371
and polymerization of cell constituents. Without pigment residues, cellulose and
other cell wall materials would be a nearly colorless white. Regardless of plant species, there is a general trajectory of color change over time. Brown pigment residues
produce the shades of light tan colors found in recently dead leaves. The residues
further degrade and become darker over winter. Eventually, they become dark
brown and chemically and spectrally indistinguishable from humus (Ziechmann
1964). The visible near-infrared shape of the absorption spectrum for recently dead
plant residues is generally monotonic, lowest at 400 nm and increasing in a polynomial curve to about 900 nm.
14.9 Leaf Water Content
The optical properties of water are better known than those of any other plant biochemical. In recent years, there have been several attempts to improve the absorption coefficients for liquid water; but since there have been no major changes, we
assume this property is mostly resolved for liquid water.
The optical properties of water are known with much greater precision than are
those of pigments or other molecules. Water absorptions are due to vibration of the
molecules, with a vibrational frequency that approximates simple harmonic motion
when excited by absorbing a quantum of energy. Because hydrogen atoms are small,
vibrations have large amplitude. Water vapor has three fundamental vibrational
modes with the dipole moments changing in the direction of the vibration
(Fig. 14.11). The first mode for liquid water at 25 °C is symmetric stretching; in this
case, both hydrogen ions vibrate simultaneously, which is at mode ν 1 with a 3050 nm
absorption feature. The second vibrational mode is bending the covalent bonds,
which occurs when the two hydrogen atoms vibrate by moving toward and away
from each other. This bending mode is at mode ν 2 and it causes a strong absorption
at 6080 nm. The third mode is asymmetric stretching that results from one hydrogen
ion being attracted toward the oxygen while the other moves away. The absorption
wavelength for this ν 3 mode is 2870 nm. Vibrational modes for liquid water at
shorter wavelengths (401–1900 nm) are combination modes. Vibrational modes are
restricted in liquid water and ice by hydrogen bonds. Libration describes the backand- forth rotation of the hydrogen ions in liquid water when its motion is restricted.
The infrared spectrum of water results from vibrational overtones and combinations with librations. Palmer and Williams (1974) conducted a detailed study,
including verification of prior authors (e.g., Hale and Querry 1973) and reported 94
optical constants between the UV and the NIR. They report liquid water values
related to those in Table 14.1 for measurements taken at 27 °C to be 769, 847, 973,
980, 1205, 1443, 1786, and 1927 nm. Kou et al. (1993) established absorption coefficients for ice between 1440 and 2500 nm, showing that these peaks are at longer
wavelengths than those of liquid water when measured at warmer temperatures.
Wieliczka et al. (1989) developed an improved measuring device, measured the real
part of the refractive index, and used it with Kramers–Kronig methods to compute
14 How the Optical Properties of Leaves Modify the Absorption and Scattering…
and polymerization of cell constituents. Without pigment residues, cellulose and
other cell wall materials would be a nearly colorless white. Regardless of plant species, there is a general trajectory of color change over time. Brown pigment residues
produce the shades of light tan colors found in recently dead leaves. The residues
further degrade and become darker over winter. Eventually, they become dark
brown and chemically and spectrally indistinguishable from humus (Ziechmann
1964). The visible near-infrared shape of the absorption spectrum for recently dead
plant residues is generally monotonic, lowest at 400 nm and increasing in a polynomial curve to about 900 nm.
14.9 Leaf Water Content
The optical properties of water are better known than those of any other plant biochemical. In recent years, there have been several attempts to improve the absorption coefficients for liquid water; but since there have been no major changes, we
assume this property is mostly resolved for liquid water.
The optical properties of water are known with much greater precision than are
those of pigments or other molecules. Water absorptions are due to vibration of the
molecules, with a vibrational frequency that approximates simple harmonic motion
when excited by absorbing a quantum of energy. Because hydrogen atoms are small,
vibrations have large amplitude. Water vapor has three fundamental vibrational
modes with the dipole moments changing in the direction of the vibration
(Fig. 14.11). The first mode for liquid water at 25 °C is symmetric stretching; in this
case, both hydrogen ions vibrate simultaneously, which is at mode ν 1 with a 3050 nm
absorption feature. The second vibrational mode is bending the covalent bonds,
which occurs when the two hydrogen atoms vibrate by moving toward and away
from each other. This bending mode is at mode ν 2 and it causes a strong absorption
at 6080 nm. The third mode is asymmetric stretching that results from one hydrogen
ion being attracted toward the oxygen while the other moves away. The absorption
wavelength for this ν 3 mode is 2870 nm. Vibrational modes for liquid water at
shorter wavelengths (401–1900 nm) are combination modes. Vibrational modes are
restricted in liquid water and ice by hydrogen bonds. Libration describes the backand- forth rotation of the hydrogen ions in liquid water when its motion is restricted.
The infrared spectrum of water results from vibrational overtones and combinations with librations. Palmer and Williams (1974) conducted a detailed study,
including verification of prior authors (e.g., Hale and Querry 1973) and reported 94
optical constants between the UV and the NIR. They report liquid water values
related to those in Table 14.1 for measurements taken at 27 °C to be 769, 847, 973,
980, 1205, 1443, 1786, and 1927 nm. Kou et al. (1993) established absorption coefficients for ice between 1440 and 2500 nm, showing that these peaks are at longer
wavelengths than those of liquid water when measured at warmer temperatures.
Wieliczka et al. (1989) developed an improved measuring device, measured the real
part of the refractive index, and used it with Kramers–Kronig methods to compute
14 How the Optical Properties of Leaves Modify the Absorption and Scattering…