INSTRUMENTATION IN RAMAN SPECTROSCOPY:
ELEMENTARY THEORY AND PRACTICE
J.Dubessy, M.C. Caumon,F. Rull, S. Sharma
EMU-CNRS International School: Applications of Raman Spectroscopyto Earth Sciences and cultural Heritage : 14-16th of june 2012
OUTLINE
• Raman instruments, elementary theory: J.Dubessy
• Calibration: M.C. Caumon
• From the laboratory to the field: F. Rull
• Coupling with other techniques: S. Sharma
EMU-CNRS International School: Applications of Raman Spectroscopyto Earth Sciences and cultural Heritage : 14-16th of june 2012
Raman instruments, elementary theory
• Initially, Raman a physics curiosity: low intensity signals
• The lasers and electronic detection (PM): crystals, gases, liquid studies in physical-chemistry-crystallography laboratories
• Raman microprobes: 1975-1978: Rosasco (USGS) and Delhaye-Dhamelincourt (LASIR, Lille, France) + instrument company.
• CCD detectors in Raman microprobes + laser rejection by filters
EMU-CNRS International School: Applications of Raman Spectroscopyto Earth Sciences and cultural Heritage : 14-16th of june 2012
Where is the information in a Raman spectrum ?
Raman shift: in relative wavenumbers withrespect to the excitation radiation
EMU-CNRS International School: Applications of Raman Spectroscopyto Earth Sciences and cultural Heritage : 14-16th of june 2012
Raman line intensities , function of:
• Intrinsic polarisation of the line• Polarisation conditions of the
excitation and signal collection• Concentration
• Raman scattering cross-section
• Molecular interactions….
RASMIN (Raman Spectra Database of Minerals and Inorganic Materials)
Raman line shift, width and shape
Musso et al. (2004) Critical line shape behavior of fluidnitrogen. Pure Applied Chem, 76, 147-155
Raman shift: in relative wavenumberswith respect to the excitation radiation
EMU-CNRS International School: Applications of Raman Spectroscopyto Earth Sciences and cultural Heritage : 14-16th of june 2012
00 1 λν =wavelength of the excitation radiation => absolute wavenumber:
jR,ν Raman wavenumber => absolute wavenumber
for a stokes Raman line: jRabs
jR ,0, ννν −=
0λ
( )jRabs
jRjR ,0,, 11 νννλ −== wavelength of the Raman line
Raman shift in wavelength: 0,, λλλ −=∆ jRjR
1 µm 10000 cm -1; 0.5 µm = 500 nm 20000 cm -1
Raman shift: in relative wavenumberswith respect to the excitation radiation
0λ (nm) 0ν (cm-1) abs
jR max,ν (cm-1) max,Rλ (nm) max,Rλ∆ (nm)
250 40000 36000 277.7 27.7 400 25000 21000 476.2 76.2 500 20000 16000 625.0 125.0 660 15151 11151 896.7 236.7 785 12739 8739 1144.3 359.3 1064 9398 5398 1852.5 788.5
Stokes Raman shift (4000 cm-1) in wavelength:
A precision of 1 cm-1 to 6.3×10-3 nm = 6.2×10-2 Å for the 250 nm excitation
A precision of 1 cm-1 6.1×10-2 nm = 0.61 Å for the 785 nm excitation
The Raman spectrum is scattered over a larger spectral interval range in wavelength for red excitations than for green or UV excitation lines
EMU-CNRS International School: Applications of Raman Spectroscopyto Earth Sciences and cultural Heritage : 14-16th of june 2012
EMU-CNRS International School: Applications of Raman Spectroscopyto Earth Sciences and cultural Heritage : 14-16th of june 2012
Raman line intensities: orders of magnitude estimated by radiometric calculations
( )( )mNd
d
A
NN
R∆Ω
Ω=−
συυυ
0
0
Number of Raman photons
Number of excitation photons
Differential Raman scattering cross
section
Solid angle of collection
Number of molecules in the excited volume
Excited area of the sample
EMU-CNRS International School: Applications of Raman Spectroscopyto Earth Sciences and cultural Heritage : 14-16th of june 2012
Raman line intensities: orders of magnitude estimated by radiometric calculations
( )( )mNd
d
A
NN
R∆Ω
Ω=−
συυυ
0
0
( )[ ]0100
)1( λνν photonEWsN =
( ) ( ) ( ) JchE photon187834
001 104105.01031062.6 −−− ×≈×××≈= λλ
-1J.s 0.1 Watt 1.00
==ν
Wphotons 104)1( 17
0×=sNν
1-233-35 .srm 10 to10−≈
Ωd
dσ sr 1=∆Ω ALNm ρ= LANm ρ=
( ) 119228-333517 10 to101010310 to101040
=×××××= −−− R
N νν
( )3-28
23
3
mmolecules. 1031002.602.0
10 ×≈×
=ρ m 01.0=L
EMU-CNRS International School: Applications of Raman Spectroscopyto Earth Sciences and cultural Heritage : 14-16th of june 2012
Raman line intensities: orders of magnitude estimated by radiometric calculations
Monochromatic luminance of the light of the sun at 0.5 µm wavelength with 1 cm-1
line width
0.02 à 2 Raman photons s -1 !
a narrow band photographic filter to createmonochromatic light (violet), and a filter(yellow-green) to block violet monochromatic light
First Raman experiment
EMU-CNRS International School: Applications of Raman Spectroscopyto Earth Sciences and cultural Heritage : 14-16th of june 2012
Hg excitation lines
Raman lines
λ λ λ λ = 488 nm
Rayleigh scattering Stokes Raman scattering
Cyclohexane
Rejection filter of the
488 nm
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Raman experiment and eyes !
blog.lib.umn.edu/chaynes/
EMU-CNRS International School: Applications of Raman Spectroscopyto Earth Sciences and cultural Heritage : 14-16th of june 2012
117 10 to100
=− RN νν
1312, 10 to10
0=RayleighNν
1715, 10 to10
0=reflectionNν
• excitation source: high power and stable monochromat ic source
• high rejection of the excitation wavelength
• high transmission of the dispersive system and high s pectral resolution
• high efficiency detector
Figures of merit of a Raman spectrometer
The different elements constitutive of a Raman (micro)-spectrometer
EMU-CNRS International School: Applications of Raman Spectroscopyto Earth Sciences and cultural Heritage : 14-16th of june 2012
The excitation sources
EMU-CNRS International School: Applications of Raman Spectroscopyto Earth Sciences and cultural Heritage : 14-16th of june 2012
Sun, Townes suggests First laser use Ar +, Kr+ Nd-Yag Laser diodes
Hg lamp the use of lasers (Porto-Weber) 532 nm
1964 (He-Ne) on
Cary spectrometer
1928 1960 1963 1969 2000 2005
End of Ar + / Kr+ lasers in 2013-2015 ?
EMU-CNRS International School: Applications of Raman Spectroscopyto Earth Sciences and cultural Heritage : 14-16th of june 2012
The excitation source: lasersLaser = Light amplification by stimulated emission of radiation: 1957-1960
Charles Hard Townes, Arthur Leonard Schawlaw (Bell labs); Gordon Gould (Columbia University); Theodore H. Maiman (Hugue Research lab)
Ground level
Excited level E2
E1
Light absorption Spontaneous
emission
Pumping: population inversion
hνhνhν
hνhν
Stimulatedemission
Defines the type of laser
+ optical resonator to promote stimulated emission rathe r than spontaneousemission with two mirrors (1 highly reflective at the rear and another partiallyreflective near 99% at the head)
EMU-CNRS International School: Applications of Raman Spectroscopyto Earth Sciences and cultural Heritage : 14-16th of june 2012
Resonator modes can be divided into two types: longitudinal modes, which differ in frequency from each other; and transverse modes, which may differ in both frequencyand the intensity pattern of the light.
The excitation source: lasers Transverse modes
Only TEM 00 mode is usedin Raman spectroscopy
and microRamanspectroscopy
Gaussian beam profile
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At the Brewster incidence angle, the windows transmit all light polarized parallel to the incident plane (P). Light polarized perpendicular to the incidence plane (S) is reflectedout the cavity.
P
S
S
P
Mirror
nθ
( ) ntg Brewster =θ
Polarization of the laser beam
Polarized incident line => access to polarization state of Raman lines
Measurements of depolarization ratio of Raman lines
Consequences of the light transmission of gratings
EMU-CNRS International School: Applications of Raman Spectroscopyto Earth Sciences and cultural Heritage : 14-16th of june 2012
Perfect Hermite Gaussianlaser beam
θ0
00
,0,02
θθ
××
=w
wM RR
The quality factor, M 2
(called the “M-squared” factor), is defined to describethe deviation of the laser beam from a theoreticalHermite-Gaussian beam.
For CW lasers and helium neon lasers, 1.1<M2<1.3 less than 1.1;
For diode lasers 1.1<M2<1.7
Divergence of the laser beam:Figure of Merit M2
EMU-CNRS International School: Applications of Raman Spectroscopyto Earth Sciences and Cultural Heritage : 14-16th of june 2012
The choice of the excitation source
Wavelength of lasers and laser choice
Ar+: 351.1; 364; 457.9; 488; 514.5 Kr+: 350.7; 406.7; 413.1; 530.9; 647.1; 676.4
Nd-YAG+: 256; 365; 532; 1064;Laser diodes: 405; 635; 660; 785
• luminescence of the usual samples;
• Consequences on optics, gratings, detector
Ar
458 nm
EMU-CNRS International School: Applications of Raman Spectroscopyto Earth Sciences and cultural Heritage : 14-16th of june 2012
Rejection of λ0
• either by a double or triple spectrometer
• either by a rejection filter with DO = 6
• super-Notch filter: well centred on λλλλ0 30 cm -1
• edge filter: high band pass filters.
6log,0
−=−=
DO
I
I
incident
dtransmitte
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Rejection of λ0 and Raman lines separation
1785 1928 1953 1972
Gratings prisms ruled precision holographic
Hairs ! gratings gratings
Refraction based:
n = f(λλλλ)
Diffraction based
Cary: double additive Czerny-Turner spectrometer
GRATINGS to separate the different radiations
• Gratings work either in transmission or in reflection
• Transmission gratings are made of parallel elongated domains whichtransmit the light and opaque domains: thus they can be considered as arrangement of parallel slits corresponding to the transmission zones
• Reflection gratings are an assembly of elongated mirrors acting as slits; the grooves are the opaque parts.
PHYSICS MODELS: ARRANGEMENT OF MANY PARALLEL EQUIDISTANT SLITS WITH THE SAME WIDTH
1 SLIT - 2 SLITS – N SLITS
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GRATING THEORY: 1 single slit: Fraunhoffer slit
2
0
sin
×=β
βII
λθπβ sin××= b
S1
S2
M1
M2
M3
m1
m2
m3
b
P
Sm
GRATING THEORY: 1 single slit
2
0
sin
×=β
βII
λθπβ sin××= b
• Symmetric distribution of maxima and minima
• Secondary maxima for ( ) ( )b
kbk
2
12sin
sin
212 max
λθλ
θππβ +=⇒=+=
• Secondary minima for ( ) ( )b
kbk
λθλ
θππβ 12sin
sin12 min
+=⇒=+=
• Width of central max ≈ 2 (λ/b) θ→ 0 if b is large with respect to λ: point source
• if (λ/b) ≈1, then at k=0, θmin ≈π/2 => large central maxima
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GRATING THEORY: 2 single slits
γβ
β 22
2
0 cossin
2 ×
×= II
Single slit termenveloppe
( ) θλπγ sind=
EMU-CNRS International School: Applications of Raman Spectroscopyto Earth Sciences and cultural Heritage : 14-16th of june 2012
( ) θλπγ sind=
EMU-CNRS International School: Applications of Raman Spectroscopyto Earth Sciences and cultural Heritage : 14-16th of june 2012
GRATING THEORY: 2 single slits
When the slits are narrow, the positions of the maxima are determined mainly by cos2γ
Maxima occur for cos 2γγγγ = 1
( )λλλλθ
πππθλπγmd
d
,...3 ,2 , ,0sin
,...3 ,2 , ,0sin
===
γβ
β 22
2
0 cossin
2 ×
×= II
Single slit termenveloppeλ
θπβ sinb= Represents the interferencesresulting from two beams of equal intensity
GRATING THEORY: N single slits
( )( )
λθπγ
γγ
ββ
sin
sin
sinsin2
2
2
2
2
0
××=
×
×=
d
NII
Single slit termenveloppeλ
θπβ sinb=Represents the interferencesresulting from N beams of equalintensity arising from N slits
EMU-CNRS International School: Applications of Raman Spectroscopyto Earth Sciences and cultural Heritage : 14-16th of june 2012
( )( )
λθπγ
γγ
ββ
sin
sin
sinsin2
2
2
2
2
0
××=
×
×=
d
NII
EMU-CNRS International School: Applications of Raman Spectroscopyto Earth Sciences and cultural Heritage : 14-16th of june 2012
GRATING THEORY: N single slits
( )( ) πγ
γγ
mN
N →±→ when 1sin
sinMaximum value
λλλλλθ mmd ,...3 ,2 , ,0sin =×=×
GRATING THEORY: grating equation
( ) λλλλλθ mmid ,...3 ,2 , ,0sinsin =×=+×
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2d
,2sinsin as ≤≤+ λθ mi
⇒= 0m Mirror, specularreflection
:m diffraction order
GRATING THEORY: grating equation
EMU-CNRS International School: Applications of Raman Spectroscopyto Earth Sciences and cultural Heritage : 14-16th of june 2012
http://hyperphysics.phy-astr.gsu.edu/
m = -2 m = -1 m = 0 m = +1 m = +2
GRATING DIFFRACTION
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Monochromaticdiffraction
Polychromaticdiffraction
Superposition of orders
222
111
sin
sin
λθλθ
×=××=×
md
md
221121 λλθθ mm =⇔=2121 2
3 3;2 λλ =⇒== mm
Red: 600 nm
bleu: 400 nm
http://h2physics.org
GRATING DIFFRACTION and Raman spectrum
λlaser = 514.535 nm Raman spectrum; m=1;
No possible overlapwith different orders
m =0
GRATING DIFFRACTION and grooves density
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( ) λθ ×=+× mid sinsin
6000(VPHG)-3000-150 G
grooves/mm ofnumber 1
:density groove
=
==d
G
λθ ××=+ mGi sinsin
at constant m and wavelength, diffraction angle is pr oportional to groove density
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RULED GRATING and HOLOGRAPHIC GRATING
GRATING DIFFRACTION and Raman spectrumAngular dispersion versus wavelength
Dispersion on the screen of the different wavelengths for a givendiffraction order results in different angular dispersion
( )
( )θθθ
λθθ
θλλθ
coscoscos
and sinsin
Gm
d
mAD
d
dmdd
d
dADmid
==⇒=
≡=+×
AD : proportional to m and G
GRATING DIFFRACTION andChromatic resolving power
mNR =∆
=λ
λ
Resolved lines Just resolved line: Rayleigh criterionThe first diffraction minimum of the image
of one source point coincides with the maximum of another.
Unresolved lines
GRATING DIFFRACTION andChromatic resolving power
mNR =∆
=λ
λ
d
LN g=
and d
LmR g=⇒ ( ) λθ mid =+× sinsin
2
best λλ
λ gL=
∆The spectral resolution is proportional to the length o f
the grating (perpendiculary to the grooves)
EMU-CNRS International School: Applications of Raman Spectroscopyto Earth Sciences and cultural Heritage : 14-16th of june 2012
GRATING DIFFRACTION andResolution in wavenumber scale
mNkk =∆=∆λλλπ2=kWave-vector
mNmNkk
λυυυπυπ 1
22 ==∆⇒∆×=∆⇒=
At the first order, if λαλ ×=≈ dd ,gLNd
ααν =×
=∆ min
cm 10
µm 5.0
==
gL
λ1-
min cm 1.010
1 ==∆⇒ νEMU-CNRS International School: Applications of Raman Spectroscopy
to Earth Sciences and cultural Heritage : 14-16th of june 2012
EMU-CNRS International School: Applications of Raman Spectroscopyto Earth Sciences and cultural Heritage : 14-16th of june 2012
GRATING DIFFRACTION and line intensity
The incident angle vs the blaze angle The polarization state of the incident radiation
Efficiency curves are function of wavelength: 1) Absolute = Incident(λ)/Diffracted(λ)
2) Relative = Diffracted(λ)/Reflected(λ) by mirror with same coating
Grating efficiency depends onWavelength , polarization in incident light, incidence angle , diffraction order , groove profile and coating material .
EMU-CNRS International School: Applications of Raman Spectroscopyto Earth Sciences and cultural Heritage : 14-16th of june 2012
GRATING DIFFRACTION and line intensity
Selection of the grating from the exciting radiation wavelength
Blaze angle, maximum efficiency
CZERNY-TURNER SPECTROMETER:Basic principles
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Sources and detectors are in a fixed position: Dv
= deviation angle
iDv −= θ
λθ ××=+ mGi sinsin
λθθθ ××=
×
+×=
−×
+× mGDiii v
2cos
2sin2
2cos
2sin2
files.chem.vt.edu
EMU-CNRS International School: Applications of Raman Spectroscopyto Earth Sciences and cultural Heritage : 14-16th of june 2012
CZERNY-TURNER SPECTROMETER:Actual optical design
opticalea.com
CZERNY-TURNER SPECTROGRAPH:Actual optical design
EMU-CNRS International School: Applications of Raman Spectroscopyto Earth Sciences and cultural Heritage : 14-16th of june 2012
CZERNY-TURNER SPECTROGRAPH:Actual optical design
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2vD=ϕ
θϕ −= i2
φωθ −=
ωϕ +=i
( ) ωθ =+ 2i
( ) ϕθ =− 2i
Angular relationships
,
λϕω ××=×× mGcossin2
ϕ
anglerotation =ω
CZERNY-TURNER SPECTROGRAPH:Sine bar for grating rotation
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DETECTORS
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1928 1940 1950 1964-1967 1978 1986 2012
Eyes, Photomultiplier Image Intensifier Intensified CCD
photographic tube Vidicon Camera Photodiode Array
plate
CCD DETECTORSInvented in 1969 at AT&T Bell Labs by Willard Boyle and George E. SmithNobel Prize of Physics in 2009
2D Array of individual « detectors » = pixels
1024 x 256 pixels (26 µm x 26 µm)
Formation of an electron/hole pair in p-dopped silico nlayer if E(photon) > Si band gap
200-1100 nm
Electrons are stored in a potential well characterized by is Full Well Capacity and read-out by an electrode
Kodak
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Cooled at -90 °C (Pelletier effect) or -130°C (liqui d N2) to eliminate thermal noise
Front or back illuminated: consequences on quantum e fficiency
CCD Detectors: quantum efficiency curves
Quantum efficiency ( )λEQ = number of electrons generated per incident photon
EMU-CNRS International School: Applications of Raman Spectroscopyto Earth Sciences and cultural Heritage : 14-16th of june 2012
Andor Company To choose as a function of the excitation source
CCD detectors: high Q E, lowdark current, low read-out noise => great input in the use of Raman spectroscopy, in Earth sciences.
CCD : intensity and wavenumber coding
Intensity: vertical binning of pixels
Linear dispersion of the different radiations: deduced from the angular dispersion produced by the grating
BWTEK
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CCD : wavenumber coding
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dx
dRLD
λ≡Average wavelength dispersion or Reciprocal Linear Dispersion
RLD is linked to AD bydx
d
d
d
dx
d ωωλλ ×=
dx
dADRLD
ω×=
Where ωωωω is the rotation angle of the grating
For a spectrometer with a focal distance f ωdfdx ×=
λθθθ ××=
×
+×=
−×
+× mGDiii v
2cos
2sin2
2cos
2sin2
For a Czerny-Turner spectrograph
mfGmf
d
dx
d
×××=××
×= ϕωϕωλ coscos2
coscos2
CCD : wavenumber coding
EMU-CNRS International School: Applications of Raman Spectroscopyto Earth Sciences and cultural Heritage : 14-16th of june 2012
mfGmf
d
dx
d
×××=××
×= ϕωϕωλ coscos2
coscos2
Numerical application: small ωωωω angle (cos ωωωω =1); f = 800 mm; grooves d = 0.5 µm;
1st order diffraction; φφφφ = 30 °
dx
d
d
d
dx
d λλνν ×=
In wavenumber scale
λν 1=dx
d
dx
d
d
d λνννλλ
ν ×−=⇒−=−=22
2
1As
,
( ) 1-3µm.mm1008.1
800
30cos5.02 −×=°××=dx
dλ
CCD of coveragecm 880cm/cm367 11 −− ⇒≈=∆∆
x
ν
At 1000 cm-1 Raman shift / 514.5 nm 111000cm
1 cm18435cm19435 −−
− =⇒= νν laser
1 pixel 26 µm corresponds to 0.95 cm -1 /pixel = pixel size resolution
EMU-CNRS International School: Applications of Raman Spectroscopyto Earth Sciences and cultural Heritage : 14-16th of june 2012
CCD : wavenumber coding
cm/cm367 1−=∆∆
x
ν 1 pixel 26 µm = 2.6 10 -3 cm
Limiting resolution of the spectrometer including the detector
33 ××
∆∆=×=∆ CCDpixelerspectromet w
xδννδν
Spectral resolution
( ) ( )22erspectrometslitSR λλλ ∆+∆= ( ) ( )22
erspectrometslitSR ννν ∆+∆=
dx
dlslit
νγν ××=∆ ( )( ) 1 and 1;
cos
cos
spectomer theofpower magnifying :
≈=×= γθ
γ
γ
B
A
B
A
L
L
L
Li
slit width=lThus , for the same numerical application
µm 100 /cm,cm367 1 == − ldx
dν -1cm 67.3=∆ slitν
-1cm 74.4=νSR
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Band Shape
( ) ( ) ( ) ( ) ( )000
,, νννννννν ALdALFx
⊗=××= ∫
Source Apparatus function
Modification of the band profile by the instrument.
Lorentzian => Gaussian, mixtures
Condition of no modification of the band profile and no enlargement:
Instrumental resolution < 1/5 FWHM of natural profile
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Coupling sampling system with spectrometer
Optimum coupling conditions: constant flux of photon s transported fromthe sample to the detector without any loss (except thos e resulting fromabsorption): Etendue or throughput is constant
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Radiometric calculations
Calculation of number of Raman photons from the source
Calculation of number of Raman photons collected by th e sampling system (lens, microscope objective)
from the value of transmission of each optical element (T= I/I0),
from the value of the QE of the detector, the number of photo-electrons canbe calculated.
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Spatial resolution of confocalRaman microspectrometers
( )..46.0 ANxy λδ =Lateral resolution
Axial resolution 2.).(
4.1
ANz
λδ =
Degradation of spatial resolution by refraction
Use of immersion objective
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10-15g.(nm 3)
1928 1950 1970 1990 2010
g (cm 3)
10-3g (mm 3)
10-12g (µm 3)
Hg lamp
Conventional laser R.S.
Raman microspectroscopy
Near-field spectroscopy
RAMAN SAMPLING VOLUME
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From Delhaye and Dhamelincourt
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