Download Remote Pulsed Laser Raman Spectroscopy System for Detecting

REMOTE PULSED LASER RAMAN SPECTROSCOPY SYSTEM FOR
DETECTING WATER, ICE, AND HYDROUS MINERALS ON PLANETARY
SURFACES
Christopher S. Garcia
Old Dominion University, Hampton Blvd, Norfolk, VA, 23529
Advisor: Dr. Hani Elsayed-Ali
For exploration of planetary surfaces, detection of
water and ice is of great interest in supporting existence of
life on other planets. Therefore, a remote Raman
spectroscopy system was demonstrated at NASA Langley
Research Center for detecting ice, water and hydrous
minerals on planetary surfaces. In this study, a 532 nm
pulsed laser is utilized as an excitation source to allow
detection in high background radiation conditions. The
Raman scattered signal is collected by a 4-inch telescope
positioned in front of a spectrograph. The Raman
spectrum is analyzed using a spectrograph equipped with
a holographic super notch filter to eliminate Rayleigh
scattering, and a holographic transmission grating that
simultaneously disperses two spectral tracks onto the
detector for higher spectral resolution. To view the
spectrum, the spectrograph is coupled to an intensified
charge-coupled device (ICCD) camera, which allows
detection of very weak Stokes line. The camera is
operated in gated mode to further suppress effects from
background radiation and long-lived fluorescence. The
sample is placed at 5.6 m from the telescope, and the laser
and telescope are arranged in a coaxial geometry to
achieve maximum performance. The system was
calibrated using the spectral lines of a Neon lamp source.
To evaluate the system, Raman standard samples such as
acetone, isopropanol, naphthalene, and calcite were
analyzed. The Raman evaluation technique is used to
analyze water, ice and other hydrous minerals, and results
from these species are presented.
INTRODUCTION
Raman spectroscopy is a powerful technique that can
provide information about the composition and structure
of a gas, liquid or solid sample.1-3 It is based on the
Raman effect, which is the phenomenon that takes place
when an incident monochromatic light of known
wavelength strikes an object and the incident light is
scattered in various manner. Although majority of the
scattered light have the same frequency as the incident
light, a small percentage will have a higher or lower
frequency. The frequency shift in the scattered light can
provide information about the composition and structure
of the target.
An energy level representation of the Raman effect is
shown schematically in Figure 1.3 When an incident
photon strikes a target, the target’s molecule is excited
from a ground vibrational state into a higher virtual state,
and then returns back to a lower-energy state. The
transition from a higher to a lower energy state causes a
release of photon referred to as the scattered light. When
the scattered light has the same energy as the incident
light, it is referred to as elastic or Rayleigh scattering.
This scattering does not give any information about the
target. When the frequency of the scattered light is shifted
relative to the incident signal, the scattering is said to be
inelastic. There are two possible forms of inelastic
scattering. If the energy of scattered light is less than the
energy of the incident light, it is referred to as Stokes
scattering. On the other hand, if the energy of the
scattered light is greater than that of the incident light, it is
referred to as Anti-Stokes scattering. The inelastic
scattering only occurs for every 1 in 107 of incident
photons, making them very weak and difficult to detect.
Excited vibrational level
Virtual levels
Ground. vibrational level
Rayleigh
Scattering
Stokes
Scattering
Anti-Stokes
Scattering
incident
light
Scattered
light
Figure 1. Energy level representation of Raman
scattering.3
For exploration of planetary surfaces, detection of
water and ice is vital in supporting the existence of life on
other planets.4 Water has been detected on Mars and
Moon in the form of ice and hydrous minerals. Various
techniques are available to detect water, ice and waterbearing
minerals. However, Raman spectroscopy stands
out in providing unambiguous identification of such
substances.
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EXPERIMENTAL SETUP
The schematic diagram of the pulsed remote Raman
spectroscopy system is shown in Figure 2.4-6 Essentially,
the Raman system consists of an excitation source to hit
the target and generate Raman scattered signal, a light
collection system to collect the Raman signal, a
spectrograph to separate the signal into its spectral
components, and a detector to detect and display the
Raman spectra.
The excitation source is a mini Nd:YAG pulsed laser
source (model UltraCFR, Big Sky Laser, 532 nm, 45
mJ/pulse, 20 Hz, pulse width 8 ns). Using two 45º prisms,
the laser beam is made collinear with the telescope’s
optical axis. This configuration, referred to as a coaxial
geometry, permits scattered light to be gathered from the
target for the 180º back scattering geometry to achieve
maximum performance.
The laser beam hits the sample placed at 5.6 meters
from the telescope, and the Raman scattered signal is
collected by the telescope (Meade ETX-105, Maksutov-
Cassegrain telescope with 4” aperture).
Telescope
Laser
Spectrograph
ICCD Camera
Computer
20X Microscope
Objective Lens
Notch Filter
VPH Grating
Slit
Sample
45o Prisms
Figure 2. Schematic diagram of the remote pulsed laser
Raman spectroscopy system.
The output from the telescope is directly coupled to
the spectrograph (Kaiser Optical Systems, Inc. Holospec
f/1.8i)) through a 20x microscope objective lens. Inside
the spectrograph, the input signal is first passed trough a
notch filter (Kaiser 532 nm Holographic SuperNotch-Plus
Filter) to eliminate the strong Rayleigh scattered photons,
then trough a 100 µm slit to improve resolution of the
spectra, and finally through a holographic transmission
grating (Kaiser VPH Transmission Grating) to decompose
the signal into its spectral components. The chosen width
of the internal slit in the spectrograph corresponds to a
spectral resolution of 4 cm-1.
The spectrograph output is detected by
thermoelectrically cooled and gated intensified CCD
(ICCD) camera (Princeton Instruments PI-MAX Camera,
1024 x 256 ICCD, 26x26 µm2 pixel size). The ICCD
detects very weak stokes line by adding a gain of up to
250 to the input signal. This is implemented by first
passing the input light signal through a photocathode that
releases electrons from incident photons. The electrons
are then accelerated through microchannel plates (MCP)
where they are amplified as they hit the channel walls. An
electron that hit the MCP walls generate additional
electrons, which in turn hit the walls and generate more
electrons, resulting in high gain of electrons. The
electrons leaving the MCP strike the phosphor coating on
a fluorescent screen causing it to release photons, which
fall on the pixels of the CCD and generate charge.
The spectrograph grating simultaneously disperses two
spectral tracks onto the CCD, with low frequency portion
of the spectra on the upper half region of the CCD and
high frequency portion on the lower half region. This
innovative scheme dramatically improves the resolution
of the spectrograph. The camera is operated using a
manufacturer-supplied software (Winspec32), which is
installed in the computer connected to the camera.
Vertical binning or summing of the signals in each
column of pixels of the CCD was performed to produce
single channel signal. The Raman spectrum is formed by
plotting the binned signal versus channel or column
numbers.
To obtain Raman spectra in lighted or high
background conditions, the signal-to-background ratio is
improved by gating the ICCD camera.5 The ICCD
detector can be operated in either continuous or gated
mode. In continuous mode, the detector is “on” and
collects light during the entire exposure time. This mode
of operation is not practical for lighted measurements
because it allows background signal such as mercury line
from room lights to be recorded on the spectra. In the
gated mode, the detector is “on” only for a very short
period of time. This ensures that the camera only collects
light whenever the laser strikes the sample. This short
period during which the camera collects light is equal to
the gate width multiplied by the number of laser pulses
within the integration time. For our measurements, the
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gate width is set to 2 µs, and the laser pulse rate is 20 Hz.
During an integration time of 1s, the detector is only on
for 40 µs; therefore, the duration for which the
background radiation is picked up by the detector in the
gated mode is only 1/25,000 of that in continuous mode.
The data presented in this paper were collected with 10s
integration time, corresponding to merely 400 µs of total
duration that the detector is “on.”
By using a pulsed laser, the intensity of the very
weak Raman signal is increased, improving the ability of
the system to detect it. The laser pulse width is only 8 ns,
and within this very short period, the sample is hit by a
large number of incident photons and Raman photons are
generated. This results in significant improvement of the
ratio of Raman photons to background photons during
that short time interval.
To evaluate the performance of the Raman system,
four standard samples (Isopropanol, Acetone,
Naphthalene, and Calcite) were analyzed. The Raman
spectra of ice, water, and various hydrous minerals
(FeSO4·7H2O, MgSO4·7H20, MgCl4·6H2O, and gypsum)
were then measured. An integration time of 10s was
chosen for all measurements.
To convert the Raman data from pixel position on the
CCD into Raman shift in wavenumber, GRAMS/32
software package from Galactic Industries was used. To
calibrate the spectra, Neon spectral lines were used.
RESULTS AND DISCUSSION
To evaluate the performance of the system, the
Raman spectra of Isopropanol, Acetone, Naphthalene and
Calcite were measured. The low frequency (0-2000 cm-1)
and high frequency (2500-4000 cm-1) spectra are shown
in Figures 3a and 3b, respectively. The peak positions
measured by the Raman system were within ±2 cm-1 of
standard values obtained from the NIST website.7
Furthermore, the advantage of operating in gated mode is
demonstrated by the absence of mercury lines from the
room lights in the low frequency spectra.
The Raman spectra of ice and water were obtained
and the high frequency regions are shown in Figure 4.
The water sample was tap water on a glass beaker, and the
ice sample was a cylindrical block with height and
diameter of about 3 inches. In liquid water, strong broad
Raman bands at 3278 and 3450 cm-1 were observed, and
were due to symmetric and antisymmetric vibrational
modes, respectively, of water molecules. The strongest
Raman bands are usually produced by the stretching
modes of vibration. The Raman spectrum of ice has a
sharper band at 3150 cm-1, which makes it easily
distinguishable from liquid water.
Hydrous minerals, melanterite (FeSO4·7H2O),
epsomite (MgSO4·7H20), bischofite (MgCl4·6H2O), and
gypsum (CaSO4·2H2O), were analyzed and their high
frequency Raman spectra are shown in Figure 5. Raman
spectra of water-bearing minerals have very sharp and
strong bands near 3500, which is an indication of a
chemically bonded water molecule. Water molecules can
exist in these minerals at temperatures significantly above
the boiling point of water because the water molecules are
chemically bonded with the minerals. The Raman system
would therefore be able to detect chemically bonded
water in dry environments which otherwise would show
no evidence of liquid water.
0 500 1000 1500 2000
Isopropanol
Acetone
Naphthalene
Calcite
Raman shift (cm-1)
Intensity (arbitrary units)
(a)
2600 2800 3000 3200 3400 3600 3800 4000
Isopropanol
Acetone
Naphthalene
Calcite
Raman shift (cm-1)
Intensity (arbitrary units)
(b)
Figure 3. Raman spectra of Isopropanol, Acetone,
Naphthalene, and Calcite in the (a) low frequency, and
(b) high frequency shift regions.
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2600 2800 3000 3200 3400 3600 3800 4000
Raman shift (cm-1)
Intensity (arbitrary units)
Ice
Water
Figure 4. High frequency Raman spectra of liquid water
and ice.
2600 2800 3000 3200 3400 3600 3800 4000
MgCl2·6H20
Raman shift (cm-1)
Intensity (arbitrary units)
MgSO4·7H20
FeSO4·7H20
Gypsum
CaSO4·2H20
Figure 5. High frequency Raman spectra of hydrous
minerals, FeSO4·7H2O, MgSO4·7H20, MgCl4·6H2O, and
gypsum.
SUMMARY
The pulsed remote Raman spectroscopy system
demonstrated its ability to effectively obtain Raman
spectra of ice, water and hydrous minerals at a target
distance of 5.6 m, in gated mode, and with room lights on.
In order to make the system suitable for application in
planetary exploration, it has to be made portable, ultra
compact, and have low power consumption.
ACKNOWLEDGEMENTS
Much thanks to my advisor, Dr Hani Elsayed-Ali,
and to my NASA technical advisor, Dr. M. Nurul Abedin,
for this incredible research opportunity. Thanks to Dr.
Anupam Misra for the valuable training. All the
experiments were performed at the Raman Spectroscopy
Lab at Passive Sensor Systems Branch of NASA Langley
Research Center. This research is also funded by
fellowship from NASA Graduate Student Researchers
Program.
REFERENCES
1. I. R. Lewis, H. G. M. Edwards, Eds., “Handbook
of Raman Spectroscopy,” Marcel Dekker, Inc.,
New York, 2001.
2. J. R. Ferraro, K. Nakamoto, “Introductory
Raman Spectroscopy,” Academic Press, Inc., San
Diego, CA, 1994.
3. Kaiser Optical Systems, Inc., “Raman
Spectroscopy Tutorial,” http://www.kosi.com/raman/resources/tutorial/index.html.
4. S. K. Sharma, S. M. Angel, M. Ghosh, H. W.
Hubble, P. G. Lucey, “Remote pulsed Raman
spectroscopy system for mineral analysis on
planetary surfaces to 66 Meters,” Applied
Spectroscopy, 56 (2002), pp. 699-704.
5. A. K. Misra, S. K. Sharma, C. H. Chio, P. G.
Lucey, B. Lienert, “Pulsed remote Raman
system for daytime measurements of mineral
spectra,” Spectrochimica Acta Part A, 61 (2005),
pp. 2281-2287.
6. A. K. Misra, S. K. Sharma, P. G. Lucey,
“Remote Raman spectroscopic detection of
minerals and organics under illuminated
conditions from a distance of 10 m using a single
532 nm laser pulse,” Applied Spectroscopy, 60
(2006).
7. NIST Standard Reference Database Website:
http://webbook.nist.gov/chemistry/.
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