Spectrochimica Acta Part B 66 (2011) 444–450

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Spectrochimica Acta Part B

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Effect of transverse magnetic field on the laser-blow-off plasma plume emission inthe presence of ambient gas

Ajai Kumar ⁎, R.K. Singh, Hem JoshiInstitute for Plasma Research, Bhat, Gandhinagar 382 428, India

⁎ Corresponding author.E-mail address: ajaiipr@yahoo.com (A. Kumar).

0584-8547/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.sab.2011.04.011

a b s t r a c t

a r t i c l e i n f o

Article history:Received 7 March 2011Accepted 25 April 2011Available online 4 May 2011

Keywords:Laser-blow-offLi plasma plumeMagnetic fieldAmbient gas

In the present paper we report the effect of variable magnetic field in the range of 0.04–0.2 T on the emissionof two neutral Lithium lines Li I 670.8 nm and Li I 610.3 nm and one ionic line Li II 548.4 nm in the presence ofambient gas on the laser-blow-off plasma plume. Enhancement in the intensity associated with structureswas observed in the temporal profile of neutrals, which is strongly dependent on the magnetic field intensity,distance from the target and background gas pressure. At 6 mm distance and 1.33 Pa argon pressure, apartfrom overall enhancement in the intensity of both the neutral lines, the results reveal some interestingfeatures, e.g. disappearance of structures and narrowing of the temporal extent of 670.8 nm line at highermagnetic fields. On the other hand, the 610.3 nm line exhibits a significant enhancement in the intensity atthe trailing part as the field is increased. At a shorter distance (2 mm) and for relatively higher pressures(133.3 Pa), the effect of field is not much prominent. Interestingly, the ionic spectral line does not exhibit anysignificant change with both, magnetic field and ambient gas.We explain the results by considering the role of various atomic processes viz. electron impact excitation,recombination and diffusion of ambient gas into plume in collisional and hydrodynamical regimes.

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© 2011 Elsevier B.V. All rights reserved.

1. Introduction

The presence of magnetic field during the expansion of laser-blow-off (LBO)/laser produced plasma (LPP) can initiate several interestingphysical phenomena, which include conversion of kinetic energy intoplasma thermal energy, plume confinement, ion acceleration/decel-eration, emission enhancement/decrease, plasma instabilities, debrismitigation for extreme ultraviolet source, increase in the detectionsensitivity of laser-induced breakdown spectroscopy etc. [1–6].

Plasma changes its physical properties during expansion across amagnetic field, and this ultimately affects its emission characteristics.An enhancement in optical emission under the influence of magneticfield has been reported in some earlier works [4–16]. Synergisticeffect of ambient gas and at fixed magnetic field has been the subjectof investigation in the recent years [17] because of its projectedapplicability in debris mitigation in vacuum ultraviolet lithographicsources. By using a transmission grating spectrograph, EUV calorim-eter and Faraday cup, Harilal et al. found that introduction of magneticfield in tin plasma in the presence of the ambient gas was quiteeffective in debris (energetic ions) mitigation. The role of ambient gasin debris mitigation was also verified from shadowgraphy results [18].

In a recent report, we have studied the emission characteristics oflithium emission lines in laser-blow-off plume in the presence of

magnetic field in vacuum [14,15] and have found that enhancement inthe intensity is due to increase in electron impact excitation processes.However, the synergistic effect of the magnetic field (with varyingfield) and the ambient environment on plume expansion by usingoptical emission spectroscopy, particularly exploring the behavior ofdifferent atomic transitions, has not been not considered so far. Thecombined effect of magnetic field and ambient gas is expected to giveadditional information, which can help understand the physicalprocesses involved in the expanding LBO plume.

In the present paper, we report the evolution of the temporalemission from LBO plume in presence of the magnetic field of differ-ent strengths and ambient argon gas. In order to get more insight intothe atomic processes, we considered two prominent neutral lithiumlines viz. resonance line Li I 670.8 nm (2s 2S1/2←2p 2P3/2,1/2) and Li II610.3 nm line ð2p 2P1=2;3=2 ← 3d 2P3=2;5=2Þ and one ionic line Li II548.4 nm (2s 3S1←2p 3P2,1,0) (Fig. 1) and studied their temporalevolution in the presence of varyingmagnetic field at different ambientpressures at 2 mm and 6 mm from the target.

2. Experimental setup

A schematic diagram of the setup is shown in Fig. 2. Theexperimental technique of time of flight emission spectroscopy usedin the present work has been described elsewhere [15]. Briefly, theplasma plume is generated inside a multi-port stainless steelchamber, which is evacuated to a base pressure of ~1×10−3 Pa. Thetarget is composed of uniform layers of 0.5 μm thick carbon and

Fig. 1. Simplified energy level diagram for Li I 670.8 nm, Li I 610.3 nm and Li II 548.4 nm transitions.

445A. Kumar et al. / Spectrochimica Acta Part B 66 (2011) 444–450

0.05 μmof LiF films on a thick quartz substrate. Target is mounted on amotorized x–y translator stage so as to expose a fresh region of thetarget for successive shots. An Nd:YAG laser (λ=1.064 μm) having apulse width of 8 ns was used to generate the plasma plume. The laserbeam was focused on the target surface. The spot size of the laserbeam was about 1 mm diameter at the target to achieve an averagepower density of~109 W/cm2. A pulsed power system consisting of acapacitor bank and a wire wound solenoid was used to produce themagnetic field in the range of 0.04–0.20 T (flat–top duration of themagnetic field profile is 40 μs; which is much larger than the plumeduration). For time and space resolved spectroscopy, the plasmaplume is viewed perpendicularly to the direction of expansion andimaged at the entrance slit of a monochromator (Δλ=12.5 Å). Themonochromator with photomultiplier tube was mounted on a single-stage translator system, which enabled space-resolved scan of theplume along its expansion axis. The time resolution in the presentexperiment is about 4 ns. Overall maximum uncertainty in ourmeasurement is less than 8%.

3. Results and discussion

In the present study, two spectral lines from neutral lithium Li I670.8 nm and Li I 610.3 nm and one ionic line Li II 548.4 nm are used

Fig. 2. Schematic diagram o

for investigating the effect of magnetic field in the presence ofambient gas.

Fig. 3 shows the variation in the neutral emissions viz. 670.8 and610.3 nm in vacuum, and at 1.33 and 133.3 Pa of argon pressure andfor different magnetic fields in the range 0.04–0.2 T observed at adistance of z=2 mm from the target . In vacuum, the 670.8 nm lineshows one fast strong peak in field free case and a broad and ratherslower shoulder appears in the temporal profile in the presence of thefield. Intensity of the slow component increases with field intensity.This effect is more prominent at 1.33 Pa pressure where the intensityof the broad shoulder is increased considerably and the maxima areshifted towards the shorter time delay with increase in the fieldstrength. However for further increase in pressure to 133.3 Pa, thisbroad shoulder completely vanishes and a moderate increase inintensity is observed when the field is increased.

On the other hand, the temporal profile of 610.3 nm line wasfound to have a double peak structure in the absence of themagnetic field. The intensity of 610.3 nm line is much larger ascompared to 670.8 nm. This is because at shorter distances, plasmatemperature and density are considerably high and hence morepopulation of the higher excited state is expected. Interestingly the610.3 nm line does not show an additional broad component in thepresence of the magnetic field. In this case the effect of magneticfield is not that significant as observed in case of 670.8 nm line.

f the experiment setup.

Fig. 3. Temporal profiles of Li I 670.8 nm (left panel) and Li I 610.3 nm (right panel) in vacuum 1.33 and 133.3 Pa and for different magnetic fields. The profiles were recorded atz=2 mm.

446 A. Kumar et al. / Spectrochimica Acta Part B 66 (2011) 444–450

However at higher pressure (1.33 and 133.3 Pa), some enhance-ment in the intensity of the second component is observed withincrease in the field.

Fig. 4 shows the variation in the neutral emissions viz. 670.8 and610.3 nm at different pressures and for differentmagnetic fields in therange 0.04–0.2 T observed at a distance of z=6 mm from the target.In vacuum, with increase in the magnetic field, the profile showsstructures as well as enhancement in the intensity. The intensityenhancement for 670.8 nm line is considerably higher as compared to610.3 nm line. However, when the ambient pressure is increased to1.33 Pa, the temporal profile of 670.8 nm line exhibits an interestingchange with increase in the field. Its temporal extent shrinks sharplyand the overall profile becomes narrower although there is anincrease in the overall intensity as compared to the field free case. Onthe other hand, 610.3 nm emission line exhibits overall enhancementin the intensity particularly at the trailing part as the field is increased,which is strikingly different to what is observed in case of 6708 nmline. However, unlike in case of vacuum, the structures in the temporalprofile are not evident at this pressure. With further increase inpressure to 133.3 Pa, there is not much pronounced effect of themagnetic field on both 670.8 nm and 610.3 nm temporal profilesunlike in case of 1.33 Pa (Fig. 4).

Here it can be pointed out that under same experimentalconditions, the difference in the temporal profiles for these twoneutral lines should originate in their photon emissivity [14,15] andhence these observations can be more qualitatively understood interms of computed photon emissivity coefficients (PEC) using atomicdata and analysis structure (ADAS) [19] for both the lines. The

intensity of a particular line can be expressed in terms of PEC [15]. InADAS, it is assumed that the collisional and radiative processesbetween all excited levels redistribute the populations and the excitedlevels are in quasi-static equilibrium with the metastable states. Theemissivity of an individual line between states j and k for electronimpact excitation is given by

εj→k excð Þ = Aj→k ∑M

σ=1F excð Þjσ NeNσ ð1Þ

and emissivity of an individual line due to recombination is given by

εj→k recð Þ = Aj→k ∑M

υ=1F recð Þjυ NeNυ ð2Þ

where Aj→k is the transition probability for transition between j and klevels, Fσexc and Fυ

rec are the effective contributions to the populations ofthe excited state from metastable σ of the atom and ν of the ion forelectron impact excitation and recombination respectively (electrondensity and temperature dependent) and Ne, Nν and Nσ are electrondensity, density of ions in metastable ν and density of atoms inmetastable σ respectively. Here it can be noted that recombinationincludes two body, three body as well as dielectronic contributions.The respective emissivity for electron impact excitation and recom-bination can be expressed in terms of

PEC’s εj→k excð Þ = PECexcNeNa ð3Þ

Fig. 4. Temporal profiles of Li I 670.8 nm (left panel) and Li I 610.3 nm (right panel) in vacuum, 1.33 and 133.3 Pa for different magnetic fields. The profiles were recorded atz=6 mm.

447A. Kumar et al. / Spectrochimica Acta Part B 66 (2011) 444–450

and

εj→k recð Þ = PECrecNeNi ð4Þ

where PECexc and PECrec are photon emissivity coefficients forelectron impact excitation and recombination respectively.

From the knowledge of PEC, the intensity of a particular line can berepresented by [15]

I = KNe PECexcNa + PECrecNi

� � ð5Þ

where K is the factor that depends on the geometry of observationand detector response. Evidently intensity will depend on (i) electrondensity, (ii) numberdensity of atoms/ions, (iii) PEC and (iv) geometricalfactor. A typical plot for the variation of PEC with electron temperatureat 1015 cm−3 electron density is shown in Fig. 5.

The observed behavior in the intensity of these two transitions in thepresence of themagnetic field and at different ambient pressures can beexplained by considering the following processes occurring simulta-neously (i) Joule heating due to the presence of the magnetic field andhence an increase in electron temperature [4], (ii) increase in electronconfinement in thepresence of themagneticfield [5], (iii) heatingdue tothe ambient gas itself [20] and (iv) increase in electron density as aresult of interaction between plume plasma and ambient gas.

Earlier we have found [14,15] that the increase in the intensity ofthese lines in the presence of the magnetic field in vacuum is due toincrease in electron impact excitation, which increases with increasein electron temperature as well as electron density. The difference in

the intensity enhancement for these two lines has been attributed tothe difference in electron impact excitation for these transitions dueto increase in Joule heating (increased electron temperature) [15].Moreover, the temporal evolution of the plume in vacuum and in thepresence of the field is correlated to the magnetic diffusion time (timeessential for Joule heating), which is ~4 to 1 μs for our experimentalparameters. This means that with increase in the field the plume willbe affected at shorter time delays.

Further, in the presence of the ambient gas, it should be noted thatcollisions start between the plume species and ambient atoms, whenthe plume dimensions are of the order of the mean free path of theejected species [20,21]. Using the Westwood model [22], theestimated mean free path for lithium atoms is ~4.8 mm at 1.33 Pa.The estimated mean free path establishes the fact that the plumeexpands into collisional regime at this pressure for z=6 mm. In thisregime, interpenetration of the plasma plume into the ambient gasoccurs, which causes an increase in the collisions between the plumespecies and the ambient gas atoms. Hence, there will be an increase inthe electron density in this pressure regime. It is also noticed that atthis pressure, plume front temperature is increased (~7 eV) [23].However, as the time evolves, plasma electrons quickly lose theirkinetic energy through elastic and inelastic collisions with theambient gas atoms. As a result, plasma with high density and lowtemperature (especially at the trailing portion of the plume) ispresumably formed. At sufficiently higher pressures (133.3 Pa), theplumematerial pushes away the background gas. The compressed gasrestricts the diffusion of the plume material and an interface isformed. On the development of the interface boundary between the

Fig. 5. Photon emissivity coefficient as a function of electron temperature for electron impact excitation and recombination processes for Li I 670.8 nm, Li I 610.3 nm and Li II548.4 nm transitions. The electron density is taken as 1015 cm−3.

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plume and the surrounding gas, the expansion transforms fromcollisional regime to hydrodynamical regime. The compressed gasrestricts the expansion of the plume, thereby confining the plume in asmaller volume [20,21].

Combined effect of the magnetic field and the ambient gas on thetemporal evolution of neutral emission lines can be understood asfollows. Apart from the heating of the plume front and increase inelectron density in presence of the ambient gas, the presence ofmagnetic field also increases the electron temperature due to Jouleheating [4] and electron density due to magnetic confinement [13].The region of maximum enhancement depends on the magneticdiffusion time, which is shortened with increase in the magnetic fieldintensity. The increase in electron temperature can cause an increasein the electron impact excitation process as the electron impactexcitation rate will increase with electron temperature (Fig. 5).

In vacuum the appearance of broad component with field can beattributed to the presence of slow neutrals emanating from the directvaporization of lithium. As is evident from Fig. 5, PECexc is muchhigher than PECrec for both the emission lines 670.8 and 610.3 nm inthe temperature range N1 eV and the electron impact excitation for670.8 nm is considerably higher in comparison to the 610.3 nm line.This explains the appearance of a broad shoulder as well as greaterenhancement for the 670.8 nm line with increasing the magnetic fieldas the electron impact excitation rate increases with electron tem-perature and electron density.

As is already mentioned at 1.33 Pa pressure, the plume expands incollisional regime for a distance zN4 mm. Therefore, at this pressureregime, plume–ambient atom collision plays an important role in theobserved differences in the temporal profiles of the neutral emissionat z=2 mm and z=6 mm. At z=6 mm, plume–atom collision causesan increase in the electron temperature as well as density and henceincrease in the electron impact excitation (dominates for 670.8 nm)which is strongly dependent on the temperature and density. Atshorter distance (z=2 mm), relatively dense phase of plasma isformed, which has little interaction with the ambient gas and alsowith the magnetic field and hence is comparatively less affected by

the environment. Further at 133.3 Pa pressure, plume already getsconfined and hence it is expected that magnetic field may not bringabout any more prominent changes in the line emission.

Again referring to Fig. 4, one of the important observation is thatthere is opposite behavior in the temporal evolution of emissionintensity for 670.8 and 610.3 nm lines at the trailing portion of theplume at z=6 mm and 1.33 Pa argon pressure. This can beunderstood as follows. Magnetic field results in Larmor motion ofthe electrons, which effectively increases the frequency of electron–atom collision and is significantly dominated in the presence of theambient gas. This leads to rapid decrease in electron temperature withtime for higher magnetic fields. This means in the presence of themagnetic field, the trailing portion of the plume has lowertemperature as compared to that of field free case and will shift toshorter delay with an increase in the field. Since PECexc is much higherthan PECrec in case of 670.8 nm, it is likely that the 670.8 nm line,which is mostly populated by electron impact excitation will show adecrease in intensity with decrease in electron temperature. One cansee from Fig. 5 that PECexc for 670.8 nm decreases by an order ofmagnitude as Te decreases from 1.5 to 1 eV. As the magnetic fieldincreases, the emission intensity of 670.8 nm at the trailing portion ofthe plume is decreased due to the decrease in electron temperature inthis region and finally disappears for 0.2 T. Thus, this explains theabsence of trailing part for 670.8 nm line with increase in themagnetic field.

Here we would like to mention that at relatively high electrondensity (≥1014 cm−3 as in the present case) and sufficiently lowelectron temperature (~0.5 eV), the dominant recombination mech-anism is three-body recombination [24]. The rates for radiativerecombination and three-body recombination are given by [25]

Rr = 2:7 × 10−19neniZ2T−3 = 4

e m−3s−1 ð6Þ

R3b = 9:2 × 10−39n2eniZ

3T−9 = 2e ln

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiZ2 + 1

pm−3s−1

: ð7Þ

449A. Kumar et al. / Spectrochimica Acta Part B 66 (2011) 444–450

Hence, for three-body recombination to be dominant, the electrondensity (ne) should follow the following condition [25]

ne≫ 3 × 1019T3:75e

� �= Z m−3 ð8Þ

In the present case, for a typical value of Te ~0.5 eV (estimatecorresponding to the trailing part), this density comes about2×1018 m−3 or 2×1012 cm−3. We would like to mention that theelectron density for the trailing portion is much higher than this valueand hence three-body recombination is expected to dominate.

Also for high electron density region (≥1014 cm−3), the plasma iscollision dominated and levels may get populated according to LTEconditions. Therefore, the higher excited states (e.g. 610.3 nm) areexpected to get more populated by three-body recombination. In caseof 610.3 nm line, the difference between PECexe and PECrec is lower ascompared to that of 670.8 nm especially at lower temperature regimeas shown in Fig. 5. It is likely that the 610.3 nm line is populated due tothe combined role of recombination and electron impact excitationwhereas in case of 670.8 nm, electron impact excitation is thedominant process. At the trailing portion of the plume, the electronimpact excitation will decrease and at the same time the three-bodyrecombination cross-section will increase with decrease in theelectron temperature. However, there will be some contribution dueto the increase in electron density resulting from magnetic confine-ment. This will exhibit a net increase in the emission of 610.3 nm. Thiscould be the reason for the increase in the intensity of 610.3 nm line atthe trailing portion of the plume with increasing the field as observedin the present experiment.

Scenario is different for the ionic spectral line. Interestingly, thetemporal behavior of Li II 548.4 nm ionic line did not show anysignificant changes in the presence of the ambient gas and magneticfield as shown in Fig. 6. It can be seen that in vacuum there is a modestincrease in intensity after 0.8 T. However, at 1.33 Pa argon pressure,the intensity increases even for 0.04 T although the increase is modest

Fig. 6. Temporal profiles of Li II 548.4 nm in vacuum and 1.33 Pa pressure and fordifferent magnetic fields. The profiles were recorded at z=2 mm.

in this case also. For further higher fields (N0.08 T) the intensitydecreases slightly.

The dominant mechanism for the excitation of 548.4 nm ionic linehas been found to be the recombination mechanism [15,16] as shownin Fig. 5. In vacuum and for low fields, the electron density is notaffected much and hence the intensity is not expected to get changedsignificantly. However, for higher fields due to increase in confine-ment a small increase in density is expected, which, of course, willresult in increased intensity. For 1.33 Pa pressure, as the confinementmay take place even for lower fields, we can expect an increase in theintensity due to increased electron density. For higher fields, electrontemperature also increases resulting in decreased recombination andhence decreased intensity.

In short the present study clearly demonstrates the role of ambientgas on the plasma plume by bringing into the plasma from freeexpansion (in vacuum) to collisional regime at 1.33 Pa and tohydrodynamical at 133.3 Pa for z=6 mm. Each of these regimesshows different behavior with respect to the plume–magnetic fieldinteraction.

It is also worthwhile to mention that simply exploring thebehavior of 670.8 nm emission profile at z=6 mm and at 1.33 Pacould have given a false impression that the neutrals are confined tohigher energy regime (shortening of temporal profile). In fact,simultaneous study of another transition (610.3 nm) clearly demon-strates that it is not correlated with confinement but to the excitationprocesses itself. The present study also underlines the importance ofsimultaneous temporal study of different transitions in order to have afiner picture of the overall mechanistic aspect in optical emissionspectroscopy.

4. Conclusion

Summarizing the present study, we studied the combined effect ofthe magnetic field and ambient gas on the temporal emission profilesof Li I 670.8 nm, Li I 610.3 nm and Li II 548.4 nm in the laser-blow-offplume. For z=6 mm, the temporal profiles for these two neutral linesshow distinct features at the tailing part at 1.33 Pa argon pressure. Atfurther higher pressures (133.3 Pa) the effect of field in not thatprominent. However for z=2 mm, the trailing potion also showsconsiderable enhancement at 1.33 Pa. On the other hand, the ionicline does not exhibit any prominent change. The difference isexplained by considering electron impact excitation and recombina-tion processes and ambient gas diffusion in collisional and hydrody-namical regimes.

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