Applications
A most widely used technique for quantitative trace analysis. Used as an adjunct to other spectrometric
techniques in the identification and structural analysis of organic materials. Relative precision 0.5–5%.
Disadvantages
Samples should be in solution. Mixtures can be difficult to analyse without prior separation of the
constituents.
Absorption of radiation in the visible and ultraviolet regions of the electromagnetic spectrum results in
electronic transitions between molecular orbitals. The energy changes are relatively large,
corresponding to about 10^5 J mol–^1. This corresponds to a wavelength range of 200–800 nm or a
wavenumber range of 12 000–50 000 cm–^1. All molecules can undergo electronic transitions, but in
some cases absorption occurs below 200 nm where atmospheric absorption necessitates the use of
expensive vacuum instrumentation.
Because of the large energy changes involved in electronic transitions there are always simultaneous
changes in rotational and vibrational energies. The effect of this upon the spectrum of a gaseous
diatomic molecule is seen by considering a potential energy diagram (Figure 9.5), which relates the
potential energy of a vibrating molecule to the internuclear distance. The two curves represent the
ground and first excited electronic states, and the horizontal lines the quantized vibrational levels
associated with each. For simplicity, rotational levels are not shown. The horizontal displacement of the
curves indicates a difference in equilibrium bond length between the electronic states. According to the
Franck-Condon principle, an electronic transition occurs very much more rapidly than vibrational or
rotational motion and may be represented in the diagram as a vertical line between
Figure 9.5
Electronic and vibrational transitions in a
gaseous diatomic molecule.