplied and Laser Spectroscopy LaboratoryInstitute of Chemistry of Organometallic CompoundsResearch Area of the National Research CouncilPisa, Italy

Jhanis GonzalezApplied Spectra, Inc.West Sacramento, CA, USAandLawrence Berkeley National LaboratoryBerkeley, CA, USA

Zongyu HouState Key Lab of Power SystemsDepartment of Energy and Power EngineeringTsinghua UniversityBeijing, ChinaandShanxi Research Institute for Clean EnergyTsinghua UniversityTaiyuan, China

Jozef KaiserCEITEC BUTBrno University of TechnologyBrno, Czech RepublicandInstitute of Physical EngineeringBrno University of TechnologyBrno, Czech Republic

Erik KépešBrno University of TechnologyBrno, Czech RepublicandInstitute of Physical EngineeringBrno University of TechnologyBrno, Czech Republic

Stefano LegnaioliApplied and Laser Spectroscopy LaboratoryInstitute of Chemistry of Organometallic CompoundsResearch Area of the National Research CouncilPisa, Italy

Hua LiKey Laboratory of Synthetic and Natural Functional Molecule of the Ministry of EducationCollege of Chemistry and Materials ScienceNorthwest UniversityXi'an, China

Noureddine MelikechiDepartment of Physics and Applied PhysicsKennedy College of SciencesUniversity of Massachusetts Lowell, MA, USA

Stefano PagnottaDepartment of Earth SciencesUniversity of PisaPisa, Italy

Vincenzo PalleschiApplied and Laser Spectroscopy LaboratoryInstitute of Chemistry of Organometallic CompoundsResearch Area of National Research CouncilPisa, Italy

Francesco PoggialiniApplied and Laser Spectroscopy LaboratoryInstitute of Chemistry of Organometallic CompoundsResearch Area of the National Research CouncilPisa, Italy

Pavel PořízkaCEITEC BUTBrno University of TechnologyBrno, Czech RepublicandInstitute of Physical EngineeringBrno University of TechnologyBrno, Czech Republic

Simona RaneriApplied and Laser Spectroscopy LaboratoryInstitute of Chemistry of Organometallic CompoundsResearch Area of the National Research CouncilPisa, Italy

Mohamad SabsabiConseil national de recherches CanadaBoucherville, Quebec, Canada

Emanuele SalernoInstitute of Information Science and TechnologiesNational Research Council of ItalyPisa, Italy

Weiran SongState Key Lab of Power SystemsDepartment of Energy and Power EngineeringTsinghua UniversityBeijing, ChinaandShanxi Research Institute for Clean EnergyTsinghua UniversityTaiyuan, China

Anna TonazziniInstitute of Information Science and TechnologiesNational Research Council of ItalyPisa, Italy

Ivan UrbinaAtomic Spectroscopy Laboratory, Centro de Investigaciones Ópticas (CIOp)La Plata, Argentina

Jakub VrábelCEITEC BUTBrno University of TechnologyBrno, Czech RepublicandInstitute of Physical EngineeringBrno University of TechnologyBrno, Czech Republic

Zhe WangState Key Lab of Power SystemsDepartment of Energy and Power EngineeringTsinghua UniversityBeijing, ChinaandShanxi Research Institute for Clean EnergyTsinghua UniversityTaiyuan, China

Kenza YahiaouiIonized Media and Lasers DivisionCenter for Development of Advanced TechnologiesAlgiers, Algeria

Tianlong ZhangKey Laboratory of Synthetic and Natural Functional Molecule of the Ministry of EducationCollege of Chemistry and Materials ScienceNorthwest UniversityXi'an, China

Preface

The laser‐induced breakdown spectroscopy (LIBS) technique was born with this name in 1981, when Loree and Radziemski for the first time published two companion papers proposing a new laser spectroscopy technique for material analysis, capable of operating at a distance, without physical contact, on gaseous, liquid, and solid samples.

Although the use of a laser for spectrochemical analysis of materials was already proposed twenty years earlier, immediately after the invention of the laser itself, and the spectral analysis of the optical emission of high temperature samples, by arks, sparks, or flames, predates the laser of a couple of centuries, at least, the 1981 papers of Loree and Radziemski presented all the characteristics of “modern” LIBS, where the laser is used for ablating a tiny amount of matter from the solid samples and, at the same time, for bringing it at a temperature high enough to observe a strong atomic emission.

The first papers on LIBS also evidenced the highly dynamic nature of the laser‐induced plasmas, which is probably the most limiting features of LIBS with respect to other similar spectro‐analytical techniques, such as inductively coupled plasma‐optical emission spectroscopy (ICP‐OES), where the optical signal is stable over a long time. In the following years, it become evident in the analytical applications of the technique the need for acquiring many LIBS spectra, for sampling a significant amount of matter from the sample and reducing the effects produced by random or systematic variations of measurement parameters such as the laser energy, for example.

At the same time, the LIBS community realized that the high spatial resolution of LIBS (in principle only limited by optical diffraction and, hence, of the order of the laser wavelength), coupled to the intrinsic speed of the technique, would have made LIBS the techniques of election for the elemental analysis and mapping of complex inhomogeneous materials, as geological or biological samples.

In both cases, either for doing significant quantitative analysis on homogeneous samples or for mapping inhomogeneous materials, the LIBS applications often lead to the accumulation of a huge quantity of spectral information, which needs to be quickly processed, with a speed adequate to the short times needed for its accumulation.

One of the many revolutions in LIBS research that have characterized the explosion of new applications of the technique at the end of the last century has been the introduction in the analysis of the LIBS data of the same statistical methods that were developed in that times in Information Science for the treatment of what would have been later defined as “Big Data.”

After many years of wide applications in LIBS of these methods, which in analytical chemistry go under the general denomination of chemometrics, it is emblematic the absence of a reference text describing the principles of chemometrics for LIBS, in relation to the problems of simplifying the spectral information, using it for classifying large numbers of samples or obtaining quantitative information about their elemental compositions.

There are many books on chemometrics, discussing methods that are also currently applied on LIBS, but none of them is specifically devoted to LIBS. There are, on the other hand, several review papers or book chapters on this topic, but they cannot be considered as exhaustive.

This book aims to fill the present gap between the diffuse use of chemometric methods in LIBS and the knowledge of the concepts at the basis of these methods. This knowledge should not imply a detailed understanding of the mathematical algorithms underlying the many different statistical methods used for the analysis of the LIBS spectra, but rather the awareness of their potential (and limits) in the typical applications. There is still some confusion on the proper application of chemometric methods in LIBS and, most of all, on the procedures that must be applied for the validation of the results obtained. The popular representation of these methods as “black boxes”, based on mechanisms as inscrutable as the ones regulating human intelligence, must not make us forget that we are just dealing with computer algorithms, whose predictions can (and must) be verified against our chemical and physical knowledge of the system under study.

The LIBS community is constantly growing, and the generation of old school spectroscopists is being quickly replaced by a new generation of young researchers that are, by nature, extremely attracted by the potential great advantages given by chemometrics in LIBS analysis. This book is thus mainly aimed to young researchers working in public or private institutions and Ph.D. students who need a clear and practical information on the application of advanced statistical methods for the analysis of LIBS spectra. This practical approach would be useful also to more experienced researchers, wanting to stay updated on the most recent development in LIBS spectral analysis, as well as to the specialists, since the number of chemometric techniques is so large that no one can honestly say of mastering all the aspects and all the methods at the same level. Lecturers of chemistry, physics, and engineering courses at university could propose the book among the teaching material in courses on spectro‐analytical techniques involving LIBS.

Introduction and Brief Summary of the LIBS Development

The reader can find the concept of laser‐induced breakdown spectroscopy (LIBS) described in almost any LIBS paper. Everyone knows that when we focus a laser beam on a sample, the irradiation in the focal volume leads to local heating of the material. When the irradiance of the laser pulse exceeds the threshold of material ablation (>MW/cm2), there is vaporization, and a hot ionized gas (called a plasma) is formed. In this plasma, atoms and ions are in excited states that emit light by radiative decay. Quantitative and qualitative analyses can be carried out by collecting and spectrally analyzing the plasma light and monitoring the spectral line emission positions and intensities. The technique based on that approach is called LIBS.

The LIBS technique is a form of atomic emission spectroscopy of plasma generated by a laser focused on the material to be analyzed. It is similar to other optical emission spectroscopy techniques based on plasmas, such as spark ablation, glow discharge, inductively coupled plasma, or arc plasma techniques. However, these techniques use an adjacent physical device (electrodes or a coil) to produce the plasma, whereas LIBS uses the laser‐generated plasma as the hot vaporization, atomization, and excitation source. This gives LIBS the advantage that it can interrogate samples at a distance and analyze the material without contact, independent of the nature of the sample, thus making it suitable for in‐the‐field and real‐time analysis of any type of material, whether in the solid, liquid, slurry, or gas phase. The capabilities of LIBS to effectively carry out fast, in situ, real‐time, and remote spectrochemical analysis with minimal sample preparation, and its potential applications to detect traces of a wide variety of materials, make it an extremely versatile analytical technique. These attributes of LIBS attracted the interest of spectroscopists, analytical chemists, and physicists since the invention of the laser in the 1960s. Indeed, the first work on “LIBS” appeared in 1962. Since then, according to the Scopus database, more than 14 000 papers have been published in the field of LIBS, covering fundamentals, instrumentation, and applications. Figure 1 reveals the significant increase in the annual number of LIBS papers in recent decades, from a few in the 1960s to an annual rate of more than 900 today. Moreover, the field is still growing.

Figure 1 LIBS papers evolution according to Scopus database using specific key words.

When we look at the development of the technique, we need to consider that the LIBS plasma is quite simple and yet complicated at the same time. You need a laser as a source of energy to generate the plasma. The plasma formed depends on the characteristics of the laser (energy, pulse duration, focusing condition, wavelength, and beam quality), on the characteristics of the sample (thermal conduction, melting and vaporization temperature, and so on), and on the ambient atmosphere (pressure, composition, and thermal conduction) where it is created. To extract the information from the light emitted, you need a spectrometer to diffract the light and a detector to convert photons to an electrical signal you can work with. It involves several fields of science, such as laser–matter interaction, plasma physics, atomic physics, plasma chemistry, spectroscopy, electro‐optics, and signal processing. The LIBS plasma is transient (it is space‐ and time‐dependent), unlike an inductively coupled plasma, arc plasma, or glow discharge plasma, which are all stationary. This characteristic dictates some restrictions on the ability to transfer tools used with other emission spectroscopy techniques to LIBS. Therefore, the development of LIBS over the years has been closely tied to the development of enabling tools (such as pulsed lasers, detectors, and spectrometers) and ongoing improvements in their performance.

We can distinguish four periods in the development and use of LIBS as technique over the last five decades. During the first period, prior to the 1990s, the plasma was generated by inadequate lasers, and the emission of the plasma was observed mostly time‐ and space‐integrated, with the limited use of single channel photomultipliers (PMT) as detectors for time‐resolved spectroscopy, so only limited analytical quantification was achievable.

During the second period, from 1990 to 2000, the arrival of the intensified charge‐coupled device (ICCD) detector after the Cold War made it possible to observe time‐resolved emission for several lines simultaneously in a given spectral window, rather than only one line as allowed by the single channel photomultiplier tube (PMT). This ability attracted some research groups to develop the understanding of the LIBS plasma and how it can be used for spectrochemistry. This development provided new capabilities for LIBS at the end of the 1990s and beginning of the 2000s, which allowed LIBS to address new emerging applications. In addition, the echelle spectrometer coupled with an intensified charge‐coupled device (ICCD) camera allowed time‐resolved broadband spectra and opened new ways to extract more information from the LIBS plasma. This capability was strengthened by the arrival of the Sony linear CCD array chip, which enabled the use of a low‐cost gated CCD camera. The combination of a gated CCD with low cost compact Czerny Turner spectrometers enabled a growth in the number of laboratories working on LIBS along with newcomers, and an increase of new applications that became feasible with the new capabilities. More importantly, it encouraged some LIBS spin-off companies to enter the market.

In the third period, from 2000 to 2010, the LIBS reached a milestone with the first conference devoted to LIBS organized in Pisa in 2000 by Vincenzo Palleschi's group. Since then, the series of LIBS International conferences has been organized every two years, alternating with the Euro‐Mediterranean symposium conference (EMSLIBS), which was started in Cairo by Mohamed Abdel Harith's group in 2001. A similar LIBS symposium began in North America in 2007 and was organized by Jagdish Singh and Andrzej Miziolek. During that period, LIBS found its way across a variety of applications and disciplines in geology, metallurgy, planetary science, defense, food, environment, industry, mining, biology, automotive, materials science, aerospace, forensics, pharmaceuticals, security, and more. In addition, more companies entered the market to commercialize LIBS systems.

In the last 10 years, the miniaturization of LIBS equipment has opened new opportunities to perform real‐time measurements and respond to emerging needs under conditions in which other spectroscopic techniques cannot be applied. In addition, the progress of laser technologies, such as the diode pumped laser and the fiber laser, with the improvement of the beam quality, led to better conditions for plasma generation and better analytical performance. Furthermore, the high repetition rate and the low cost of ownership of these devices have met the requirements of acceptance for several industrial applications in terms of speed of analysis and cost. Big players entered the market and now offer handheld LIBS systems. Nowadays, as an example, the operating lifetime of a fiber laser is around 100 000 hours, or 11 years, of 24/7 use without any consumables, which is better than the TV in our houses. We have seen some growth as well in R&D reflected by several regional symposium that has been organized in Asia (ASLIBS) and Latin America (LASLIBS).

To summarize, during the last three decades, extensive research has been carried out on the influence of the parameters affecting the analytical signal, to improve LIBS performance. Meanwhile, dynamic technological development in the field of solid‐state lasers, electro‐optical detectors, and signal processing was successfully harnessed for LIBS. The analytical performance of LIBS for a multielement analysis now achieves a level that is equal to, or even better than in some cases, that of classical methods. LIBS is currently considered one of the most active research areas in the field of analytical spectroscopy.

After the brief history and the introduction, this the first part of this book provides a brief explanation of the physics involved in plasma generation and the features of this plasma in LIBS (Chapter 1), then followed by a description of the basic components (Chapter 2), which compose a LIBS instrumentation. These devices are described associating their features with the properties of the laser‐induced plasma. Finally, some key LIBS applications is described in Chapter 3.

This part will introduce the reader to the basic of LIBS, its fundamentals, instrumentation, and applications. It is not intended to be exhaustive survey of LIBS literature nor the state of art of the technique. It will bring generally to the reader a brief overview for the necessary ingredients needed to use the LIBS technique as analytical method for a given application and help understanding how to correlate spectra to composition and the factors affecting that correlation. It will provide a brief explanation of the physics involved in plasma generation and the features of this plasma in LIBS, followed by a description of the basic devices, which compose a LIBS instrumentation. These components will be described associating their features with the properties of the laser‐induced plasma. Moreover, different kinds of parameters that affect the plasmas and its use for spectrochemistry will be highlighted in order to help the users understand different analysis algorithms and chemometrics method that go beyond the “spectral signatures” obtained with the technique. Finally, some key LIBS applications will be described, and the main research challenges that this approach face at the moment will be discussed.

Part IIntroduction to LIBS

1LIBS Fundamentals

Mohamad Sabsabi

Conseil national de recherches Canada, Boucherville, Quebec, Canada

From laser ablation to nuclear fusion, from welding and drilling to surface treatment, and from laser deposition to selective chemical reactions in nanotechnologies, laser plasma physics has generated an enormous fallout of applications. By measuring the emission spectrum from the laser‐induced plasma, qualitative and quantitative information about the sample’s chemical composition can be obtained. Laser ablation means using laser light energy to remove a portion of a sample by melting, fusion, sublimation, ionization, erosion, and/or explosion. Laser ablation results in the formation of a gaseous vapor, luminous plasma, and in the production of fine particles. This is the base of laser‐induced breakdown spectroscopy (LIBS) technique. LIBS is only a tiny fraction of all the application domains in which these physical mechanisms have found useful applications. Furthermore, the laser ablation process (a term that includes the processes of evaporation, ejection of atoms, ions, molecular species, and fragments; generation of shock waves; plasma initiation and expansion; plasma–solid interactions; etc.) influences the amount and composition of the ablated mass and must be understood and controlled in order to achieve accurate and sensitive quantitative analysis. The LIBS technique is at a crossroad of several fields of science, such as laser–matter interaction, plasma physics, atomic physics, plasma chemistry, spectroscopy, electro‐optics, and signal processing. Readers with no substantial prior knowledge on this subject are invited to thoroughly read the review of Hahn and Omenetto [1–2] as well as the references of LIBS books and review papers contained herein. Also, expert readers can find more detailed information on those topics in the books and review paper contained herein [1–25]. This part of Chapter 1 is devoted to LIBS fundamentals. It will introduce the reader to the basics of LIBS plasmas and their fundamentals in order to understand their use for analytical chemistry. It will bring generally to the reader a brief overview of the necessary ingredients needed to use the LIBS technique as analytical method for a given application. We will discuss the fundamental principles of the laser–matter interaction, plasmas physics, atomic physics and optical emission spectroscopy involved in the LIBS technique and relevant for its use for spectrochemical analysis. By using optical means, the spectroscopist tries to correlate the LIBS signal to the chemical composition of the sample they would like to analyze. The spectral emission intensity in the plasma depends not only on the concentration of the element in the sample but it is also affected by the properties of the plasma itself. The latest depends on factors such as the characteristics of the laser excitation source (energy, pulse duration, focusing condition, wavelength, and beam quality), the sample characteristics (thermal conductivity, melting and evaporation temperature, density, and chemical composition), and the surrounding gas (nature, composition, thermal conduction, and ambient atmosphere). Furthermore, the laser ablation process (which includes the processes of evaporation, ejection of atoms, ions, molecular species, and fragments; generation of shock waves; plasma initiation and expansion; plasma–solid interactions; etc.) influences the amount and composition of the ablated mass and must be understood and controlled in order to achieve qualitative or accurate and sensitive quantitative analysis. The complexity of the phenomena involved can be assessed by resorting to a simple derivation of the dependence of the LIBS signal upon the various processes leading from the (solid) sample to the measured signal photons emitted from the (gas phase) atoms and ions excited in the plasma volume. The fundamental parameters governing the overall process can then be explicitly written (see Chapter 3 by Palleschi and Sabsabi Reference [4]):

(1.1)

It can be concluded that the signal is influenced by three interrelated functions, describing the initial interaction between the sample and the laser, finteraction (leading to ablation/vaporization of solid material); the excitation/ionization mechanism leading to atomic (ionic) emission, fexcitation; and the characterization of the radiation environment, fdetection (thin or thick plasmas), while Amn is the transition probability of the transition chosen (photons/s). The finteraction and the fexcitation will be developed in the following part of Chapter 1, while the fdetection will be detailed in Chapter 2.

In summary, the quality of correlation between the LIBS analytical signal and the element concentration in the sample depends on the features of the laser‐induced plasmas and their generation conditions. In addition, the plasmas’ generation conditions should be reproducible and controlled in order to be useful for spectrochemistry.

1.1 Interaction of Laser Beam with Matter

The interaction of high‐power laser light with a target material has been an active topic of research not only in plasma physics but also in the field of material science, chemical physics, and particularly in analytical chemistry [3–8]. The high intensity laser beam focused on a target (solid, liquid, or gas) may dissociate, excite, and/or ionize the constituent atomic species of the solid and produces plasma, which expands either in the vacuum or in the ambient gas depending on the experimental conditions. As a result of laser–matter interaction and depending on the laser characteristics (in particular its irradiance), various processes may occur such as ablation of material (the processes of evaporation, ejection of atoms, ions, molecular species and fragments; generation of shock waves; plasma initiation and expansion; plasma–solid interactions; etc.) that influences the amount and composition of the ablated mass, high‐energy particle emission, generation of various parametric instabilities, as well as emission of radiation ranging from the infrared (IR) to X‐rays. By measuring the emission spectrum from the laser‐induced plasma, qualitative and quantitative information about the sample’s chemical composition can be obtained. This is the base of the LIBS technique. These processes have many applications, but we are mainly interested in the one that is related to the study of optical emission from the plasma. For other applications of laser‐produced plasma, the readers can find detailed information in a series of books [26–29]. Laser ablation is the first step in the LIBS process, and its influence will be reflected in the “figures of merit,” temporal and spatial resolution, sensitivity, precision, and accuracy. The influence of laser ablation on LIBS has been studied extensively in several books, and we refer the reader particularly to Chapter 3 by Russo et al. in the book edited by Singh and Thakur [3].

1.2 Basics of Laser–Matter Interaction

When a high‐power laser pulse is focused onto a material target (solid, liquid, gas, and aerosols), the intensity in the focal spot produces rapid local heating and intense evaporation followed by plasma formation. The interaction between a laser beam and a solid is dependent on many characteristics of both the laser and the solid material. It is a complicated process and not fully understood phenomenon, which is still under intensive investigation. Various factors affect ablation of material, which includes the laser pulse width, its wavelength, its spatial and temporal fluctuations, as well as its power fluctuations. The mechanical, physical, and chemical properties of the target material also play an important role in laser‐induced ablation. The phenomena of laser–target interactions have been reviewed by several authors [30, 31], while the description of melting and evaporation at metal surfaces has been reported by Ready [30]. The ablated material compresses the surrounding atmosphere and leads to the formation of a shock wave. During this process, a wide variety of phenomena including rapid local heating, melting, and intense evaporation are involved. Then, the evaporated material expands as a plume above the sample surface. The plasma expands normal to the target surface at a supersonic speed in vacuum or in the ambient gas. The hot expanding plasma interacts with the surrounding gas mainly by two mechanisms: (i) the expansion of high‐pressure plasma compresses the surrounding gas and drives a shock wave and (ii) during this expansion, energy is transferred to ambient gas by the combination of thermal conduction, radiative transfer, and heating by shock wave. The evolution of plasma depends on the intensity of laser, its wavelength, size of focal spot, target vapor composition, ambient gas composition, and its pressure. It has been found that the plasma parameters such as radiative transfer, surface pressure, plasma velocity, and plasma temperature are strongly influenced by the nature of the plasma. Since vaporization and ionization take place during the initial fraction of laser pulse duration, the rest of the laser pulse energy is absorbed in the vapor and expanding plasma plume. This laser absorption in the expanding vapor/plasma generates three different types of waves as a result of different mechanisms of propagation of absorbing front into the cool transparent gas atmosphere. These waves are (i) laser‐supported combustion (LSC) waves, (ii) laser‐supported detonation (LSD) waves, and (iii) laser‐supported radiation (LSR) waves [26, 30]. Each wave is distinguished based on its velocity, pressure, and on the effect of its radial expansion during the subsequent plasma evolution, which is strongly dependent on the intensity of irradiation. See more details for low irradiance [31, 32] and intermediate and high irradiance [33, 34].

At low irradiation, LSC waves are produced, which comprise of a precursor shock that is separated from the absorption zone and the plasma. The shock wave results in an increase in the ambient gas density, pressure, and temperature, whereas the shock edges remain transparent to the laser radiation. The front edge of the expanding plasma and the laser absorption zone propagate into the shocked gas and give rise to LSC wave.

At intermediate irradiance, the precursor shock is sufficiently strong, and the shocked gas is hot enough to begin absorbing the laser radiation without requiring additional heating by energy transport from the plasma. The laser absorption zone follows directly behind the shock wave and moves at the same velocity.

At high irradiance, the plasma is so hot that, prior to the arrival of the shock wave, the ambient gas is heated to temperatures at which laser absorption begins. In the ideal condition, laser absorption is initiated without any density change, and the pressure profile results mainly from the strong local heating of the gas rather than a propagating shock wave.

1.3 Processes in Laser‐Produced Plasma

As discussed in the previous section, the interaction of a high intensity laser light with solid target initially increases the surface temperature of the sample such that material transfer across the surface becomes significant (vaporization). As a result of material vaporization and plasma formation, target erosion appears in the form of craters on the sample surface. The theoretical considerations on plasma production and heating by means of laser beams have been proposed by several authors [32–35]. The initiation of plasma formation over a target surface begins in the hot target vapor. First of all, absorption of laser radiation takes place via electron‐neutral inverse Bremsstrahlung, but when sufficient electrons are generated, the dominant laser absorption mechanism makes a transition to electron‐ion inverse Bremsstrahlung. Photoionization of excited states can also contribute in the case of interactions with short wavelength radiations. The same absorption processes are responsible for the absorption by the ambient gas also. The laser‐produced plasma expands into the vacuum or into the surrounding gas atmosphere, where the free electrons present in the plasma [25–29] modify propagation of laser light. The plasma formed by a high intensity or short time duration laser has a very steep density and temperature gradient in comparison to the plasma formed by the low intensity or long time duration laser. The density gradient in the plasma plays a very important role in the mechanism of light absorption and in the partition of absorbed energy between thermal and nonthermal particle distribution. There are three basic mechanisms through which intense laser light may interact with plasma [29]. The first mechanism is an inverse Bremsstrahlung, where electric field of the incident light rattles electrons, which then lose this energy in collision with ions. This mechanism is important with shallow density gradients in the plasma. The parametric processes also take part most efficiently, when the density gradient is shallow. There are three wave parametric interaction processes in which intense laser field drives one or more longitudinal plasma waves out of the noise and also parametric decay processes where laser light decays into a high‐frequency electron acoustic wave and a low‐frequency ion acoustic wave conserving energy and momentum. Another important short‐pulse laser absorption mechanism is the resonance absorption. With a p‐polarized light obliquely incident on plasma surface, the radial component of electric field resonates with plasma frequency and causes large transfer of energy to electrons near critical density Nc surface. Critical density for a given laser wavelength is

(1.2)

where λ is in micron. Energy absorbed at or below the critical density in plasma is then conducted toward the target surface by various transport processes. The study of energy coupling to the target has many subareas such as laser light absorption, nonlinear interaction, electron energy transport, and ablation of material from the target surface. One of the important processes, in laser–plasma interaction, is emission of radiation from the plasma ranging from visible to hard X‐rays [3], and it is very relevant for the understanding of LIBS. It has been found that X‐rays are emitted from all parts of the absorption, interaction, and transport regime. At densities near and slightly above the critical, nearly 70% of the incident laser energy may be re‐emitted as X‐rays with energy ranging from 50 eV to 1 keV or above depending on the temperature of the plasma. However, as the plasma expands away from the target surface, its density as well as the temperature decreases. As the plasma temperature decreases, the wavelength of emission from the plasma increases, that is, emission shifts from X‐rays to visible region.

1.4 Factors Affecting Laser Ablation and Laser‐Induced Plasma Formation

1.4.1 Influence of Laser Parameters on the Laser‐Induced Plasmas

The laser is the most important variable affecting the characteristics of the plasma since the effects of its parameters are twofold: first, during its interaction with the targeted sample and then with the plasma plume itself. In general, photons are coupled within the available electronic, or vibrational, states in the material depending on its wavelength. During this coupling, the material is heated to a particular temperature depending on the mechanism of interaction of the laser pulse with that, and the onset of ablation (either thermal or photochemical) occurs if the fluence is above a particular threshold. Once the plasma plume is generated, its density may obstruct (“plasma shielding” as explained in the previous section) partially or entirely laser radiation, depending on the laser wavelength and pulse length. Consequently, not the full energy is transferred from the laser pulse to the original material. With all these key aspects affecting the whole chain of possible events occurring during plasma formation, it seems obvious to the reader that different laser parameters may affect the physics of the plasma plume. In order to bring more clarification to the reader, an understandable framework devoted to the influences of laser parameters will be addressed as follows.

1.4.2 Laser Wavelength (λ)

The wavelength influence on LIBS can be explained from two points of view; the laser–material interaction (energy absorption) and the plasma development and properties (plasma–material interaction).

When photon energy is higher than bond energy, photon ionization occurs and nonthermal effects are more important. For this reason, the plasma behavior depends on wavelength in nanosecond LIBS setups. In the same way, the optical penetration is shorter for ultraviolet (UV) lasers, providing higher laser energy per volume unit of material. In general, the shorter the laser wavelength, the higher the ablation rate and the lower the elemental fractionation [36].

The plasma ignition and its properties depend on wavelength. The plasma initiation with nanosecond lasers is provoked by two processes; the first one is inverse Bremsstrahlung by which free electrons gain energy from the laser during collisions among atoms and ions. The second one is photoionization of excited species and excitation of ground atoms with high energies. Laser coupling is better with shorter wavelengths, but at the same time the threshold for plasma formation is higher. This is because inverse Bremsstrahlung is more favorable for IR wavelengths [37].

In contrast, for short wavelengths (between 266 and 157 nm), the photoionization mechanism is more important. For this reason, the shorter the wavelength in this range, the lower the fluence necessary (energy per unit area) to initiate ablation [38]. In addition, when inverse Bremsstrahlung occurs, part of the nanosecond laser beam reheats the plasma. This not only increases the plasma lifetime and intensity but also increases the background at the same time. Longer wavelengths increase inverse Bremsstrahlung plasma shielding but reduce the ablation rate and increase elemental fractionation (elemental fractionation is the redistribution of elements between solid and liquid phases that modifies plasma emission) [39].

1.4.3 Laser Pulse Duration (τ)

Irrespective of their duration, laser pulses usually reach the required conditions for ablation of targets since the rate of energy deposition greatly exceeds the rate of energy redistribution and dissipation, thus resulting in extremely high temperatures in those regions where energy absorption occurs. However, as a consequence of the different mechanisms of energy dissipation in the sample, differences in pulse duration result in fundamental differences of the ablation process. Indeed, interaction of nanosecond (ns) pulses with materials is substantially different from those of femtosecond (fs) pulses since the rate of energy deposition is significantly shorter in this last instance. Thus, for ns pulses, the material undergoes transient changes in the thermodynamic states from solid, through liquid, into a plasma state. Furthermore, the leading edge of the laser pulse creates plasma, and the remaining part of the pulse heats the plasma instead of interacting with the target. In the case of ultrashort laser pulses, at the end of the laser pulse, only a very hot electron gas and a practically undisturbed lattice are found.

If the selected laser is a femtosecond one, nonthermal processes will dominate the ionization. The pulse is too short to induce thermal effects; hence, other effects should ionize the atoms, depending on the kind of sample. The pulse has a huge amount of energy and effects like multiphoton absorption and ionization, tunneling, and avalanche ionization excite the sample. With this amount of energy, the electron–hole created will induce emission of X‐rays, hot electrons, and photoemission. This will create highly charged ions through a process called Coulomb explosion [5]. The absence of thermal effects creates a crater with highly defined edges without melted or deposited materials.

In contrast, nanosecond lasers induce other effects. The electron‐lattice heating time is around 10−12 second, much shorter than the pulse time. This causes thermal effects to dominate the ionization process. Briefly, the laser energy melts and vaporizes the sample, and the temperature increase ionizes the atoms. If the irradiance is high enough, nonthermal effects will be induced too and both will ionize the sample. Between 10−9 and 10−8 seconds, plasma becomes opaque for laser radiation, thus the last part of the laser pulse interacts with plasma surface and will be absorbed or reflected; hence, it will not ionize much more material. This effect is called plasma shielding and is strongly dependent on environmental conditions (surrounding gases or vacuum) and experimental conditions (laser irradiance and wavelength) [40, 41]. This shielding reduces the ablation rate because the radiation does not reach the sample surface. This induces a crater with melted and deposited material around it, but at the same time the plasma is reheated, and the lifetime and size of plasma is higher [40, 41].

1.4.4 Laser Energy (E)

The energy parameters related to laser material interaction are fluence (energy per unit area, J/cm2) and irradiance (energy per unit area and time, W/cm2). Ablation processes (melting, sublimation, erosion, explosion, etc.) have different fluence thresholds [5]. The effect of changes in the laser energy is related to laser wavelength and pulse time. Hence, it is difficult to analyze the energy effect alone. In general, the ablated mass and the ablation rate increase with laser energy. The typical threshold level for gases is around 1011 and 1010 W/cm2 for liquids, solids, and aerosols [3]