Spectrometric Measurement of Plasma Parameters Utilizing the Target Ambient Gas O I & N I Atomic Lines in LIBS Experiment

doi:10.4236/opj.2012.24035

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Optics and Photonics Journal, 2012, 2, 286-293

http://dx.doi.org/10.4236/opj.2012.24035 Published Online December 2012 (http://www.SciRP.org/journal/opj)

Spectrometric Measurement of Plasma Parameters

Utilizing the Target Ambient Gas O I & N I Atomic

Lines in LIBS Experiment

Ashraf M. El Sherbini1,2, Abdel Aziz Saad Al Amer1, Ahmed T. Hassan1,2, Tharwat M. El Sherbini2

1Department of Physics, Collage of Science, Al Imam Muhammad Ibn Saud Islamic University (AIMSIU),

Al Riyadh, KSA

2Laboratory of Lasers and New Materials (LLNM), Department of Physics, Faculty of Science, Cairo University, Giza, Egypt

Email: [email protected]

Received August 16, 2012; revised September 23, 2012; accepted October 12, 2012

ABSTRACT

In this article, we shall report the results of the spectroscopic measurements of the plasma parameters utilizing the spec-

tral lines emitted from the air atoms surrounding plasma (O I line at 777.19 nm and N I at 746.83 nm). The plasma was

created via irradiation of plane solid aluminum target in open air by a high peak power Nd: YAG laser pulses at funda-

mental wavelength of 1064 nm. The emission spectra were recorded using Echelle type spectrograph in conjunction

with a time gated ICCD camera at different delay time from 1 to 5 μs and at a fixed gate time of 1 μs. The plasma elec-

tron density was measured utilizing the Stark broadening of the N I and O I lines and then compared to the reference

density as deduced from the optically thin Hα-line at 656.27 nm appeared in the same emission spectra. The results

show that under our experimental conditions the air lines are subjected to moderate absorption. The plasma electron

temperature was measured utilizing the relative spectral intensity of the air (O I to N I) lines after correcting their spec-

tral radiance against absorption. The standard temperature was measured utilizing the Al II ionic lines. A comparison to

the reference temperatures shows a very close agreement after correcting the emission spectral radiance of the air lines

against self absorption, which emphasizes the importance of correction process.

Keywords: LIBS; Self Absorption; O I-N I Lines; Plasma Parameters

1. Introduction

The laser induced breakdown spectroscopy (LIBS) is the

spectral analysis to the emitted light from the plasma

generated via the interaction of pulsed laser with matter.

Over the past decades the LIBS-technique becomes a

distinguished applied analytical technique used in dif-

ferent application fields, because of its reliability, non-

contact optical nature and its freedom from sample pre-

paration [1]. Different fields of applications were already

established e.g. elemental analysis [2-4], the quality of

steel manufacturing [5], characterization of jewellery

products [6,7], in soil studies [8,9], pulsed laser thin film

deposition [10,11], the quality of pharmaceutical prod-

ucts [12], cleaning [13], culture heritage (cleaning of old

papers, wood preservers, old paintings) [14-16] and in

situ planetary exploration [17].

However, there are more investigations to improve the

technique from the physical point of view; e.g. correction

of the emitted spectral lines against optical thickness

[18-22], improvement of the limit of detection [23], ef-

fect of ambient gas pressure on laser ablation process

[24,25] and finally an enhancement of the LIBS signals

through different suggested schemes e.g. Double pulse

LIBS [26].

Two major parameters in LIBS namely electron den-

sity and temperature has to be given by experiment in

what is called the plasma diagnostics. Different diagnos-

tic techniques were established and utilized [27,28]. The

most direct method is the optical emission spectroscopy

(OES) in which the light emerging from the plasma is

studied using a standard experimental setup.

Spectroscopically, the plasma electron temperature can

be measured from the relative intensity of two or more

optically thin lines having small separation in the emis-

sion wavelength and large separation in their upper ex-

cited states (Boltzmann plot method) [27]. Moreover, the

ionization temperature can be calculated from the relative

intensity of two or more lines originating from two con-

secutive ionization stages. This method enhances the

precession in measuring temperature due to enhancement

of the separation of the upper excited states by the ioni-

zation energy (Saha-Boltzmann plot) [28]. As an exten-

A. M. EL SHERBINI ET AL. 287

sion to the method, the relative intensity of two spectral

lines emerging from different elements but belongs to the

same ionization stage can be utilized in measurement of

the plasma electron temperature provided that the relative

abundance is known [29].

On the other hand, the electron density can be meas-

ured utilizing the Stark broadening of optically thin

emitted spectral lines from neutral atoms or ions with the

help of the precise Stark broadening parameters that can

be found in different standard tables [30-33]. In a previ-

ous publication, it was suggested that the use the H line

at a wavelength of 656.27 nm can garantee a standard

measure to the electron density in the laser-plasma ex-

periments in open air because of its inherent properties,

i.e. the line was found optically thin under similar ex-

perimental conditions [34-36].

It is worth noting that, the lines emitted from the air

atoms surrounding the plasma e.g. O I, N I, were previ-

ously utilized in the measurement of electron density but

not the temperature [37-40].

In this research paper we will demonstrate the ability

to assess the plasma diagnostics (measurement of elec-

tron density and temperature) via utilizing the lines emit-

ted from the plasma surrounding air atoms, N I at 746.8

nm and O I at 777.2 nm after getting rid of the effect of

self absorption.

2. Measurements of Plasma Parameters and

the Effect of Absorption

2.1. Plasma Parameters

The plasma state (LTE-local thermodynamical equilib-

rium) can be determined knowing two plasma parameters

namely the electron temperature and density. Both pa-

rameters should experimentally be measured utilizing the

principle of the optical emission spectroscopy technique,

assuming that the emitted light from the plasma is suffi-

ciently influenced by the plasma parameters.

At an intermediate electron density range at concentra-

tion of (~1017 cm–3) in the laser-plasma experiments, the

electron density can be measured utilizing the Stark

broadening of the spectral lines. This effect manifests

itself on the form of Lorentzian broadening to the emitted

radiation line of full width at the half of maximum

(FWHM) Δλo For neutral atoms, the following expres-

sion can be used, after neglecting the ion contribution to

the broadening, to calculate the electron density, [31,32].





















(1)





is the temperature dependant Stark broad-

ening parameter of the line under investigation which can

be found in tables [30,33]. Nr is the reference electron

density; in the case of neutral atoms is 1016 cm–3.

On the other hand, one can easily shows that at the

LTE plasma state the electron temperature can be spec-

troscopically measured from the relative intensity of two

lines belonging to two different elements in the same

ionization stage, provided that the relative abundance

(relative concentration) is known. In our case, the elec-

tron temperature will be evaluated utilizing the relative

spectral radiance of O I line at 777.19 nm to the N I line

at 746.83 nm, utilizing the good knowledge of the rela-

tive abundance of nitrogen to the oxygen in open air. The

following expression can be used [29];





log3.72

OOOOOON NNNN N

IZCAg IZCAg













(2)

Whereas, 3.72 is the relative abundance of the nitrogen

to oxygen in the open air [29], ,

I are the spectral

line intensities measured at the central emission wave-

lengths, ,



are the transition wavelengths,

are the correction coefficients against relative response of

the detection system (camera and spectrograph), while

Z are the partition functions. ON ON

,&,

Agg

and are the transition probabilities, statistical

weights and level energies of the upper states of both O I

and N I lines used in the calculation, respectively. A

generalization to Equation (2) to include several lines is

known as multi-elemental Boltzmann plot [29].

2.2. Effect of Self Absorption on the Spectral

Line Shape

Two major effects of self absorption on the emission

spectral line shape can be recognized [30-33]. First, it

acts to decrease the spectral line intensity because of the

re-absorption to the number of the emerging photons

from the inner core of the plasma medium. The second

effect is that it acts to distort (enlarge) the Lorentzian

FWHM component of the line [32]. In that case we say

that the plasma become optically thick to the line [33].

Quantification to the amount of the plasma absorption

(optical depth) to different emission spectral lines was

already defined in terms of the coefficient (SA) [35]. It

was expressed in terms of the ratio of the Lorentzian

components of the line,





in the case if the line is

optically thick (actually measured) to that if the line is

optically thin i.e. in the limit of the extremely small con-

centration o





[35]. Or better in terms of electron den-

sity derived from the optically thin hydrogen H-line to

that measured from the optically thick suspected line





line

n [35];



with 0.54

nline

SA nH











 







(3)

A. M. EL SHERBINI ET AL.

288

In this expression, is the Lor-

entzian FWHM of the spectral line subjected to absorp-

tion and ose se



2lin









2line2nn













10SA

is the Lor-

entzian FWHM of the same line, in the limit of optically

thin condition. These FWHM’s were replaced by the

electron densities as measured from the line subjected to

absorption (which will yield an extra large density value)

to that from the optically thin Hα-line. It is vital to note

that if a certain line is optically thin, it will yield the

same electron density value as deduced from the Hα-line.

It is worth noting that in Equation (3) we have been util-

ized the properties of the Hα-line present in the emission

spectra to calculate the amount of absorption to any sus-

pected spectral line. Moreover, this expression shows

that this coefficient . SA reaches unity in the

case of optically thin line and decreases to the limit of

zero in the case of complete self absorbed line [35].

On other hand, the self absorption acts to decrease the

number of the emerged photons contained in the line i.e.

the spectral intensity of the emission line will be de-

creased. Therefore, the coefficient self absorption (SA)

was expressed in terms of the relative spectral intensities

of the same spectral line in the limits of optically thin





to that of completely optically thick value





;



1exp

SA I





















 



(4)

where ; is the optical depth of the













plasma of length at the line center













is the linear absorption coefficient of the plasma at the

line centre





[41]. Equation 4 indicates that the SA

varies from unity in case of pure optically thin line to the

limit of zero in case of completely self absorbed line [35]

and can be used to correct the spectral line intensity

against the effect of self absorption.

3. Experimental Details

3.1. Experimental Setup

A Nd-YAG laser working at the wavelength of 1.06 μm

was used to irradiate a polished solid aluminum target

contains some traces of magnesium in a humid air and is

shown in Figure 1. The data was acquired using an

echelle type spectrograph (type Catalina, model SE200)

with a resolving power of 2400 in conjunction with time

integrated ICCD-camera (type Andor iStar) at the

binning mode of 1 × 1. The laser energy was kept fixed

at the level of 670 mJ at the target surface and was

brought into focus with a 10 cm quartz convex lens. The

emitted light from the plasma was collected using a 25

μm diameter quartz optical fiber cable positioned at dis-

tance of 12.5 mm normal to the laser axis and arranged to

observe a circle of cross section of only 2 mm in front of

the target. The identification of the different lines was

carried out by spectrum analyzer software version 1.6.

Other calculations were done using home-made routines

under the MATLAB® package (Ver. 7). The relative re-

sponse of the detection system was measured using a

standard Deuterium Tungsten-Halogen calibration light

source (type DH-2000-CAL). Over the entire experiment

the gate time was kept at 1 μs and the delay time was

changes in equal steps from 1 - 5 μs. The instrumental

bandwidth was measured from the FWHM of the Hg

lines and was found to be, at the wavelengths of interest,

0.12 ± 0.02 nm.

3.2. Calculation Procedures

An example of the plasma emission is given at Figure 2,

at different delay times. One can notice the existence of

the continuum emission appeared under the lines that is

decreases in advance of the delay time. Therefore, at the

start of the calculations, one should remove such com-

ponent. This was achieved via utilizing a home-made

Figure 1. The experimental setup.

Figure 2. The emitted spectra from the air plasma at the

long wavelength side recorded at a delay time of 1 μs (a),

delay of 3 μs (b) and delay of 5 μs (c).

A. M. EL SHERBINI ET AL. 289

software that can fit the continuum to a polynomial func-

tion of the 8th order over the entire wavelength scale

(200 - 1000 nm) and then we have carefully subtracted

the resulted polynomial function (new base line) from the

emitted spectrum.

On the other hand, in order to evaluate the electron

density from a spectral line, a Voigt fitting procedures to

the line was adopted. This Voigt profile contain the con-

tribution from three effects, one is the Stark broadening

which manifested itself on the form of a Lorentzian dis-

tribution of emitted light across the line profile. While

the two other mechanisms are of Gaussian nature, one is

resulted from the thermal motion of the emitting species

at a kinetic temperature (Doppler effect) and the other,

which can’t be neglected, is the instrumental band-width

which was measured using a low pressure Hg-calibration

lamp and was found as ~0.12 nm. A convolution between

these three functions was carried out using the MATLAB

processing and the resulted Voigt line shape was com-

pared to the experimentally measured line profile. The

fitting is automated via a home-made computer program

(Using MATLAB 7) and we have followed the principle

of least square fitting between the theoretical profile and

the measured line profile. The fitting is repeated at dif-

ferent electron density values used as an input independ-

ent free running parameter until the best fitting was

achieved. At this step the program is automatically ter-

minated and the values of the electron density from the

three lines are recorded.

4. Results and Discussion

The emitted spectrum at the longer wavelength region

from the O I lines at 777.2 nm and the N I lines at 746.8

nm as well as the Hα line at 656.27 nm are shown in

Figures 2(a)-(c) at different delay times namely 1, 3 and

5 μs as indicated at the vertical scale.

Also, one can notice the existence of the continuum

component under the lines of interest. This component is

resulted from free-free transition in the plasma (brems-

strahlung) and free-bound transitions and displaying a

decreasing trend as shown in Figures 2(a)-(c) as the de-

lay time is changed from 1 to 5 μs. This continuum was

carefully removed from each spectrum before utilizing

the spectral lines shapes in the measurements of the

plasma parameters.

The measurement of the electron density utilizing the

Stark component of emission spectral line requires ex-

traction of the Lorentzian component (FWHM) from the

lines together with knowledge of the Stark broadening

parameters for the different lines as given in Table 1.

This was done with the help of fitting of the emitted line

shape to the Voigt function. An example of the Voigt

profile fitting to the emitted lines of the O I at 777.19 and

Table 1. Atomic parameters of the used spectral lines.

(nm) ωs × 10–3 (nm)Eu

(eV) g A × 107 (sec–1)

O I 777.193.15 10.74 15 3.69

N I 746.834.75 11.99 4 1.93

the N I at 746.8 nm together with the Hα line at 6565.27

nm is shown in Figures 3(a)-(c) at different delay times

namely 1, 3 and 5 μs. The fitting is automated via a

home-made computer program (Using MATLAB 7). It is

worth noting that, because of the finite symmetry of the

Hα-line, especially at the early delay times, the fitting to

the symmetric Voigt function is almost limited as shown

at a delay of 1 μsec.

ωs is the Stark broadening parameter, g is the statisti-

cal weight of the upper state and A is the transition

probability

With the help of Equation (1), the electron densities

from the different lines were calculated. The temporal

variation of the measured electron densities with delay

time is plotted and shown at Figure 4. We noticed that

the measured electron density utilizing the H line gives

the lowest values, which means that the lines originated

from the ambient air atoms O I, N I are subjected to some

absorption. Because of the nature of the spatially inte-

grated method of detection of the light one can attribute

this absorption to passage of at least half of the photons

from the air lines across the plasma body in its way to the

spectrograph. Moreover, the deduced values of the elec-

tron densities from the O I and N I atoms is shown to

display a decreasing trend with delay time at different

rates and finally approaching the standard density values

as measured utilizing the Hα line, which means that the

lines are drifting toward the optically thin values with

delay time or better with the cooling of the plasma. This

fact can better understand in terms of the weak absorp-

tion of the cold plasma to the lines at the infrared region

as given in Ref. [41].

Figure 5 demonstrates the variation of the calculated

coefficients of the self absorption of the plasma to the air

O I, N I lines with delay time. It is noticed that the lines

are showing a less absorption in advance with delay time.

Moreover, the amount of absorption to the O I line is a

bit larger than that of the N I by a factor of 2, regardless

that the relative abundance of the nitrogen to oxygen gas

is nearly 4. This can be explained in terms of the absorp-

tion oscillator strength of both lines (0.468 for the O I at

777.2 nm and 0.109 at the N I 746.8 nm).

Figure 6 shows the Boltzmann plot utilizing the Al II

ionic lines appeared in the same spectra at an arbitrary

delay time of 1 μs, indicating a reference electron tem-

perature of 1.14 eV. The overall variation of reference

electron temperature utilizing the Al II lines is shown in

A. M. EL SHERBINI ET AL.

290

Figure 3. Voigt fitting to different spectral lines O I, N I and Hα at a delay time of 1 μs (a), delay of 3 μs (b) and delay of 5 μs

(c).

Figure 4. The temporal variation of the measured electron

density from the Hα-line (squares), from the N I line (Trian-

gles) and from the O I (solid circles).

Figure 5. Shown is the temporal variation of the coefficients

of self absorption to the N I line (triangles) and O I line

(solid circles).

A. M. EL SHERBINI ET AL. 291

Figure 6. This figure demonstrates the Boltzmann plot util-

izing the Al II ionic lines (281.6 nm (square), 466.2 nm (in-

verted triangle) and 358.6 nm (solid circle) taken at an ar-

bitrary delay and gate times of 1 μs. The slope of the best fit

straight line indicated temperature of 1.14 eV.

Figure 7 at the different delay times (solid squares).

In our analysis we have assumed the existence of the

plasma species in a local thermodynamical equilibrium

(LTE) with the ambient air spices. Therefore, we have

compared the measured electron temperature evaluated

from the air atoms (O I & N I-atomic lines) utilizing

Equation (2) to the reference temperature values as

measured from the Al II ionic lines. Figure 7 shows the

result of this comparison at different delay times without

correction of the spectral lines intensities from the air

atoms (O I, N I) against effect of absorption of the

plasma ( solid red inverted triangles) and after correction

as made utilizing Equation (4) (red open inverted trian-

gles). One can notice the relatively insignificant large

values of the electron temperature as measured from the

air atoms without correction against absorption. But after

applying the correction procedures to the spectral line

intensities, there is a very close agreement between the

temperatures from the air atoms and the standard refer-

ence temperatures as deduced utilizing the spectral inten-

sities emitted by the Al II lines.

It is worth noting that, a limitation on the use of this

method can be provided as following;

1) If the Hα-line becomes optically thick. This condi-

tion might exist if the concentration of the water vapor

around the target becomes relatively large [34].

2) At the early delay time of 0 μs, the Hα-line was in-

spected I a separate publication and was found to contain

some optical thickness, and in that case the line cannot be

used any more [36].

5. Conclusion

These results confirms the reliability of using of the ei-

ther of O I, N I lines at wavelengths of 777.2, 746.8 nm

Figure 7. The temporal variation of the plasma electron

temperature utilizing the relative spectral intensity of the O

I line at 777.2 nm to the N I line at 746.6 nm in comparison

to the standard reference temperature utilizing the Al II

ionic lines (solid black squares) both before correction

(solid inverted red triangles) and after correction against

the effect of self absorption (open inverted triangles).

respectively, to measure to the electron density and tem-

perature to a plasma generated via the interaction of laser

with solid target in air. This will gives rise to the ques-

tion upon the coupling of the plasma generated from

aluminum target and the surrounding air atoms. This

mixing was confirmed for the hydrogen in a previous

study at Ref. [40]. This mechanism concerning the origin

of the emission from the oxygen and nitrogen atoms will

be subjected to further experimental investigation in the

future.

6. Acknowledgements

The experimental part of this work was conducted at the

Lab. of Lasers and New Materials, Physics Dep., Cairo

Univ., Egypt. The authors express their gratitude to the

valuable discussions with Prof. S. H. Allam and Dr. H.

Hegazy.

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