Chemistry, Third edition

20 · LIGHT AND SPECTROSCOPY

mole of absorbing molecules. As a consequence of infrared absorption, the atoms in

the molecule move farther in each vibration (the amplitude of the vibration

increases). Emission of infrared light causes the atoms in a molecule to vibrate with a

reduced amplitude. In Fig. 20.4, transition ‘b’ involves the absorption of infrared

light, and transition ‘d’ involves the emission of infrared light. All hot objects emit

infrared light. The warming of a beaker of water placed close to a heated electric fila-

ment occurs partly by absorption of infrared light.

Spectrometers

Instruments which measure and record the wavelengths at which samples emit or

absorb light are called spectrometers. The basic components of a UV–visible or

infrared spectrometer are shown in Fig. 20.5. The components include

●Asourcewhich continuously emits a broad range of wavelengths of light.

●Adiffraction gratingor prism which separates the wavelengths of light by spread-

ing them at different angles and which (ideally) allows only one wavelength of

light to strike the cell.

●Acellwhich is transparent to the incident light (quartz for UV, glass or perspex

for visible light and solid sodium chloride for infrared). The cell contains the

sample whose spectrum is required.

●An electronic detector(or photographic emulsion) which measures the intensity

of the light beam emerging from the cell. Commercial spectrometers are ‘com-

puter driven’, and spectra are displayed on a monitor.

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Fig. 20.5(a) Obtaining the emission spectrum of the source. (b) If an absorbing sample is

placed in the light beam, intensity measurements over the wavelength range of the source show

that only certain wavelengths are absorbed. These appear as troughs in the graphical profile or

as lines in the photographic emulsion.