incoming radiation, it ordinarily returns to the ground state after some
short amount of time due to one of several relaxation processes. For some
molecules, fluorescence is a common relaxation process in which the
molecule relaxes by reemitting light, generally of a lower energy (longer
wavelength) than the absorbed light. Fluorescence is typically observed in
the ultraviolet–visible region of the electromagnetic spectrum and is
depicted schematically in Figure 6.2. Nonradiative relaxation is the most
common relaxation process, in which the excited molecule relaxes
without reemitting any electromagnetic radiation. This process typically
occurs in small steps by the conversion of the excited energy into kinetic
energy through collisions with nearby molecules, producing heat.
Nonradiative relaxation is also depicted in Figure 6.2.
Phosphorescence is another relaxation process that typically occurs in
the UV–vis region. It is observed when an excited electron undergoes nonradiative intersystem crossing to a lower-energy excited state in which the
electron in the excited state and the remaining electron in HOMO have
the same spin (a triplet state). Relaxation is much slower since the electron needs to flip its spin again to return to the ground state while obeying
the Pauli exclusion principle. When the excited electron then relaxes back
to a state of lower energy, a photon is produced at a longer wavelength
(lower frequency) than the absorbed light. Phosphorescence is a much
rarer phenomenon than fluorescence and is consequently less important
to our discussion of spectroscopic techniques.
In summary, molecules may exist only at discrete, quantized energy
states. By examining the ways in which a given molecule interacts with
light, one can gather information about its energy states, thus providing
valuable insight into the identity of the molecule, the strength and type of
its chemical bonds, and the concentration of the molecule in the substance being studied. For our elementary purposes, those observations
serve as the basis of spectroscopy. Let’s consider several important types
of spectroscopy and their applications to the study of nanomaterials.
6.1.2 UV–visible spectroscopy
6.1.2.1 Principles of UV–visible spectroscopy
UV–vis spectroscopy uses the transmission of visible and/or ultraviolet
light through a sample to determine the presence and/or the amount of
material that absorbs light within the sample. As mentioned in our discussion of the interactions between light and matter, absorption of a
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short amount of time due to one of several relaxation processes. For some
molecules, fluorescence is a common relaxation process in which the
molecule relaxes by reemitting light, generally of a lower energy (longer
wavelength) than the absorbed light. Fluorescence is typically observed in
the ultraviolet–visible region of the electromagnetic spectrum and is
depicted schematically in Figure 6.2. Nonradiative relaxation is the most
common relaxation process, in which the excited molecule relaxes
without reemitting any electromagnetic radiation. This process typically
occurs in small steps by the conversion of the excited energy into kinetic
energy through collisions with nearby molecules, producing heat.
Nonradiative relaxation is also depicted in Figure 6.2.
Phosphorescence is another relaxation process that typically occurs in
the UV–vis region. It is observed when an excited electron undergoes nonradiative intersystem crossing to a lower-energy excited state in which the
electron in the excited state and the remaining electron in HOMO have
the same spin (a triplet state). Relaxation is much slower since the electron needs to flip its spin again to return to the ground state while obeying
the Pauli exclusion principle. When the excited electron then relaxes back
to a state of lower energy, a photon is produced at a longer wavelength
(lower frequency) than the absorbed light. Phosphorescence is a much
rarer phenomenon than fluorescence and is consequently less important
to our discussion of spectroscopic techniques.
In summary, molecules may exist only at discrete, quantized energy
states. By examining the ways in which a given molecule interacts with
light, one can gather information about its energy states, thus providing
valuable insight into the identity of the molecule, the strength and type of
its chemical bonds, and the concentration of the molecule in the substance being studied. For our elementary purposes, those observations
serve as the basis of spectroscopy. Let’s consider several important types
of spectroscopy and their applications to the study of nanomaterials.
6.1.2 UV–visible spectroscopy
6.1.2.1 Principles of UV–visible spectroscopy
UV–vis spectroscopy uses the transmission of visible and/or ultraviolet
light through a sample to determine the presence and/or the amount of
material that absorbs light within the sample. As mentioned in our discussion of the interactions between light and matter, absorption of a
CHAPTER 6: Bulk Characterization Techniques for Nanomaterials
188