22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Spectroscopic studies of sonoluminescence of the N 2 /Ar/H 2 O system to probe the
non-equilibrium plasma in cavitation bubbles
T. Ouerhani, R. Pflieger and S.I. Nikitenko
Institut de Chimie Séparative de Marcoule, UMR5257, UM2-CEA-CNRS, Centre de Marcoule, Bat. 426, P.O. Box
17171, FR-30207 Bagnols-sur-Cèze cedex, France
Abstract: Spectroscopy of sonoluminescence and follow up of the sonochemical products
are two ways of studying the SC plasma. Though, their links are poorly understood. This
work focuses on the N 2 /Ar/H 2 O system using both approaches at various ultrasonic
frequencies. Besides, the temperature characteristics of the plasma (vibrational and
rotational temperatures of NH) are estimated using Specair software.
Keywords: plasma, spectroscopy, sonoluminescence, sonochemistry
Sonochemistry, or in other words the chemical effects
of ultrasound, originates from acoustic cavitation:
nucleation, growth and implosive collapse of gas bubbles
in liquids submitted to an ultrasonic field. The implosion
occurs on the microsecond time scale and induces
extreme local conditions of temperature and pressure. A
non-equilibrium plasma is formed that emits light, the socalled sonoluminescence (SL). Spectroscopic studies of
SL are a major tool to investigate the plasma conditions
(temperatures and pressure [1, 2]) created by acoustic
cavitation in liquids. The spectra of SL typically show an
intense continuum ranging from the UV to the NIR,
usually
attributed
to
bremsstrahlung,
radical
recombination and water molecule emission, on top of
which emission peaks from excited species are present,
like OH (A-X) and OH (C-A) in water sparged with Ar.
The shape of these molecular emissions reflects the
vibrational and rotational temperatures of the considered
species.
In the present work the SL spectra of water sparged
with Ar/N 2 gas mixtures are systematically studied in a
large range of ultrasonic frequencies (20, 204, 362, 613
and 1057 kHz) using the multifrequency sonoreactor
described recently [3]. At 20 kHz, the sole molecular
emission is that of OH (A-X). On the contrary, at high
frequency a second emission is present, namely that of
NH(A-X) system (Fig. 1). Comparison of experimental
NH(A-X) spectra with emission spectra obtained using
Specair software [4] allow to estimate NH(A-X)
vibrational and rotational temperatures, and therefore to
assess the plasma conditions.
Besides, the rate of formation of sonolysis products was
followed during the sonolysis of water sparged with
various Ar/N 2 mixtures: W H2 by mass spectrometry,
W H2O2 and W NO2- by absorption spectroscopy and W NO3by ion chromatography method. A comparison of NH
emission in MBSL and sum of formation rates of NO 2 and NO 3 - as a function of N2 content in Ar is shown in
Fig. 2. Based on the comparison of the SL and the
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sonochemical results, the mechanisms of the reactions
occurring in the cavitation bubbles are proposed (Fig. 3).
Fig. 1. SL spectrum of water sparged with Ar/ 14% N 2 at
359 kHz, P ac = 50 W, T = 11 °C.
Fig. 2. Peak height of NH emission in SL and sum of
formation rates of NO 2 - and NO 3 - as a function of N 2
content in Ar at 359 kHz, P ac = 50 W and T = 11 °C.
1
Fig. 3. Mechanism of formation of NO 2 - and NO 3 -.
References
[1] E.B. Flint and K.S. Suslick.
Science, 253,
1397-1399 (1991)
[2] M.V. Kazachek and T.V. Gordeychuk. Techn.
Phys. Lett., 35, 193-196(2009)
[3] N.M. Navarro, R. Pflieger and S.I. Nikitenko.
Ultrason. Sonochem., 21, 1026-1029 (2014)
[4] C.O. Laux, T.G. Spence, C.H. Kruger and
R.N. Zare. Plasma Sources Sci. Technol., 12,
125-138 (2003)
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