PerkinElmer Spotlight 400 Series, User Manual

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Techniques for Collecting IR Images and Spectra . 75
Collecting Images Using the Infrared Polarizer
Accessory
An absorption band in the infrared range occurs when a vibration is accompanied by a
change in dipole moment. The electric vector of the incident radiation must have a
component in the direction of the dipole moment change.
In polarization spectroscopy, the absorption bands of greatest interest are those in which the
direction of dipole moment change is related to a bond direction, for example, the nitrile
stretching vibration. If, in a particular sample, all the bonds of a particular type are aligned in
a specific direction, the strength of the absorption depends on the polarization of the incident
radiation, that is, whether the electric vector is parallel to or perpendicular to the bond
direction.
For example, stretching an acrylic fiber aligns the molecules with the general direction of the
polymer chains parallel to the fiber axis, and the nitrile groups tend to be oriented
perpendicular to the axis. If the spectrum is collected with the infrared radiation polarized
perpendicular to the axis, the nitrile absorption peak is much stronger than if the spectrum is
collected with radiation polarized parallel to the axis. The ratio of the two intensities (called
the
dichroic ratio)
is a measure of the extent of alignment of the nitrile groups and thus of
the polymer chains.
The Spotlight imaging system enables you to collect polarization spectra of very small
samples. These include:
•
Single filaments (typically 14 × 70 µm);
•
Films;
• Single crystals;
• Liquid crystals.
Equipment
The optional infrared polarizer has a rotatable silver bromide element in an aluminum mount
(Figure 28).
Figure 28 The infrared polarizer