**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 **.

**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 Schrödinger´s
equations for inelastic víbration 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 shows the principle of a
system for Raman, and Brillouin spectroscopy**.

As mentioned above, the Raman signals are constant in the axis of energy (cm

*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 spectra, which may
appear interfering. *

*G**raph R2-4: Raman Spectra of some
solid state materials. *

**R2.2.1 Design of Raman Spectrometers
**

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: Raman Spectrometer with Notch
Filter. **

R2.2.3.2 Double Spectrometer

*G***raph R2-6 compares the Raman Spectrum of L-Cystine,
with and without the Range of 0 - 100 cm ^{-1}**

**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 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 shows three Brilloiun spectra, requiring
different instrument performance**

** 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

*G**raph 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 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 The impact of Rayleigh scatter in some
typical Raman spectrometer configurations*. The curves demonstrate the
part of output signal, originated by Rayleigh scattering.

R2.3.1 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

The spectra in graphs R2-9, R2-12, showing H

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**