Notes
equal to some energy level difference in the atom or molecule. In figure 8.2 the
red light does not have enough energy to excite the electron to the higher energy
level. The blue light has too much energy and therefore does not excite the
electron to the higher energy level. But the green light has exactly the right
amount of energy to excite the electron to the higher energy level. This molecule
will absorb green light but will not absorb red or blue light.
Ultraviolet-Visible Absorbance Spectroscopy
Many different wavelengths (IR,
microwave, x-ray, etc.) of light can
be used in absorbance
spectroscopy. One of the most
widely used wavelength ranges for
absorbance spectroscopy is light in
the ultraviolet and visible (UV-Vis)
regions of the electromagnetic
spectrum. The UV-Vis region has
energies of photons that are often
similar to energies required to
promote an electron from one
energy level to another energy
level. In UV-Vis absorption
spectroscopy we measure how
much light passes through a sample
at all of the different wavelengths.
If we look at a liquid sample and
observe it to have a specific color
with our eye, the light that is being
absorbed by the sample is the
complement of that color on a color
wheel. Using figure 8.4 as a guide,
we can demonstrate how this
works. A solution that appears
yellow to our eye, for example, is absorbing the complementary color purple. We
would expect a yellow solution to show strong absorbance for photons near 435
nanometers.
61
Figure 8.3: Derived from "File:Light wave harmonic
diagram.svg" by Rubber Duck (☮ • ✍) is licensed
under CC BY-SA 3.0.
Figure 8.4: "File:Color wheel wavelengths.png"
by Tem5psu is licensed under CC BY-SA 4.0.