22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Laser-induced photodetachment in a magnetically confined low-pressure
argon-acetylene plasma destined for nanoparticles formation
G. Al Makdessi1, A. Hamdan1, J. Margot1 and R. Clergereaux2
1
Groupe de physique des plasmas, Université de Montréal, C.P. 6128, succ. centre-ville,
Montréal, Québec, H3C 3J7, Canada
2
Université de Toulouse-LAPLACE-CNRS, 118 Route de Narbonne, Bât. 3R3, 31062 Toulouse Cedex, France
Abstract: We present the investigation of negative ions in magnetically confined Ar-C 2 H 2
plasmas, using the laser photodetachment technique. The negative ion density n - is
observed to increase with B and to slightly decrease with the C 2 H 2 percentage. In addition,
n - decreases with increasing gas pressure. The photodetachment cross section is deduced
from the laser photodetachment signal as a function of laser energy. It is shown to be
significantly higher than the cross-section expected from C 2 H - ion only. This is attributed
to the negatively charged nanoparticles synthesized and confined in the plasma.
Keywords: laser photodetachment, acetylene, negative ions, magnetic field, nanoparticles
2. Experimental setup
The experimental setup is schematically shown in
Fig. 1. The reactor is composed of a cylindrical stainless
steel chamber (20 cm of diameter and 96 cm of length)
connected to a 15 cm-diameter quartz tube. The plasma
was generated by a surface wave whose power was
injected by a Ro-box [5]. The frequency of the surface
wave is 200 MHz and the absorbed power was fixed at
350 W. The stainless steel chamber was exposed to a
magnetic field created by four coils connected in series
and spatially distributed to form a magnetic mirror with a
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magnetic mirror ratio of 1.9.
L2
M1
Baratron
gauge Coils
Coaxial stub
Ionization
gauge
Grid
Quartz tube
L3
Quartz
windows
Platinum tip
Langmuir probe
Alumina
M2
Ro-box
Faraday cage
Valve
Turbomolecular
pump
Mechanical
pump
100 Ω
Mass flow
controller
Photodiode
L1
VDC
Oscilloscope
C2H2
XeCl laser
308 nm
Ar
1. Introduction
Dusty plasmas are plasmas containing charged nanoand micro-sized particles. Beside their dominant presence
in the universe, dust particles are also observed in
laboratory plasmas especially those used in material
processing [1] and nuclear fusion [2, 3]. When reactive
gases (silane, acetylene, etc.) are introduced in the
plasma, dusty particles can spontaneously form in the
reactor chamber independently of the power coupling
mode provided the residence time of the precursors is
large enough for allowing volume interactions dominate
over surface interactions. In RF discharges at pressures
below than 150 mTorr, anions are considered to be the
species being the most likely to form dust particles [4].
This has been linked to their largest residence time
(confinement in the ambipolar electric field) among all
species present in the plasma. For this reason, the
characterization of negative species remains an important
issue for further understanding of dusty plasmas kinetics.
In this work, we are interested in the determination of the
negative ion density in magnetized dusty plasmas
operated at very low pressure. This density is measured
using a laser photodetachment technique in plasmas
generated in argon-acetylene mixture. The influence of
the operating plasma parameters is investigated.
Trigger signal
Fig. 1. Schematic of the experimental setup.
Photodetachment was achieved by using a pulsed XeCl
excimer laser at 308 nm. The laser frequency was fixed at
10 Hz and its temporal pulse width is ~10 ns. The laser
beam was focused in the plasma using a lens of 5 cm in
diameter and 2 m in focal length. A few centimeters
ahead of the focal region, the beam was crossed by an
electrostatic planar probe formed of a platinum disk
(3 mm of diameter). The disk is connected to a tungsten
wire (0.5 mm of diameter) and both are surrounded by an
alumina tube except the disk surface. The probe was
positioned at the chamber center by ensuring that its
surface was completely immersed in the laser beam and
that it was not ablated. A DC voltage of 55 V was applied
to the probe in order to attract all the electrons present in
the plasma around the probe. This voltage value was
selected based on Langmuir probe measurements ensuring
that we operate in the electron saturation region. The
value of the current (before and after each laser pulse)
was deduced by measuring the potential drop across a
calibrated resistor of 100 Ω connected in series with the
voltage source (batteries). The electrical signal was
1
visualized and saved using a Tektronix
oscilloscope (TDS 2014C, 100 MHz-2GS/s).
digital
3. Laser induced photodetachment
When the energy of an incident photon is greater than
the electron affinity of a negative ion, the photon can
detach the electron in excess from this ion. This
phenomenon leads to a time-dependent increase of the
electron density. Quantitatively, the fraction ρ of negative
ions lost through photodetachment in the laser beam
volume is given by [6]:
1 − exp(−
ρ=
EL σ p (e L )
)
SL e L
(1)
For large enough laser fluence (E L /S L ), ρ tends to 1,
which means that all the negative ions present in the
plasma-laser interaction volume are photodetached. In
such a case, the negative ions density n ‒ can be linked to
the electron density n e through the equation:
n− ∆ne
=
ne ne 0
(2)
Δn e represents the instantaneous increase of n e due to the
photodetached electrons and n e0 is the initial electron
density (i.e., without laser perturbation).
Considering that the probe electron saturation current is
proportional to the electron density, equation 2 can be
expressed as follows:
∆I e n−
=
I e 0 ne
1.52
probe-lens distance
0.12
∆Ie/Ie0
133 cm
98 cm
0.10
0.08
0.06
6
9
12 15 18
EL/SL (mJ/cm2)
21
24
Fig. 3. ΔI e /I e as function of E L /S L for two probe-lens
distances for 55 V, 2 mTorr, 20% C 2 H 2 and 350 W.
1.48
∆Ie
Ie (mA)
0.14
0.04
3
1.56
1.40
1.36
-1
Ie0
0
1
2 3 4
Time (ms)
5
6
7
Fig. 2.
Example of photodetachment signal.
Pressure = 2 mTorr, power = 350 W, the percentage of
C2H2 = 80% and the probe-lens distance = 98 cm.
2
4. Results and discussion
In order to optimize the laser parameters (fluence, focal
point position, etc.) and make sure that all the negative
ions are detached in the interaction region, we have
performed first the following study. The fluence of the
laser at the probe position was varied by changing the
laser energy E L and the focal point position (which
changes the beam section S L at the probe position). This
was achieved by moving the lens with respect to the
entrance of the vacuum chamber. The ratio ΔI e /I e0 was
then determined and plotted in Fig. 3 as a function of the
fluence E L /S L for two lens positions (the distance lensprobe being equal to 98 cm and 133 cm).
(3)
where I e0 corresponds to the electron density of the
undisturbed plasma and ΔI e to the disturbed electron
current increment. In Fig. 2, we present a typical
photodetachment signal observed in Ar-C 2 H 2 (80-20%)
mixture.
1.44
After each laser pulse, the current undergoes an
increase due to the photodetached electrons. This first
peak is followed by a series of secondary peaks. These
oscillations are explained by the formation of a dynamic
sheath at the probe surface due to the fast motion of
electrons as compared to ions. As we can see, these
oscillations are rapidly damped and they disappear after
several microseconds as the plasma relaxes to its
equilibrium state. In this study, attention is exclusively
focused on the ratio ΔI e /I e0 .
We note that when E L /S L is lower than ~18 mJ cm-2,
the ratio ΔIe /I e0 increases with the fluence independently
of the lens-probe distance. Then it saturates when the
fluence reaches ~14 to 18 mJ cm-2 as expected from
theory. In addition, for a laser fluence higher than ~18
mJ cm-2, ΔIe /I e0 starts to increase again. This second
increase could be related to phenomena other than
photodetachment such as multiphoton ionization. In the
following, E L /S L has been fixed at ~15 mJ cm-2 by setting
the laser energy at 100 mJ and the lens-probe distance at
98 cm. In these conditions, Eq. 3 can be used to directly
calculate the ratio of the negative species density to the
electron density (n ‒ /n e ).
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Moreover, fitting the data of Fig. 3 by Eq. 1 and Eq. 3
enables to determine the photodetachment cross section σ.
It is found to be ~1.4×10ˉ16cm2. The dominant negative
species in acetylene discharges have been found to be
C 2 Hˉ, C 4 Hˉ and C 6 Hˉ [7] and their corresponding
photodetachment cross-sections, for a photon energy of
~4 eV, are 5.05×10-18, 7.72×10-18 and 1.04×10-18 cm2,
respectively. The discrepancy between these values and
that obtained in our case (two orders of magnitude higher)
suggests that the observed photodetachment signal cannot
be related only to the negative ions but to other species.
These species could be carbon nanoparticles that are
known to be negatively charged in acetylene plasmas
[8, 9]. If the electron affinity of these nanoparticles is
relatively low and their photodetachment cross section is
relatively high, they could significantly contribute to the
photodetachment signal.
3.1. Effect of the magnetic field and Ar-C 2 H 2 mixture
The variation of the ratio n ‒ /n e as a function of the
magnetic field intensity is shown in Fig. 4 for two
acetylene percentages; 20% and 50%.
0.12
20% C2H2
50% C2H2
n-/ne
0.10
The variation of the n - /n + ratio with the magnetic field
and the acetylene percentage is very similar to n ‒ /n e .
Their dependence on B can be attributed to an increase in
the confinement of positive ions in the center of the
plasma as the intensity of the magnetic field augments.
This can also be related to the confinement of electrons by
the magnetic field. Indeed, the ionization rate increases in
the plasma leading thus to the formation of more electrons
and positive ions without denying an enhancement in the
electron dissociative attachment process which creates
negative ions. Moreover, another reason can explain the
decrease in the ratio n ‒ /n + . Note that the positive ions
can be created from argon as well as from hydrocarbon
radicals. As a consequence, their creation rate increases
more rapidly with B than the creation rate of the negative
ions. Indeed negative ions are only created from
hydrocarbon negative ions and from nanoparticles.
3.2. Effect of gas pressure
The effect of gas pressure on the n ‒ /n + ratio is
presented in Fig. 5 for 20% of C 2 H 2 . It shows a decrease
when the pressure increases from 2 to 10 mTorr. Clearly,
the decrease rate depends on the magnetic field intensity.
For instance, without magnetic field (B = 0) n ‒ /n +
decreases from ~0.1 at 2 mTorr to ~0.06 at 10 mTorr,
while at 140 G the decease is only ~0.01 (four times
lower).
0.12
0.08
0G
140 G
0.10
n-/n+
0.06
0.04
0
20 40 60 80 100 120 140
Magnetic field (Gauss)
Fig. 4. n - /n e as function of B for two C 2 H 2 percentages
for 2 mTorr and 350 W.
First of all, let us note that the negative species density
does not exceed 10% of the electron density. For
instance, n ‒ /n e is about 0.1 without magnetic field (B = 0)
and it decreases with the magnetic field to reach ~ 0.04 at
140 G. This indicates that the electronegativity of the
plasma is rather weak. The decrease rate does not depend
much on the acetylene percentage and, for 20% of C 2 H 2 ,
n ‒ /n e is slightly higher than that obtained for 50% of
C2H2.
Using the plasma quasi-neutrality condition (i.e.,
n + = n e + n ‒ ), the negative-to-positive ion density ratio
can be deduced from the n - /n e ratio as:
n−
1
=
n+ 1 + ( n− ) −1
ne
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(4)
0.08
0.06
0.04
0.02
2
4
6
8
Pressure (mTorr)
10
Fig. 5. n - /n + as function of pressure for two values of B
for 20% C 2 H 2 and 350 W.
Without magnetic field and at low gas pressure, one can
assume that the charged species, electrons and positive
ions diffuse easily outside the plasma axis due to the
ambipolar diffusion to the walls. At high gas pressure,
they can no longer spread so easily and they remain
confined in the central plasma which results in a decrease
of the n ‒ /n + ratio. When applying a magnetic field
(140 G) at low pressure (2m Torr), the charged species
are already confined and prevented from diffusing outside
the plasma center which decreases the effect of the
pressure observed without B, as we notice in Fig. 5.
3.3. Absolute density of negative species
3
The absolute value of the negative ion density can be
deduced from the n ‒ /n + ratios taking the positive ion
density determined from probe measurements. The
variation of n ‒ as a function of the magnetic field
intensity is shown in Fig. 6 for three C 2 H 2 percentages.
3,0
20% C2H2
50% C2H2
80% C2H2
n- (109cm-3)
2,5
2,0
1,5
1,0
0,5
0,0
0
20 40 60 80 100 120 140
Magnetic field (Gauss)
Fig. 6. Variation of n - with B for three percentages of
acetylene for 2 mTorr and 350 W.
First, n ‒ shows a drop off from ~ 1.5×10 cmˉ at 0G to
~1×109 cmˉ3 at 35 G in the case of 20% of C 2 H 2 . When
augmenting further the intensity of the magnetic field, n ‒
increases from ~ 1×109 cmˉ3 reaching a value of
~ 3×109 cm-3 at 140 G. In the other hand, the dependence
of n - on the C 2 H 2 percentage is less important except at
high magnetic field intensity. For instance, at 140 G n ‒ is
higher when the C 2 H 2 percentage is lower. The variation
of n ‒ , for 20% C 2 H 2, as a function of the gas pressure is
presented in Fig. 7. In general, n ‒ decreases with the gas
pressure with a rate depending on the magnetic field
intensity. When the gas pressure increases from 2 to
10 mTorr and without a magnetic field, the decrease is
~ 22% while it is ~ 45% when applying a magnetic field
(140 G).
9
3
3,0
0G
140 G
2,7
n- (109cm-3)
2,4
2,1
1,8
1,5
1,2
2
4
6
8
Pressure (mTorr)
10
Fig. 7. Variation of n - with pressure for two values of B
for 2 mTorr, 20% of C 2 H 2 and 350 W.
4
In acetylene discharges, the nucleation of carbon
nanoparticles process strongly depends on the
concentration of C 2 H- ions that are expected to be
efficiently generated in Ar/C 2 H 2 plasmas [10].
Increasing the magnetic field intensity will induce two
important phenomena: i) increase in the dissociative
attachment rate of C 2 H 2 molecules leading to the
formation of more nanoparticles through chain reactions
and ii) increase of the negative species residence time in
the plasma, by compensating the gravitational force,
leading to an increase in their density. On the other hand,
when increasing the percentage of argon in the plasma,
the absolute value of the negative species increases
signifying that the presence of argon enhances the
formation of carbon nanoparticles, through the
enhancement of the dissociative attachment over other
processes such as for example ionization.
As pressure increases, the collision frequency between
plasma species increases and therefore the probability of
ion-ion recombination rises. This induces a global
decrease in the negative species in the plasma.
5. Conclusion
In this work, laser photodetachment was used to
measure the density of negative species n ‒ in Ar/C 2 H 2
magnetized dusty plasma. The negative ion density n varies non monotonously with B, while it decreases with
increasing acetylene percentage and pressure. In addition,
the photodetachment cross section is likely to be related
to the negatively charged carbon nanoparticles
synthesized in the plasma volume.
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