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: elsherbinia@yahoo.com
Received August 16, 2012; revised September 23, 2012; accepted October 12, 2012
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-
opyright © 2012 SciRes. OPJ
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].
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];
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
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
n [35];
with 0.54
 
Copyright © 2012 SciRes. OPJ
In this expression, is the Lor-
entzian FWHM of the spectral line subjected to absorp-
tion and ose se
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
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).
Copyright © 2012 SciRes. OPJ
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
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
Copyright © 2012 SciRes. OPJ
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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
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).
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
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.
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