When an atom is excited from one
energy state to a higher stage by the absorption of a photon, it may
return to the lower level in a series of two (or more) jumps if an
energy level exists in between. The photons emitted will,
consequently, have lower energy and frequency than the absorbed
photon. This phenomenon is called fluorescence; common fluorescent
rocks and paints can emit visible light after absorbing UV light.
Fluorescence is responsible for the appearance of objects under the
so-called "black light," which is a source of ultraviolet radiation.
Photons in the ultraviolet region, invisible to the human eye, have
higher energies than those in the visible region, and hence, if an
ultraviolet photon is absorbed by an atom, the outer electron (which
is responsible for the visible transitions) can be excited to high
levels. These electrons make transitions back to their ground state
accompanied by the emission of photons in the visible region.
Objects seen in ultraviolet light often show colors in the blue or
violet end of the spectrum that are not present when the objects are
viewed in sunlight. We can understand this effect by considering the
composition of sunlight and the optical excited states of a typical
atom. The intensity of sunlight is concentrated in the center of the
visible spectrum, in the yellow region; very little intensity is
present in the red or blue ends of the visible spectrum. The
"yellow" photons have enough energy to excite an atom up to its
lower levels, but not enough to reach the higher levels. However,
the higher-energy uV photons do have sufficient energy to reach the
higher level, so the light emitted by the atom has a stronger blue
component when that atom is excited by ultraviolet light than when
excited by sunlight.
The
wavelength at which fluorescence will occur depends on the energy
levels of the particular atoms. Because the frequencies are
different for different substances, and because many substances
fluoresce readily, fluorescence is a powerful tool for
identification of compounds.
Sulfur Analysis by
uVF |
