matches the energy difference between the vibrational states of the
molecule as in infrared absorption. However, Raman scattering is a distinct process from infrared absorption—the vibrational modes are the
same, but the rules for when a scattering event can occur are different.
Instead of requiring a change in dipole moment, Raman scattering
requires a change in the polarizability (see Chapter 5) of the molecule.
This allows vibrational frequencies that cannot be excited using infrared
spectroscopy, such as the stretching of homonuclear diatomics such N 2 or
the symmetric stretching mode of CO 2 , to be excited.
In our diagram (see Figure 6.12), Raman scattering is represented as
follows. Suppose the molecule happened to be in the first vibrational state
above the ground state at the time of interaction with the photon. The
molecule would be excited to the virtual excited state and then immediately relax, producing a scattered photon as described previously.
Suppose, however, that the molecule would relax to either the second
vibrational state or to the ground state rather than the first vibrational
state. In this case, the reemitted photon would have either gained or lost
energy according to the exact difference in energies between the vibrational states, represented by ΔE in the figure. And, because the energy of
light is a function of its frequency, the higher or lower energy light has a
frequency that is slightly shifted from its original value and Raman scattering has occurred. Not only has the Raman scattered light changed
direction, but it has also shifted its frequency.
6.2.3 Raman spectroscopy
Because the frequency shifts of Raman scattered light are a direct result
of changes in vibrational states of the molecule, it seems reasonable to
assume that one could gather similar information to IR spectroscopy by
simply measuring the shifts in frequency of Raman scattered light.
Indeed, this is the basic idea behind Raman spectroscopy, and frequency
shifts in a Raman spectrum are directly analogous to IR absorption
frequencies. In practice, shifts in frequency of Raman scattered light are
monitored and those shifts are matched to specific bond vibrations
within the molecule being studied. At a basic level, this allows one to
identify the functional groups of the molecule being studied as well
as their local chemical environment. Combining data from a Raman
experiment and an infrared experiment on a particular molecular system
can provide even more detailed information about the structure of the
molecule.
LIGHT SCATTERING METHODS 203
molecule as in infrared absorption. However, Raman scattering is a distinct process from infrared absorption—the vibrational modes are the
same, but the rules for when a scattering event can occur are different.
Instead of requiring a change in dipole moment, Raman scattering
requires a change in the polarizability (see Chapter 5) of the molecule.
This allows vibrational frequencies that cannot be excited using infrared
spectroscopy, such as the stretching of homonuclear diatomics such N 2 or
the symmetric stretching mode of CO 2 , to be excited.
In our diagram (see Figure 6.12), Raman scattering is represented as
follows. Suppose the molecule happened to be in the first vibrational state
above the ground state at the time of interaction with the photon. The
molecule would be excited to the virtual excited state and then immediately relax, producing a scattered photon as described previously.
Suppose, however, that the molecule would relax to either the second
vibrational state or to the ground state rather than the first vibrational
state. In this case, the reemitted photon would have either gained or lost
energy according to the exact difference in energies between the vibrational states, represented by ΔE in the figure. And, because the energy of
light is a function of its frequency, the higher or lower energy light has a
frequency that is slightly shifted from its original value and Raman scattering has occurred. Not only has the Raman scattered light changed
direction, but it has also shifted its frequency.
6.2.3 Raman spectroscopy
Because the frequency shifts of Raman scattered light are a direct result
of changes in vibrational states of the molecule, it seems reasonable to
assume that one could gather similar information to IR spectroscopy by
simply measuring the shifts in frequency of Raman scattered light.
Indeed, this is the basic idea behind Raman spectroscopy, and frequency
shifts in a Raman spectrum are directly analogous to IR absorption
frequencies. In practice, shifts in frequency of Raman scattered light are
monitored and those shifts are matched to specific bond vibrations
within the molecule being studied. At a basic level, this allows one to
identify the functional groups of the molecule being studied as well
as their local chemical environment. Combining data from a Raman
experiment and an infrared experiment on a particular molecular system
can provide even more detailed information about the structure of the
molecule.
LIGHT SCATTERING METHODS 203