INSTRUMENTATION IN RAMAN SPECTROSCOPY: ELEMENTARY THEORY AND PRACTICE

INSTRUMENTATION IN RAMAN SPECTROSCOPY: ELEMENTARY THEORY AND PRACTICE J.Dubessy, M.C. Caumon, F. Rull, S. Sharma EMU-CNRS International School: Appli...
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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



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

(



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





0 , Rayleigh

0 −ν R

= 1012 to 1013

= 107 to 1011



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 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

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