case the thickness of the film. We can estimate l using techniques
such as ellipsometry, DPI, or SPR (discussed in Chapter 8), and if we
know ε (molar absorptivity), we can obtain the concentration, c.
6.1.4 Molecular fluorescence spectroscopy
6.1.4.1 Principles of fluorescence and fluorescence quantum yield
As described in Section 6.1.1 on the interactions between light and
matter, fluorescence is the process whereby a molecule that has been
excited by the absorption of radiation relaxes to a lower energy state by
emitting a photon of lower energy than the photon that was absorbed. Not
all molecules exhibit fluorescence; molecules that do fluoresce are called
fluorophores. Each fluorophore has a characteristic absorption profile,
which is identical to the UV–vis absorption spectrum for the molecule.
For most fluorophores, fluorescence is typically observed in the ultraviolet to visible range of the electromagnetic spectrum, meaning that it
corresponds to transitions in the electronic state of the molecule (much
like UV–vis absorption spectroscopy). As with UV–vis spectroscopy, the
wavelength of light (generally nm) is typically used to describe fluorescence spectra rather than wavenumber (cm
–1 ) or frequency (Hz) units.
Wavelengths that produce fluorescence when absorbed are referred to as
excitation wavelengths. However, not all excitation wavelengths cause the
fluorophore to fluoresce to the same degree or even to produce fluorescent light of the same wavelength. Therefore, for each excitation
wavelength, the fluorophore has a characteristic fluorescence emission
profile, which is the range and intensity of wavelengths that are produced
when that excitation wavelength is used to excite the fluorophore.
Alternatively, for each emission wavelength there also exists an excitation
profile, which is the range of wavelengths that produce fluorescence of
that emission wavelength and the intensities of emission associated with
each of those excitation wavelengths. Both emission and excitation profiles provide useful information about the fluorophore and its local
environment, so most fluorometers can be set to determine either the
emission spectrum at a set excitation wavelength or the excitation spectrum at a set emission wavelength. It should be noted that because
fluorescence-emitted photons are typically of lower energy (i.e., longer
wavelength) than their excitation photon counterparts, the emission
spectrum for a given fluorophore usually occurs at longer wavelengths
than its excitation spectrum. Typical emission and excitation spectra for
an optically active polymer are shown in Figure 6.8. The polymer is a
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such as ellipsometry, DPI, or SPR (discussed in Chapter 8), and if we
know ε (molar absorptivity), we can obtain the concentration, c.
6.1.4 Molecular fluorescence spectroscopy
6.1.4.1 Principles of fluorescence and fluorescence quantum yield
As described in Section 6.1.1 on the interactions between light and
matter, fluorescence is the process whereby a molecule that has been
excited by the absorption of radiation relaxes to a lower energy state by
emitting a photon of lower energy than the photon that was absorbed. Not
all molecules exhibit fluorescence; molecules that do fluoresce are called
fluorophores. Each fluorophore has a characteristic absorption profile,
which is identical to the UV–vis absorption spectrum for the molecule.
For most fluorophores, fluorescence is typically observed in the ultraviolet to visible range of the electromagnetic spectrum, meaning that it
corresponds to transitions in the electronic state of the molecule (much
like UV–vis absorption spectroscopy). As with UV–vis spectroscopy, the
wavelength of light (generally nm) is typically used to describe fluorescence spectra rather than wavenumber (cm
–1 ) or frequency (Hz) units.
Wavelengths that produce fluorescence when absorbed are referred to as
excitation wavelengths. However, not all excitation wavelengths cause the
fluorophore to fluoresce to the same degree or even to produce fluorescent light of the same wavelength. Therefore, for each excitation
wavelength, the fluorophore has a characteristic fluorescence emission
profile, which is the range and intensity of wavelengths that are produced
when that excitation wavelength is used to excite the fluorophore.
Alternatively, for each emission wavelength there also exists an excitation
profile, which is the range of wavelengths that produce fluorescence of
that emission wavelength and the intensities of emission associated with
each of those excitation wavelengths. Both emission and excitation profiles provide useful information about the fluorophore and its local
environment, so most fluorometers can be set to determine either the
emission spectrum at a set excitation wavelength or the excitation spectrum at a set emission wavelength. It should be noted that because
fluorescence-emitted photons are typically of lower energy (i.e., longer
wavelength) than their excitation photon counterparts, the emission
spectrum for a given fluorophore usually occurs at longer wavelengths
than its excitation spectrum. Typical emission and excitation spectra for
an optically active polymer are shown in Figure 6.8. The polymer is a
CHAPTER 6: Bulk Characterization Techniques for Nanomaterials
194