Page 319 - Instant notes

General features of spectroscopy 305

radiation associated with a transition between a pair of energy levels E 1 and E 2 is given
by:
hv=|E 1−E 2|=|∆E|

Spectroscopy that probes different magnitudes of energy level separation is associated
with different regions of the electromagnetic spectrum.
The analysis of atomic spectra (see Topics G5 and G7) yields information about the
electronic structure of the atom. Molecules also possess energy due to rotation and to the
vibration of the bonds between the atoms, so molecular spectra are more complicated,
since they include rotational and vibrational transitions as well as electronic transitions.
However, the analysis of molecular spectra provides a wealth of information on
molecular energy levels, bond lengths, bond angles and bond strengths. The characteristic
spectral frequencies associated with particular atoms and molecules also means that
spectroscopy is widely used for identification of species and monitoring of specific
reactants or products in, for example, kinetic measurements (see Topic F1).

The electromagnetic spectrum
Electromagnetic radiation is a propagating oscillation of interconnected electric and
magnetic fields. The fields oscillate in phase along the direction of propagation as sine
waves with frequency, v, and wavelength, λ, related by c=vλ, where c is the speed of light
8
−1
in a vacuum (c=2.9979246×10 m s ). All electromagnetic waves travel at this speed.
The frequency of electromagnetic radiation is often specified as wavenumber, :

−1
with units of reciprocal centimeters, cm . Individual photons of electromagnetic
radiation have energy, E:

i.e. photon energy is proportional to frequency and inversely proportional to wavelength.
The wavelength range of electromagnetic radiation encompasses many orders of
3
magnitude, from ~10 m at the low frequency, low energy end, to ~10 −12 m at the high
frequency, high energy end (Fig. 1). Different regions of the spectrum correspond to
different types of radiation. For example, radiation visible to the human eye occurs over a
very narrow range of wavelengths between about 700 and 400 nm.
The interaction of the oscillating electric and magnetic fields with the electrical and
magnetic properties of atoms and molecules gives rise to various forms of spectroscopy.
Because of the direct relationship between energy and radiation frequency (or
wavelength), spectral transitions between different types of atomic or molecular energy
levels are associated with different regions of the electromagnetic spectrum. For
example, the rotational energy levels of molecules are more closely spaced than the