Laser Induced Breakdown Spectroscopy

Diane M Wong, A3 Technologies LLC, Aberdeen, MD, USAAlexander A Bol’shakov, A3 Technologies LLC, Aberdeen, MD, USA and Applied Spectra Inc., Fremont, CA, USARichard E Russo, A3 Technologies LLC, Aberdeen, MD, USA, Applied Spectra Inc., Fremont, CA, USA andLawrence Berkeley National Laboratory, Berkeley, CA, USA

& 2010 Elsevier Ltd. All rights reserved.

AbbreviationsIB inverse bremsstrahlung

ICP-AES inductively coupled plasma-atomic

emission spectrometry

ICP-MS inductively coupled plasma-mass

spectrometry

LIBS laser-induced breakdown

spectroscopy

LSD laser-supported detonation

LTE local thermodynamic equilibrium

MPI multiphoton ionization

UV ultraviolet

Introduction

Laser-induced breakdown spectroscopy (LIBS) is apowerful analytical technique that can be used for thedetection and characterization of materials. In LIBS, afocused laser beam is used to generate a plasma plume onthe surface of solid and liquid samples or inside thesample volume of gases, liquids, and aerosols. Each ex-cited atom in the plasma emits a unique set of spectrallines, particularly in the optical region of the spectrum.Therefore, this optical emission can be collected andanalyzed to determine the chemical composition of asample. A LIBS plasma can be generated as a single eventusing just one laser pulse or using repetitive laser pulses.As a result, localized microanalysis with lateral and depthprofiling information is easily obtained. Remote sensingof materials is also possible with LIBS, since only photonsneed to come in direct contact with the sample. Theability to perform analyses on samples at a standoffdistance is especially important when dealing withhazardous materials, samples located in a dangerousenvironment, or in physically inaccessible locations.

LIBS has a distinct advantage over many other tech-niques since little or no sample preparation is neededbefore analysis, and it can be used for rapid real-timeanalysis in field operations. Quantitative analytical resultscan be obtained from LIBS by using conventional (one-element) or multivariate calibration. Recently, cali-bration-free LIBS analysis has been demonstrated as thesemiquantitative determination of elements, based ontheoretical plasma models. With LIBS, there are no

specific requirements for the sample to fluoresce, beRaman-active or infrared-sensitive. It is truly a universaltechnique, where any sample type can yield a LIBSspectrum.

Theory of Laser-Induced BreakdownSpectroscopy

In practice, LIBS is a very simple spectroscopic techni-que to implement. A high-powered laser beam (com-monly 10-ns pulsed infrared 1.06 mm radiation) is focusedthrough a lens to produce a plasma. The emitted light iscollected, often through a fiber optic cable, and directedinto a spectrometer (Figure 1). However, the physics andchemistry of the plasma initiation, formation, lifetime,and decay are very complicated. Much progress has beenmade in recent years toward elucidating the physicsof the plasma generation processes. Studies that havebeen performed to characterize the LIBS plasma includelocal thermodynamic equilibrium (LTE) models, hydro-dynamic and kinetic models, nonuniform plasmas, andplasmas generated in a vacuum.

There are two main processes that can initiate ion-ization of atomic and molecular species in laser-inducedbreakdown. The first is the direct ionization of thesample by multiphoton ionization (MPI), and the secondis the inverse bremsstrahlung (IB) absorption processes.In MPI, atoms or molecules undergo simultaneous ab-sorption of sufficient numbers of photons to cause ion-ization (or the ejection of electrons from the valence tothe conduction band, in the case of metals) (eqn [1]).

Mþ mðhvÞ-Mþ þ e� ½1�

If EI is the ionization potential, then m, the number ofphotons, must be greater than the integer part of (EI/hnþ 1). MPI is only significant at wavelengths that areshorter than B1 mm and at high laser power, that is,greater than 1010 W/cm2. When the wavelength is sub-stantially longer than B1 mm, it is statistically unlikelyfor an atom or molecule to absorb enough photons toincrease the energy of the neutral above its ionizationpotential. This process is also important at low pressures,when few collisions are occurring due to the low particledensity of the medium.

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IB processes involve the absorption of a photon by oneor more seed electrons present in the focal volume at thebeginning of the laser pulse. These initial free electronsmay be generated through the presence of cosmic rays, orby the Earth’s natural radiation. The first few photons ina laser pulse can also produce seed electrons from dust,negative ions, such as O2

–

, organic vapors, or from atomsand molecules present in the atmosphere via MPI. Seedelectrons are not necessary for ‘pure’ MPI processes.

In IB, the absorption of a photon raises the electronenergy to a higher state in the continuum. This processmust occur in the vicinity of a heavy particle, such as anatom, ion, or molecule so that momentum is conserved.In normal bremsstrahlung processes, high-energy elec-trons emit radiation as they slow down upon interactingwith a gas or a solid. This term comes from the Germanwords, bremsen, which means to slow down, and strah-lung, which means radiation. Electrons will typically loseenergy by rotational and vibrational excitation of neutralmolecules, excitation of electronic states, and by attach-ment of electrons. However, in IB processes, electronsacquire energy from the absorbance of photons andcollisions with atoms, ions, and molecules. If the energyof the free electron is greater than the ionization po-tential of a neutral species, it can ionize a molecule (M)by colliding with it. This produces two lower-energy freeelectrons, which can gain more energy from the electricfield, causing ionization of other neutrals and two moreelectrons (eqn [2]).

e� þM-2e� þMþ ½2�

With the increase in the population of ions and electronsin the focal volume, the probability of electron–photonneutral collisions also increases, resulting in electronmultiplication and cascade growth. As more and moreelectrons repeat the process, there is a geometric growth

in the number of free electrons, resulting in cascadeionization. IB processes can be so dramatic that all thespecies ablated from the surface of the substrate canbe ionized causing such an increase in plasma growththat the entire laser pulse can be coupled into the plasma.This results in the plasma becoming so optically opaquethat the substrate becomes shielded from the remainderof the laser pulse.

Both MPI and IB absorption can contribute to cascadeionization. The dominant process will depend on thewavelength of the laser radiation, laser intensity, anddensity of the medium in which the laser breakdownoccurs. IB-dominated breakdown is important at highpressures, when collisional effects are strong, and atwavelengths longer than 1 mm. At shorter irradiationwavelengths (o1 mm) or at low densities of molecules,the possibility of electrons colliding with neutral speciesis small. Therefore, MPI dominates usually at theseshorter wavelengths and in low-density media. Cascadeionization continues throughout the duration of the laserpulse and results in the ionization and dielectric break-down of gases, vaporized particles, and the creation of aplasma.

Plasma Life Cycle

The plasma generated by a focused laser beam in LIBShas a distinct, time-dependent life cycle. For a plasma tobe created, the breakdown threshold must be reached.This breakdown threshold is usually defined qualita-tively as the minimum irradiance needed to generate avisible plasma. With the exponential increase in theproduction of free electrons and ions by IB and MPIprocesses, the plasma begins to expand from the initialfocal volume. Ablated materials, such as particulates,ions, molecules, neutrals, and electrons, are also present

Laserpower supply

Laser head Focusing lens

Input optical fiberCollection lenses

Spectrograph

Delay generator

Detector controller

PC

Laser spark

Unknown material

Laser-induced breakdown spectroscopy(LIBS)

Figure 1 Schematic diagram of a typical lab bench LIBS setup.

1282 Laser Induced Breakdown Spectroscopy

in the plasma that forms at the surface of the sample insolids and liquids. The ionization potential of gases isgenerally higher than that of liquids and solids, but theytoo can be readily analyzed by LIBS. Using higher-energy densities, gases can be ionized with a tightlyfocused laser to produce a plasma in the sample volume.A sonic boom is produced from the initial expansion ofthe plasma. The compression wave front, or shock wave,emanates from the focal volume of the plasma. The wavefront is traveling well above the speed of sound, on theorder of 105 ms–1. For comparison, the speed of sound inair is B345 ms–1 at 211C. As the plasma expands, itcontinuously emits spectroscopic signals from all con-stituent components in the sample. When the plasmacools through radiation and other loss mechanisms, theionized species recombine to form neutral atoms andmolecules.

Throughout the lifetime of the plasma, the emissionspectrum changes as a function of time. In the earliestphase, there is a strong white light component, whichcontains little useful spectroscopic information. Thiscontinuum light consists mainly of bremsstrahlungand recombination radiation. Recombination radiation iscaused by the recombination of free electrons and ions.As stated earlier, bremsstrahlung radiation is caused bythe decrease in the translational energy that occurs whenions and electrons slow down upon collision with a gas orsolid. This is in contrast to IB processes, which causecascade ionization during the initial breakdown event.

After the laser pulse, the continuum gradually fadesallowing the weaker spectroscopic signals from theelements of interest to be detected. This occurs in a timeframe that is much longer than the laser pulse duration,which in a nanosecond laser is on the order of 10 ns.Since the weaker signals only appear as the continuumfades, the collection of the spectroscopic signal of interestshould be delayed.

Time parameters in LIBS are important consider-ations when obtaining emission spectra. At low laserpulse energies, that is, several millijoules, the timedelay may need to be less than 1 ns in duration to ob-tain an optimal signal. With higher laser powers, thedelay may be several microseconds. Too short agate width will not allow enough signal to be processedacross the detector. Too long a gate width will allowtoo much ambient light to enter the detector thus ad-versely affecting the signal to noise ratio. Optimizationof the time delay and gate width should be performedon the individual samples to maximize the relativeintensities of the emission lines for the signaturesof interest. Sample properties, such as substrate, matrix,and concentration, and experimental conditions such aslaser power, wavelength, and pulse width, can all influ-ence the temperature and energy density of the plasma(Figure 2).

LIBS Emission Signal Enhancement –Background Gas Effects

Different background gases are often utilized to enhancethe quality and intensity of the LIBS plasma, especiallyfor the analysis of complicated or difficult samples. Theuse of argon gas is a common technique to effectivelyenhance the LIBS emission signal. In this method, asteady stream of gas is directed toward the focal volumeof the plasma. This increases both the intensity andquality of the signal by displacing the background air andmaking it easier to generate a plasma. Calculations ofelectron densities and plasma temperatures of plasmascreated in difference background gases show that argonproduces the highest temperatures and electron densities,followed by ambient air. Helium is associated with thelowest temperatures and densities.

When laser ablation is performed in a vacuum, lowertemperatures are attained and lower concentrations ofelectrons are seen in the plasma volume than in a gaseousenvironment. Since there is no pressure counteractingfree plasma expansion in the vacuum, the expansion rateis increased whereas the cooling rate is decreased ascompared to expansion in a gas. This decrease in coolingrate of the excited electrons can be explained by thedominance of elastic collisions. Since cooling is inverselyproportional to the mass of the background gas, heaviergases like argon have a less efficient cooling capacity thanhelium or air. Therefore, using argon will produce higherplasma temperatures with greater electron density thanone produced in ambient air or gases with lower atomicmasses, like helium. Argon also displaces the nitrogenand oxygen molecules in the air. This effectively elim-inates any interference from these molecules in the an-alysis of species that contain nitrogen and oxygen atoms,including organic compounds.

When argon gas is utilized, a laser-supported det-onation (LSD) wave can be observed at the tip of thevapor plume 3 ns after the laser pulse. An LSD waveoccurs when a shock wave front, expanding at supersonicspeeds, is transmissive enough to allow the incominglaser energy to penetrate the boundary of the plasmaand the surrounding atmospheric gases. The shock waveis powerful enough to compress and heat the gasessurrounding the plasma, thus allowing for strong opticalabsorption of the incoming laser radiation. The com-pression of background gases by the shock wave en-hances the conversion of kinetic energy into thermalenergy in the vapor plume by slowing the rate of vaporplume expansion. As a result, the vapor plume generatedwith a background gas of argon continues to increase intemperature and electron number density after the laserpulse ends. The LSD is thought to create an area of hightemperature and low gas density above the vapor plume,which allows the expansion of the plume to continue, in

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