Raman Spectroscopy of Laser-Shocked Nitrobenzene, CHEMIA I PIROTECHNIKA, Chemia i Pirotechnika
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336
Propellants, Explosives, Pyrotechnics
27
, 336 ± 339 (2002)
Raman Spectroscopy of Laser-Shocked Nitrobenzene
Naoshi Kozu*
Ammunition R & D Department, Chugoku Kayaku Co., Ltd., Yoshii Plant, Iwasaki, Yoshil, Gunma 370-2131 (Japan)
Toshihiko Kadono
Institute for Frontier Research on Earth Evolution, Japan Marine Science and Technology Center, Yokosuka, Kanagawa
237-0061 (Japan)
Reiko I. Hiyoshi, Jun Nakamura
National Research Institute of Police Science, Kashiwa, Chiba 277-0882 (Japan)
Mitsuru Arai, Masamitsu Tamura
Graduate School of Frontier Sciences, University of Tokyo, Bunkyo, Tokio 113-8656 (Japan)
Masatake Yoshida
National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8565 (Japan)
Summary
the amount of explosive samples is several tens of mg at
most, the experiments can be carried out safely.
In this paper, we apply this technique to the study of the
properties of NB: The measurement system of Raman
spectroscopy for laser-shocked NB is developed and the
Raman spectra of NB at high pressures are obtained.The
data are compared with those under isothermal compres-
sion.It is also investigated how the thermodynamic
parameters such as temperature and density affect the
spectrum.
3.5 GPa, and peak shifts at
particular frequencies are observed.The shifts are plotted as a
function of density and compared with the data under isothermal
compression.Both data provide the same results.This indicates
that the numbers of peak shifts of nitrobenzene depend solely on
material density.
1 Introduction
2Experiments
The reaction mechanism of energetic materials is very
important for understanding their reactivity and safety.
Since nitrobenzene (NB) has the simplest structure in
aromatic nitro compounds, which are widely known as
energetic materials, its behavior and properties such as
decomposition processes
(1±4)
, the Hugoniot
(5±7)
, isotherm
(6)
,
and Raman spectra
(8)
have been investigated under various
conditions.
Recently, the techniques for measuring Raman spectra of
shocked materials using laser-driven shock waves often have
been used
(9,10)
.These techniques have some advantages.
First, they allow us to control the timing of measurements
with high resolution.Also, shocks can be reproducibly
generated and probed at a high repetition rate.Therefore,
efficient signal averaging is possible, producing high quality
spectra.Moreover, since the apparatus is tabletop size and
The system developed here is shown in Figure 1(a).The
laser beam produced by a Nd:YAG laser system is divided
into two paths by semireflecting mirror.One is the
fundamental beam (1064 nm, 500 mJ) used for generating
shock waves.The wavelength of the other path is 532 nm
produced by passing through a second harmonic generator
(SHG) and is used as the probe light for Raman.The probe
light is finally focused on the sample in a spot of 800
, and the energy is about 130 mJ.The length of the probe-
light path can be changed to adjust the arrival time on the
target.
The scattered probe light is collected by L1 (
f
50 mm)
m
at the entrance of the optical fiber.The focus point of L1 is
50
50 mm), and the diameter is 200
* Corresponding author; e-mail: n-kozu@chugokukayaku.co.jp
m apart from the rear side surface of the driver, i.e. in the
¹ 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
0721-3113/02/2706-0336 $ 17.50+.50/0
A technique for
in situ
Raman spectroscopy of laser-shocked
nitrobenzene was developed.Raman spectra of shocked nitro-
benzene are obtained up to
m
diameter using the lens with 150 mm of the focal length
f
.
The angle between the incident probe beam and the target is
60
and focused by L2 (
f
Propellants, Explosives, Pyrotechnics
27
, 336 ± 339 (2002)
Raman Spectroscopy of Laser-Shocked Nitrobenzene 337
Figure 2. Raman spectrum of NB under shock compression at
32 ns after arriving of the shock wave at the sample.The label ™U∫
denotes unassigned peaks.
Figure 1. (a) Experimental setup, (b) Target setup.
middle of the sample.The point on which the probe light
focuses in the sample is adjusted to the same point as the
focus point of L1.The light radiating from the other end of
the optical fiber enters into a spectrometer, and it goes
through the notch filter that cuts a Rayleigh scattering light,
and is dispersed by holographic grating.Thus, a spectrum is
obtained at the exit of the spectrometer and finally focused
on the charge-coupled device detector cooled by liquid
nitrogen.Vibrational peak positions are obtained with an
uncertainty of
m thick
aluminum (Al) foil is glued as a driver to a 5 mm thick glass
plate (BK7) with epoxy resin.The beam for shock
generation irradiates the driver foil perpendicularly in a
focal spot diameter of about 1 mm.The sample (NB) is
confined in the 100
Figure 3. Temporal pressure profile at the middle point of the
sample obtained by the numerical simulation.
m thick space between the Al driver
and another 5 mm thick glass window (BK7).
The experiments are carried out with constant laser input
energy with various delay times by changing the length of
the probe-light path.
profile in the sample by using a one-dimensional hydro-
dynamic code
(11)
, which has been often used for the analysis
of laser-driven shock experiments and provided the con-
sistent results
(12±14)
.
Figure 3 shows the temporal pressure profile at the middle
point of the sample (50
m from the driver plate) obtained
by the numerical simulation, in which the Hugoniot of NB
obtained by Kozu et al.
(6)
is used and the Gr¸neisen
parameter is assumed to be 2.0. It can be seen that the
sample is compressed to 3 GPa by direct shock wave.Then
the reflecting shock wave from the glass window with a
higher shock impedance than that of the sample increases
the pressure up to 4 GPa.The horizontal bars indicate the
timing of the probe-light irradiation.The length of the bars
means the full width at half maximum (FWHM) of the probe
light (10 ns).The average pressure within FWHM at 32 ns
after the shock wave arrival is 2.44
3 Results and Discussion
Figure 2 shows a Raman spectrum of NB under shock
compression at 32 ns after arriving of the shock wave at the
sample.The spectrum was measured with a single probe
laser pulse.Several bands can be assigned, but some
unassigned bands exist (labeled by ™U∫ in Figure 2).Since
the positions of these unassigned bands do not change at any
shock pressure and these are also observed without NB
sample, it was concluded that these peaks were not
originated from NB.
The pressure change in the sample is rather complex: The
shock wave in the sample should be attenuated and the
reflective shock wave is generated at the interface between
the sample and the glass window.We estimate the pressure
0.09 GPa and is
0.13 GPa at 54 ns.
Figure 4 shows the Raman frequency shifts against
density.For simplicity, only the Raman bands of NO
2
symmetrical stretching and C
H stretching modes are
discussed here.The data from Ref.8 are also shown in
3.5 cm
1
.
The target setup is shown in Figure 1(b).A 50
3.62
338 Kozu, Kadono, Hiyoshi, Nakamura, Arai, Tamura, Yoshida
Propellants, Explosives, Pyrotechnics
27
, 336 ± 339 (2002)
4 Conclusion
Insitu
Raman spectroscopy of laser-shocked NB is carried
out.Raman spectra of shocked NB are obtained and shock-
induced peak shifts are observed.It is found from the
comparison with the data obtained under isothermal
compression that the peak shifts of NB depend solely on
material density up to at least 3.5 GPa.
5 Future Works
Figure 4. Peak shifts of C
H stretching mode and NO
2
sym-
metric stretching mode against density.
Using the laser with higher input energy and the
spectrometer with higher performance, more precise
measurements and analyses of Raman spectra of NB are
planned in the future.It is indicated that detonation occurs
under shock compression at higher than 15 GPa
(6)
.Hence,
by measuring Raman spectra of NB at higher shock
pressures with high time resolution, the initiation processes
of its decomposition are expected to be clarified.Also, the
dependence of the processes on thermodynamic quantities
such as temperature, pressure, and density could be known
by comparison with the data in static conditions.These
would allow us to establish a model of the detonation of
aromatic nitro compounds.
Figure 4, where the pressure values are converted to density
values using the Hugoniot of NB
(6)
.The data from Ref.8
seem to have a good agreement with our results.In this
figure, using an isothermal compressibility obtained by
fitting the data of pressure-volume relation
(6)
to a Murna-
ghan equation, the Raman peak shifts in isothermal static
compression
(15)
are also plotted as a function of density.The
uncertainty of the peak positions in static compression is
0.5 cm
1
.It seems that the data in shock compression and
those in static compression provide the same results.This
suggests that the peak shifts of NB in both static and shock
compression conditions depend not on temperature but
only on material density.It should be noted that the similar
dependence of the peak shifts on density is also observed for
anthracene
(16)
.
The data points around normalized density 1.5 obtained
by Kobayashi and Sekine
(8)
deviate from the data in static
compression.Since it has been suggested that NB undergoes
a polymerization at static high pressures over 7 GPa
(6)
,i.e.
normalized density about 1.5, the peak shifts in static
compression over 1.5 should be affected by this polymer-
ization reaction.Hence, the peak shifts in static compression
are expected to be different from those in shock compres-
sion.
The peak shifts in C
H stretching mode are about four
times larger than that of NO
2
symmetric stretching mode in
both shock and static compression.Kobayashi and Sekine
(8)
explained the small shifts of the NO
2
stretching mode as the
cancellation of two factors, that is, the pressure-induced
softening mechanism in hydrogen-bonded materials and the
general pressure-induced hardening mechanism.For quan-
titative discussion, the quantum mechanical calculations of
electronic states in NB molecules are necessary, but those
are beyond the scope of this paper.
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Acknowledgements
The authors would like to thank K.Wakabayashi, K.Kondo,
K.G.Nakamura, Y.Fujimoto, N.K.Mitani, H.Fujihisa, and K.
Aoki for their cooperation with the experiments and helpful
comments.
(Received July 16, 2002; Ms 2002/110)
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