INSTRUMENTATION IN RAMAN SPECTROSCOPY: ELEMENTARY THEORY AND PRACTICE J.Dubessy, M.C. Caumon, F. Rull, S. Sharma
EMU-CNRS International School: Applications of Raman Spectroscopy to 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 Spectroscopy to 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-chemistrycrystallography 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 Spectroscopy to Earth Sciences and cultural Heritage : 14-16th of june 2012
Where is the information in a Raman spectrum ? RASMIN (Raman Spectra Database of Minerals and Inorganic Materials)
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….
Raman shift: in relative wavenumbers with respect to the excitation radiation
Raman line shift, width and shape Musso et al. (2004) Critical line shape behavior of fluid nitrogen. Pure Applied Chem, 76, 147-155
EMU-CNRS International School: Applications of Raman Spectroscopy to Earth Sciences and cultural Heritage : 14-16th of june 2012
Raman shift: in relative wavenumbers with respect to the excitation radiation
λ0
wavelength of the excitation radiation => absolute wavenumber: 1 µm
ν R, j
10000 cm-1; 0.5 µm = 500 nm
Raman wavenumber => absolute wavenumber for a stokes Raman line:
λR, j = 1 ν
abs R, j
(
= 1 ν 0 −ν R, j
Raman shift in wavelength:
)
ν 0 = 1 λ0
20000 cm-1
ν
abs R, j
= ν 0 −ν R, j
wavelength of the Raman line
∆λ R , j = λ R , j − λ 0
EMU-CNRS International School: Applications of Raman Spectroscopy to Earth Sciences and cultural Heritage : 14-16th of june 2012
Raman shift: in relative wavenumbers with respect to the excitation radiation Stokes Raman shift (4000 cm-1) in wavelength: λ0 (nm)
ν 0 (cm-1)
ν Rabs, j max (cm-1)
λR , max (nm)
∆λ R , max (nm)
250 400 500 660 785 1064
40000 25000 20000 15151 12739 9398
36000 21000 16000 11151 8739 5398
277.7 476.2 625.0 896.7 1144.3 1852.5
27.7 76.2 125.0 236.7 359.3 788.5
The Raman spectrum is scattered over a larger spectral interval range in wavelength for red excitations than for green or UV excitation lines 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
EMU-CNRS International School: Applications of Raman Spectroscopy to Earth Sciences and cultural Heritage : 14-16th of june 2012
Raman line intensities: orders of magnitude estimated by radiometric calculations
Number of excitation photons
Number of Raman photons
Nυ0 −υ R
Excited area of the sample
Differential Raman scattering cross section
N υ0 dσ = (∆Ω)( N m ) A dΩ Solid angle of collection
Number of molecules in the excited volume
EMU-CNRS International School: Applications of Raman Spectroscopy to Earth Sciences and cultural Heritage : 14-16th of june 2012
Nν
0
Raman line intensities: orders of magnitude estimated by radiometric calculations N υ0 dσ Nυ0 −υ R = (∆Ω)( N m ) A dΩ W = 0.1 Watt = 0.1 J.s -1 (1s ) = W [E1 photon (λ0 )] 17 ν0
ν0
E1 photon (λ0 ) = h(c λ0 ) ≈ 6.62 × 10
dσ −35 -33 2 -1 ≈ 10 to 10 m .sr dΩ
−34
(3 ×10
8
0.5 × 10
∆Ω = 1 sr
−7
) ≈ 4 ×10
−18
J
Nν (1s ) = 4 × 10 photons 0
N m = ρAL
103 28 -3 ρ= ≈ 3 × 10 molecules. m 0.02 6.02 × 10 23
(
Nν
0 −ν R
(
)
N m A = ρL
L = 0 . 01 m
)
= 4 ×1017 × 10 −35 to 10-33 × 3 ×10 28 ×10 −2 = 109 to 1011
EMU-CNRS International School: Applications of Raman Spectroscopy to 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 create monochromatic light (violet), and a filter (yellow-green) to block violet monochromatic light
EMU-CNRS International School: Applications of Raman Spectroscopy to Earth Sciences and cultural Heritage : 14-16th of june 2012
First Raman experiment
Hg excitation lines
Raman lines
EMU-CNRS International School: Applications of Raman Spectroscopy to Earth Sciences and cultural Heritage : 14-16th of june 2012
Raman experiment and eyes ! blog.lib.umn.edu/chaynes/
Rayleigh scattering
Stokes Raman scattering
λ = 488 nm
Cyclohexane
Rejection filter of the 488 nm
EMU-CNRS International School: Applications of Raman Spectroscopy to Earth Sciences and cultural Heritage : 14-16th of june 2012
Figures of merit of a Raman spectrometer • excitation source: high power and stable monochromatic source
Nν
Nν
0 , Rayleigh
0 −ν R
= 1012 to 1013
= 107 to 1011
Nν
0 , reflection
= 1015 to 1017
• high rejection of the excitation wavelength • high transmission of the dispersive system and high spectral resolution • high efficiency detector
EMU-CNRS International School: Applications of Raman Spectroscopy to Earth Sciences and cultural Heritage : 14-16th of june 2012
The different elements constitutive of a Raman (micro)-spectrometer
EMU-CNRS International School: Applications of Raman Spectroscopy to Earth Sciences and cultural Heritage : 14-16th of june 2012
The excitation sources
1928
Sun, Hg lamp
1960
1963
Townes suggests the use of lasers
First laser use
1969
Ar+, Kr+
(Porto-Weber)
2000
2005
Nd-Yag Laser diodes 532 nm
1964 (He-Ne) on Cary spectrometer
End of Ar+ / Kr+ lasers in 2013-2015 ?
EMU-CNRS International School: Applications of Raman Spectroscopy to Earth Sciences and cultural Heritage : 14-16th of june 2012
The excitation source: lasers Laser = 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) Excited level
E2 hν
Ground level Light absorption
hν
hν
hν
hν E1 Spontaneous emission
Pumping: population inversion
Stimulated emission
Defines the type of laser + optical resonator to promote stimulated emission rather than spontaneous emission with two mirrors (1 highly reflective at the rear and another partially reflective near 99% at the head) EMU-CNRS International School: Applications of Raman Spectroscopy to Earth Sciences and cultural Heritage : 14-16th of june 2012
The excitation source: lasers Transverse modes 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 frequency and the intensity pattern of the light.
Only TEM00 mode is used in Raman spectroscopy and microRaman spectroscopy Gaussian beam profile
EMU-CNRS International School: Applications of Raman Spectroscopy to Earth Sciences and cultural Heritage : 14-16th of june 2012
Polarization of the laser beam 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 reflected out the cavity.
tg(θBrewster) = n
S
θ
P S
n P
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 Spectroscopy to Earth Sciences and cultural Heritage : 14-16th of june 2012
Mirror
Divergence of the laser beam: Figure of Merit M2
Perfect Hermite Gaussian laser beam θ0 The quality factor, M2 (called the “M-squared” factor), is defined to describe the deviation of the laser beam from a theoretical Hermite-Gaussian beam.
M = 2
w0, R × θ 0, R w0 × θ 0
For CW lasers and helium neon lasers, 1.1 Gaussian, mixtures Condition of no modification of the band profile and no enlargement: Instrumental resolution < 1/5 FWHM of natural profile
EMU-CNRS International School: Applications of Raman Spectroscopy to Earth Sciences and cultural Heritage : 14-16th of june 2012w
Coupling sampling system with spectrometer
Optimum coupling conditions: constant flux of photons transported from the sample to the detector without any loss (except those resulting from absorption): Etendue or throughput is constant
EMU-CNRS International School: Applications of Raman Spectroscopy to Earth Sciences and cultural Heritage : 14-16th of june 2012w
Radiometric calculations
Calculation of number of Raman photons from the source
Calculation of number of Raman photons collected by the 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 can be calculated.
EMU-CNRS International School: Applications of Raman Spectroscopy to Earth Sciences and cultural Heritage : 14-16th of june 2012w
Spatial resolution of confocal Raman microspectrometers
1. 4λ ( N . A.) 2
Axial resolution
δz =
Lateral resolution
δxy = 0.46λ (N . A.)
EMU-CNRS International School: Applications of Raman Spectroscopy to Earth Sciences and cultural Heritage : 14-16th of june 2012w
Degradation of spatial resolution by refraction Use of immersion objective
EMU-CNRS International School: Applications of Raman Spectroscopy to Earth Sciences and cultural Heritage : 14-16th of june 2012w
RAMAN SAMPLING VOLUME 1928
1950
g (cm3) 10-3g (mm3)
1970
1990
2010
Hg lamp Conventional laser R.S. Raman microspectroscopy
10-12g
(µm3) Near-field spectroscopy
10-15g.(nm3)
From Delhaye and Dhamelincourt
EMU-CNRS International School: Applications of Raman Spectroscopy to Earth Sciences and cultural Heritage : 14-16th of june 2012w