Laser Induced Breakdown Spectroscopy

Submitted to: WWW.AASK24.COM

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

1. Introduction

1.1. Laser Induced Breakdown Spectroscopy

1.2. Laser Matter Interaction

1.2.1. Multi-photon Ionization

1.2.2. Inverse Bremsstrahlung

1.2.3. Cascade Growth

1.3. Fundamental Of Plasma Physics

1.3.1. Laser Ablation

1.3.2. Laser Plasma

1.3.3. Interaction Of Plasma With Ambient Environment

1.3.4. Shock Wave

1.3.5. 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

3. 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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Myers Kigre, Inc., 100 Marshland Road, Hilton Head Island, SC 29926.

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Department of Physics ,The Ohio State University ,FSC Summer School 11-15 July 2011 UCSD

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Heidelberg, Germany June 22, 2006.

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Lockyer, J.C. Vickerman; International Journal of Mass Spectrometry 197 (2000) 197–209.

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Michael Keidar2 and Iain D. Boyd3, University of Michigan, Ann Arbor, MI 48109, USA.

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an ambient gas, C Boulmer-Leborgne, J Hermann and B Dubreuil.

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Scott Sensors (Basel) 2010; 10(5): 4907–4925, published online May 14, 2010.

[28] Arp Z.A., Cremers D.A., Harris R.D., Oschwald D.M., Parker G.R., Wayne D.M.

Feasibility of generating a useful laser-induced breakdown spectroscopy plasma on rocks at high

pressure: Preliminary study for a Venus mission. Spectrosc. Acta Pt. B-Atom. Specter. 2004; 59:

987–999.

[29] L.M. Cabalin, Experimental determination of laser induced breakdown thresholds of metals

under nanosecond Q-switched laser operation, 1998.

[30] Laser-induced breakdown spectroscopy (LIBS) – an emerging field-portable sensor

technology for real-time, in-situ geochemical and environmental analysis Russell. S. Harmon,

Frank C. De Lucia, Andrzej W. Miziolek, Kevin L. McNesby, Roy A. Walters and Patrick

D. French ,2005.

[31] Shiwani Pandhija ,Laser-induced breakdown spectroscopy , a versatile Tool for monitoring

traces in materials, 2007.

[32] Laser-Induced Breakdown Spectroscopy, Elemental Analysis Environment: Trace Gas

Monitoring, David A. Cremers, Fang-Yu Yueh, Jagdish P. Singh, Hansheng Zhang, 2012.

[33] Laser-Induced Breakdown Spectroscopy: Fundamentals, Applications, and Challenges F.

Anabitarte, A. Cobo, and J. M. Lopez-Higuera, 2012.

[34] Remote laser-induced breakdown spectroscopy (LIBS) for lunar exploration, J. Lasue, R. C.

Wiens, S. M. Clegg, D. T. Vaniman, K. H. Joy, S. Humphries, A. Mezzacappa, N. Melikechi, R.

E. McInroy, S. Bender1, Article first published online: 6 JAN 2012.

[35] Laser induced breakdown spectroscopy (LIBS) as a rapid tool for material analysis, T Hussain1 and M A Gondal.

[36] “Plasma Physics” Richard Fitzpatrick.

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Radzemski Copyright© 2006 John Wiley & Sons Ltd.

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Nassisi Radiation Effects & Defects in Solids, 163, 429–433, 2008.

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[48] Laser-Induced Breakdown Spectroscopy ,LIBS2500plus ,Installation and Operation Manual.

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 breakdown at 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

∆ E . ∆ t ≥ ⱨ

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].

Fig 2: 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

M +m ( hv )=M +¿+ e−¿¿¿

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

relation for longer wavelengths,

E=hcʎ

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].

Fig 3: 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

e−¿+M⇨M+¿+2 e−¿¿¿ ¿

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.

Fig 4: 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 cascade

ionization. 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.

Fig 5: 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].

1.3.3 Interaction 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 previous experiments 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.

In the presence of an ambient gas the complexity of laser ablation process is increased by the

occurrence of shock waves and plume confinement [20]. Laser-matter interaction are depicted in

Figure 6, which include laser absorption in the surface and material excitation, temperature rise

and surface melting, ablation and plasma formation, laser-plasma interaction, shock wave

formation, and finally, in cases with sufficiently high ambient pressure, plume collapse.

All these processes can be broadly classified into three regimes separated by different time zones

(shown in dotted lines in Figure 6):

(i) Laser-target and laser-plasma interaction occurring during the laser pulse

(ii) Plasma expansion and confinement

(iii) Plume condensation.

The characteristics of laser induced plasmas (LIPs) depend on numerous parameters, such as

target material, laser wavelength, pulse duration, and irradiance, as well as ambient gas pressure

and composition [21].

1.3.4 Shock Wave

Shock waves can be defined as high pressure, high density waves travels with supersonic speed.

It absorbs most of the laser energy thereby shields the bulk of plasma from further interaction

and absorption of radiation. Shock waves can be caused by explosions or by objects moving

through a fluid at a speed greater than the speed of sound [22].

The shock wave is one of several different ways in which a gas in a supersonic flow can be

compressed. Some other methods are isentropic compressions. The method of compression of a

gas results in different temperatures and densities for a given pressure ratio, which can be

analytically calculated for a non-reacting gas. A shock wave compression results in a loss of total

pressure, meaning that it is a less efficient method of compressing gases for some purposes, for

instance in the intake of a scramjet. The appearance of pressure-drag on supersonic aircraft is

mostly due to the effect of shock compression on the flow.

1.3.5 Effect of Ambient Environment

1.3.5.1 Ambient Vacuum

If the plasma is induced in a vacuum, the plasma-plume expands adiabatically and the expansion

of the ablated material can be described by the Euler equations of hydrodynamics [23]. In

contrast, if the surrounding medium is a gas or a liquid, the plume will compress the surrounding

medium and produce shockwaves. In this situation the plasma plume is a mixture of atoms and

ions from both vaporized material and ambient gas [24.

1.3.5.2 Ambient Gas

The plasma size, propagation speed, energy, and emission properties are related to the ambient

gas into which the plasma expands. The ambient gas can help or prevents the plasma shielding.

For example, the gas can shield the sample from the laser beam if a gas breakdown occurs before

sample vaporization [25]. These undesirable effects are less important for gases and aerosols, but

they can be important for solid samples.

Gas pressure will influence plasma expansion. Low pressures increase energy losses and

uniformity of the plasma energy distribution. In addition, different gases have different behaviors

at different pressures [26].

1.3.5.3 Influence of Pressure

1.3.5.3.1 Low Pressure, <760 Torr

Performing LIBS on a surface at reduced pressures (pressures below atmosphere) can result in

enhanced spectra and improved ablation. Specifically, these enhancements are an increase in

spectral intensity, spectra signal-to-noise (S/N), spectra resolution, increased ablation, and more

uniformed ablation craters.

When nanosecond laser is employed in surface LIBS experiments, there are some advantages in

lowering the surrounding pressure. These advantages are higher spectral resolution, greater S/N,

and increased spectral intensity. For ablation, it appears that only lasers with nanosecond pulse

lengths can take advantage of lower pressures. With the nanosecond pulse lasers, a significant

portion of the photons are able to interact with the expanding plume. At reduced pressures,

plasma generated with a nanosecond pulse is expanding in a less dense atmosphere, which

results in a less dense shock wave. The reduced density in the shock wave results in reduced

plasma shielding; thus, allowing more photons to reach the sample. Increasing the number of

photon interacting with surface results in increased sample ablation, which can also lead to a

more intense spectrum. During LIBS plasma expansion, energy is lost to the surrounding

atmosphere. This loss of energy reduces the lifetime of the laser plasma. Therefore, reducing the

pressure increases the lifetime of the plasma, allowing for more light from the laser plasma to be

collected. If pressures are too low (<∼7 Torr), there is a steep loss in LIBS spectral intensity.

This loss in intensity is likely due to disordered plasma, which results from the lack of sufficient

atmosphere to provide adequate confinement [27].

1.3.5.3.2 High Pressure, >760 Torr

The effect of background high-pressures on LIBS shows that as pressure increases, the LIBS

intensity and S/N are greatly reduced. Under closer examination using a higher resolution

spectrometer, spectra show significant peak broadening and self-absorption as pressures

increased the plasma and or the laser pulse could have been defocused or misaligned due to the

increase of pressure, resulting in reduced intensity and S/N [28].

2. Literature Survey

L.M. Cabalin et al [29] has studied Experimental determination of laser induced breakdown

thresholds of metals under nanosecond Q-switched laser operation,1998 that is Laser-induced

breakdown thresholds for several pure metals were determined using a nanosecond laser. A Q-

switched pulsed Nd: YAG laser operating at infrared (1064 nm), visible (532 nm) and ultraviolet

(266 nm) wavelengths have been used. The plasma was generated by focusing the Nd: YAG

laser on the target in air at atmospheric pressure. The dispersed plasma light was detected using a

two-dimensional intensified charge-coupled device (CCD) detector. The studied elements were

chosen according to their different thermal and physical properties, particularly boiling point,

melting point and thermal conductivity. The effect of wavelength on the plasma threshold has

been discussed. Laser fluency thresholds in the ultraviolet were larger than those obtained using

visible and infrared radiation, while the energy threshold is larger using infrared radiation.

Correlations between the plasma threshold of metal targets and the melting point and boiling

point at 266, 532 and 1064 nm have been established. The results indicate that thermal effects

have an important influence on the ablation behavior of metals at the three wavelengths used.

Russell. S. Harmon et al [30] has studied laser-induced breakdown spectroscopy (LIBS) – an

emerging field-portable sensor technology for real-time, in-situ geochemical and environmental

analysis,2005 that is Laser-induced breakdown spectroscopy (LIBS) is a simple spark spectral

chemical sensor technology in which a laser beam is directed at a sample to create a high-

temperature micro plasma. A spectrometer/array detector is used to disperse the light emission

and detect its intensity at specific wavelengths. LIBS have many attributes that make it an

attractive tool for chemical analysis. A recent breakthrough in component development, the

commercial launching of a small, high-resolution spectrometer, has greatly expanded the utility

of LIBS and resulted in a new potential for field-portable broadband LIBS because the technique

is now sensitive simultaneously to all chemical elements due to detector response in the 200 to

980 nm range with 0.1 nm spectral resolution. Other attributes include small size and weight,

technologically mature, inherently rugged, and affordable components, in-situ analysis with no

sample preparation required, inherent high sensitivity, real-time response; and point sensing or

standoff detection. LIBS sensor systems can be used to detect and analyze target samples by

identifying all constituent elements and by determining either their relative or absolute

abundances.

Shiwani Pandhija et al. [31] has studied Laser-induced breakdown spectroscopy , a versatile tool

for monitoring traces in materials, 2007 is that Laser-induced breakdown spectroscopy (LIBS) is

an emerging technique for simultaneous multi-elemental analysis of solids, liquids and gases

with minute or no sample preparation and thus revolutionized the area of on-line analysis

technologies. The foundation for LIBS is a solid state, short-pulsed laser that is focused on a

sample to generate a high-temperature plasma, and the emitted radiation from the excited atomic

and ionic fragments produced within the plasma is characteristic of the elemental composition of

the sample that can be detected and analyzed using a suitable optical spectrograph. The

applicability of LIBS for deferent solid samples have homogeneous (silver ornament, aluminum

plate) or heterogeneous composition (soil) using nanosecond laser pulses is discussed.

Nanosecond pulse laser makes plasma at the sample surface even at very low pulse energies and

also allows for precise ablation of the substrate material with little damage to the surrounding

area. We have also studied the penetration of different heavy metals inside the soil surface.

David A. Cremer’s et al [32] has studied Laser-Induced Breakdown Spectroscopy, Elemental

Analysis, 2012 is that Laser-induced breakdown spectroscopy (LIBS) is a laser-based technique

that can provide nonintrusive, qualitative, and quantitative measurements of elements in gas,

liquid, and solid samples. Two major advantages of LIBS, a method of atomic emission

spectroscopy (AES), compared with other emission techniques, are that time-consuming sample

preparation is not necessary and measurements can be done rapidly. LIBS use the plasma

generated by a powerful laser pulse to prepare and excite the sample in one step. It also has the

ability to perform simultaneous multi-element analysis. The small amount of sample material

used in LIBS analysis also limits detection sensitivity for some elements compared with

inductively coupled plasma atomic emission spectrometry (ICPAES) and atomic absorption

spectrometry (AAS). The potential of LIBS to detect toxic metals in harsh environments was

recognized in the early 1970s. Recent developments toward improving its analytical capability

have led to additional applications. This article reviews some applications of LIBS with an

emphasis on environmental monitoring. A brief review of some fundamentally directed LIBS

studies is also presented. The analytical abilities of LIBS are compared with other spectroscopic

techniques commonly used in the laboratory, such as AAS, ICPAES, and X-ray fluorescence

spectroscopy (XRF).

F. Anabitarte et al [33] has studied Laser-Induced Breakdown Spectroscopy: Fundamentals,

Applications, and Challenges, 2012 is that Laser-induced breakdown spectroscopy (LIBS) is a

technique that provides an accurate in situ quantitative chemical analysis and, thanks to the

developments in new spectral processing algorithms in the last decade, has achieved a promising

performance as a quantitative chemical analyzer at the atomic level. These possibilities along

with the fact that little or no sample preparation is necessary have expanded the application fields

of LIBS. In this paper, we review the state of the art of this technique, its fundamentals, and

algorithms for quantitative analysis or sample classification, future challenges, and new

application fields where LIBS can solve real problems.

T. Hussain et al [34] has studied Laser induced breakdown spectroscopy (LIBS) as a rapid tool

for material analysis that is Laser induced breakdown spectroscopy (LIBS) is a novel technique

for elemental analysis based on laser-generated plasma. In this technique, laser pulses are applied

for ablation of the sample, resulting in the vaporization and ionization of sample in hot plasma

which is finally analyzed by the spectrometer. The elements are identified by their unique

spectral signatures. LIBS system was developed for elemental analysis of solid and liquid

samples. The developed system was applied for qualitative as well as quantitative measurement

of elemental concentration present in iron slag and open pit ore samples. The plasma was

generated by focusing a pulsed Nd: YAG laser at 1064 nm on test samples to study the

capabilities of LIBS as a rapid tool for material analysis. The concentrations of various elements

of environmental significance such as cadmium, calcium, magnesium, chromium, manganese,

titanium, barium, phosphorus, copper, iron, zinc etc., in these samples were determined. The

laser-induced breakdown spectroscopy (LIBS) results were compared with the results obtained

using standard analytical technique such as inductively couple plasma emission spectroscopy

(ICP). This study demonstrates that LIBS could be highly appropriate for rapid online analysis

of iron slag and open pit waste.

3. Methodology3.1 Instrumentation of LIBS

The LIBS setup generally include,

1. The pulsed Laser system that generates the powerful optical pulses.

2. The focusing system of mirror and lens that directs and focuses the laser pulse on the target

sample.

3. Sample chamber and a target holder.

4. The light collection system that collects the spark light and transport the light to the detection

system.

5. Detection system consisting of spectrometer, Photomultiplier tube, CCD, ICCD to study the

plasma emission spectrum

6. Computer and electronics to gat e the detector, store the spectrum etc.

Fig 7: Schematic Diagram of LIBS

A typical setup for LIBS is shown in Figure 7. When an intense pulsed laser beam is focused

onto the target which is kept inside the vacuum chamber , breakdown of the sample occurs,

which eventually results in the creation of spark, frequently called laser-induced plasma, and

rapidly heats the sample region to extremely high temperature. The light emitted from the plasma

was collected and analyzed using spectrometer interfaced with CCD camera or Photomultiplier

Tube [36].

3.2 Laser System

3.2.1 ND: YAG Laser

The Nd:YAG laser(Quanta Ray Spectra Physics) used for plasma generation is pulsed laser

system emitting at a wavelength of 1064nm ,delivering an energy up to 600 milli-joules per pulse

with pulse width 7ns and repetition rate 10Hz [35]. The gain medium is the YAG crystal doped

with around 1% neodymium by weight, which is optically pumped by a flash lamp. The triply

ionized neodymium typically replaces yttrium in the crystal structure of the yttrium aluminum

garnet (YAG), since they are of similar size [36]. The laser oscillator has two pump chambers

assembled with one flash lamp on each chamber for high power [37]. The controller employs

conventional knobs and switches for setting and controlling the various system parameters. The

laser comprising of an active media and resonator will emit a pulse of laser light each time the

flash lamp fires. The pulse duration will be long and its peak power will be low. When a Q

switch is added to the resonator to shorten the pulse the output peak power will increase. In the

simplest case, the 1064nm Nd: YAG fundamental interacts with the crystal to produce a

secondary wave with half the fundamental wavelength [38].

3.2.2 Sample Chamber

A sample chamber is an essential part for the plasma generation for most of the experiments

performed by LIBS at laboratory scale. This is due to the reason that experiments by LIBS can

be performed at various ambient gas pressures by introducing different gases in the chamber or

in vacuum. Sample chamber can be evacuated by a rotary mechanical vacuum pump. This

sample chamber was specially designed for producing high vacuum inside it and was made of

stain less steel. The target sample inside the chamber can be rotated by a step motor with a

sample holder to get the fresh surface for each LIBS measurement. A vacuum gauge is used to

measure the pressure inside the vacuum chamber [39]. Lower pressures are measured by using a

cold cathode gauge, which has a measuring range of 7.5x10-3Torr to 1.5x10-9Torr.

Fig 8: Vacuum chamber used for LIBS experiment

Vacuum quality is subdivided into different pressure ranges as Rough vacuum (760-10-3torr).

High vacuum (10-4 - 10-8torr) and Ultrahigh vacuum (10-9 to 10-12 torr). Rough vacuum can be

obtained by using mechanical pumps like rotary vane pumps and high vacuum can be attained by

using turbo molecular pumps in series with the mechanical pump. For ultrahigh vacuum gyro

pumps or ion pumps in series with the mechanical pump can be used [40].

3.2.3 Optical Fiber and Focusing Lens

The branch of optics that deals with the transmission of light through transparent fibers, as in the

form of pulses for the transmission of data or communications, or through fiber bundles for the

transmission of images.

A fiber optic (FO) laser-induced breakdown spectroscopy (LIBS) sensor that measures the on-

line, in situ elemental composition of a molten alloy inside the melt in a furnace is described.

The sensor is based on the transmission of laser energy through a multimode optical fiber.

Atomic emission from sparks from the laser plasma is collected by the same stainless steel lens

holder and transmitted back through the optical fiber and finally fed into the entrance slit of the

spectrograph [41]. The present design of the stainless steel holder is useful for obtaining a

collimated LIBS signal over a long distance (the distance between the focusing and collimated

lenses is more than 200 cm). Parametric studies such as sample-to-lens distance and the effect of

the angle of incidence of the laser beam on the sample surface were performed. The performance

of the probe was also tested by inserting the stainless steel holder into the melt at a 45° angle,

which is necessary for collecting LIBS data [42].

3.3 Imagining and Detection

The radiation emitted by the plasma was collected and focused on to the slit of the spectrometer

for detection by two convex lenses and a mirror which is placed right angle to the direction of the

laser beam. The beam collimated by the first lens is focused by the other and is allowed to fall on

the slit of the spectrometer by using a mirror.

3.3.1 Spectrometer

A spectrometer is an instrument used to measure properties of light over a specific portion of the

electromagnetic spectrum, typically used in spectroscopic analysis to identify materials.

Simultaneous recording of spectra at multiple locations in plasma can provide critical information

about spatially varying phenomena. The Czerny-Turner spectrometers, which has enhanced

capabilities for use with a CCD or Photomultiplier tube [43].

Fig 9: Schematic of grating monochromatic,

The radiation emitted by the plasma was collected and focused on to the entrance slit of the

spectrometer. The slit is placed at the effective focus of a curved mirror so that the light from the

slit reflected from the mirror is collimated (focused at infinity). The collimated light is diffracted

from the grating and then is collected by another mirror which refocuses the light, on the exit slit.

At the exit slit of the spectrometer a compact CCD detector or a photomultiplier tube is

interfaced for recording many lines simultaneously [44].

3.3.2 Photomultiplier Tube

The photomultiplier is a very versatile and sensitive detector of radiant energy in the ultraviolet,

visible, and near infrared regions of the electromagnetic spectrum. Photomultipliers are constructed

from a glass envelope with a high vacuum inside, which houses a photo-cathode, several dynodes,

and an anode. Incident photons strike the photocathode material, which is present as a thin deposit on

the entry window of the device, with electrons being produced as a consequence of the photoelectric

effect. These electrons are directed by the focusing electrode toward the electron multiplier, where

electrons are multiplied by the process of secondary emission. The electron multiplier consists of a

number of electrodes called dynodes. Each dynode is held at a more positive voltage than the

previous one. The electrons leave the photocathode, having the energy of the incoming photon minus

the work function of the photocathode [45].

3.3.3 ICCD Camera

An image intensified high speed camera basically consists of a high performance CCD camera and

an image intensifier that is mounted in front of it. The incoming light is first amplified by the image

intensifier. The intensified image is then transmitted from the intensifiers phosphor screen and

projected onto the CCD sensor by means of a coupling lens [46].

Fig10: Schematic showing the working of ICCD

Spectral sensitivity is from UV to NIR region. It offers a high resolution of 1360x1024 pixels.

We have used ICCD in our experiment to study evolution of plasma plume at different time

instants.

3.3.4 OOLIBS2500 Plus

The Ocean Optics LIBS2500plus Laser-induced Breakdown Spectrometer is a detection system

that permits real-time, qualitative measurements of trace elements. This broadband, high-

resolution instrument allows for spectral analysis from 200-980 nm, with resolution of ~0.1 nm

(FWHM). Sensitivity has been reported to parts-per-billion and pictogram levels.

The LIBS 2500plus consists of 1 to 7 spectrometer channels and operates with any 32-bit, USB-

compatible Windows PC. We provide OOILIBS plus Application Software with spectral-saving

and data-logging capabilities for operating the LIBS2500plus and for firing the laser. Correlation

Software provides instant material identification when using the LIBS2500plus and a spectral

library consisting of 2500 atomic emission lines from the NIST (National Institute of Standards

and Technology) tables for elemental identification [47].

LIBS-CH-A 200-305 nm wavelength range

LIBS-CH-B 295-400 nm wavelength range

LIBS-CH-C 390-525 nm wavelength range

LIBS-CH-D 520-635 nm wavelength range

LIBS-CH-E 625-735 nm wavelength range

LIBS-CH-F 725-820 nm wavelength range

LIBS-CH-G 800-980 nm wavelength range

The LIBS Imaging Module is available for use with the LIBS system to enable you to precisely

adjust the laser to focus on the exact spot on the sample that you wish to analyze. The LIBS

Imaging Module also comes with special software for the camera to capture high quality images

on your PC. A high-intensity, 10 nanosecond-wide laser pulse beam is focused on the sample

area. When the laser is fired, the high temperature of the laser ablates the surface of the sample

and creates a plasma. As the plasma decays or cools, excited atoms in the plasma emit light of

characteristic wavelengths distinct to the elements present. All elements have emission spectra in

the 200-980 nm region. The detection system uses up to seven of our HR2000+ High-resolution

Miniature Fiber Optic Spectrometers, each with a 2048-element linear CCD array. All

spectrometers are triggered to acquire and read out data simultaneously. The detectors in the

broadband (200-980 nm) LIBS2500plus collect the signal; software included with the system

displays and identifies the emission spectrum [48].

Conclusion This project includes the study of Laser-Induced Breakdown Spectroscopy having wavelength of

1064 nm. 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. The expansion dynamics of the

material is investigated by ICCD camera. The spectrum produced by the plasma plume is

identified by the NIST database.