SOURCES
DESIGNS OF OPTICAL INSTRUMENTS
Typical spectroscopic instruments contain five components:
1. A stable source of radiant energy
2. A transparent container for holding the sample
3. A device that isolates a restricted region of the spectrum for measurement
4. A radiant detector, which converts energy to a usable signal
5. Signal processor and readout
Figure 7-1, p. 144 describes the setup of various types of instruments
SOURCES OF RADIATION
In order to be suitable for spectroscopic studies, a source must generate a beam of radiation
with sufficient power for easy detection and measurement. Figure 7-3a lists the most widely
used spectroscopic sources. Continuum sources emit radiation that changes in intensity only
slowly as a function of wavelength. Line sources emit a limited number of lines or bands of
radiation each of which spans a limited range of wavelengths.
EMISSION OF RADIATION
Electromagnetic radiation is produced when excited particles (atoms, ions, or molecules) relax
to lower energy levels by giving up their excess energy as photons. This excitation can be
brought about by a variety of means, including:
1. Bombardment with electrons or other elementary particles
a. Generally leads to X-radiation
2. Exposure to an electrical current (ac), spark or the heat of a flame
a. Produces UV, visible or IR radiation
3. Irradiation with a beam of electromagnetic radiation
a. Produces fluorescent radiation
LINE SPECTRA
Line spectra in the UV and visible regions are produced when the radiating species are
individual atomic particles that are well separated, in a gas phase. The energy-level diagram in
figure 6-17a shows the source of two of the lines in a typical emission spectrum of an element.
BAND SPECTRA
Often encountered in spectral sources when gaseous radicals or small molecules are present.
Bands arise from numerous quantized vibrational levels that are superimposed on the ground-
state electronic energy level of a molecule. Vibrational levels associated with the two excited
states have been omitted because the life-time of an excited vibrational state is brief compared
with that of an electronically excited state.
CONTINUUM SPECTRA
Thermal radiation of this kind, blackbody radiation, is characteristic of the temperature of the
emitting surface rather than the material of which that surface is composed. Blackbody radiation
is produced by the innumerable atomic and molecular oscillations excited in the condensed solid
by the thermal energy.
, ABSORPTION OF RADIATION
Absorption is a process in which electromagnetic energy is transferred to the atoms, ions, or
molecules composing the sample. Absorption promotes these particles to one or more higher-
energy excited states. The nature of a spectrum is influenced by such variables as the
complexity, the physical state, and the environment of the absorbing species. More fundamental,
however, are the differences between absorption spectra for atoms and those for molecules.
ATOMIC ABSORPTION
The passage of polychromatic UV or visible radiation through a medium that consists of
monoatomic particles, such as gaseous mercury or sodium, results in the absorption of but a few
well-defined frequencies. The relative simplicity is due to the small number of possible energy
states for the absorbing particles. Several other narrow absorption lines, corresponding to other
allowed electronic transitions, are also observed. Ultraviolet and visible radiation has sufficient
energy to cause transitions of the outermost for bonding electrons only. X-Ray frequencies, on
the other hand, are several orders of magnitude more energetic and are capable of interacting
with electrons that are closest to the nuclei of atoms.
MOLECULAR ABSORPTION
Absorption spectra for polyatomic molecules, particularly in the condensed state, are
considerably more complex than atomic spectra because the number of energy states is
generally enormous when compared with the number of energy states for isolated atoms. The
energy difference between the ground state and an electronically excited state is large relative to
the energy differences between vibrational levels in a given electronic state.
HOLLOW CATHODE LAMPS
These types of lamps are the most commonly used lamps for atomic absorption measurements.
It consists of a tungsten anode and a cylindrical cathode sealed in a glass tube that is filled with
neon or argon at a pressure of 1-5 torr. The cathode is constructed of the metal whose spectrum
is desired or serves to support a layer of that metal. Ionization of the inert gas occurs when a
potential on the order of 300 V is applied across the electrodes, which generates a current of
about 5 to 15 mA as ions and electrons migrate to the electrodes. If the potential is sufficiently
large, the gaseous cations acquire enough kinetic energy to dislodge some of the metal atoms
from the cathode surface and produce an atomic cloud in a process called sputtering. A portion
of the sputtered metal atoms are in excited states and thus emit their characteristic radiation as
they return to the ground state. Eventually, the metal atoms diffuse back to the cathode surface
or to the glass walls of the tube and are redeposited.
SELF-ABSORPTION
The greater currents produce an increased number of unexcited atoms in the cloud. The
unexcited atoms, in turn, are capable of absorbing the radiation emitted by the excited ones. This
self-absorption leads to lowered intensities, particularly at the center of the emission band.
