7.22 Light Absorption by Biological Matter
245
Figure 7.10 shows some vibrational modes found in organic molecules,
along with their photon absorption wave-numbers. (These wave-numbers
vary when nearby molecules affect the bond being distorted.)
(c) Molecular orbital excitations: A valence electron in a molecular orbital (i.e.
a quantum state of an electron not confined to a single atom) can absorb
a photon if there is sufficient energy carried by that photon to excite the
electron to a higher energy molecular orbital. Transition to this new electron
state may cause the molecule to change its configurational state.
12. Atomic excitations: Electrons in inner-shell atomic orbitals can be excited
to unoccupied higher energy states by light. The energies necessary can be
estimated by the Coulomb binding of electrons to nuclei, which is of the order
of Z eff e 2 /r, where r is the distance to the positively charged nucleus and Z eff
is the effective charge number for the core of the atom (Z, the nuclear charge
number, minus the average number of electrons inside the orbit of the excited
one). Using e 2 / ¯
hc = 1/137.0, we find the wavelengths of the light needed is of
the order λ = 10 −5 cm. We expect, then, that ultraviolet light frequencies will
be necessary to excite these levels.
13. Fluorescence and Phosphorescence: A ‘phosphor’ is a substance which emits
light by atomic or molecular transitions some time after being excited. Phosphors can emit light by being exposed to a source of particle or light radiation.
If the time delay in emission is perceptibly long (greater than about a microsecond), then the material is ‘phosphorescent’. By selection of the phosphor, the
length of light persistence on the old cathode-ray tubes used to generate images
could be adjusted to reduce flickering, since the electron beam took about 2 ms
to return to the same spot on the screen. X-ray phosphorescent screens are
used to monitor X-ray sources and view images produces by X-rays. Some
phosphors can “glow in the dark” seconds and even minutes after being exposed
to a radiation source.
If there is no perceptible time delay between energy absorption and light
emission, but not all the energy is re-emitted, then the material is ‘fluorescent’
The coating inside a fluorescent tube converts ultraviolet light generated by
a current passing through a vaporized gas of mercury (together with an inert
noble gases) into visible light. Commercial tubes use non-toxic halophosphatebased phosphors to coat the inside of the tube, but the tubes still contain small
amounts of toxic mercury.
Phosphors stimulated by light must have at least two special unoccupied
excited states in the atoms or molecules. One for electrons to go when first
excited by the incoming radiation. A second intermediate energy state is needed
into which an excited electron can ‘fall’. Transitions from the first to the second
state must have a high probability relative to transition from the first back to
the initial ground state. In turn, if the intermediate state has a high probability
to ‘decay’ into the ground state, the material will be fluorescent. Typical such
decay times are less than a microsecond range. If the intermediate state has a
low probability of decay, then the electron will take some time before falling
245
Figure 7.10 shows some vibrational modes found in organic molecules,
along with their photon absorption wave-numbers. (These wave-numbers
vary when nearby molecules affect the bond being distorted.)
(c) Molecular orbital excitations: A valence electron in a molecular orbital (i.e.
a quantum state of an electron not confined to a single atom) can absorb
a photon if there is sufficient energy carried by that photon to excite the
electron to a higher energy molecular orbital. Transition to this new electron
state may cause the molecule to change its configurational state.
12. Atomic excitations: Electrons in inner-shell atomic orbitals can be excited
to unoccupied higher energy states by light. The energies necessary can be
estimated by the Coulomb binding of electrons to nuclei, which is of the order
of Z eff e 2 /r, where r is the distance to the positively charged nucleus and Z eff
is the effective charge number for the core of the atom (Z, the nuclear charge
number, minus the average number of electrons inside the orbit of the excited
one). Using e 2 / ¯
hc = 1/137.0, we find the wavelengths of the light needed is of
the order λ = 10 −5 cm. We expect, then, that ultraviolet light frequencies will
be necessary to excite these levels.
13. Fluorescence and Phosphorescence: A ‘phosphor’ is a substance which emits
light by atomic or molecular transitions some time after being excited. Phosphors can emit light by being exposed to a source of particle or light radiation.
If the time delay in emission is perceptibly long (greater than about a microsecond), then the material is ‘phosphorescent’. By selection of the phosphor, the
length of light persistence on the old cathode-ray tubes used to generate images
could be adjusted to reduce flickering, since the electron beam took about 2 ms
to return to the same spot on the screen. X-ray phosphorescent screens are
used to monitor X-ray sources and view images produces by X-rays. Some
phosphors can “glow in the dark” seconds and even minutes after being exposed
to a radiation source.
If there is no perceptible time delay between energy absorption and light
emission, but not all the energy is re-emitted, then the material is ‘fluorescent’
The coating inside a fluorescent tube converts ultraviolet light generated by
a current passing through a vaporized gas of mercury (together with an inert
noble gases) into visible light. Commercial tubes use non-toxic halophosphatebased phosphors to coat the inside of the tube, but the tubes still contain small
amounts of toxic mercury.
Phosphors stimulated by light must have at least two special unoccupied
excited states in the atoms or molecules. One for electrons to go when first
excited by the incoming radiation. A second intermediate energy state is needed
into which an excited electron can ‘fall’. Transitions from the first to the second
state must have a high probability relative to transition from the first back to
the initial ground state. In turn, if the intermediate state has a high probability
to ‘decay’ into the ground state, the material will be fluorescent. Typical such
decay times are less than a microsecond range. If the intermediate state has a
low probability of decay, then the electron will take some time before falling