information about nanoscale systems. X-ray spectroscopy is a collection
of techniques that use x-ray excitation in order to probe the electronic
structure of molecules in a material. These techniques are particularly
useful methods for determining a material’s composition, probing the
ordering of individual atoms in a crystal, or observing nanoscale molecular processes. Each technique is based on the measurement of
absorption, emission, or scattering of electromagnetic radiation caused
by x-rays.
6.3.1 Absorption
When x-rays pass through a thin layer of matter, the intensity of the x-rays
is diminished as a result of absorption and scattering. The effect of
scattering can be ignored in wavelength regions at which significant
absorption occurs. Each element has its own absorption spectrum with
well-defined x-ray absorption peaks that can be used to identify it. In the
x-ray region of light, enough energy is available to ionize the molecules
that the photons interact with. When x-ray energy corresponding to the
binding energy of a core (nonvalence) electron is absorbed, the core
electron is ejected from the atom. This results in an excited ion. There is a
higher probability of this happening when an x-ray beam with energy
equal to the binding energy of the core electron is used. As the x-ray beam
increases in energy away from the core electron binding energy, the
probability of the corresponding wavelength being absorbed diminishes,
and so the amount of the x-ray beam absorbed will decrease. If the x-ray
beam is too low in energy, the corresponding wavelength will not be
appreciably present and will not be able to eject the core electron. This
will cause an abrupt decrease in the amount of the x-ray beam absorbed.
The wavelength of x-rays ranges from about 10 to 10
–6 nm, but conventional x-ray spectroscopy generally uses only wavelengths between 2.5
and 0.001 nm because this range contains the x-rays with energies corresponding to core electron-binding energies, which differ between
elements.
6.3.2 Fluorescence
The excited ion, which is a result of the x-rays’ ejecting of a core electron,
will fluoresce through transitions of electrons in higher energy levels to
the vacancy left by the ejected core electron. These transitions allow the
excited ion to return to its more stable ground state. This fluorescence is
measurable and is often used in concert with nanoscience techniques
CHAPTER 6: Bulk Characterization Techniques for Nanomaterials
212
of techniques that use x-ray excitation in order to probe the electronic
structure of molecules in a material. These techniques are particularly
useful methods for determining a material’s composition, probing the
ordering of individual atoms in a crystal, or observing nanoscale molecular processes. Each technique is based on the measurement of
absorption, emission, or scattering of electromagnetic radiation caused
by x-rays.
6.3.1 Absorption
When x-rays pass through a thin layer of matter, the intensity of the x-rays
is diminished as a result of absorption and scattering. The effect of
scattering can be ignored in wavelength regions at which significant
absorption occurs. Each element has its own absorption spectrum with
well-defined x-ray absorption peaks that can be used to identify it. In the
x-ray region of light, enough energy is available to ionize the molecules
that the photons interact with. When x-ray energy corresponding to the
binding energy of a core (nonvalence) electron is absorbed, the core
electron is ejected from the atom. This results in an excited ion. There is a
higher probability of this happening when an x-ray beam with energy
equal to the binding energy of the core electron is used. As the x-ray beam
increases in energy away from the core electron binding energy, the
probability of the corresponding wavelength being absorbed diminishes,
and so the amount of the x-ray beam absorbed will decrease. If the x-ray
beam is too low in energy, the corresponding wavelength will not be
appreciably present and will not be able to eject the core electron. This
will cause an abrupt decrease in the amount of the x-ray beam absorbed.
The wavelength of x-rays ranges from about 10 to 10
–6 nm, but conventional x-ray spectroscopy generally uses only wavelengths between 2.5
and 0.001 nm because this range contains the x-rays with energies corresponding to core electron-binding energies, which differ between
elements.
6.3.2 Fluorescence
The excited ion, which is a result of the x-rays’ ejecting of a core electron,
will fluoresce through transitions of electrons in higher energy levels to
the vacancy left by the ejected core electron. These transitions allow the
excited ion to return to its more stable ground state. This fluorescence is
measurable and is often used in concert with nanoscience techniques
CHAPTER 6: Bulk Characterization Techniques for Nanomaterials
212