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Optik 124 (2013) 4776– 4779

Contents lists available at ScienceDirect

Optik

j o ur nal hom epage: www.elsev ier .de / i j leo

avelength Shift Investigation of Optical Emission from Nanosecond Pulsedaser Ablated Metal Al

un Gao ∗, Jingquan Linchool of Science, Changchun University of Science and Technology, Changchun, 130022, China

r t i c l e i n f o

rticle history:eceived 11 September 2012ccepted 25 January 2013

eywords:ulsed laser ablation

a b s t r a c t

The wavelength shift of optical emission from pulsed laser ablation of metal Al in 1atm air has beeninvestigated by using the time-resolved optical emission spectroscopy (OES). The wavelength shift ofOES has a second order exponential decay with time and was also found to be significantly influencedby the laser ablated pulse energy. By taking account of the shielding effect of free electrons inside of theplasma plume, the relation between the electron density and wavelength shift has been discussed.

© 2013 Elsevier GmbH. All rights reserved.

ptical emission spectroscopyavelength shift

ACS:2.38.Mf

2.50.Lp2.70.Kz

. Introduction

Pulsed laser induced plasma has a very short temporal exist-nce and is transient in its nature with a fast evolution of theharacteristic parameters that are heavily dependent on irradia-ion conditions such as incident laser energy, beam size, ambientas composition and pressure et al. It is also true that these param-ters vary drastically with axial or radial distance from the targeturface under the same experimental conditions. Pulsed laser abla-ion(PLA) describes the strong explosive laser-material interaction,hich has many applications in science and engineering, such

s pulsed laser deposition [1], nano-particle preparation[2], lasernduced breakdown spectroscopy (LIBS) [3], chemical analysis[4]nd precise micromachining[5] in the last two decades. Since therere many dynamic processes involved, the dynamics of PLA is veryomplicated, especially at its early expansion stage. These dynamicrocesses include pulsed laser interaction with materials, explo-ive removal of substrate fragments, and laser plasma interactionsmong plasma species. Plasma properties are very closely relatedith many parameters such as the electron temperature, the elec-

ron density, and the self-built magnetic field in the plasma plume.

t the early stage of the plasma plume expansion, the electron

emperature and density are in the order of 104K and 1017/cm3

espectively, which would change the behavior of plasma such

∗ Corresponding author. Tel.: +86 43185582761; Fax: +86 43185582761.E-mail address: lasercust@yahoo.com.cn (X. Gao).

030-4026/$ – see front matter © 2013 Elsevier GmbH. All rights reserved.ttp://dx.doi.org/10.1016/j.ijleo.2013.01.096

as the broadening and the shifting of observed spectral line [6].Recently, the spectra line shift has been observed and there existtheoretical calculations to predict and explain the shift during theprocess of pulsed laser ablation [7–9]. There is exist elementalmisinterpretation due to broadening and wavelength shift of anelemental emission spectral line in application of LIBS[10].

In the present work, the wavelength shifts of the plasma opticalemission induced by Nd3+:YAG laser ablation of metal Al were stud-ied by using of the time-resolved OES and the calculated electrondensity from Stark broadening. Finally, taking account of the shiel-ding effect of free electrons inside of the plasma plume, the relationbetween the electron density and wavelength shift has been dis-cussed. Research results can help understanding the mechanismsof PLA and making LIBS more attractive for industrial applications.

2. Experimental setup

The 355 nm frequency tripled Q-switched Nd3+:YAG laser (Con-tinuum, model Precision 8000) delivering 5 ns pulse duration ata repetition rate of 10 Hz was perpendicularly focused onto thesurface of pure metal Al (size was 50mm×50mm×3 mm)whichwas fixed in a rotated stage to produced plasma. The flat-Gaussianbeam was focused by a plano-convex quartz lens with 100 mmfocal length and the beam size was 100 �m on the sample sur-

face. The pulse energy was continuous changed by the laser energyattenuator composition of a half-wave plate and a gland prism.In the direction perpendicular to the incident laser beam, a cylin-drical lens is used to image the plasma plume onto the fiber of

X. Gao, J. Lin / Optik 124 (2013) 4776– 4779 4777

tcegdisatlAs

3

3

sts3dte

rdce

Fig. 1. Schematic of the Experiment Setup of optical emission spectroscopy of Al.

he spectrometer with a 1:1 magnification. The spectrometer wasoupled to an ICCD detector (model:PI-MAXII) with 1024×256 pix-ls, which spectral resolving power is less than 0.05 nm with 2400rooves/mm. The delay time which defined the time interval of theetector ICCD gate opening time to the time of laser pulse trail-

ng edge arriving at Al surface was controlled by the spectrometeroftware and monitored by an oscillator (Tektronix, TDS644B). Tocquire the time-resolved emission spectroscopy of plasma plume,he ICCD was synchronous trigged using the Q-switch signal of nsaser. Gate width was setted 20 ns during the whole experiments.ll the measurements were carried out in air at one normal atmo-pheric pressure.

Fig. 1.

. Results and discussion

.1. Plasma optical emission spectroscopy

The time resolved OES of nanosecond pulsed ablation of Al ishown in Fig. 2. The laser fluence is 91.7GW/cm2, and the delayime are 7, 57, 107, 157, 207 ns respectively. There are individualpectral lines such as AlI394.4nm(3s23p-3s24s), AlI396.1nm(3s23p-s24s) superposition of the continue emission in the 380—420 nmetection range. Furthermore there also exists a NII 399.5 nm spec-ral line which is from cascade ionization at the early plasma plumexpansion stage,but its life time is shorter than 100 ns.

The continuum spectrum generated from bremsstrahlung and

ecombination radiation from laser–plasma interaction [11] isominant as delay time less than 100 ns.The high intensity of theontinuum spectrum indicates that there are a lot of free electrons,xcited atoms and ions, and clusters in the ejected vapor near the Al

Fig. 2. Optical emission spectroscopy of Al plasma plume.

Fig. 3. Line shift of Al spectra at different time delays.Laser fluence is 91.7GW/cm2.

surface at the early stage of plasma expansion. The continuum spec-trum intensity drops gradually to zero when delay time higher than400 ns, which results from the surround air cooling and electron-ion recombination in the plasma plume. During expansion processof the plasma plume, electron density and plasma temperaturedecrease, which leads to a reduction of the number of atoms inthe excited state and the intensity of the corresponding discreteatomic spectral lines.

At the early stage of plasma expansion, the full width of halfmaximum (FWHM) of spectral line because of the stark broaden-ing is large, there are two Al lines in close proximity that overlapless than 50 ns and then can distinguish each other, so the FWHMand wavelength shift of plasma emission spectral line were studiedafter delay time 100 ns.

3.2. Wavelength shift

The wavelengths of characteristic line of plasma has a shift var-ied with delay time during the plasma plume expansion at 2.78 mmdistance from target surface, as shown in Fig. 3. There is a redshift phenomenon after laser ablation, which the OES wavelengthdecreases as the delay time increasing and the spectral broadeningwidth simultaneous became narrow at the plasma expansion pro-cess. Take AlI396.1nm(3s23p-3s24s) for example (see Fig. 4), thewavelength of OES approaches to the NIST spectral value when

the delay time is longer than 1500 ns.Wavelength shift has a sec-ond order exponential decay vary with delay time for error rangeunder the 10%. The shift dramatically reduces when the delay timeis below 500 ns, then slowly changing.

Fig. 4. The wavelength shift of 396.15 nm and electron density variation with delaytime. Laser fluence is 91.7GW/cm2.

4778 X. Gao, J. Lin / Optik 124 (2013) 4776– 4779

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eeptebts

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Fig. 6. AlI396.1 nm line shifts versus the electron density. The dots are the exper-

diagnosed by observing the red-shift of spectral lines. We focus

ig. 5. The wavelength shift of 396.15 nm variation with laser fluence at differentaser delay time.

Wavelength shift of OES at different delay time of 115, 515,95 ns varied with laser ablated energy as shown in Fig. 5. The laseruence were 40.7, 66.2, 78.9, 91.7 GW/cm2 respectively in exper-

ment. Wavelength shift values increased with laser fluence,andncreased significantly at the early stage of plasma plume expan-ion, but changed slowly after 500 ns, which indicated that laseruence is an important factor to the wavelength shift of OES.

The broadening of spectral lines is a complicated function of thenvironment of the radiating atoms and ions, and there are sev-ral possible broadening mechanisms[12]. The pressure of plasmalume is up to some GPa during the PLA process [13,14], which leadso the collisions among charged particles in the plume. A stronglectric field resulting from charged particles produces a spectrumroadening of the transitions between the split atomic levels, sohe Stark broadening is the dominant broadening mechanisms ofpectral line.

Assumption the local thermodynamics equilibration (LTE) con-ition, we can get the number density of the electrons in the plasmay measuring the FWHM of the spectral line. The FWHM of Starkroadening ��1/2 is related with the electron density Ne [12]

�1/2 = 2W(

Ne

1016

)+ 3.5A

(Ne

1016

)1/4

×[

1 − 34

N−1/3D

]W

(Ne

1016

)Å (1)

Here W(nm) is the electron width parameter, which is a weakunction of temperature, A(nm) is the ion broadening parameter,nd ND is the number of particles in the Debye sphere. The contri-ution due to the ion broadening is very small and can be neglected.hus only the first term on the right in equation (1) remains, whicheans that the electron density is approximately proportional to

he FWHM of Stark broadening. So we measured FWHM of themission line AlI396.1 nm that can give the evolution of the elec-ron density. The obtained results are shown in Fig. 4. It can be seenhat the electron density have the second order exponential decayith the delay time and decreases swiftly at the early stage of thelasma expansion,

The plasma emission spectra red-shift phenomenon is growingoncerned in the study of pulsed laser ablation. Currently the maineasons of the spectrum red-shift are considered: the Stark broad-ning effect by the self- established electric field of the free electronithin the plasma plume, the pressure induced by the shock waveuring the plume expansion process, the electric dipole formattedy electron - ion within the plasma plume interactions with the

ound electron of atoms, etc. Based on the experimental result weeduced the electron density is up to 1017 • cm-3 from the spec-ral broadening, and thus it plays dominant role in the red-shiftf the spectral lines. The electric field produced by the high free

imental data, the line is the simulation considering the Debye sphere numbers ND

unrelated to electron density.

electrons density induced the Stark broadening of spectral line,and caused atomic-level interval changing, which induced the OESwavelength shift. Considering the time evolution laws of the wave-length shift and the electron density, we show that the wavelengthshift increases with the plasma electron density, as shown in Fig. 6.This result can be explained as follows. The wavelength shift value��shift is [15]

��shift = D(

Ne

1016

)± 2A

(Ne

1016

)1/4

×[

1 − 34

N−1/3D

]W

(Ne

1016

)Å (2)

Where D is independent of electron density while influenced bythe plasma temperature Te. The results show that the experimentalresults of wavelength shift variation with the electron density isconsistent with theory simulation if assumption that the numberparticles ND is unrelated to the electron density (see Fig. 6).

Preliminary analysis indicates that, when the electron density ofplasma plume increases, the difference of the atomic energy levelis reduced, and then the red shift is raised. The Al atomic levelsembedded in plasma plume are perturbed by the self-sustainedelectric field produced by the high free electrons density, whichscreen the Coulomb potential of the Al atomic nuclear. Then theelectrical field’s attractive force between the nucleus and boundelectrons are diminished, while the repulsion force of the free tobound electrons is enhanced, so that the energy levels out of thenucleus are raised and the intervals of excited energy level 3s23pto 3s24s are diminished. Therefore, the greater the electron densityof the plasma plume, the larger the electric field shielding of thenucleus. The electrical attractive force between the nucleus and itsbound electrons weaken significantly, which induced the gap ofnuclear level reducing and the wavelength shifted.

4. Conclusion

In summary, the 355 nm frequency tripled Q-switched Nd:YAGlaser is focused onto metal Al surface mounted in ambient air.Wavelength shift of OES was found by time resolved diagnostictechnique and has a second order exponential decay with delaytime. Thus, some parameters of the plasma plume such as spec-troscopy lines broadening and the electronic density can be indirect

on only wavelength shift of OES produced by laser ablation in1atm atmosphere, then next step study the wavelength shift phe-nomenon on vacuum condition and other ambient atmosphere tosee the relation between the wavelength shift and gas pressure.

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X. Gao, J. Lin / Optik

cknowledgements

This work was supported by the National Natural Scienceoundation of China (Grant No. 11074027, 60978014, 61178022),unds from the Natural Science Foundation of Jilin Province (Granto. 20100168 and 201215132) and the specialized research fund

or the doctoral program of high education of China (Grant No.0112216120006)

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