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Module 7: Scattering Techniques
Lecture 42: Raman spectroscopy, fluorescence
Fluorescence measurement
Raman Scattering
Videos
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Fluorescence measurement - Optical Measurement Techniques in Thermal Sciences - Lecture Slides, Slides of Mechanical Engineering
Some concept of Optical Measurement Techniques in Thermal Sciences are Absorption Techniques, Alternative Approaches, Calibration Details, Computerized Tomography, Convolution Backprojection. Main points of this lecture are: Fluorescence Measurement, Scattering Techniques, Raman Spectroscopy, Fluorescence, External Light, Light Source, Outermost Electrons, Undergo Transition, Atom, Energy Levels
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Module 7: Scattering Techniques
Lecture 42: Raman spectroscopy, fluorescence
Fluorescence measurement
Raman Scattering
Videos
Module 7: Scattering Techniques
Lecture 42: Raman spectroscopy, fluorescence
Fluorescence measurements
Under the influence of an external light source, outermost electrons of an atom undergo transition to the first (and possibly higher) energy levels of the material. On returning to the ground level, an electron is emitted with a frequency that corresponds to the difference in energy levels between the concerned quantum states. This frequency is, thus, purely a material property, independent of the wavelength of the incident radiation. Since the laser power required to raise electrons to the higher energy level needs to
be necessarily higher, the incident frequency v is greater than the emission at frequency v 1 and
scattering can be classified here as inelastic.
Figure 7.9: Energy transitions during fluorescence measurement.
Module 7: Scattering Techniques
Lecture 42: Raman spectroscopy, fluorescence
Raman Scattering
Raman scattering is seen in materials composed of molecules (as opposed to single atoms as in monatomic gases). Here, the molecules may have rotational and vibrational modes of motion, apart from the translational. These modes are quantized and, as a result, all energy levels are split further into rotational and vibrational states. Specifically, the ground state is split as well, shown schematically in Figure 7.10. Thus, electrons may undergo transition from one of the higher ground states to the first energy level. After a certain residence time, these may return back to the true ground state. The difference between the two frequencies is proof of the Raman effect. In experiments, the frequency shift is the signal of interest and is material specific. Thus, Raman scattering is most commonly used for identifying species in a certain medium – solid, liquid, or gas. The intensity of the scattered signal is a measure of the number density of the species, and hence its concentration. If concentration is known, the signal can be used to determine pressure and temperature.
Raman signals can be expected from spontaneous emissions within the medium. Such signals tend to be very weak and stimulated emission is more often used in engineering measurements. The stimulant is the laser and one can detect Raman shifts in frequency with reference to that in the incident laser beam. Since Raman signals generally weak, a high power laser is necessarily required, adding to the cost of the experimental apparatus.
Figure 7.10: Energy states in molecules showing quantization of the ground
state into vibrational and rotational levels.
When the emitted photon has a frequency less than the one absorbed, the frequency shift is called a red shift and is shown schematically in Figure 7.11 (left). When the frequency of the emitted radiation is greater than the one absorbed, one obtains the blue shift (Figure 7.11, right).
Figure 7.11: Schematic drawing of transitions in Raman spectroscopy. Left: red
shift, also called Stokes red shift; Right: blue shift, also called the anti-Stokes
blue shift.
Figure 7.12: A laser-Raman experimental set-up for species concentration
measurement, adapted from Mayinger (1994).
Module 7: Scattering Techniques
Lecture 42: Raman spectroscopy, fluorescence
Raman Scattering (contd...)
Typical Raman spectra, in the form of frequency shifts, are presented in Figure 7.13 for dry air (inhaled) as well as the air exhaled as human breath. Dry air is mainly nitrogen and oxygen, with traces of moisture (H 2 O), indistinguishable levels of CO 2 , and other gases that the measurement has not been
able to resolve. The intensity peaks depend on pressure, temperature and species concentration. Since the nitrogen content in air is an invariant and so is pressure, the nitrogen peak can be used to determine air temperature. With this information, oxygen levels in air being inhaled by an individual can be determined.
Note that Raman spectroscopy has determined concentration of all species in a single step; it points towards the power of the technique in engineering applications.
The second spectrum in Figure 7.13 is that of air exhaled. As expected it is rich in moisture and CO 2 ,
while the oxygen content has decreased. Nitrogen data can once again be used to fix temperature while other peaks will provide information on the respective concentrations.
Multiple peaks (such as CO 2 , Figure 7.12) are not uncommon. In fact, the identification of an intensity
peak to a species is enabled by the vast amount of spectroscopic data available in the open literature.
Figure 7.13 Raman spectra and species detection in air inhaled by an individual
and air exhaled. Figure adapted from Mayinger (1994).
Module 7: Scattering Techniques
Lecture 42: Raman spectroscopy, fluorescence
Closure
The present module is a short introduction to optical methods that rely on photon-particle interaction and scattering for signal generation. Signals can be generated
i. when the particle is large enough to cast a shadow, ii. particle influences the wave-like parameters of light, frequency and phase, for example, or iii. when the electrons are pumped to one of the (quantized) higher energy levels.
Correspondingly, signal strengths can be strong, mild, or very weak. The measurement costs are, in general, inversely proportional to the signal strength.
The connection between the signal intensity and the property to be measured has not been discussed in this chapter. The subject is quite vast and the reader is advised to consult the specialist literature on subject.
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Module 7: Scattering Techniques
Lecture 42:
These images show flow from a synthetic jet into a stationary fluid as a function of aspect ratio of a rectangular slot
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Module 7: Scattering Techniques
Lecture 42:
These images show flow from a synthetic jet into a stationary fluid as a function of aspect ratio of a rectangular slot
file:///G|/optical_measurement/lecture42/42_12.html[5/7/2012 12:44:49 PM]
Module 7: Scattering Techniques
Lecture 42:
These images show flow from a synthetic jet into a stationary fluid as a function of aspect ratio of a rectangular slot