Anisotropic emission in laser-produced aluminum plasma in ambient nitrogen

Applied Surface Science 253 (2007) 3113–3121 www.elsevier.com/locate/apsusc

Anisotropic emission in laser-produced aluminum plasma in ambient nitrogen A.K. Sharma *, R.K. Thareja Department of Physics and Centre for Laser Technology, Indian Institute of Technology Kanpur, Kanpur 208016, India Received 7 February 2006; received in revised form 12 June 2006; accepted 2 July 2006 Available online 8 August 2006

Abstract We report on the dynamical expansion of pulsed laser ablation of aluminum in ambient pressure of nitrogen using images of the expanding plasma. The plasma follows shock model at pressures of 0.1 Torr and drag model at 70 Torr, respectively, with incident laser energy of 265 mJ. The plasma expansion shows unstable boundaries at 70 Torr and is attributed to Rayleigh–Taylor instability. The growth time of Rayleigh–Taylor instability is estimated between 0.09 and 4 ms when the pressure is varied from 1 to 70 Torr. The pressure gradients at the plasma–gas interface gives rise to self-generated magnetic field and is estimated to be 26 kG at 1 Torr ambient pressure using the image of the expanding plasma near the focal spot. The varying degree of polarization of Al III transition 4s 2S1/2–4p 2P83/2 at 569.6 nm gives rise to anisotropic emission and is attributed to the self-generated magnetic field that results in the splitting of the energy levels and subsequent recombination of plasma leading to the population imbalance. # 2006 Elsevier B.V. All rights reserved. PACS: 52.30.-q; 52.35.Py; 52.35.Tc; 52.50Jm Keywords: Ablation; Shock wave; Rayleigh–Taylor instability; Self-generated magnetic field; Degree of polarization

1. Introduction Laser-ablated plasmas have been the subject of interest from the point of view of fundamental physics and various applications in the field of material processing, biology and medicine, and chemical analysis [1–4]. Interaction of plasma with an ambient gas gives rise to interesting features such as shock wave formation, instability, plume splitting/bifurcation, and self-generated magnetic field, to name a few [5–8]. Expansion of plasma has been investigated in detail and various theoretical models have been proposed [9–12]. Fast photography using intensified charge coupled device (ICCD), shadowgraphy, and Schlieren photography have been used to study the plume dynamics of the expanding plasma in vacuum and ambient atmosphere [13–15]. Using equations of mass,

* Corresponding author. Present address: Institut fu¨r Physik und Physikalische Technologien, Technische Universita¨t Clausthal, Leibnizstrasse 4, D 38678, Clausthal-Zellerfeld, Germany. Tel.: +49 5323 72 3628; fax: +49 5323 72 3600. E-mail address: [email protected] (A.K. Sharma). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.07.014

momentum, and energy conservation [16] physical parameters of interest like vapor density, vapor pressure, and vapor temperature just behind the shock front can be estimated [17]. Two-dimensional imaging has been extensively used to study the distribution of atoms, molecules, clusters, and nanoparticles in laser-ablated plasmas [18–20]. In presence of an ambient gas expanding plasma may show instabilities at the plasma–gas interface, namely Rayleigh– Taylor (RT) instability [5] and is reported in the carbon plasma expanding in presence of an external inhomogeneous magnetic field [21]. The dynamics of laser-produced aluminum plasma in the presence of an external magnetic field is also discussed in the literature [22]. Laser-produced plasma is also a source of both electric [23] and self-generated magnetic fields. Selfgenerated magnetic fields in laser-ablated plasmas have been studied using magnetic probes [24,25], Faraday rotation method [26,27], polarization measurements of higher order VUV laser generated harmonics during interaction with polished glass target [28], and interaction of femtosecond pulse with aluminum as the target [29]. These magnetic fields have an important role in laser induced fusion studies and

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transport properties [30,31]. It has also been shown theoretically that these fields can absorb light via excitation of upper hybrid oscillations and gives rise to second harmonic emission [32]. Interaction of short pulses result in the emission of X-rays from the laser-ablated plasmas [33,34] and in order to understand the energy transport properties of these high density plasmas X-ray line polarization studies are being done extensively [35,36]. Due to non-equilibrium nature of these plasmas the degree of polarization measurements have been used to understand the anisotropy that contribute to polarized emission [37]. We report on ablation characteristics of the expanding aluminum plasma in nitrogen ambient using ICCD images (a) to discuss the plume dynamics in terms of shock and drag models, (b) to estimate the growth time of RT instability and its pressure dependence, (c) to estimate the initial strength of selfgenerated magnetic field, which to the best of our knowledge has been attempted for the first time, and (d) to study the effect of ambient gas on the degree of polarization, an indication of anisotropic emission, using space- and time-resolved optical emission spectroscopy. 2. Experimental We used a Q-switched Nd:YAG (DCR-4G, Spectra Physics) laser with a pulse width of 8 ns at full width half maximum (FWHM) at a repetition rate of 10 Hz, operating in the fundamental mode (l = 1.064 mm) for creating plasma in vacuum and in presence of nitrogen gas. The laser beam was focused on the aluminum target in a vacuum chamber to a spot size of 260 mm. The target was continuously rotated so that the laser beam falls on the fresh target surface. The vacuum chamber was evacuated to a pressure 105 Torr and was flushed with the gas several times before introducing it in a controlled manner. The nitrogen pressure in the chamber was varied from 0.01 to 70 Torr. The images of the expanding plasma were recorded using a gated CCD (DH 720, Andor Technology) at various time delays with respect to the ablating laser pulse. ICCD consisted of 696  256 active CCD array. To study the degree of polarization in the plasma a Wollaston prism was used in the path between the plasma emission and the monochromator (HR-320, Jobin Yvon) slit to record the spaceand time-resolved spectrum of two different polarized states simultaneously. The output was detected using ICCD as the detector at the exit slit of the monochromator with spatial response in the region 190–850 nm. 3. Results and discussion 3.1. Plume dynamics Interaction of expanding plasma with the background gas results in attenuation, thermalization, and scattering of the plasma. As the background pressure is increased plasma emission increases due to collisions between the plasma and background gas at the plasma–gas interface and also within the plasma due to the collisions amongst the constituent particles.

UV radiations emitted due to the interaction of laser with the target interacts with the ambient gas and results in an increase in the density in a very narrow region which propagates in the ambient atmosphere with the speed more than that of the local ion sound speed given by cs = (hZikTe/mi)1/2 as a shock wave. UV radiation may also modify the front of the shock wave. hZi is the average ion charge, k is the Boltzmann constant, Te is the electron temperature, and mi is the ion mass. According to the Taylor–Sedov (T–S) theory of spherical blast waves emanating from strong point explosions, the shock position is defined by [16]  R ¼ j0

E0 r0

1=5

t2=5 ;

(1)

where t is the time with respect to the laser pulse, r0 the ambient gas density, E0 the amount of energy released during the explosion, and j0 is a constant given by j0 = [(75/ 16p)(g  1)(g + 1)2/(3g  1)]1/5. Due to the complex spatial and temporal nature of the plasma parameters like temperature, density, velocity, and pressure behind the shock front in the expanding plasma [16] the position where the intensity maximum of the light emitted from the plasma occurs does not coincide with the position of the plume front [38]. In order to look for mismatch of axial distance, d and plume front, R from the target surface we consider the dependence of temperature and density on d and R. A typical plot for the ratio of temperature, T(d)/T(R) = (d/R)3/(g1), and density, r(d)/ r(R) = (d/R)3/(g1), for varying values of d and R is shown in Fig. 1(a) for g = 1.23. The parametric dependence of intensity profile is given by the relation I(d) / r(d) exp[E/kBT(d)], where r(d) and T(d) are the temperature and density at a distance d, Fig. 1(a), E is the energy of the upper level of the transition and kB is the Boltzmann constant, respectively. Assuming the plasma in thermal equilibrium we can get temperature of various species in the plasma at varying pressures from the observed optical emission spectrum of species [39]. Fig. 1(b and c) shows the emission intensity distribution, I(d) for N II and Al III at various electron temperatures at an ambient nitrogen pressure of 70 Torr. The temperature values were found to be 3.5  0.5 and 2.5  0.2 eV for N II and Al III, respectively, at 70 Torr. With electron temperature as high as 4.4 and 2.7 eV for N II and Al III, respectively, the intensity maximum is observed at a distance (R  d)/R = 0.11 and 0.13 with respect to the plume front position (d/R = 1). Therefore, the maximum intensity emission and plume front mismatch is about 300 and 390 mm with respect to the plume front for N II and Al III considering the spatial expansion of the plume front of 3 mm, as observed in the images of the expanding plume in our experiment. This shift is reasonably small and to a good approximation, we can consider plasma–gas interface as the position of the plume front for the analysis. We also observe that the change in temperature from 3.4 to 3.9 or 3.9 to 4.4 eV for N II, and 2.3 to 2.5 or 2.5 to 2.7 eV for Al III accounts for a shift in the intensity maximum of around 0.01 which is only 1% of the plume front position with respect to the target surface.

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Fig. 1. (a) Density and temperature profile for an ideal blast wave for g = 1.23, and emission intensity profile for (b) N II and (c) Al III at various electron temperatures.

During the expansion plasma also decelerates and eventually comes to a stop as a result of plume confinement due to the ambient atmosphere [40] and the distance at which this happens is called the stopping distance. Due to high density of the ambient gas at high pressures the gas atoms exert force on the expanding plume proportional to the expansion velocity. The equation of motion describing this dynamics is of the form dv=dt ¼ bv, where v is the plume expansion velocity and b is the stopping coefficient. The solution of this equation is given by R ¼ R0 ð1  ebt Þ;

(2)

where R0 is the stopping distance (distance at which the plume comes to rest). Fig. 2(a and b) shows a typical set of ICCD images of the expanding aluminum plasma at 0.1 and 70 Torr ambient nitrogen pressures at the incident laser energy of 265 mJ. In the case where the plume front is smooth (as in Fig. 2(b) at 60 ns), R is taken as the distance measured from the target surface to the smooth edge of the plume whereas in the case of non-spherical plume front (as in Fig. 2(b) at 1200 ns), R is taken as the distance measured from the target surface to the protruding edge of the plume. Fig. 3 shows the R–t plot at 0.1 and 70 Torr with laser energy 265 mJ, respectively. Since Eq. (1) holds for a point source explosion, R(t) = atn is used [5] to model the dynamics of the expanding plasma plume at 0.1 Torr where a is a constant and n = 0.4 for a perfect shock wave. At 0.1 Torr and at early times, the plasma boundary is

smooth and continuous and takes the form of a shock wave but beyond 160 ns the plume front becomes unstable indicating RT instability. On the other hand at 70 Torr the plume remains almost spherical and close to the target surface and shows instability beyond 350 ns. In our recent work [5] we observed only shock wave formation at 0.1 Torr nitrogen ambient, and both shock wave formation as well as instability at 1 Torr at 88 mJ incident laser energy whereas with 265 mJ, both features are observed at 0.1 Torr. At low energy the amount of material ablated is less which results in a weak interaction with the ambient gas atoms/molecules. At high energy more material is ablated resulting in strong interaction between the ablated mass and the gas atoms/molecules. 3.2. Rayleigh–Taylor instability The difference in the density of two immiscible fluids at the interface manifests in the form of some kind of perturbation and gives rise to RT instability. For laser-ablated plasmas the growth of instability occurs in the region of maximum acceleration and can be derived from the derivative of momentum conservation equation d dt



  4 3 m0 þ pR rg u ¼ 0; 3

(3)

where m0 is the mass of the ablated material, u the plasma front velocity, R the distance of the plume front from the target, and

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Fig. 3. R–t plot of aluminum plasma expanding at 0.1 and 70 Torr of ambient nitrogen.

where rp and rg are the plasma and background gas densities, K the wavenumber, and a is the acceleration of plasma front. The plasma front is stable when n2 > 0 (rp < rg) and unstable when n2 < 0 (rp > rg). We have shown that [5,13] at pressures 1 Torr and above and at later times Eq. (4) implied n2 < 0, justifying the occurrence of RT instability. Since similar features are also observed with 265 mJ incident laser energy at later times at 0.1 Torr therefore we attribute the instability to RT instability. The growth time of RT instability can be calculated from the intensity plots of ICCD images of the plasma. The maximum growth rate of instability is given by [21]  gg ¼

Fig. 2. ICCD images of the expanding aluminum plasma at 265 mJ incident laser energy at (a) 0.1 Torr and (b) 70 Torr of ambient nitrogen. T: Target.

rg is the gas density, respectively. Solving Eq. (3), we get rg ¼

6m0 28pR3

(4)

A narrow interface formed at the plasma–gas interface may be stable or unstable depending upon whether the acceleration is from low density to high density region or vice versa. The growth of RT instability is given by [41]   rp  rg 2 n ¼ Ka ; (5) rp þ rg

geff s

1=2 (6)

where geff is the effective deceleration due to the magnetic field, and s is the density gradient scale length (distance from the plume front upto which the density is nearly constant). In the present experiment though no source of external magnetic field is present but as shown in Section 3.3, there exists self-generated magnetic field that may induce RT instability in the plasma. Fig. 4 shows the ICCD images along with the corresponding intensity plots recorded at ambient pressures of 1, 10, and 70 Torr at 120, 200, and 280 ns delay time at which instability is observed with laser incident energy 88 mJ. The values of s were therefore found to be 520, 350, and 290 mm. The value of s was calculated as follows from the intensity plots shown in Fig. 4. With our calibration of each pixel of ICCD, one pixel corresponds to 57.2 mm. As shown in Fig. 4, for example at 10 Torr, the number of pixels between the two lines which represent the distance where the density is constant, are 6. Therefore, in terms of distance it is 350 mm. Similarly, value of s was calculated at other pressures also. Thus, the gradient scale length is more at low pressure as compared to that at high pressure indicating a steep rise in the density gradient at high pressures. From the velocity–time graph, geff at a delay time of 120, 200, and 280 ns is found to be 6.7, 0.17, and 0.002 cm ms2 at 1, 10,

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Fig. 4. ICCD images of the aluminum plasma along with the corresponding intensity plots at delay times beyond which RT instability is observed at ambient nitrogen pressures of 1, 10, and 70 Torr, respectively. T: Target.

and 70 Torr, respectively. The growth time of instability, which is the inverse of growth rate of instability, is 0.09, 0.45, and 4 ms at the respective ambient nitrogen pressures. At 1 Torr growth rate of instability compares reasonably well with the onset time of the instability of 100 ns though at other pressures there is a significant deviation. This deviation could be understood as follows. The magnetic pressure calculated using the relation PB = B2/2m0 is 2.7  106 Nm2 (with B = 2.6 T at 1 Torr as discussed in Section 3.3). The plasma vapor pressure (Pg) as calculated from the ICCD images at 1 Torr is 1  107 Nm2 [5]. Therefore, the plasma parameter b = Pg/PB = 3.7 is >1 and indicates that the effect of drag is more dominant than that of the selfgenerated magnetic field. In presence of an external source of magnetic field the deceleration of the plasma plume arises due to the J  B force (B is the applied magnetic field and J is the electron conduction current in the plasma) acting on the plasma and is discussed in detail in the literature [21,22]. 3.3. Self-generated magnetic field The laser radiation incident on a target surface gives rise to large electric field due to the thermoelectric process and hence large current [24] resulting in the generation of magnetic field close to the target surface. The primary plasma produced as a result of laser-target interaction is a source of strong UV radiation which interacts with the surrounding ambient gas and photoionizes it resulting in the formation of secondary plasma, the characteristics of which are pressure dependent [42]. The expanding secondary plasma interacts with the ambient gas and generates magnetic field at the plasma–gas interface. The self-generated magnetic field could originate due to (a) ion-electron separation at the front of the expanding plasma [25], (b) electron and ion currents from the target [43], (c) density and temperature gradients in the plasma [44], (d) light pressure on the plasma [45], and (e) ablation waves or shocks propagating in inhomogeneous plasma [46].

According to the generalized Ohm’s law, the following relation gives the current density of the plasma     1 J ¼ s E þ Ve  B þ rPe ; (7) ene where s is the electrical conductivity, E the electric field, B the magnetic field, Ve the fluid velocity of the electrons, ne the electron density, and Pe is the pressure. Using @t@ B ¼ r  E, the equation describing the development of magnetic field becomes     @ 1 k B ¼ r  ðVe  BÞ þ rT e  rne ; r2 B þ @t m0 s ene (8) First term on the right hand side is the convection term and the second term is the diffusion term. Since there is no external magnetic field, initially B = 0. Therefore, the last term in this equation S ¼ enke rT e  rne is the source term and for the generation of B it should be non-zero. The magnetic field arises

Fig. 5. Variation of shock thickness d (measured at different delay times using Eq. (10)) with cube root of ambient pressure P1/3 of nitrogen.

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due to the modulations in the density (ne) and temperature (Te) perpendicular to ne and Te gradients resulting in an unstable growth of 5Te  5ne. The expanding plasma is axisymmetric about the expanding direction and there is no azimuthal density or temperature gradients. Therefore, magnetic field is generated entirely in the azimuthal direction. At the focal point of the focused laser radiation, the selfgenerated magnetic field is given by [47]   kT e B ; (9) er 0 v where k is the Boltzmann constant, Te the electron temperature, e the electronic charge, r0 the distance from the target at which B is to be calculated, and v is the expansion velocity of the plume, respectively. In order to arrive at Eq. (9), let us assume negative radial temperature gradient and axial density gradient i.e., gradient directions are into the plasma and the plasma is expanding in the opposite direction. Then, expand 5Te  5ne term considering the gradients in the radial (r) and axial (z) directions. With the assumeddirections of temperature  and density gradients

@T  @T e @ne e we have @r > @z or @z > @n@re . With these simplifica  1 i tions and replacing @T@re  Tr e and n1e @n@ze  n1 @n @z  v t L , where 0

i

i

tL is the pulse duration, we arrive at Eq. (9). At 1 Torr and 20 ns delay time, with typical parameters deduced from our experiment using ICCD images T = 8.3  106 K, r0 = 2.3 mm, and v ¼ 1:2  107 cm s1 , we get B around 26 kG. Self-generated magnetic field close to the focal spot has been reported to be as high as 20 kG [47]. Edwards et al. [25] studied the dependence of the self-generated magnetic field at distances away from the target normal (axial distance z). They showed that the magnetic field has two components: one that showed r2 dependence and found to be independent of background gas pressure, and the other component that showed r3 dependence indicating that this component is due to the interaction of the expanding plasma with the background gas long after the termination of the laser pulse and it decreases with z. In our experiment therefore we expect r3

dependence for the self-generated magnetic field. Source term h i kðrT r Þ , where 5Tr is the radial temperacan be written as S ¼ ed 1 e ture gradient in the negative r direction, and rn ne  d in the negative z direction and d is the characteristic length over which density changes occur. The strongest field is therefore expected to occur at the edge of the focal spot where 5Tr is largest and at the front of the expanding plasma where, is 1d largest. As the background pressure increases d decreases which results in high pressure gradients [5] at the plasma-background gas interface leading to the generation of spontaneous magnetic fields. Fig. 5 shows the dependence of shock thickness, calculated using the relation [48]

# 8 " 1=3 > 2g > > 1 R g1 > > : 3 gþ1

for conical expansion (10) for spherical expansion

with cube root of background pressure at different delay times. R is the distance of the plasma front from the target surface, and g is the specific heat ratio of the ambient gas. At 60 ns, d decreases linearly with P1/3 till 1 Torr, and at 160 ns till 10 Torr. Beyond 10 Torr (if 70 Torr is also taken into consideration) we observe deviation from the linear behavior. The same also holds at 260 and 360 ns delay times, respectively. Bird et al. [49] have reported the linear dependence of shock thickness with cube root of background pressure beyond 30 mTorr, however, beyond what background pressure this validity breaks down is not discussed. It has been shown [50] that self-generated magnetic field varies as P1/3, therefore, linear decrease in d with P1/3 is an indication of increase in self-generated magnetic field, which is consistent with our previous report [5] where we showed that the pressure gradient is the cause for self-generated magnetic field and gradient attained maximum value at 70 Torr. The deviation could be due to one or all the following reasons: (a) experimentally it is difficult to estimate d from ICCD images

Fig. 6. Schematic of the experimental setup used to study the polarization of aluminum plasma.

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Fig. 7. Energy level diagram showing splitting of the levels for the transitions (a) Al III (4s 2S1/2–4p 2P83/2) at 569.6 nm and (b) Al III (4s 2S1/2–4p 2P81/2) at 572.3 nm.

and hence cannot be compared with values calculated from Eq. (9), (b) at pressures 10 Torr and above the plasma front almost attains stopping distance at very early times and therefore d value shows hardly any variation at various time intervals, and (c) RT instability at high pressures may also result in a poor estimation of plasma front position.

could be spatially anisotropic velocity distribution arising due to Maxwellian distribution with different temperature in different directions. Therefore, degree of polarization is a measure of anisotropy in the electron distribution function in laser-induced plasmas and is defined as P¼

3.4. Polarization in laser-ablated plasma Fig. 6 shows the experimental setup used to study the polarization in the laser-produced aluminum plasma in vacuum and in ambient nitrogen. An attempt is made to study the polarization of plasma at various background pressures and incident laser energies. The polarization alignment of the ions in specific upper level results from spatial anisotropy of the plasma. Since our plasma is optically thin, the origin of this alignment

I II  I ? I II þ I ?

(11)

where III is the intensity of the emitted light whose component is parallel to the plane of laser incidence and I? the intensity perpendicular to the plane of laser incidence. We used Al III transition 4s 2S1/2–4p 2P83/2 at 569.6 nm to measure P (Fig. 7(a)) and a non-polarized Al III transition 4s 2S1/2–4p 2P81/2 at 572.3 nm (Fig. 7(b)) was used to calibrate the intensities [37]. The transition 4s 2S1/2–4p 2P81/2 at 572.3 nm is non-polarized because it has four multiplets having two s polarizations and two

Fig. 8. Polarization resolved images of horizontal and vertical components (shown as III and I?) of Al III transitions 4s 2S1/2–4p 2P83/2 at 569.6 nm and 4s 2S1/2–4p 2 P81/2 at 572.3 nm at different laser energies and ambient pressure of nitrogen.

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Fig. 9. Variation of P with delay time at (a) 44 mJ and (b) 265 mJ incident laser energy at different ambient pressure of nitrogen.

p polarizations. The contribution from each of these polarizations between the two magnetic sub levels of the upper and lower states of this transition is equal in strength and hence P vanishes. The Partial Grotrian diagram for Al transitions observed in the polarization resolved images is available in the literature [51]. Fig. 8 shows the polarization resolved images of horizontal and vertical components of Al III plasma emission in vacuum and at 0.1, 1, and 70 Torr ambient pressure of nitrogen at different incident laser energies. The Al II transition 4p 1P8–4d 1D at 559.3 nm is observed distinctly in the images at 70 Torr and in the energy range between 18 to 265 mJ used in the present experiment whereas at low pressures and at 44 and 88 mJ it does not appear. The measurement of the ratio of intensities I?/III (where I? implies I? (596.6)/I? (572.3) and III implies III (596.6)/III (572.3)) corresponding to Al III at 569.6 and 572.3 nm at different delay times with respect to ablating pulse at an incident energy of 88 mJ for different background pressures showed oscillatory behavior at pressures 1 Torr attributed to RT instability [5,13]. Fig. 9 shows the variation of P with delay time at different ambient pressures. P is found to be sensitive at 0.1 Torr and at low energies (44 mJ in this work and 88 mJ as in [5]) whereas at high energy (265 mJ) almost no variation is

observed. At 0.1 Torr, plasma follows shock wave model and the shock wave is initiated somewhere in the time interval 100– 140 ns (Fig. 3). Initially, P will evolve both in space and time due to change in the density of atoms in the excited state. Since the self-generated magnetic field is present in the plasma since its inception, the energy levels of Al III at 569.6 nm which split in the presence of this field will give rise to anisotropic emission. Decrease in density both spatially and temporally will result in a decrease in P. The shock wave initiation at the time interval 100– 140 ns will however again result in an increase in plasma density in the compressed region (region between the plasma and the ambient gas) of the plasma that will lead to further imbalance in the upper magnetic sublevels of Al III at 569.6 nm. This will increase P as shown in Fig. 9(a). At pressures 1 Torr, no variation in P is observed. The occurrence of RT instability at these pressures indicates that the density of atoms in the excited state is not the same at various positions along the plasma–gas interface. Thus, at these positions the intensity of the excited species of one kind (Al III at 596.6 nm in the present case) may increase or decrease randomly. Since P depends upon I?/III, we expect it to fluctuate thus showing no variation in P. P measurements were made close to the target (100 mm). Laser polarization, spatial anisotropy of electron distribution function (EDF), and radiation trapping have been reported to be the cause of observed polarizations [35,52,53]. These experiments used ps and sub-ps pulses with intensities in excess of 1011 W cm2. In the present experiment laser intensity used is 109 W cm2. The electron–electron collision time is given by the relation  3=2  4 Te tee ¼ 1:66  10 (12) ne and electron–ion collision time by  3=2  ATe 8 tei ¼ 2:52  10 2 ne Z¯ ln L

(13)

where A is the atomic mass number, Te (eV) the electron temperature, ne (cm3) the electron density, Z¯ the effective ion charge, and ln L  10 represents the Coulomb logarithm for the laser plasma. These collision times being much shorter than the pulse duration and the time of investigation in the experiment therefore collisions do not contribute in anisotropy in EDF. The presence of Al III ions in the emission spectrum suggests that more of Al IV ions might have recombined to populate the upper magnetic sublevel thus giving rise to polarized emission as speculated by Kim et al. [37]. It is also interesting to note that aluminum plasma is dominated by Al III as compared to Al I and Al II species at high pressures (>30 Torr). As discussed in Section 3.3, the pressure gradient at the plume front is the source of self-generated magnetic field. We also noticed that the behavior of P is due to the shock wave initiation at low pressures and due to RT instability at high pressures as discussed in the literature [5]. In the light of these results we expect that the polarization could have been induced either due to the self-generated magnetic field or recombination

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leading to population build-up in the upper magnetic sublevel of Al III. However, several possible mechanisms like heavy particle collision, various recombination mechanisms (radiative, Bremsstrahlung, dielectric), forbidden line emission due to electric field, self-alignment due to radiation trapping etc have been proposed that result in the polarization [54]. 4. Conclusions In the present work we used ICCD images of the expanding aluminum plasma in ambient nitrogen to discuss the dynamics of the plasma at various pressures. We showed that at low pressure plasma expansion followed shock wave model whereas at high pressure it followed drag model. The plasma–gas interface was observed to be unstable at high pressure and later times due to RT instability. We estimated the growth rate of instability which was found to be reasonably close to the onset time at 1 Torr and discussed the discrepancy at 10 and 70 Torr in terms of the plasma parameter b. We discussed the self-generated magnetic field in the plasma and estimated its strength to be around 26 kG in our experiment very close to the target surface. We studied the degree of polarization and found it to be sensitive at 0.1 Torr and at low incident laser energies. We expect that the observed polarization may have been induced due to the self-generated magnetic field in the plasma. Acknowledgement Work is partly supported by Department of Science and Technology, New Delhi. References [1] E. Fogarassy, S. Lazare (Eds.), Laser Ablation of Electronic Materials, North-Holland, 1992. [2] D.B. Chrisey, G.K. Hubler (Eds.), Pulsed Laser Deposition of Thin Films, John Wiley & Sons, Inc., New York, 1992. [3] J.C. Miller, R.F. Haglund, Jr. (Eds.), Laser Ablation: Mechanisms and Applications, Springer-Verlag, Berlin, 1991. [4] L.J. Radziemski, D.A. Cremers (Eds.), Laser-Induced Plasmas and Applications, Marcel Dekker, Inc., New York, 1989. [5] A.K. Sharma, R.K. Thareja, Appl. Phys. Lett. 84 (2004) 4490. [6] V.Yu. Baranov, O.N. Derkach, V.G. Grishina, M.F. Kanevskii, A.Yu. Sebrant, Phys. Rev. E 48 (1993) 1324. [7] A. Neogi, A. Mishra, R.K. Thareja, J. Appl. Phys. 83 (1998) 2831. [8] J.A. Stamper, K. Papadopoules, R.N. Sudan, S.O. Dean, E.A. McLean, J.M. Dawson, Phys. Rev. Lett. 26 (1971) 1012. [9] P. Mora, Phys. Rev. Lett. 90 (2004) 185002. [10] K.R. Chen, J.N. Leboeuf, R.F. Wood, D.B. Geohegan, J.M. Donate, C.L. Liu, A.A. Puretzky, Phys. Rev. Lett. 75 (1995) 4706. [11] N. Arnold, J. Gruber, J. Heitz, Appl. Phys. A 69 (1999) S87. [12] H. Strehlow, Appl. Phys. A 72 (2001) 45. [13] A.K. Sharma, R.K. Thareja, Appl. Surf. Sci. 243 (2005) 68. [14] K.H. Wong, T.Y. Tou, K.S. Low, J. Appl. Phys. 83 (1998) 2286. [15] P.L.G. Ventzek, R.M. Gilgenbach, C.H. Ching, R.A. Lindley, J. Appl. Phys. 72 (1992) 1696. [16] Y.B. Zeldovich, Y.P. Raizer, Physics of Shock Waves and High-Temperature Hydrodynamic Phenomena, Academic Press, New York, 1966.

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