Optical Spectroscopy with dispersive Spectrometers
Basics - Building Blocks - Systems - Applications

This page summarizes chapter 6 of the book
"Applications of Dispersive Optical Spectroscopy Systems",
ISBN 9781628413724, SPIE monographs, Bellingham, WA, USA

Application R2
Raman and Brillouin Spectroscopy

This page presents the directory, the signs and symbols, conversions, and equations of the book, while the details are an exclusive part of  the book
.

R2.0 Introduction
The interactive effects of light scattering at molecules can be separated into two main groups: the inelastic, and the elastic scattering. Rayleigh scattering is an elastic one, because the energy of the scattered photons remains unchanged, while the direction of travel is modified. In inelastic scattering [Raman] processes, the photon energy/frequency/wavelength of the light after the interaction with the sample is different, and the distribution is spherical. Thus, it is possible to measure at the original wavelength [Rayleigh] only the intensity and 3-D angular distribution of the elastic scattering. The Raman photons, on the other hand, will carry a slightly different energy/frequency/wavelength after interaction. The reason is, that the molecule may absorb a certain amount into its rotational structure, and vary the vibrational state (stretch) - that is measured at the Raman Stokes side of the spectrum. In turn, the molecule will also add some energy from its own rotation, resulting in spectral features at the Raman Antistokes side. Commonly, the Stokes spectrum is stronger than the Antistokes part, and is more often used for analysis. The effect was proposed by Adolf Smekal in 1923 for solid state structures. C.V. Raman added the theoretical base on liquids. In 1928, the effect was proven first time by Raman and Krishnan. Raman received the Nobel price in 1930 for his work, and is the popular spender of the name. Sometimes, one also finds the name Raman-Smekal effect. If the inelastic scattering happens in the range of optical phonons, it is called Brillouin scattering, and the spectrum will appear largely symmetric. That effect was predicted by Leon Brillouin in 1922, based on Schr÷dinger┤s equations for inelastic vÝbration scattering. The sense of both effect is different, but complementary, and the spectra may overlap. Both effects, Raman and Brillouin, are very weak, typically only 10-14....10-10 of the photons present in the sample volume, are converted. And those are even spectrally and spatially distributed. The spectral distribution reaches from 0...+/- 4500 cm-1 or 0...+/- 0,6 eV.
In contrast to absorption/reflection, luminescence, emission, and polarization spectroscopy, the scattering features appear, in absolute difference of energy/wavenumbers, always at the same difference. That is independent from the wavelength of excitation. It shall be kept in mind, that all scattering effects depend on the excitation photon energy with the 4th power. It shall also be realized, that the relative bandwidth and dispersion of spectrometers are inverse to that in principal. The distribution of the photons, scattered by the Raman, or Brillouin effect is spherical and statistical, while elastic scattering carries information in the direction of the scattered photons. The bandwidth of Raman signals can appear rather narrow, typically in the order of 10 cm-1 or 0.001 ev. The Brillouin lines are even narrower, mainly in the range of <1 cm-1 or 10-4 eV, and they lie rather close to the excitation. Raman and Brillouin are quantitative methods, and both are complementary to IR absorption spectroscopy, which senses the rotation of the molecules. There are samples, which do not absorb in the IR, but produce Raman/Brillouin signals, and vice versa.

Graph R2-1 the basic set-up of a Raman spectrometer

Graph R2-1 shows the principle of a system for Raman, and Brillouin spectroscopy.
R2.1 Requirements for a Raman spectrometer
As mentioned above, the Raman signals are constant in the axis of energy (cm-1 or eV) in their relative position versus the excitation and bandwidth. Therefore, almost always the scale of wavenumbers is used. Converting the data into the nm domain may lead to strong variations.

Graph R2-2 the relation between cm-1 and nm

Graph R2-2 demonstrates the conversion of wavenumber versus wavelength in the UV-Vis-NIR range.

R2.1.1 Beam Travel and spectral Interferences
R2.2 Exemplary Raman, and Brilloun Spectra
as many samples are solved in aqueous solutions, the Raman spectrum of water is of interest. The same applies to the spectra of materials used in typical equipment, which may be the source if interferences:

Graph R2-3 Raman curves of water, glass, quartz, sapphire

Graph R2-3: Raman spectra, which may appear interfering.

Graph R2-4 Raman curves of CaF2, Diamond, ClD, Graphite

Graph R2-4: Raman Spectra of some solid state materials.

R2.2.1 Design of Raman Spectrometers
R2.2.2 The Wavelength of Excitation

Often used Laser excitation sources are

Wavelength Wavenumber Type of Laser typ. Power Output
nm cm-1   W  
       
244 40984 Argon 0.01 cw
248 40323 Excimer 2 5-ns Pulses/500 Hz
266 37594 Nd:YAG 0.1 5-ns Pulses/ cw
275 36364 Argon 0.1 cw
308 32468 Excimer 2 5-ns Pulses/500 Hz
325 30769 Helium-Cadmium 0.02 cw
351 28490 Krypton 0.25 cw
355 28169 Nd:YAG 20 ps Pulses / cw
442 22624 Helium-Cadmium 0.2 cw
488 20492 Argon 2 cw
514.5 19436 Argon 2 cw
534 18727 Nd:YAG 0.35 5-ns Pulses / cw
633 15798 Helium-Neon 1 cw
780 12821 Diode Laser 200 div pulsed / cw
1064 9398 Nd:YAG 0.7 5-ns Pulses / cw

 

R2.2.3 How close, in Relation to the Rayleigh line, must the system be able to detect spectral features?
R2.2.3.1 Single stage spectrometer, with Notch Filter
 

graph R2-5 single stage spectrometer with notch filter

Graph R2-5: Raman Spectrometer with Notch Filter.

R2.2.3.2 Double Spectrometer
Grafik R2-6 spectral inpact of Notch Filters
Graph R2-6 compares the Raman Spectrum of L-Cystine, with and without the Range of 0 - 100 cm-1
Graph R2-7 Double spectrometer

Graph R2-7 demonstrates one of the many possible double spectrometer set-ups, that one is optimized for efficiency, because it has a minimum of reflections.

R2.2.3.3 A short Consideration on Straylight
Graph R2-8 General behaviour of Straylight in a double spectrometer

Graph R2-8 demonstrates the general impact of Rayleigh scattering to the signal of interest, in an additive double spectrometer.
R2.2.3.4 Spectrometer for Measurements extremly close to the Rayleigh Line, Brillouin Spectrometer
To analyse samples, which show the scatter from phonons, the Brillouin signal, data between roughly +/- 150 cm-1. Incorporated is the extra challenge to acquire useful data within +/- 1 cm-1.

Graph R2-9 three Brillouin Spectra

Graph R2-9 shows three Brilloiun spectra, requiring different instrument performance
Grafik R2-10 Doppelpass-Doppelspektrometer

Graph R2-10 represents a high performance double spectrometer for extreme low wavenumber differences.
2.2.3.4.1 The free spectral Range of Echelle Spectrometers
2.2.3.4.2 Alternative Solution for the ultra near Wavenumber range and History
R2.2.3.5 Triple spectrometers, the work Horses of Raman, and Brillouin Spectroscopy
Graph R2-11 Triple Raman Spectrometer

Graph R2-11 The modern, flexible triple spectrometer, equally well suited for the use as spectrograph, and as monochromator. In addition, the first two stages can be configured to disperse additive or subtractive. The focal length of the stages typically is 0,5....0,75 m, whereas the pre-stages may have a shorter length than the third one.
Graph R2-12 Nearfield-Raman with and without Rayleigh

Graph R2-12 The performance of a modern Triple spectrometer, with and without Rayleigh scattering.
 

R2.2.6 Estimation on the Impact of Rayleigh Scattering in different Systems

Graph R2-13 Impact of Straylight to several set-ups

Graph R2-13 The impact of Rayleigh scatter in some typical Raman spectrometer configurations. The curves demonstrate the part of output signal, originated by Rayleigh scattering.
R2.3 Special Methods
R2.3.1 Raman and Fluorescence
R2.3.2 NIR-Raman
R2.3.3 UV-Raman
R2.3.4 Microscopy Raman
R2.3.3.5 Resonance Raman, RR
R2.3.3.6 Surface enhanced Raman Scattering, SERS
R2.3.3.7 Coherent Antistokes Raman Spectroscopy, CARS
External Source Indication:
The spectra in graphs R2-6, and R2-12, showing L-Cystine, are courtesy of Spectroscopy & Imaging (S&I) GmbH, D-59581 Warstein, Germany, recorded by instruments of the "Tri-Vista" series.
The spectra in graphs R2-9, R2-12, showing H2O, Si, and LiF, are courtesy of SOPRA SA, F-92270 Bois-Colombes, France, recorded by instruments of the "DMDP 2000" series.
The other spectra are either from own measurements, or from different open university sources, available in the internet.

All copyrights on "spectra-magic.de" and "Optical Spectroscopy with dispersive Spectrometers Basics - Building Blocks - Systems - Applications " are reserved by Wilfried Neumann, D-88171 Weiler-Simmerberg.
Status April 2012