Bull. Mater. Sci., Vol. 21, No. 5, October 1998, pp. 433-438. © Indian Academy of Sciences.
A low cost laser-Raman spectrometer
A K BANDYOPADHYAY*, NITA DILAWAR, ARUN V I J A Y A K U M A R , D E E P A K VARANDANI
and DHARAMBIR SINGH
Pressure Standards and Pressure Physics, National Physical Laboratory, Dr K S Krishnan Marg,
New Delhi 110 012, India
MS received 9 June 1998
Abstract. A Jobin Yvon-Spex (HR640) monochromator with a notch filter (514-5 nm) and Ar ÷ ion laser
has been used to set up a low cost laser Raman spectrometer. The selection and setup of the various optical
components used in the present work has been solely carried out in our laboratory. The calibration of the
monochromator was established from the studies of various standard mercury lines and the obtained data
fitted with the reported data. Raman signals have been recorded for a number of samples e.g. diamond,
ruby, carbon tetrachloride (CC14), benzene (C6H~) and ethanol (C2HsOH). The obtained results are found
to be in excellent agreement with the reported values for these materials in the literature.
Keywords.
1.
Raman spectrometer; laser Raman spectroscopy.
Introduction
Since its discovery in 1928, Raman spectroscopy has
proved to be an invaluable, powerful and widely used
optical method for investigating the dynamics of different
states of matter. In the early years, experiments were
carried out using mercury sources, prism spectrographs
and photographic detection techniques (Bansal et al 1976).
However, the techniques and instrumentation in Raman
spectroscopy have undergone considerable changes during
the last few decades. The availability of lasers as intense,
monochromatic and polarized excitation sources enables
one to record weak scattering spectra and its applications
are attached widely over the fields of physics, chemistry
and bio-science (Tonino and Okushi 1990). Conventional
laser-Raman spectrometer employs a monochromator for
spectral dispersion and photo-multiplier for detection of
light signal (Gardiner and Graves 1989; Roy et al 1992),
which corresponds to a single channel detection system.
However, several improvements have been incorporated
for ease in handling and operation of the instrument.
Typical setups consist of (i) a CW laser which emits
monoenergetic beam, (ii) steering or source sampling
optics which directs the beam towards the sample, (iii)
a single stage monochromator with computer controlled
scanning for spectrally analyzing the inelastically scattered
radiation from the sample and (iv) PMT or CCD for
detecting Raman signal with associated photon counting
system linked to computer for data acquisition and analysis. This type of setup allows for a fast acquisition of
Raman spectra over a large spectral range but its inherent
*Author for correspondence
limitation is the rather low resolving power (Roy and
Bansal 1988; Panitz et al 1994).
With fast advancement in technology, multiple grating
monochromators ~vith charged coupled device (CCD)
arrangement or multichannel signal acquisition spectrometers are being used. Although these instruments provide
higher resolving powers, however, their use is limited
by their expensive components. The throughput of double
or triple monochromators is smaller in comparison to
the single monochromators. Moreover, in high resolution
experiments, the intensity is spread over a large number
of channels (Engert et al 1994). CCDs provide increase
in quantum efficiency and very low dark count rates
resulting in increase in sensitivity (Carter and Pemberton
1995). However, the use of multichannel detection system
also suffers from some inherent disadvantages as compared to single channel detection system such as small
dynamic range of the array detector and restricted density
of measurement points depending upon the dimensions
of single detector element. In addition, the spectral range
is limited by both the overall dimension of the detector
and the dispersion of the monochromator (Deckert and
Kiefer 1992; Engert et a l 1994).
In this context we have used a low cost single stage
monochromator with single channel signal acquisition
system whose optical steering system has been indigenously setup. The room temperature Raman spectra
were recorded using 514.5 nm line of 4 W argon ion
laser in backscattering geometry. The Rayleigh line was
filtered from the exciting line by a notch filter. The
laser light was steered through a long optical path with
the help of a series of highly polished mirrors and
lenses. The various components of this low cost laser433
A K Bandyopadhyay et al
434
Raman
in the
spectra
in later
2.
2.1
spectrometer and their advantages are described
following sections. The calibration as well as
of some of the materials studied are described
sections of this paper.
The spectrometer
Laser source
An argon ion laser is an important and powerful excitation
source for light scattering. It can provide several discrete
lines. The most important of these lines are centred at
488 nm and 514.5 nm (Roy and Bansal 1988). A Coherent
Innova 70 argon ion laser is used to conduct our experiments. It can be operated in the single-line or multiline
mode with an all line power output of 4 W. We have
used an excitation wavelength of 514.5 nm (green radiation) with a beam diameter of 1.5 mm and a beam
divergence of 0.5 mrad. An SCR (Silicon-controlled rectifier) is used to provide DC current to the plasma tube,
magnet and power supply electronics. The laser head
cooling is done by a PolarPure 12 laser chiller.
2.2
Focussing optics
The optical components used for focussing the laser
beam, are used in a configuration which helps to minimize
the intensity loss from the incident as well as the
scattered radiation from the sample. These components
have been purchased from M/s Melles Griot, Holland.
The selection of these optical components was based on
a detailed estimation of several optical parameters, in
particular, focal length of the combination of collecting
lenses, the dispersivity of the prism, mirror reflectivity
etc with a view to obtain maximum efficiency. As shown
in the schematic diagram (figure 1), the output of the
laser source is allowed to fall on a beam steerer which
comprises of two vertically displaced, highly reflecting
L2
plane mirrors (average reflectance - 9 2 % ) placed at 90 °
to each other and at 45 ° to the vertical axis, which
result in a vertical displacement of the beam to the level
of the entry slit of the monochromator. The beam is
now directed towards a right angled prism which has a
surface flatness of - 9 5 % . This prism reflects the beam
at an angle of 90 ° towards a small mirror (5 mm diameter).
Another reflection at this mirror surface (at 45 ° to the
incident beam) allows the beam to fall on the sample
in a 180 ° or back scattering geometry. Due to small
size of the mirror the Raman signal back scattered from
the sample which is usually very weak (1 in 105), is
not hindered. This scattered beam from the sample is
imaged on to the entrance slit of the monochromator
using a collecting and a focussing lens with focal lengths
1 5 0 m m and 200mm, respectively. In case of liquid
samples a spherical mirror is placed behind the sample
so as to minimize losses of the scattered radiation from
the sample.
2.3
The monochromator
A single stage Jobin Yvon-Spex HR 640 Czerny-Turner
monochromator is used for our measurements. It has a
focal length of 0-64 m. Its low degree of deviation
confers high performance characteristics on the instrument
in terms of ultimate resolution. In addition, a good
throughput is achieved due to large useful size
( 8 0 x 110 mm 2) of the holographic grating which has
1200 grooves/mm blazed at 500 nm. The grating is moved
by a stepper motor controlled by the software described
below. The mechanical scanning range of the monochromator is from 0-1500 nm. Two identical input and output
slits with straight blades are used. The dispersion of the
instrument is 12 A/mm at 500 nm and the resolution is
0-16A at 5 4 6 n m and 2 mm high, 5-10 I.tm wide slits.
The programmable scanning speed is continuous from
0.02 to 3000 A/mm. The stray light level is less than
L1
100%
80%
60%
40%
I--
.%
SPECTP-..A LINK
E'qIPLrf~)UTPUT MODULE
S'IE!I~ER MOTORCONTROL
]- -I
~
s
.
I~U_ATt41~6
L2- CONVER-GNGLENS
RAM 8MB
G: GRATING1200U ~
_
_
2O%
0%
400
PMT: PHCCFO.MULT~L~gR TL~E
Figure 1. Schematic diagram of laser Raman spectrometer
setup used for the present work.
I
I
p
4so
soo
sso
Goo
Wavelength (nm)
Figure 2. Transmission profile of the notch filter used for the
present work.
Low cost laser-Raman spectrometer
10-5 at 10 A with slits 20 p.m wide and 2 m m high. The
precision is + 5 A from 0 to 15000 A with a repeatability
of 0-2 A.
A notch filter is used just before the entry slit of the
monochromator to attenuate the Rayleigh line. It has an
optical density of 4.3 and a spectral bandwidth of 575
wave numbers. Figure 2 shows the transmission characteristics of the filter used. The use of Rayleigh line
rejection filters is essential since the stray light
suppression of the single monochromator is often not
sufficient for Raman spectroscopy, especially at low
Raman shifts. Rayleigh filters represent an efficient and
low cost alternative to the otherwise necessary filter
stage in the expensive triple monochromator. It has been
reported that efficient filters with high optical density
at the laser frequency and sharp onset of absorption can
replace the prefilter stage of the triple monochromator.
The result is not only a large optical throughput but
also a compact Raman instrument (Shulte 1992).
A photomultiplier tube (PMT) is used for the detection
of the signal at the exit slit. The spectrally analyzed
light emerging through the slit is detected using a R928
S PMT phototube in photon counting mode. The photon
counter consists of HV supply, fast amplifier, discriminator and TTL converter controlled through the 'Spectralink' described later.
Thus the laser light scattered from the sample passes
through the entry slit of the single monochromator which
is placed in the focal plane of the spherical mirror M1
(figure 1) and is collimated to the grating G which
disperses the signal. The diffracted beam is focussed by
the mirror M2 onto the exit slit. Prior to falling onto
the mirror M1, the light is made to pass through the
notch filter described earlier. Low stray light levels are
achieved due to this filtration despite using a single
grating monochromator.
2.4
The spectralink
The monochromator control and data acquisition are
executed by a modular system known as 'Spectralink'.
It consists of many modular sub-units. There is a single
high voltage power supply programmable from 0-2000 V
(through the software) for the PMT. An acquisition
module for monochannel detection with an integration
time of 1 ms to 65 s is also provided. This module
consists of a programmable photon counter and a dc
amplifier. This module is used for acquisition of analog
signals coming from the photo-multiplier tube. A computer interface module provides communication between
the spectralink and the computer. In addition, spectralink
also has a input-output module. A stepper motor control
module is also a part of the spectralink. It is used for
manually setting the monochromator position via a .joystick, though it is programmable via the interface board
435
and the computer. This module is used to access built-in
scanning functions (scanning speed, number of cycles
and range of wavelength etc).
2.5
Software
2.5a Data acquisition: The data acquisition and
parameter control is carried out through the 'Spectramax'
software written in ' C ' language provided by the JobinYvon company. The spectramax software is a completely
integrated series of programs and spectral routines for
the acquisition and treatment of spectroscopic data. It
has been written to work with MS DOS versions 5.0,
6.0 or higher. Spectramax utilizes the GEM graphical
user environment as an operating environment for its
windowing capability. The software allows for a flexible
use of most standard modes of acquisition and helps in
creation, storage and modification of acquisition routines.
2.5b Data analysis: The analysis of the experimental
data obtained from the Raman measurements is carried
out using the 'Peakfit' commercial non-linear curve fitting
package. This package provides for data editing, conversion of units, deconvolution of the peaks, calculation
of peak area as well as the line width. The observed
Raman peaks in the present study have been fitted with
a Voigt fit using Voigt function given by the equation:
a 0 exp(-
y2)
-= 2 + [ ( x - a, ) yl
f=
dy
2
,
~ exp(-Y2) dy
where a 0 is the peak amplitude, a~ the peak centre, and
a 2 and a 3 determine the width of the peak (Dilawar
1997).
For a sharp peak and an isolated band the fitting
procedure produces same peak characteristics with a
decimal point accuracy. However, for broad bands, the
uncertainty in position and width is within + 5 cm -~.
3.
Calibration
The calibration of the instrument was carried out using
a low pressure Hg light source. The entry and the exit
slits of the monochromator were opened to a size of
50 gm each. The wavelength counter was set to zero to
verify that maximum light flow leaves the instrument
via the exit slit. The grating and the mirror position
were adjusted in order to bring zero order into the exit
slit. Thereafter the Hg spectrum was obtained in the
wavelength range 500 nm-850 nm. A number of lines
A K Bandyopadhyay et al
436
were obtained which have been found to be in excellent
agreement with the standard Hg lines (Jobin Yvon-Spex
1996) as shown in table 1. A plot of obtained wavelengths
versus the standard wavelengths is shown in figure 3
which shows a linear correlation between the two.
It has been established by a number of workers that a
natural type IIa diamond shows a sharp transition at
- 1332cm -1 (Wada and Sotin 1981; Knight and White
1989). Thus our result is in excellent agreement with
the reported value. From the data analysis it is also seen
that the width of the peak at half maximum is 14-1 cm -1.
4. Sample study
4.2
Subsequent to the installation and set up of the instrument
as detailed in the above sections, a number of samples
were studied which include solid as well as liquid
samples. The results obtained for a few representative
samples are discussed in the following sub-sections.
4.1
Diamond
Diamond is one of the few materials which has revolutionized many a technology e.g. high pressure physics,
thin films, electronics etc. We have studied a type IIa
natural diamond with 0-5 watt laser power at the source.
The spectrum so obtained is shown in figure 4. It is
clear from the figure that a very sharp Raman transition
is obtained which is found to be centred at 1333.1 cm-L
Ruby (Cr 3+ doped AI203) is also an important material
technologically which is used extensively for high
pressure calibration, fabrication of ruby laser etc. Ruby
primarily shows two lines which in our study have been
found to be centred at 693.9 nm (RI line corresponding
to transition E~/z --->A2) and 692.5 nm (R2 line corresponding to transition E3/2--+A 2) which correspond to
5026.18 crn-~ and 4996.4cm -~, respectively. The R lines
of ruby are quite intense and the doublet R1 and R2 have
been reported to have wavelengths 694.2 and 692-7 nm,
respectively (Jayaraman 1983). The experimentally obtained
peaks, deconvoluted using the 'Peakfit' package are shown
in figure 5 and are found to be in good agreement with
the reported values (Jayaraman 1983). The data analysis
Table 1. Comparison between the standard Hg Raman lines
and the experimentally obtained lines.
S1.
No.
Expt. observed Hg lines
(position in nm)
1
2
3
4
5
6
7
3e+04
Std. Hg lines
(position in nm)
545.4
576.8
580.2
626.2
668.2
729.4
809-4
i
2.5e+04
~ ".
o
546.00
576-90
579.70
626-30
668.30
730.02
809.30
Ruby
1.5e404
E
le+04
5000
800
/.-"
HI
!
i
i
tiL
i!
i
i
!
L...... ...... i
/-"
i
1300
1400
1500
Romon shift ( c m - 1 )
j.-
t75e+05
1600
[
/.-
1.5e+05
..-
~1.25e+05
650
j-
600
..'"
!
le+05
7"
"~ 7.5e+04
//"
. . . . . . .
I
i. . . . . . . . . . . . . . . . . . . . . . . .
I
5e#04
"
5O0
50O
L
t
700
550
i li •
./" .11
./."
750
t-
6_
X
LM
i
Figure 4. Raman spectrum of natural type IIa diamond crystal.
850
t-
i
i
L[....
0
1200
E
i
I.I. . !. . . . . . nA, . . . . . .i
i
2e404
:"
i
5;0
6;0
8;0
7;0
7;0
8bo
850
Std. W a v e l e n g t h (nm)
Figure 3. Plot of observed vs standard Hg peaks showing
linear correlation.
.. '
.
~
2.5e+04
4900
4950
5000
5050
Roman S h i f t ( e r a - I )
Figure 5. Raman spectrum of ruby crystal.
5100
5150
437
Low cost laser-Roman spectrometer
results yield the linewidth of RI and R2 peaks as
16-82 cm -1 and 14.35 cm -~, respectively.
The peak analysis results for diamond and ruby are
summarized in table 2.
4.3
Benzene
Benzene has been studied in the range 7 5 0 - 1 2 5 0 c m -~
and two peaks centred at 9 9 2 . 3 6 c m -~ and 1 1 7 6 . 8 c m -~
have been obtained. These peaks correspond to the
fundamental vibrations o f benzene as reported in literature
and agree well with the reported peak positions at 992
and 1178 c m t, respectively (Grassmann and Weiler 1933;
Gerrity et al 1985). The peak at 992-36 c m -1 corresponds
to c a r b o n - t o - c a r b o n vibration while a hydrogen bending
motion is found in the vibrations of the hydrogen atoms
perpendicular to the C - H bond yielding the 1176.8 c m -~
vibration (Hibben 1939). The deconvoluted spectrum of
benzene obtained in our studies is shown in figure 6.
The peak at 992.35 is found to be quite sharp with a
linewidth o f 15,18 cm -t, while the other peak has a width
of 21.62 cm -].
4.4
Carbon tetrachloride
The Raman spectrum for carbon tetrachloride depicting
Table 2. Comparison between the reported and observed Roman spectra of solid samples,
SI.
No.
Sample
1
Diamond
2
Ruby
/
6e+05
d
4e+o5
.................... ] .............
~
_~ ~e,OS ....
le+05
0
!
i
i
i .............. It[~
II
t
1333.10
14.10
1332
4996.40(692.5 nm)
5026.18(693-9 nm)
14.35
16-82
5000(692.7 nm)
5031 (694-2 nm)
Sample
Observed
peak values
(cm-l)
Line width
(cm-l)
Reported
peak values
(cm-~)
1
Benzene
(C6H6)
992-36
1176-80
15-18
21-62
992
1178
2
Carbon
tetrachloride
(CC 14)
218.66
313.49
458 -08
19.34
21-07
20-63
218
313
458
3
Ethanol
(C2HsOH)
882.43
1052.98
1095.15
19.32
27.04
32.34
883
1051
1096
...................................
....
..... i ....... tt
i ......
!
i
i
!
i
i
SI.
No.
!......
1
. . . . . . . . . . . . . . . . . . . . . . . . . . .
700
Reported peak
values (cm-1)
Table 3. Comparison between reported and observed Roman
spectra of liquid samples studied.
i
.........
Line width
(cm-])
i
,I
I
1
5e+05 ..................................................
~I
q
Observed peak
values (cm-I)
I
"
"
900
1100
Roman Sl'fif t(cm-1)
1300
Figure 6. Roman spectrum of benzene molecule.
3e-1-05
t
i
..........
2.5e+05 ............it ...........................
f
":"
q 2e-l-05o
>- 1.5e+05 c
le+05
...... , ........
i
],
i-
~I
i ........................
i
i
I
,
7e+04
6e+04
_se,o,
!
....... i ........................... , .................
i
[
i
'-
t
•
................................
t
"ll I......................... i.............................
..................... ! ..........
.........
•
.I
~ 2e+04
.................... 1 ...........
l!!
HI
................
~ ..............
i ..............
!
i
'................... t
i ..........
..............................
5e+04
0
100
,
.........
200
300
400
Roman Shift(cm-1)
Figure 7. Roman spectrum of CCI4 molecule.
580
700
900
1100
Roman Shift(cm-1)
Figure 8, Raman spectrum of ethanol molecule,
1300
A K Bandyopadhyay et al
438
deconvoluted peaks is shown in figure 7. The carbon
tetrachloride molecule corresponds to a symmetrical
tetrahedron (symmetry Td). In the present study, a Raman
spectra for the molecule has been obtained from 100 to
5 5 0 c m -l and peaks have been observed at 218.66cm J,
313-49cm -l and 458.08cm -~. These correspond to the
fundamental vibrations of C C I 4 a s reported in the literature
and are quite close to the reported values at 218, 313
and 4 5 8 c m -t (Hibben 1939). The observed peaks are
found to be wide with peak widths of 19.34cm -1,
21.07 cm -~ and 20.63 cm -~, respectively.
4.5
Ethanol
The Raman spectrum of ethyl alcohol has been obtained
in the range 7 5 0 c m - t - 1 2 5 0 c m -~. Peaks have been
observed at 882.43 cm -t, 1052.98 cm -~ and 1095.15 cm -~.
These agree well with the fundamental vibrations for
ethyl alcohol reported at 883, 1051 and 1096 crn-1 (Hibben
1939; Bolla 1934). Figure 8 shows the deconvoluted
peaks for the Raman spectrum of the molecule. The
linewidth for the respective peaks are 19.32 c m j, 27.04
c m -1 and 32.34 cm ~.
The peak positions and linewidths of the liquid samples
studied are summarized in table 3.
5.
Conclusions
A low cost laser Raman spectrometer is described in
the present study. The signal collection and focussing
optics used lead to a sufficiently high resolution of the
spectrometer as is evidenced by the spectra of the
different samples. The high throughput of a single stage
monochromator, a large grating and lower integration
time required by a single channel detector also help in
lowering the acquisition time. This low cost spectrometer
is also being used for ultra-high pressure studies of
materials using a diamond anvil cell (DAC), the details
of which will be published elsewhere.
The spectra of some of the well studied solid and
liquid samples are obtained. The obtained peak positions
are in good agreement with the reported peak values in
the literature.
Acknowledgements
The authors are thankful to Prof. A K Ray Chaudhuri
and Mr A C Gupta for their kind help at every stage
of this programme. Special thanks are due to Prof. E S
Raja Gopal and Prof. R Srinivasan. T h e financial help
from the Department of Science and Technology, New
Delhi, is especially acknowledged.
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