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Applied Surface Science 253 (2007) 3113–3121

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 photo-

graphy 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 fur Physik und Physika-

lische Technologien, Technische Universitat Clausthal, Leibnizstrasse 4, D

38678, Clausthal-Zellerfeld, Germany. Tel.: +49 5323 72 3628;

fax: +49 5323 72 3600.

E-mail address: a.sharma@pe.tu-clausthal.de (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. Self-

generated 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

A.K. Sharma, R.K. Thareja / Applied Surface Science 253 (2007) 3113–31213114

transport properties [30,31]. It has also been shown theoreti-

cally 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 self-

generated 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 10�5 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 space-

and 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. hZiis 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 tempera-

ture, 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 tem-

perature and density on d and R. A typical plot for the ratio of

temperature, T(d)/T(R) = (d/R)�3/(g�1), and density, r(d)/

r(R) = (d/R)3/(g�1), 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 spec-

trum 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.

A.K. Sharma, R.K. Thareja / Applied Surface Science 253 (2007) 3113–3121 3115

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 R0ð1� e�btÞ; (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

��m0 þ

4

3pR3rg

�u

�¼ 0; (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

A.K. Sharma, R.K. Thareja / Applied Surface Science 253 (2007) 3113–31213116

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

nitrogen.

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 ¼6m0

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]

n2 ¼ �Ka

�rp � rg

rp þ rg

�; (5)

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�

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 mag-

netic 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 pres-

sures 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 ms�2 at 1, 10,

A.K. Sharma, R.K. Thareja / Applied Surface Science 253 (2007) 3113–3121 3117

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.

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.

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

Nm�2 (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 Nm�2 [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 self-

generated 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

J ¼ s

�Eþ Ve � Bþ

�1

ene

�rPe

�; (7)

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

@

@tB ¼ r� ðVe � BÞ þ

�1

m0s

�r2Bþ

�k

ene

�rTe �rne;

(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 kene

� rTe �rne is the source term and for the

generation of B it should be non-zero. The magnetic field arises

A.K. Sharma, R.K. Thareja / Applied Surface Science 253 (2007) 3113–31213118

due to the modulations in the density (ne) and temperature (Te)

perpendicular to ne and Te gradients resulting in an unstable

growth of5Te � 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 self-

generated magnetic field is given by [47]

B ��

kTe

er0v

�; (9)

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., gra-

dient directions are into the plasma and the plasma is expanding

in the opposite direction. Then, expand 5Te � 5ne term con-

sidering the gradients in the radial (r) and axial (z) directions.

With the assumed directions of temperature and density gradients

we have @Te@r

�> @Te

@z

� or @ne

@z

� > @ne

@r

�. With these simplifica-

tions and replacing @Te@r �

Ter0

and 1ne

@ne@z

� � 1

ni

@ni@z

� � 1

vitL, where

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� 107cm s�1, 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 r�2 dependence and

found to be independent of background gas pressure, and the

other component that showed r�3 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 r�3

Fig. 6. Schematic of the experimental setup used

dependence for the self-generated magnetic field. Source term

can be written as S ¼ kðrTrÞed

h i, where5Tr is the radial tempera-

ture gradient in the negative r direction, and rnene� 1

din 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 where5Tr 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]

d ¼R

2g

g þ 1

� �1=3

� 1

" #for conical expansion

R

3

g � 1

g þ 1

� �for spherical expansion

8>>><>>>:

(10)

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

to study the polarization of aluminum plasma.

A.K. Sharma, R.K. Thareja / Applied Surface Science 253 (2007) 3113–3121 3119

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 there-

fore d value shows hardly any variation at various time inter-

vals, and (c) RT instability at high pressures may also result in a

poor estimation of plasma front position.

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

Fig. 8. Polarization resolved images of horizontal and vertical components (shown2P81/2 at 572.3 nm at different laser energies and ambient pressure of nitrogen.

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

P ¼ III � I ?III 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

as III and I?) of Al III transitions 4s 2S1/2–4p 2P83/2 at 569.6 nm and 4s 2S1/2–4p

A.K. Sharma, R.K. Thareja / Applied Surface Science 253 (2007) 3113–31213120

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 polariza-

tions 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 experi-

ment 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 measure-

ments 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 experi-

ments used ps and sub-ps pulses with intensities in excess of

1011 W cm�2. In the present experiment laser intensity used is

�109 W cm�2. The electron–electron collision time is given by

the relation

tee ¼ 1:66� 104

�T

3=2e

ne

�(12)

and electron–ion collision time by

tei ¼ 2:52� 108

�AT

3=2e

neZ2

ln L

�(13)

where A is the atomic mass number, Te (eV) the electron

temperature, ne (cm�3) 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

A.K. Sharma, R.K. Thareja / Applied Surface Science 253 (2007) 3113–3121 3121

leading to population build-up in the upper magnetic sublevel

of Al III. However, several possible mechanisms like heavy

particle collision, various recombination mechanisms (radia-

tive, 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.

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