that the molecule almost immediately returns to its ground state,
reemitting the photon. The photon is reemitted in a random direction.
It is important to note that this model of the scattering process should
not be confused with absorbance and reemission of the photon. The
incoming photon is not absorbed by the molecule according to our
model. The promotion to the virtual excited state is a result of a
momentary interaction with the photon and the entire process is not
quantized (meaning it can happen for any energy of light). Absorbance
only occurs when the frequency of the incoming light exactly matches the
difference between allowed energy states of the molecule.
6.2.2 Rayleigh and Raman scattering
Most commonly, light is scattered elastically (or without losing or gaining any energy), in a process described as Rayleigh scattering. From
Einstein’s equation (Equation 6.1), we know that the energy of light is
directly related to its frequency (or wavelength). Therefore, since Rayleigh
scattering is elastic, the scattered light has the same frequency (or
wavelength) as before it was scattered. However, due to the wave nature
of light, Rayleigh scattering is a wavelength-dependent process, meaning
that some wavelengths are scattered to a greater extent than others.
Indeed, it is this aspect of Rayleigh scattering that accounts for the blue
color of the sky. As light from the sun interacts with particles in the
atmosphere, it undergoes Rayleigh scattering. Blue light is scattered
more than the other wavelengths of light, because Rayleigh scattering is
strongest for the shorter wavelengths of light that are closer in size to the
air molecules responsible for scattering, and so it appears as though the
sky is a blue color. Rayleigh scattering is depicted schematically using our
model of light scattering in Figure 6.12.
Raman scattering occurs when light is scattered inelastically by a molecule. In other words, Raman scattering happens when the scattered light
is of a higher or lower energy after it has been scattered than before. This
increase or decrease in energy is generally due to a change in the
vibrational energy of the molecule. According to our model of light
scattering, this type of Raman scattering might occur when the molecule
relaxes from the virtual excited state to a vibrational state that is higher or
lower in energy than the state at which the molecule was previously. In
this case, the reemitted (or scattered) photon is a slightly different frequency (or wavelength) than before it interacted with the molecule. The
shift in the frequency of the scattered light from its original value directly
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reemitting the photon. The photon is reemitted in a random direction.
It is important to note that this model of the scattering process should
not be confused with absorbance and reemission of the photon. The
incoming photon is not absorbed by the molecule according to our
model. The promotion to the virtual excited state is a result of a
momentary interaction with the photon and the entire process is not
quantized (meaning it can happen for any energy of light). Absorbance
only occurs when the frequency of the incoming light exactly matches the
difference between allowed energy states of the molecule.
6.2.2 Rayleigh and Raman scattering
Most commonly, light is scattered elastically (or without losing or gaining any energy), in a process described as Rayleigh scattering. From
Einstein’s equation (Equation 6.1), we know that the energy of light is
directly related to its frequency (or wavelength). Therefore, since Rayleigh
scattering is elastic, the scattered light has the same frequency (or
wavelength) as before it was scattered. However, due to the wave nature
of light, Rayleigh scattering is a wavelength-dependent process, meaning
that some wavelengths are scattered to a greater extent than others.
Indeed, it is this aspect of Rayleigh scattering that accounts for the blue
color of the sky. As light from the sun interacts with particles in the
atmosphere, it undergoes Rayleigh scattering. Blue light is scattered
more than the other wavelengths of light, because Rayleigh scattering is
strongest for the shorter wavelengths of light that are closer in size to the
air molecules responsible for scattering, and so it appears as though the
sky is a blue color. Rayleigh scattering is depicted schematically using our
model of light scattering in Figure 6.12.
Raman scattering occurs when light is scattered inelastically by a molecule. In other words, Raman scattering happens when the scattered light
is of a higher or lower energy after it has been scattered than before. This
increase or decrease in energy is generally due to a change in the
vibrational energy of the molecule. According to our model of light
scattering, this type of Raman scattering might occur when the molecule
relaxes from the virtual excited state to a vibrational state that is higher or
lower in energy than the state at which the molecule was previously. In
this case, the reemitted (or scattered) photon is a slightly different frequency (or wavelength) than before it interacted with the molecule. The
shift in the frequency of the scattered light from its original value directly
CHAPTER 6: Bulk Characterization Techniques for Nanomaterials
202