12.3 Rayleigh Scattering
265
the scattered light being inversely proportional to the fourth power of the wavelength
of the laser, it is advantageous to work with a short wavelength, particularly with
ultraviolet lasers.
12.4 Raman Scattering
Raman scattering is an interaction in which light from a laser beam is elastically
dispersed by the gas at wavelengths shifted from that of the incident light. The cross
section of the Raman scattering is much smaller than that of the Rayleigh scattering,
making observation of the radiated signal more difficult. With this technique, it is
possible to determine the species concentration, the temperatures of vibration and
rotation of the particular species and the gas velocity.
The principle of the method is as follows (see Fig. 12.1b). When a photon hits a
molecule, it leaves a fraction of its energy to the molecule that is raised to a higher
energy level. When de-excitation occurs, a photon is emitted at lower energy than
that of the incident photon (longer wavelength). This process is called Stokes-Raman
scattering. The scattered light has a spectrum whose frequencies are characteristic of
the molecule. In particular, the distribution of the intensities between the frequencies
of the radiated signal depends on the initial distribution of the energy states in the gas
under test. Thus, the analysis of the emitted spectrum constitutes a means of measuring the vibration and rotation temperatures in this particular state. The concentration
of the species is determined from the amount of light contained in a certain narrow
spectral band.
If the photon emerges from the interaction at a shorter wavelength, hence with
higher energy, the process is called anti-Stokes-Raman scattering (see Fig. 12.1c).
This diffusion process takes place with molecules of a higher energy level whose
relative number is small, the anti-Stokes Raman signal being thus weaker than the
Stokes signal. The use of stimulated Raman scattering mitigates this disadvantage
(see Sect. 12.5 below). The technique of basic Raman scattering, or spontaneous
Raman scattering, can be implemented with a laser having an arbitrary wavelength.
However, it is advantageous to use short wavelengths, as for Rayleigh scattering.
Pulsed lasers are often used because they can provide high power in a small measurement volume. The Raman signals, being at a wavelength different from that of
the laser, are very little affected by a scattering background. In addition, the Raman
effect is an interaction due to the radiation field and, for this reason, is not affected by
quenching (a non-radiative energy exchange by collision between atoms or molecules
which tends to decrease the radiated energy, see Sect. 9.3). The main disadvantage
of this technique is the weakness of the signal. In addition, the laser beam being
dispersed in all directions in space, the spatial resolution is obtained by observing
the signal from a small region of the beam. Thus, the efficiency of spontaneous
Raman scattering at visible wavelengths is generally very low (about 1000 times less
than Rayleigh scattering). Stimulated Raman spectroscopy has been developed to
eliminate this major drawback.
265
the scattered light being inversely proportional to the fourth power of the wavelength
of the laser, it is advantageous to work with a short wavelength, particularly with
ultraviolet lasers.
12.4 Raman Scattering
Raman scattering is an interaction in which light from a laser beam is elastically
dispersed by the gas at wavelengths shifted from that of the incident light. The cross
section of the Raman scattering is much smaller than that of the Rayleigh scattering,
making observation of the radiated signal more difficult. With this technique, it is
possible to determine the species concentration, the temperatures of vibration and
rotation of the particular species and the gas velocity.
The principle of the method is as follows (see Fig. 12.1b). When a photon hits a
molecule, it leaves a fraction of its energy to the molecule that is raised to a higher
energy level. When de-excitation occurs, a photon is emitted at lower energy than
that of the incident photon (longer wavelength). This process is called Stokes-Raman
scattering. The scattered light has a spectrum whose frequencies are characteristic of
the molecule. In particular, the distribution of the intensities between the frequencies
of the radiated signal depends on the initial distribution of the energy states in the gas
under test. Thus, the analysis of the emitted spectrum constitutes a means of measuring the vibration and rotation temperatures in this particular state. The concentration
of the species is determined from the amount of light contained in a certain narrow
spectral band.
If the photon emerges from the interaction at a shorter wavelength, hence with
higher energy, the process is called anti-Stokes-Raman scattering (see Fig. 12.1c).
This diffusion process takes place with molecules of a higher energy level whose
relative number is small, the anti-Stokes Raman signal being thus weaker than the
Stokes signal. The use of stimulated Raman scattering mitigates this disadvantage
(see Sect. 12.5 below). The technique of basic Raman scattering, or spontaneous
Raman scattering, can be implemented with a laser having an arbitrary wavelength.
However, it is advantageous to use short wavelengths, as for Rayleigh scattering.
Pulsed lasers are often used because they can provide high power in a small measurement volume. The Raman signals, being at a wavelength different from that of
the laser, are very little affected by a scattering background. In addition, the Raman
effect is an interaction due to the radiation field and, for this reason, is not affected by
quenching (a non-radiative energy exchange by collision between atoms or molecules
which tends to decrease the radiated energy, see Sect. 9.3). The main disadvantage
of this technique is the weakness of the signal. In addition, the laser beam being
dispersed in all directions in space, the spatial resolution is obtained by observing
the signal from a small region of the beam. Thus, the efficiency of spontaneous
Raman scattering at visible wavelengths is generally very low (about 1000 times less
than Rayleigh scattering). Stimulated Raman spectroscopy has been developed to
eliminate this major drawback.