CP620, Shock Compression of Condensed Matter - 2001
edited by M. D. Furnish, N. N. Thadhani, and Y. Horie
© 2002 American Institute of Physics 0-7354-0068-7/02/$ 19.00
SHOCK-INDUCED ORIENTATION OF BENZENE MOLECULES
STUDIED BY NANOSECOND TIME-RESOLVED RAMAN
SPECTROSCOPY
Kunihiko Wakabayashi1, Kazutaka G. Nakamura2, Ken-ichi Kondo2
1
National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba 305-8565, JAPAN
Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama,
Kanagawa 226-8503, JAPAN
2
Abstract. Nanosecond time-resolved Raman spectroscopy has been used to study a molecular
response of benzene under shock compression in the pressure range up to 1.6 GPa. S-polarized light
against the target surface was used for excitation of Raman scattering. Shock wave was generated by
pulsed-laser irradiation with repetition rate at 1.25 Hz. Here we used a multi-lens array for smoothing
and homogenizing the intensity distribution of a shock driver laser on the surface of a target. This
technique made it possible to generate a spatially uniform shock wave. The change in vibrational
frequency of 993 cm'1 (breathing vibrational mode) has been observed with respect to applied shock
pressure, and a vibrational frequency shift increased with increasing shock pressure. Temporal changes
of the integrated intensity of Raman band at 993 cm"1 indicated the existence of mechanical processes,
which were associated with uniaxial nature of shock compression. By comparing the Raman spectra
under shock compression to that under ambient state, it is concluded that the temporal change of its
intensity implies the occurrence of the shock-induced orientation of benzene molecules.
information of molecular dynamics. In particular,
Raman or infrared spectroscopy is well suitable for
examination of molecular changes. In this work,
we performed nanosecond time-resolved Raman
spectroscopy of Benzene, using a pump-probe
technique. Shock wave was generated repetitively
by laser irradiation.
INTRODUCTION
Shock wave experiments have provided reliable
information on mechanical and thermo-physical
properties of materials under high-pressure and
high-temperature conditions. However, in order to
understand the transient states of excitation and
relaxation processes of materials under shock
compression, it is important to investigate the
microscopic1'2 and dynamic behavior3 of materials.
Vibrational spectroscopy has been recognized as a
powerful
tool for extracting microscopic
EXPERIMENTS
The repetitive experimental system4 for shock
compression with Table-Top laser was used, and
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glass-confinement geometry5. The target assembly
in glass-confinement geometry (Fig.2) was
fabricated with a back-up glass substrate (FL3, 100
XI00X3 mm3) on which aluminum foil (25 \Jtm
thick) was glued with epoxy adhesive dissolved in
Toluene, a spacer made of PTFE sheet (130 ^im
thick), and a cover glass substrate (FL3, 100X100
X 3 mm thick). The PTFE spacer was sandwiched
between glass substrates. Liquid-Benzene was
filled in the space held by the PTFE spacer.
The target assembly was mounted on computercontrolled X-Z stage. The movement of X-Z stage
was synchronized to the laser pulse reduced the
repetition rate at 1.25 Hz with mechanical shutter,
so that fresh target's surface was exposed after each
shot. The driver laser homogenized with the MLA
was focused on the aluminum foil at normal
incidence through a glass window with laser energy
up to 512 mJ/pulse, with a 1.25 mm spot diameter.
Confined plasma was generated near the aluminumglass interface, which drove a shock wave through
the aluminum foil into the Benzene sample.
Optically delayed S-polarized Raman probe laser
( A = 532 nm, 0.8 mJ/pulse) was focused on the rear
side of the Benzene sample with a diameter of 1000
\im at the center of the region irradiated by the
shock driver laser. Raman scattered light was
collected with a camera lens, and led to a
polychromator (Kaiser Co.,) with optical-fiber
coupling (400 jmi core). Two Raman notch filters
(bandwidth of 350 cm"1) were placed in front of the
fiber, and inside a polychrometer for reducing the
Raman scattered-light from fiber and Raman
exciting laser. The spectrally resolved signal was
detected with an intensified charge coupled device
(ICCD) camera (ANDOR). Spectral resolution
was about 3 cm"1. Raman spectrum was obtained
by accumulating data of 60 laser shots.
applied it to pump-probe Raman spectroscopy.
The Q-switched Nd3+:YAG laser (Continuum Co.,
Powerlite Plus ) is operated at repetition rate of 10
Hz. The maximum outputs of fundamental (1064
nm) and second harmonic (532 nm) radiations are 3
and 1.5 J/pulse, respectively. A temporal profile
of the laser beam is Gaussian. The full widths at
half maximum (FWHM) of the fundamental and
second harmonic are 10 and 8 ns, respectively.
The output energies in different shots were constant
within 2.5 %. The fundamental radiation was
directed through a multi-lens array (MLA) and a
focusing lens, and used to irradiate a target for
shock wave generation. We used a MLA for
smoothing and homogenizing the intensity
distribution of a shock driver laser on the
target surface, by its overlapping effect of
many beamlets. This technique made it
possible to obtain a flat-top beam, so that it
could be supposed to genetare a spatially
uniform shock wave. The second harmonic
light was directed through a variable optical delay
line and a polarized beam splitter. Only Spolarized component of the second harmonic light
against a target surface was used for excitation of
Raman scattering.
Fig. 1 shows a target assembly, which is called
Nd:YAG
Shock driver laser
E=512
spectrometer
Nd:YAG 2o>
Raman probe laser
E=0.8 mJ
Glass(3mmt)
Glass(3mmt)
Alfoil(25//mt)\
for back up
Benzene(130Mmt)
Fig. 1 Schematic drawing of the target assembly.
The sample is liquid-Benzene filled between both
glasses.
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by time-resolved techniques were made up by the
superposition of Raman scattered light from both
the compressed area (behind the shock front) and
the uncompressed area (in front of the shock front).
By assuming that the frequency distribution of
Raman spectrum scattered from both areas is
Lorentzian shape, temporal change of integrated
intensity of both spectrum (Fig. 3) was obtained.
Fig. 3 showed that integrated intensity of Raman
peak from the compressed area was increasing with
RESULTS AND DISCUSSIONS
Fig. 2(a) shows a Raman peak of Benzene (ringbreathing mode) under ambient state. Fig. 3 (b)(f) show time-resolved Raman spectra of shocked
Benzene at delay time of 23.4, 28.2, 34.2, 40.2, 46.2
ns, respectively. The zero delay time (td=0.0 ns)
denoted an arrival of the driver laser pulse at the
glass-aluminum interface.
750 •
0
10
20
30
40
50
Time [nsl
Fig. 3 The integrated intensity of the peak
960
980
1000
1020
at 993 cm"1 was plotted against the delay time.
1040
1
Raman shift [cm" ]
the increase of the delay time. It has been
considered that this temporal change of intensity
has well associated with a propagation of shock
wave. However, this behavior could not been
explained on the basis of the intensity change which
accounted for increase of the number of Benzene
molecules inside the compressed area (dotted line in
Fig. 3). It was considered that there were three
factors which caused the increase of the intensity, as
follows,
1. Geometrical configuration of experimental
set up.
2. Changes of the polarizability of Benzene
molecules.
3. The effect between the polarization of probe
laser and the orientation of the Benzene
molecules.
Fig. 2 Time-resolved Raman spectra of Benzene
under laser induced shock compression.
As shown in Fig. 2, vibrational frequency shift
which supposed to be due to the effects of
compression was observed.
Under present
experimental condition, temporally and spatially
resolved particle velocity measurements of Benzene
has been also performed with the use of the lineORVIS6 system. As a result of the line-ORVIS
measurements, it was revealed that spatial and
temporal profile of shock wave generated by laser
irradiation with confined geometry target was
relatively flat (SOO^m & ) and steady (about 40 ns).
Generated shock pressure was about 1.6 GPa.
Based on the results of line-ORVIS measurements,
it has been supposed that Raman spectra obtained
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Taking into account of the present experimental
condition, it was reasonable to suppose that the
reason why the intensity increased was the results
of orientation of Benzene molecules along with a
propagation of shock wave. Because rotational
energy of Benzene molecules was relatively small
(0.005 eV), then it was excited easily at room
temperature (0.026 eV). This effect could be
expected under shock condition. Up to this time, it
has been reported that Benzene molecules were tend
to orient to the particular direction under static
compression.
Based on this assumption that
intensity change depends solely on the molecular
orientation, Raman intensity I0 could be described
as a function of the angle 9 between the
polarization vector of probe laser and the
polarization vector of the polarizability of Benzene
ring-stretching vibrational mode by following
expression;
angle of Benzene tends to small with the increase of
the delay time.
This results indicated that
molecular rotation were depressed against the
particular direction.
Temporal change of the
molecular orientation and the Raman shift indicated
that there were something mechanical dynamics
that the vibrational energy state of Benzene
molecules were led to lower state with relaxing the
Raman shift inside the compressed area.
CONCLUSION
Nanosecond time-resolved Raman spectroscopy
has been performed to investigate the microscopic
behavior of Benzene shocked about 1.6 GPa. Peak
intensity observed were increasing with the increase
of the delay time. It was supposed that the
temporal change of the peak intensity was due to
the orientation of Benzene molecules. It was
concluded that the temporal change of its intensity
implied the occurrence of the shock-induced
orientation of Benzene molecules.
where IL is probe laser intensity, P0 the Raman cross
section of Liquid-Benzene, p 0 the initial density.
A is a probed area, / [ M m ] the sample thickness.
Fig. 4 showed the temporal change of the
molecular-angle inside the compressed area with
the use of both the temporal change of Raman
intensity (Fig. 3) and the equation. Molecular
ACKNOWLEDGEMENTS
This work has been supported by the CREST
(Core Research for Evolutional Science and
Technology) program organized by the Japan
Science and Technology Corporation (1ST).
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Time [ns]
Fig. 4 The changes of the mean molecular angle was
plotted as a function of the delay time.
matter (1991)767.
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