www.elsevier.com/locate/apsusc
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: [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. 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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