On the Rydberg transitions and elemental compositions in the laserproduced Al (6063) plasma

M. A. Baig,a) M. A. Fareed, B. Rashid, and R. AliAtomic and Molecular Physics Laboratory, Department of Physics, Quaid-i-Azam University,45320 Islamabad, Pakistan

(Received 10 June 2011; accepted 22 July 2011; published online 22 August 2011)

We present new studies on the optical emission spectra of the laser produced Al 6063 alloy plasma

generated by the 1064 nm Nd: YAG laser. The spectrum reveals Rydberg transitions; nd 2D3=2,5=2

! 3p 2P1=2,3=2 (n¼ 3 – 8), ns 2S1=2! 3p 2P1=2,3=2 (n¼ 4–6), and the dominant spectral lines of the

other constituent elements. We have extracted the relative abundance of the impurities using the

relative intensity ratio method. Besides, we have calculated the electron temperature (�7580 K)

from the Boltzmann plot method and the electron number densities (�1.4� 1017=cm3) from the

Stark widths of the aluminum spectral lines. The plasma parameters determined in the present

work are in agreement with that reported in the literature. The molecular vibrational transitions of

the AlO free radical associated with the B 2P!X 2P band system have also been identified.VC 2011 American Institute of Physics. [doi:10.1063/1.3625552]

I. INTRODUCTION

Laser-induced breakdown spectroscopy (LIBS), also

known as laser-induced plasma spectroscopy (LIPS), is a

type of atomic emission spectroscopy (AES) which is a most

sensitive and reliable optical diagnostic technique for mate-

rial analysis.1,2 Among the available elemental analysis tech-

niques, LIBS exhibit very attractive features and is a very

useful technique for variety of applications like environmen-

tal monitoring, material analysis, and thin film deposition.3

In this technique, a high power laser pulse is focused on a tar-

get by using a lens and when the laser irradiance exceeds the

breakdown threshold of the material aluminous plasma is

created. The plasma cools down within few milliseconds.

The radiations are emitted from this plasma, which is regis-

tered for the spectroscopic analysis and to determine the

plasma parameters such as composition, electron number

density, and electron temperature quantitatively and qualita-

tively. Different experimental studies are reported in the lit-

erature about the effects of laser parameters in the

evaporation process, plume, and plasma characteristics of

aluminum. Sabsabi et al.4 analysed the aluminum alloys

plasma characteristics quantitatively. Karandikar et al.5

reported the composition of the Al 6063 sample using energy

dispersive x-ray spectroscopy (EDX). Rai et al.6 demon-

strated the effectiveness of a fiber-optic LIBS probe to mea-

sure the concentration of minor constituents of molten

aluminum alloys. Francois et al.7 discussed the comparative

study of the laser-induced breakdown spectroscopy and spark

optical spectroscopic study of aluminum. Sherbini et al.8

evaluated the self-absorption coefficients of the neutral and

ionic aluminum lines. Freedman et al.9 used a microchip-

laser to analyze the aluminum alloys. Shen et al.10 discussed

the confinement effects in LIBS. Shaikh et al.11 studied the

spatial evolution of the aluminum plasma at 1st, 2nd, and 3rd

harmonics of Nd:YAG Laser. Li et al.12 reported the quanti-

tative analysis of impurities in aluminum alloys using LIBS

without internal calibration. Herrera et al.13 presented a com-

parative study of calibration free (CF-LIBS) and Monte-

Carlo simulation annealing optimizing method (MC-LIBS)

for the quantitative analysis of aluminum alloy. Goueguel

et al.14 investigated the resonance-enhanced laser induced

breakdown spectroscopy (RELIBS) to improve the limit of

detection of trace elements analysis of aluminum alloys.

More recently, Luo et al.15 reported the spectroscopic analy-

sis of element concentrations in aluminum alloy using LIBS.

The motivations following the present work were two

fold; first to determine the compositions of the constituent

elements in the Al 6063 alloy using the LIBS technique and

secondly to use the optical emission spectroscopic technique

to extract more accurate plasma parameters. In the present

work, we present the spectroscopic studies of the laser pro-

duced Al 6063 alloy plasma using a Nd:YAG laser at 1064

nm and determined the relative compositions of the elements

through the intensity ratio method. The optical emission has

been recorded spatially by scanning the plume along its

expansion revealing the nd 2D3=2,5=2 and ns 2S1=2 Rydberg

levels of aluminum. The electron temperature Te and elec-

tron number density Ne have been determined using the

Boltzmann plot method and from the Stark broadened spec-

tral line of aluminum.

II. EXPERIMENTAL SETUP

The experimental details are the same as described in

our earlier papers.16–18 A schematic diagram of the experi-

mental arrangement used is depicted in Fig. 1. A Q-switched

Nd:YAG laser (1064 nm, Pulse duration 5 ns and repetition

rate 10 Hz) was used as an energy source. The laser was ca-

pable of delivering energy �400 mJ. The pulse energy was

varied from �20 mJ to 150 mJ monitored by an energy meter

(Quantel NOVA-QTL). The laser beam was focused througha)Electronic mail: baig@qau.edu.pk.

1070-664X/2011/18(8)/083303/6/$30.00 VC 2011 American Institute of Physics18, 083303-1

PHYSICS OF PLASMAS 18, 083303 (2011)

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a 20 cm focal length quartz lens and the radius of the spot af-

ter focusing the laser beam on the target was determined as

ro ¼ 2:7� 10�3cm using the relation: r0 ¼ kp

fr1

; here r1 is the

radius of the incident laser beam on the lens and f is the focal

length of the lens. The sample placed on a 3-D rotating

holder, which was rotated to avoid surface damage and to

provide a fresh surface after each laser pulse. The lens was

adjusted slightly less than the focal length to avoid break-

down of ambient gas and to provide maximum energy on the

material surface. The optical emission was collected through

a collecting lens of 1 cm (0–45� field of view) using a 5 cm

diameter silica window, placed at right angle to the direction

of the plasma plume expansion. The emitted photons were

registered by the LIBS2000 (Ocean optics Inc.) detection sys-

tem in conjunction with an optical fiber (high-OH, core diam-

eter: 600 lm). Five spectrometers equipped with 2400 lines

gratings cover the region from 220 nm to 720 nm with an op-

tical resolution �0.05 nm. The data detection system was

triggered by the Q-switch of the Nd:YAG laser. The data

were acquired with a delay time 3.5 micro seconds, integra-

tion time of 2.1 ms and stored by the OOI LIBS software. The

data were further corrected by subtracting the dark signal and

averaged for 10 laser shots to minimize the statistical errors.

III. RESULTS AND DISCUSSION

A. Elemental composition

We have recorded the emission spectrum from the Al

6063 alloy plasma generated by focusing the first harmonic

(1064 nm) of a Q-switched Nd:YAG laser. The plasma

expands perpendicular to the target surface and when it cools

down, characteristics emission of the constituent elements

occurs. In Figs. 2–5, we present the emission spectra of the

Al 6063 plasma plume covering the wavelength region from

200 to 720 nm. The major part of Fig. 2 consists of the spec-

tral lines of aluminum and magnesium. Fig. 3 contains the

spectral lines of aluminum and calcium. Since Al 6063 alloy

mainly contains aluminum, therefore, most of the observed

spectral lines belong to neutral and singly ionized aluminum

besides the strong lines of the other constituent elements.

Fig. 2 covers the spectral region from 210 nm to 320 nm

clearly showing the Rydberg series of aluminum originating

from the decay of electrons from the nd 2D3=2,5=2 (n¼ 3, 4,

5, 6, 7, and 8) levels to the 3p 2P1=2,3=2 lower levels. The

optically allowed transitions from the upper 2D levels to the2P lower levels are 2D5=2! 2P3=2, 2D3=2! 2P1=2, and 2D3=2

! 2P3=2, and their relative intensities are predicted to be in

the ratio; 9:5:1 in the LS-Coupling scheme. The 2D3=2

! 2P3=2 transitions are too weak to be detected in the present

work. The 2D5=2 ! 2P3=2 lines are indeed observed much

stronger than the 2D3=2 ! 2P1=2 transition lines. Further-

more, the intensities of the lines decrease monotonically

with the increase of the principal quantum number. The Ryd-

berg series consists of two lines and their spacing remains

constant for all the series members. Since the spin-orbit

interaction in the nd upper levels decrease by �1=n3, there-

fore, the doublet splitting reflects the spin-orbit splitting of

the 3p lower level, which is 112.04 cm�1. The ns 2S1=2

FIG. 1. (Color online) Schematic dia-

gram of the experimental arrangement.

FIG. 2. (Color online) The emission spectrum generated by the 1064 nm

laser; pulse energy¼ 120 mJ and distance from the target surface¼ 0.05

mm, showing the Rydberg series of aluminum along with the foremost mag-

nesium lines.

083303-2 Baig et al. Phys. Plasmas 18, 083303 (2011)

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(n¼ 4, 5 and 6) ! 3p 2P1=2,3=2 series have also been

observed. The 5s 2S1=2 ! 3p 2P1=2,3=2 doublet is marked in

Fig. 2. We believe it is for the first time that a Rydberg series

have been observed in the LIBS spectrum of aluminum. The

magnesium resonance line 3s 3p 1P1! 3s2 1S0 is detected at

285.29 nm, whereas the resonance lines of the singly ionized

magnesium 3p 2P3=2! 3s 2S1=2 and 3p 2P1=2! 3s 2S1=2 are

observed at 279.55 nm and 280.27 nm, respectively.

Fig. 3 shows a selected portion of the LIBS Spectrum

recorded using the 1064 nm laser having pulse energy �120

mJ while the detector was placed at �0:05 mm from the tar-

get surface along the plasma expansion. The dominating alu-

minum lines are identified as 4s 2S1=2 ! 3p 2P3=2 at 396.15

nm and 4s 2S1=2 ! 3p 2P1=2 at 394.40 nm. In the LS-cou-

pling scheme, the intensities ratio for these lines is 2:1; how-

ever, the observed intensity ratio is �1.5:1. The nearby two

lines are the singly ionized calcium identifies as 4p 2P3=2

! 4s 2S1=2 at 393.36 nm and 4p 2P1=2 ! 4s 2S1=2 at 396.85

nm. Nevertheless, their intensities are nearly five times lower

than the neutral aluminum lines. We estimated the widths of

all these lines using a subroutine to fit the Lorentzian line

profiles to the observed data. The width of the Ca II lines

corresponding to the 4p 2P3=2 ! 4s 2S1=2 and 4p 2P1=2

! 4s 2S1=2 transitions have been extracted as 0.097 nm and

0.061 nm, respectively, whereas that of the Al I lines corre-

sponding to the 4s 2S1=2! 3p 2P1=2 and 4s 2S1=2! 3p 2P3=2

transitions have been determined as 0.147 nm and 0.173 nm,

respectively. It is noted that the widths of the Al I lines are

nearly twice that of the Ca II lines.

In Fig. 4 we show the spectrum covering the region

from 420 to 550 nm showing the resonance transition line of

calcium, 4s 4p 1P1 ! 4s2 1S0 at 422.67 nm and the triplet

structure of magnesium around 518 nm due to the 3s 4s 3S1

! 3s 3p 3P0,1,2 transitions. The appearance of these spectral

lines is the signature of the presence of magnesium in any

sample. The molecular structure in this region is attributed to

AlO free radical associated with the B2P!X2P band sys-

tem. The (0,0), (1,0) (0,1), (2,0), and (0,2) bands are clearly

resolved whereas the adjacent bands belong to the

Dv¼ 0, 6 1, and 6 2 vibrational progressions. The intensities

of the molecular transitions are very low as compared with

the atomic lines used in the analysis.

In Fig. 5, we show the spectrum covering the wave-

length region from 630 nm to 720 nm. The multiplet of neu-

tral calcium lines around 645 nm are identified as 3d4p3F2,3,4 ! 3d4s 3D1,2,3 transitions whereas an isolated line

3d4p 1D2 ! 3d4s 1D2 at 714.81 nm also belongs to neutral

calcium. The strong doublet around 670 nm belongs to neu-

tral aluminum identified as 5p 2P1=2, 3=2 ! 4s 2S1=2 transi-

tions. Interestingly, the Balmer Ha line at 656 nm appears

quite strong in this region. The appearance of the Ha line is

the evidence of the presence of hydrogen in the laser pro-

duced aluminum plasma.

FIG. 3. (Color online) The emission spectrum generated by the 1064 nm

laser; energy 120 mJ and at d �0.05 mm from the target surface, showing

predominantly the spectral lines of neutral Al and singly ionized calcium

along with the Lorentzian fit to determine the widths of the lines.

FIG. 4. (Color online) Showing the resonance line of calcium, multiplet

structure of calcium, magnesium, and the vibrational structure of the AlO

free radical recorded at different distances from the target surface. The inten-

sities of the spectra increase with the decrease of distance from the target

surface.

FIG. 5. (Color online) The emission spectrum generated by the 1064 nm

laser, energy 120 mJ at a distance 0.05 mm, showing the spectral lines of

aluminum, calcium, and Ha line of hydrogen.

083303-3 Rydberg transitions and elemental compositions Phys. Plasmas 18, 083303 (2011)

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It is evident that the spectral lines of Al, Mg, and Ca are

present in the emission spectrum of the sample. The reso-

nance lines of these elements are selected for the composi-

tion elemental analysis; Al I 394.40 nm, Mg I 285.21 nm,

Ca I 422.67 nm. The local thermodynamic equilibrium

(LTE) condition was verified by registering the intensities of

the spectral lines of the constituent elements as a function of

laser energy. If LTE exists, the intensities of the spectral

lines of all the elements increase linearly with the increase of

the laser energy.

In Fig. 6 we show the variations in the intensities for

some of the selected spectral lines of the elements as a func-

tion of laser energy; varied from 25 mJ to 40 mJ. The lines are

Al I at 396.15 nm, Ca I at 422.67 nm, Mg I at 285.21 nm. Evi-

dently, the lines intensity increase linearly with the increase of

the laser energy. The spectrum selected to determine the com-

position was recorded at a distance �0.25 mm from the target

surface and the laser energy was adjusted at 120 mJ. The rela-

tive compositions of major elements Al, Ca, and Mg have

been determined using the resonance lines of these elements:

Al I at 396.15, Ca I at 422.67, and Mg I at 285.21 nm. In the

commercially available Al 6063 alloy in Pakistan, the domi-

nant elements quoted are Ca, Fe, Mg, and Si. However, we

have not been able to detect any spectral line of Fe and Si

which shows that their contents are very small and the emis-

sion lines of these elements are too weak to be detected in the

present experiments. The lines of magnesium and calcium

have been detected with good intensities along with the domi-

nant aluminum lines. We estimated about 8% calcium, 4%

magnesium, and about 88% aluminum in the Al-6063 alloy.

The relative compositions of the elements as determined by

this technique are shown in Fig. 7.

B. Electron temperature and number density

Since we have observed the well resolved spectrum of

aluminum, it is tempting to calculate the plasma parameters

from this data. The electron temperature is determined by

using the Boltzmann’s plot technique, assuming the LTE.

The aluminum spectral lines used for the Boltzmann’s plot

are 256.79 nm, 257.59 nm, 265.24 nm, 266.03 nm, 305.71

nm, 308.21 nm, 309.27 nm, 394.40 nm, and 396.15 nm. A

larger energy difference between the upper and lower level

is opted for the accurate determination of the plasma temper-

ature Te. The corresponding Boltzmann plot is shown in Fig.

8, based on the following equation:10,19

lnkmiImi

hcAmigm

� �¼ ln

NðTÞQðTÞ

� �� Em

kT(1)

where kmi, Ami, and gm are the wavelength, the transition

probability, and the statistical weight of the upper level,

respectively, Em is the energy of the upper level, k is the

Boltzmann constant, and Te is the excitation temperature.

Q(T) is the partition function and N(T) is the total number

density. The relevant parameters to calculate the electron

temperature are listed in Table I.

FIG. 6. (Color online) Line intensities of the spectral lines as a function of

the laser energy. Al I 394.40 nm, Ca 422.67 nm, and Mg 285.21 nm.FIG. 7. (Color online) Relative compositions of elements present in the Al

6063 Alloy.

FIG. 8. (Color online) Boltzmann plot of aluminum lines recorded at

0.05 mm from the target using the1064 nm laser at 120 mJ.

083303-4 Baig et al. Phys. Plasmas 18, 083303 (2011)

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The spectroscopic data is taken from National Institute

of Standards and Technology (NIST) database. The experi-

mental data points are fit to the linear function and the

plasma temperature is calculated from slope of this straight

line which is equal to 1=kT. The plasma temperature is deter-

mined as 7580 6 250 K. The uncertainty in the plasma tem-

perature is about 15% which is attributed to the uncertainty

attached to the transition probabilities, determination of the

line intensities and the area under the curve.

The number density can also be estimated from the

intercept of the line of Equation (1) but we have calculated

the number density from the broadening of the spectral lines.

In LIBS the major causes of line broadening are Doppler

broadening and Stark broadening. The emission lines of the

laser produced plasma are noticeably broadened; therefore,

the electron number density can be estimated from the

widths of the spectral lines. The Doppler broadening can be

estimated from the relation,20,21

Dk ¼ km

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi8kT ln 2

Mc2

r;

where km is the wavelength, k(JK�1) is the Boltzmann constant,

T(K) is the temperature, m(Kg) is the atomic mass, and c(ms�1)

is the velocity of light. At a temperature of �10 000 K, the

Doppler width is estimated � 0.005 nm for the 394.40 nm Al

line, however, the experimentally observed broadening of this

line is 0.10 nm to 0.15 nm. Thus we can safely neglect the

Doppler broadening11 and the Ne is estimated from the Stark

broadening, using the following relation:17

Dk1=2 ¼ 2xNe

1016

� �; (2)

here x is the electron impact parameter, Dk1=2 is the FWHM

of the observed spectral line, and Ne is the electron number

density. The value of x¼ 0.0037 nm is reported in the litera-

ture22 and Dk1=2 is extracted by fitting the Lorentzian line

shape to the observe data. Fig. 9 shows the experimental data

along with the Lorentzian fit to the line profile of the isolated

aluminum line at 394.40 nm.

The observed line shape has been corrected by subtract-

ing the contribution of the instrumental width,4,23

Dk1=2 ¼ Dkobserved � Dkinstrumental:

The instrumental width of the LIBS 2000 spectrometer has

been determined as 0.05 nm using a narrow line width dye

laser.16

The LTE condition is verified using the McWhirter

criterion,24

Neðcm�3Þ � 1:6� 1012T1=2ðDEÞ3;

where T(K) is the electron temperature and DE(eV) is the

energy difference between the level. Using the 394.40 nm

aluminum line, DE¼ 3.13 eV and Te � 8000 K (calculated

from the Boltzmann), the electron number density turns out

to be Ne� 4.39� 1015 which is much lower than our esti-

mated value hence confirms the validity of the LTE in the

present work. The plasma temperature estimated from Al

6063 alloy spectra recorded at distances from 0.05 mm to 1.6

mm varies from 7580 K to 6250 K whereas the electron

number density Ne varies as 1.35� 1017–1.11� 1017 (cm�3)

at these distances. The variations in the plasma temperature

and electron number density are shown in Fig. 10. More over

the combined variation of Te and Ne is checked and it is

observed that the order of magnitude of variations of both

these parameters is identical as is evident from Fig. 10.

In conclusion, we have presented new data on the opti-

cal emission of the Al 6063 alloy revealing Rydberg

TABLE I. Spectroscopic parameters of the observed neutral aluminum lines.

Statistical weight

Transition probability A (s�1) Upper level Energy Em (cm�1)Wavelength k (nm) Transitions gm gi

256.79 4d 2D3=2! 3p 2P1=2 4 2 2.3� 107 38929.41

257.50 4d 2D5=2! 3p 2P3=2 6 4 2.8� 107 38933.97

265.24 5s 2S1=2! 3p 2P1=2 2 2 1.3� 107 37689.41

266.03 5s 2S1=2! 3p 2P3=2 2 4 2.6� 107 37689.47

305.71 3s3p4s 4P5=2! 3s3p2 4P5=2 6 6 7.5� 107 61843.54

308.21 3d 2D3=2! 3p 2P1=2 4 2 6.3� 107 32435.45

309.27 3d 2D5=2! 3p 2P3=2 6 4 7.4� 107 32436.79

394.4 4s 2S1=2! 3p 2P1=2 2 2 4.9� 107 25347.75

396.15 4s 2S1=2! 3p 2P3=2 2 4 9.8� 107 25347.75

FIG. 9. (Color online) Lorentzian fit to the Stark broadened profile of the Al

I line at 394.40 nm.

083303-5 Rydberg transitions and elemental compositions Phys. Plasmas 18, 083303 (2011)

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structure of aluminum and dominant lines of the other con-

stituent elements present in the sample. We have determined

the compositions of the constituent elements from the rela-

tive line intensities, whereas the plasma temperature and

electron number densities have been extracted as 7580 K and

�1.4� 1017=cm3, respectively, from the Al Rydberg struc-

ture. Molecular structure of the AlO free radical associated

with the B 2P!X 2P band system has also been identified.

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FIG. 10. (Color online) Spatial variations of electron number density and

temperature.

083303-6 Baig et al. Phys. Plasmas 18, 083303 (2011)

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