understand these methods, it is appropriate to begin with a brief discussion of the interactions between light and matter.
6.1.1 Interactions between light and matter
As described earlier (see Section 4.2.1), in many instances light is best
thought of as an oscillating electromagnetic wave with a characteristic
energy E. The energy of a photon of this electromagnetic radiation is a
function of its frequency n (or its wavelength l), and can be calculated by
Einstein’s famous equation
E = hn =
hc
l
(6.1)
where h is Planck’s constant and c is the speed of light. This equation was
introduced in Chapter 4; because of its importance we repeat it here.
As previously discussed in Chapters 4 and 5, the energy states of molecules are quantized; molecules may only exist in a finite number of discrete states. These allowable energy states are the sum of several
quantized aspects of the molecule, such as the energies of its electrons
around their respective nuclei, the interatomic vibrations that exist in the
molecule, and the rotations of the molecule around its center of mass. We
can also say that a given molecule has quantized (or discrete) electronic,
vibrational, and rotational energy states and that it can only exist at those
energy states (or at a sum of those energy states). The lowest energy state
of a molecule is termed the ground state and higher energy states are
referred to as excited states. When a molecule gains energy by absorbing
radiation or by transfer of energy between molecules from collisions
(heat) or various electronic processes, it enters an excited state. Following
this excitation, the molecule may relax from the excited state to a lower
excited state or to the ground state. This relaxation is often accomplished
by the emission of electromagnetic radiation (and consequently the frequency and the wavelength) of the emitted light is the exact difference
between the upper and lower energy states. Using Einstein’s equation
(Equation 6.2), this process can be written as
ΔE = E 1 − E 0 = hv =
hc
l
(6.2)
where ΔE is the difference in energy between the higher and lower energy
states. Therefore, by measuring the wavelength of the emitted light, one
can calculate the energy difference between the higher and lower states of
the molecule.
SPECTROSCOPIC METHODS 183
6.1.1 Interactions between light and matter
As described earlier (see Section 4.2.1), in many instances light is best
thought of as an oscillating electromagnetic wave with a characteristic
energy E. The energy of a photon of this electromagnetic radiation is a
function of its frequency n (or its wavelength l), and can be calculated by
Einstein’s famous equation
E = hn =
hc
l
(6.1)
where h is Planck’s constant and c is the speed of light. This equation was
introduced in Chapter 4; because of its importance we repeat it here.
As previously discussed in Chapters 4 and 5, the energy states of molecules are quantized; molecules may only exist in a finite number of discrete states. These allowable energy states are the sum of several
quantized aspects of the molecule, such as the energies of its electrons
around their respective nuclei, the interatomic vibrations that exist in the
molecule, and the rotations of the molecule around its center of mass. We
can also say that a given molecule has quantized (or discrete) electronic,
vibrational, and rotational energy states and that it can only exist at those
energy states (or at a sum of those energy states). The lowest energy state
of a molecule is termed the ground state and higher energy states are
referred to as excited states. When a molecule gains energy by absorbing
radiation or by transfer of energy between molecules from collisions
(heat) or various electronic processes, it enters an excited state. Following
this excitation, the molecule may relax from the excited state to a lower
excited state or to the ground state. This relaxation is often accomplished
by the emission of electromagnetic radiation (and consequently the frequency and the wavelength) of the emitted light is the exact difference
between the upper and lower energy states. Using Einstein’s equation
(Equation 6.2), this process can be written as
ΔE = E 1 − E 0 = hv =
hc
l
(6.2)
where ΔE is the difference in energy between the higher and lower energy
states. Therefore, by measuring the wavelength of the emitted light, one
can calculate the energy difference between the higher and lower states of
the molecule.
SPECTROSCOPIC METHODS 183