Laser Induced Breakdown Spectroscopy

Submitted to:

Abstract:

Laser-induced breakdown spectroscopy (LIBS) is a laser-based technique that can provide qualitative and quantitative measurements of elements in gas, liquid, and solid samples. It is a technology in which a laser beam is directed at a sample to create a high-temperature micro plasma. . A determination of the background gas on spectral lines and the influence of plasma parameters like temperature and number density are studied. The life-time of the plasma is strongly dependent on the pressure of the background gas. A spectrometer is used to disperse the light emission and detect its intensity at specific wavelengths.

CONTENTS

Introduction
Laser Induced Breakdown Spectroscopy
Laser Matter Interaction
Multi-photon Ionization
Inverse Bremsstrahlung
Cascade Growth
Fundamental Of Plasma Physics
Laser Ablation
Laser Plasma
Interaction Of Plasma With Ambient Environment
Shock Wave
Effect of Ambient Environment

1.3.5.1. Ambient Vacuum

1.3.5.2. Ambient Gas

1.3.5.3. Influence of Pressure

1.3.5.3.1. Low Pressure, <760 Torr

1.3.5.3.2. High Pressure, >760 Torr

2. Literature Survey

Methodology

3.1. Instrumentation of Lib

3.2. Laser System

3.2.1. ND: YAG Laser

3.2.2. Sample Chamber

3.2.3. Optical Fiber and Focusing Lens

3.3. Imaging and Detection

3.3.1. Spectrometer

3.3.2. Photomultiplier tube

3.3.3. ICCD Camera

3.3.4. OOLIBS2500 Plus

References:

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1. INTRODUCTION

1.1 Laser Induced Breakdown Spectroscopy

LIPS (laser plasma spectroscopy) also known as Laser Induced Breakdown Spectroscopy (LIBS), is a relatively new type of atomic emission line Spectroscopy made possible with the advent of the laser in 1961. LIBS was originally Coined by Leon Radziemski and David Cremer’s at LANL (Los Alamos National Laboratory) in New Mexico (USA) in 1981 [1].

Laser Induced Breakdown Spectroscopy (LIBS) is a technology that uses a short laser pulse to create a micro-plasma on the sample surface. LIBS technology is the formation of high-temperature plasma, induced by a short laser pulse. When a short laser pulse (with typical duration from ns to fs), is focused on a portion of matter, a significant amount of energy is transferred to the lattice, which can result in the formation of a plasma of the irradiated material, a phenomenon usually referred as breakdownat the material surface. The breakdown can occur only if the pulse irradiance exceeds a threshold value which depends on the state of aggregation of the material, an irradiance value of ~1 GW/cm2 is generally considered as an appropriate reference value to yield high-temperature and high-electron density plasma from virtually any kind of irradiated solid targets.

When the laser pulse terminates, the plasma starts to cool down. During the plasma cooling process, the electrons of the atoms and ions at the excited electronic states fall down into natural ground states, causing the plasma to emit light with discrete spectral peaks [2].The LIB spectra consist of spectral lines which give information about all the constituting elements as well as elements in trace amount present in that sample [3].

1.2 Laser Matter Interaction

The laser-plasma interaction is the transition of the target material into plasma state due to the presence of the laser electric field [4]. Experimentally, laser-induced ionization was observed shortly after the invention of the laser in the sixties already. With advances in laser technology, however, higher intensities, different wavelengths and shorter pulse durations became available. Depending on the laser parameters used, different ionization processes such a Multi-photon ionization, above-threshold ionization and Barrier suppression ionization. The two mechanisms for electron generation and growth which produce breakdown are called multi-photon ionization and inverse bremsstrahlung [5].

Fig 1: Laser matter interaction

1.2.1 Multi-photon Ionization

The term multi-photon ionization is a generic one. It includes many individual processes that depend on the wavelength, field strength, and the polarization state of the laser pulse. In essence, more than one photon participate in the ionization process, in sequence or simultaneously. Coherence between photons becomes an important factor, not only because this makes it possible to achieve very high intensity levels, but also because certain MPI involve resonant processes. The electronic structure of the material is also very important. MPI processes are also very selective in terms of which atomic or molecular species is ionized[6].

The probability for multi-photon absorption is dependent on the number of photons incident per unit time on the atom or molecule. The lifetime of the system after absorption of the photon is governed by the Heisenberg uncertainty principle

Where is the uncertainty in the energy of the state after absorption of the photon (which is very large for a virtual state, and hence is small)? The absorption of a second photon, then, depends on it arriving within the time so that the system can effectively make a transition to the ``two photon'' absorbed state. Similarly the absorption of the next photon depends upon the third photon arriving before the ``de-excitation'' of the atom or molecule. The probability of the photon absorption is then proportional to, where is the intensity (number of photons per unit time, per unit area) of the incident laser field [7].

Fig2:Multi-photon Ionization

A sufficient number of photons are absorbed by atoms or molecules which results in the ejection of electron from the valence band and transferred to the conduction band cause the ionization of molecules or atoms. It can be defined by the equation

This process is dominant at short wavelengths (ʎ ˂ 1µm) and requires high irradiances [8]. The relation for longer wavelengths,

Shows the low absorption of photons by an atom or molecule which increases the energy of the neutral atoms to its ionization potential. The ionization rate Wm is directly proportional to irradiance Im.

Where

m = number of simultaneously absorbed photons to ionize the gas.

This process generates a few free electrons as receptors of energy through three body collisions with photons and neutrals.

1.2.2 Inverse Bremsstrahlung

Inverse bremsstrahlung (IB), an important process in the laser–matter interaction, involves two different kinds of interaction-the interaction of the electrons with the external laser field and the electron–ion interaction. . In this process, an electron absorbs energy from the laser beam during a collision with a nucleus. From a classical viewpoint, the electron oscillates in the electric field of the laser beam. During a collision with a nucleus, the electron is knocked out of phase with the help of electric field, and the oscillatory energy of the electron is converted to random thermal energy. From a quantum viewpoint, the electron can gain energy only in units of ħω, where ω is the frequency of the laser radiation [9].

Fig3: Inverse Bremsstrahlung

The term bremsstrahlung is a German word stands for, (Brems) slowing down and (strahlung) the radiation. So, the normal bremsstrahlung process can be defined as the high energy electron slowing down upon interaction with the atoms of solids or gas emitting radiation at the same time [10]. However, IB is the inverse of bremsstrahlung in which the electrons attain energy by absorption of photons colliding with the atoms, molecules or ions and releases two free electrons of lower energy. The electron energy can ionize a molecule (M), if it is greater than the ionization potential of neutrals. The IB process can be described by the reaction

Inverse bremsstrahlung process is dominant at longer wavelengths (ʎ ˃ 1µm) and at low irradiances, because at shorter wavelength the possibility for the collision of electrons with neutrals is negligible. IB is avalanche ionization in focal volume [11].

1.2.3 Cascade Growth

The probability of electron-photon neutral collisions increases as the number of ions and electrons increases, results in the multiplication of electrons which is called cascade growth [12]. The interaction of high energy electrons, positrons, and photons with intense laser pulses is shown that electrons and/or positrons undergo a cascade-type process involving multiple emissions of photons. These photons can consequently convert into electron-positron pairs.

Fig4: Cascade Growth

The final distributions of electrons, positrons, and photons are calculated for the case of a high energy e-beam interacting with a counter-streaming, short intense laser pulse. The energy loss of the e-beam, which requires a self-consistent quantum description, plays an important role in this process, as well as provides a clear experimental observable for the transition from the classical to quantum regime of interaction [13]. MPI and IB both processes can play a role in cascadeionization. The influential process depends on laser wavelength, density of the medium and laser irradiance [14].The electron-photon-ion collision increases, as the population of ions increases results further multiplication of electrons. After breakdown, plasma spread out in the surroundings of focal volume.

1.3 Fundamentals of Plasma Physics

1.3.1 Laser Ablation

Laser ablation (LA) is a process in which a laser beam is focused on a sample surface to remove material from the irradiated zone [15]. This process involves sequence of steps, initiated by the laser radiation interacting with the solid target, absorption of energy and localized heating of the surface, and subsequent material evaporation. The properties and composition of the resulting ablation plume may evolve, both as a result of collisions between particles in the plume and through plume-laser radiation interactions. Finally the plume come into contact to the substrate to be coated; incident material may be accommodated, rebound back into the gas phase, or induce surface modification (via sputtering, compaction, sub-implantation, etc.). Such a separation has conceptual appeal but, inevitably, is somewhat over-simplistic.

Fig5: Laser ablation

Furthermore, the laser-target interactions will be sensitively dependent both on the nature and condition of the target material, and on the laser pulse parameters (wavelength, intensity, fluency, pulse duration, etc.). Subsequent laser-plume interactions will also be dependent on the properties of the laser radiation, while the evolution and propagation of the plume will also be sensitive to collisions and thus to the quality of the vacuum under which the ablation is conducted and/or the presence of any background gas. Obviously, the ultimate composition and velocity distribution (or distributions, in the case of a multi-component ablation plume) of the ejected material is likely to be reflected in the detailed characteristics of any deposited film [16]. LA generates bright plasma on the sample's surface. The light emitted from this plasma can be analyzed to determine the presence and concentration levels of elements in the period table.

Laser ablation is very complex, involving many simultaneous processes during and following the laser pulse such a heat transfer, electron-lattice energy exchange, material melting and evaporation, plasma plume formation and expansion, laser energy absorption, etc.

1.3.2 Laser plasma

A numerical model has been developed to describe the laser induced ablation of metal surfaces and the subsequent expansion of plasma above the surface. The heating of the target and subsequent ablation of material forms a plume in the vacuum above the surface that continues to develop with increasing temperature and density, causing the ionization of the vapor and the formation of plasma. This rise in temperature and density means the collisions between particles becomes frequent enough for the assumption of local thermodynamic equilibrium to be adopted for the description of the plume expansion. This assumption implies that thermal equilibrium is established between neutrals, ions and electrons in a sufficiently small region of the vapor and a common temperature can be used to characterize them [17].

The plasma is initiated after the production of primary electrons by multi-photon absorption and thermionic photoemission mechanisms. Vaporization of the surface then occurs at a lower threshold than theoretically deduced, due to surface defects and impurities. Vapor ionization is first a thermal process, primary electrons gaining energy by inverse Bremsstrahlung then electron cascade growth occurs. The plasma propagates with absorption waves; laser-supported combustion or detonation waves depending on the laser irradiance regime.

Fig 6: Process of Ablated Plasma

The ablation process is favored with the shortest wavelengths, whereas ambient gas breakdown occurs with the largest wavelengths [18].

1.3.3Interaction of Plasma with Ambient Environment

The physics of laser ablation remains incompletely understood due to complex laser-matter as well as plasma-ambient interaction processes [19]. Many previousexperiments have focused on the adiabatic expansion of the laser generated plasma in vacuum, despite the fact that most applications of PLA are performed in the presence of an ambient gas. The presence of an ambient gas dramatically affects the laser-target and laser plasma coupling, as well as plasma expansion features.