Page 324 - Instant notes

Physical Chemistry 310

Figure 2 in Topic I1 illustrates the basic experimental arrangements for emission,
absorption and Raman spectroscopic measurements. The three basic components of a
spectroscopic apparatus are: a radiation source (not required for emission spectroscopy),
a frequency dispersing element, and a detector.

Radiation source
The radiation source for absorption spectroscopy must emit over a range of frequencies
and depends on the region of the electromagnetic spectrum under investigation. For the
ultraviolet region, a discharge through deuterium or xenon gas in a quartz cell is used as a
broad-band source; for the visible region a tungsten-iodine lamp is used. Far infrared
radiation is provided by a mercury discharge inside a quartz tube, and near infrared
radiation is usually provided by a heated ceramic filament. Microwave radiation is
generated by a device called a klystron. Radio-frequency radiation (as required for
nuclear magnetic resonance spectroscopy) is generated by oscillating an electric
current through coils of wire at the appropriate frequency.
The intense monochromatic light source required for Raman spectroscopy is usually
provided by a visible or ultraviolet laser.

Dispersing element
The dispersing element separates the emitted, transmitted, or scattered radiation into its
constituent frequencies after interaction with the sample under investigation. (In some
applications of absorption spectroscopy, the dispersing element is used to separate the
broad band source into frequency-resolved radiation before it is incident on the sample.)
The simplest dispersing element is a glass or quartz prism. Diffraction gratings are also
widely used. These consist of parallel lines etched into the surface of a glass or ceramic
plate at spacings comparable with the wavelength of the radiation being dispersed.
Radiation incident onto the surface of the grating is reflected (dispersed) at different
angles according to incident frequency because of destructive and constructive wave
interference.

Detector
The detector is a device that produces an electrical voltage or current in response to the
intensity of incident radiation. In the visible and ultraviolet regions photomultiplier tubes
are widely used. An incident photon ejects an electron from a photosensitive surface, the
electron is accelerated by a potential difference to strike another surface, and the shower
of secondary electrons from this collision is accelerated towards another surface, and so
on. Thus, each photon creates an amplification electron cascade which is converted into
an electric current. Infrared detectors often consist of a mixture of metal alloys or a
mixture of solid oxides whose electrical resistances change as a function of temperature.
Throughout the visible and infrared regions of the spectrum, specific radiation-sensitive
semiconductor devices have been developed which convert incident photons directly into
an electrical signal.

Sample presentation
In absorption spectroscopy, the extent of absorption depends, amongst other factors, on
the path length of the incident radiation through the sample. The path length required for