The Absorption and Emission of Electromagnetic Radiation
If a substance is irradiated with electromagnetic radiation, the energy of the incident photons may be
transferred to the atoms or molecules raising them from the ground state to an excited state. This
process, known as absorption, is accompanied by attenuation of the incident radiation at a particular
frequency and can occur only when the energy difference between the two levels is exactly matched by
the energy of the photons. The frequency of the radiation is given by
where E 1 and E 2 are the energies of the two levels and ∆E is the difference between them. The absorbed
energy is rapidly lost to the surroundings by collisions allowing the system to revert or relax to the
ground state. Sometimes the energy is not dissipated in this way but is re-emitted a few milliseconds
later – a process known as fluorescence.
By heating substances to high temperatures in a flame, plasma or in an electric discharge some of the
kinetic energy imparted to the atoms is utilized in exciting electrons into higher energy levels, from
which they relax to the ground state with the spontaneous emission of radiation. The frequency of the
emitted radiation corresponds to the difference in energy between the excited and ground states,
equation (7.4).
Even for the simplest compounds, the number of energy levels and hence the number of possible
transitions is large. It has already been seen that these transitions are related to a variety of atomic and
molecular processes involving energy changes of quite different magnitudes. Radiation can thus be
absorbed or emitted over the entire range of the electromagnetic spectrum from low-energy radiowaves
to high-energy γ-rays. Depending on the nature of the analytical information required, and for
instrumental reasons, only transitions related to a particular atomic or molecular process and confined to
a narrow region of the electromagnetic spectrum are studied at any one time. The information is
presented in the form of a spectrum, a graphical representation of the degree of absorption or the
intensity of emission of electromagnetic radiation as a function of frequency, wavelength or
wavenumber.
The Complexity of Spectra and the Intensity of Spectral Lines
The number and intensities of lines which may appear in a spectrum are determined by three factors:
(1) the populations of the energy levels from which the transitions originate
(2) the values of individual transition moments or transition probabilities
(3) quantum mechanical selection rules.
In brief, the statistical probability for the occurrence of a transition can be calculated and lies between
zero and one. It can be shown that weak