Chapter 12
Laser Spectroscopy and Electron Beam
Excitation
12.1 Basic Principles
The advent of laser sources in the 1960s gave considerable impetus to the development of non-intrusive methods for the in situ determination of gas properties,
including the velocity. These methods are based on physical processes related to the
interaction between light and matter. The analysis of the resulting phenomena makes
it possible to deduce the characteristics of the atoms and/or molecules composing the
gas being tested and to measure properties such as its nature, concentration, energy
levels, etc. Although these methods are not commonly used in aerodynamic applications because of their sophistication and limitations, they are powerful tools for
studying complex flows. In particular, they give access to species concentration, local
pressure, temperature and are of great interest for the study of very high temperature
flows or flows containing chemically active combustion products.
Laser spectroscopic measurements are based on the interaction of a laser beam
with some of the physical quantities in fluid. Depending on the interaction process,
the laser light is either absorbed or dispersed by species active at the particular
wavelength employed. The interactions between light and matter can be classified
into three categories (see Fig. 12.1):
– In elastic scattering, light is diffused instantaneously without exchanging energy
with the internal states of the molecules of the medium, the incident and scattered
photons having the same energy. This is the case of Rayleigh scattering and Mie
scattering.
– In inelastic scattering, the light exchanges energy with some of the molecules of
the medium accordingly with the wavelength of the incident light. The molecule
absorbs some of the energy and hence the scattered photon has less energy than
the incident photon: this is the case of absorption, fluorescence or Raman Effect.
– In Raman scattering with anti-Stokes shift, the molecule loses energy and the
scattered photon has more energy than the incident photon.
© Springer Nature Switzerland AG 2020
B. Chanetz et al., Experimental Aerodynamics,
Springer Tracts in Mechanical Engineering,
https://doi.org/10.1007/978-3-030-35562-3_12
261
Laser Spectroscopy and Electron Beam
Excitation
12.1 Basic Principles
The advent of laser sources in the 1960s gave considerable impetus to the development of non-intrusive methods for the in situ determination of gas properties,
including the velocity. These methods are based on physical processes related to the
interaction between light and matter. The analysis of the resulting phenomena makes
it possible to deduce the characteristics of the atoms and/or molecules composing the
gas being tested and to measure properties such as its nature, concentration, energy
levels, etc. Although these methods are not commonly used in aerodynamic applications because of their sophistication and limitations, they are powerful tools for
studying complex flows. In particular, they give access to species concentration, local
pressure, temperature and are of great interest for the study of very high temperature
flows or flows containing chemically active combustion products.
Laser spectroscopic measurements are based on the interaction of a laser beam
with some of the physical quantities in fluid. Depending on the interaction process,
the laser light is either absorbed or dispersed by species active at the particular
wavelength employed. The interactions between light and matter can be classified
into three categories (see Fig. 12.1):
– In elastic scattering, light is diffused instantaneously without exchanging energy
with the internal states of the molecules of the medium, the incident and scattered
photons having the same energy. This is the case of Rayleigh scattering and Mie
scattering.
– In inelastic scattering, the light exchanges energy with some of the molecules of
the medium accordingly with the wavelength of the incident light. The molecule
absorbs some of the energy and hence the scattered photon has less energy than
the incident photon: this is the case of absorption, fluorescence or Raman Effect.
– In Raman scattering with anti-Stokes shift, the molecule loses energy and the
scattered photon has more energy than the incident photon.
© Springer Nature Switzerland AG 2020
B. Chanetz et al., Experimental Aerodynamics,
Springer Tracts in Mechanical Engineering,
https://doi.org/10.1007/978-3-030-35562-3_12
261