7.22 Light Absorption by Biological Matter
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8. Excitation of valence electrons: Electrons at the surface of atoms, the valence
electrons, take part in chemical bonds, and are typically excited by ultraviolet
radiation, but sometimes by visible light. If the latter, then the material will
show color.
9. Ionization: An incoming photon may have sufficient energy to release a bound
electron in a material. Atomic ionization processes typically requires UV or
X-ray rays. Apart from the valence electrons, the other electrons in atoms are
held tightly, and require tens to many thousands of electron-volts of energy
to release them. Photons with those energies have frequencies (f = E/ h) in
the ultraviolet to X-ray part of the spectrum. Ions created by radiation inside
cells easily make free radicals. These can alter the chemistry of the cell, almost
always in unhealthy ways.
Light can also release electrons from the surface of metals. This is the
photoelectric effect. Although the conduction-band electrons are not bound to
individual metal atoms, it does take energy to pull one of those electrons from
the metal. Part of this energy comes from the work done in pulling the electron
away from the opposite charge the electron creates on the surface of the metal
by its repulsion of nearby conduction electrons. Einstein, in 1905, used energy
conservation and the photonic properties of light to write hf = (1/2)mv 2 + W ,
where v is the speed of the ejected electron, and W is the work needed to get the
electron away from the metal. Einstein could explain all the properties observed
in the process, while classical theory had failed to do so.
10. Absorption through atomic or molecular electron-orbital distortions: Electrons
in bound orbitals can be perturbed by the action of an oscillating electric
field (of an electromagnetic wave). This may distort the orbitals rather than
cause quantum transitions to other bound states. This distortion occurs in
optical materials as light passes through. The oscillating electrons generate new
radiation of the same frequency as the wave passing by. As we have described,
this accounts for the index of refraction of transparent materials.
11. Absorption by molecular excitations: When the frequency of the incoming light
is close to the natural resonant frequency for charge centers in the molecule,
the probability for absorption is enhanced and the photon energy can be used
to excite a particular atomic or molecular state. Because this light energy is
stored in the molecular system for a time longer than typical scattering times
(e.g. 10 −6 s vs. 10 −9 s), the index of refraction is significantly larger at this
frequency and the “absorption coefficient” (a ratio of outgoing vs. incoming
intensity along the direct beam) also increases. There are several general types
of molecular excitations:
(a) Molecular rotational excitations: Small molecules with a large moment of
inertia and electric dipole moment can be induced to rotate by exposure
to light of an appropriate frequency. This frequency can be calculated as
follows: The energy stored in a rotating system is given by E = L 2 /(2I ) =
l(l + 1) ¯
h 2 /(2I ) where I is the moment of inertia of the molecule about
its axis of rotation, ¯
h is Planck’s constant over 2π , L is the angular
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8. Excitation of valence electrons: Electrons at the surface of atoms, the valence
electrons, take part in chemical bonds, and are typically excited by ultraviolet
radiation, but sometimes by visible light. If the latter, then the material will
show color.
9. Ionization: An incoming photon may have sufficient energy to release a bound
electron in a material. Atomic ionization processes typically requires UV or
X-ray rays. Apart from the valence electrons, the other electrons in atoms are
held tightly, and require tens to many thousands of electron-volts of energy
to release them. Photons with those energies have frequencies (f = E/ h) in
the ultraviolet to X-ray part of the spectrum. Ions created by radiation inside
cells easily make free radicals. These can alter the chemistry of the cell, almost
always in unhealthy ways.
Light can also release electrons from the surface of metals. This is the
photoelectric effect. Although the conduction-band electrons are not bound to
individual metal atoms, it does take energy to pull one of those electrons from
the metal. Part of this energy comes from the work done in pulling the electron
away from the opposite charge the electron creates on the surface of the metal
by its repulsion of nearby conduction electrons. Einstein, in 1905, used energy
conservation and the photonic properties of light to write hf = (1/2)mv 2 + W ,
where v is the speed of the ejected electron, and W is the work needed to get the
electron away from the metal. Einstein could explain all the properties observed
in the process, while classical theory had failed to do so.
10. Absorption through atomic or molecular electron-orbital distortions: Electrons
in bound orbitals can be perturbed by the action of an oscillating electric
field (of an electromagnetic wave). This may distort the orbitals rather than
cause quantum transitions to other bound states. This distortion occurs in
optical materials as light passes through. The oscillating electrons generate new
radiation of the same frequency as the wave passing by. As we have described,
this accounts for the index of refraction of transparent materials.
11. Absorption by molecular excitations: When the frequency of the incoming light
is close to the natural resonant frequency for charge centers in the molecule,
the probability for absorption is enhanced and the photon energy can be used
to excite a particular atomic or molecular state. Because this light energy is
stored in the molecular system for a time longer than typical scattering times
(e.g. 10 −6 s vs. 10 −9 s), the index of refraction is significantly larger at this
frequency and the “absorption coefficient” (a ratio of outgoing vs. incoming
intensity along the direct beam) also increases. There are several general types
of molecular excitations:
(a) Molecular rotational excitations: Small molecules with a large moment of
inertia and electric dipole moment can be induced to rotate by exposure
to light of an appropriate frequency. This frequency can be calculated as
follows: The energy stored in a rotating system is given by E = L 2 /(2I ) =
l(l + 1) ¯
h 2 /(2I ) where I is the moment of inertia of the molecule about
its axis of rotation, ¯
h is Planck’s constant over 2π , L is the angular