Intracavity-Absorption Spectroscopy
Introduction
Intracavity-absorption spectroscopy is an ultrasensitive method for measuring
very small absorptions. Some examples are
very low concentrations of analytes, and very weak absorption such as infrared
overtone bands. The technique takes advantage of the very high light intensities
within a laser resonance cavity, and the sensitivity of the lasing output
to losses within the cavity. Small losses within the laser cavity due to
absorption results in large changes in the intensity of the laser output.
Schematic of an intracavity-absorption experiment
Instrumentation
To record a spectrum the laser output must be tunable, therefore the most
common lasers used in intracavity-absorption measurements are dye
lasers.
Samples can be solids, liquids, or gases. Liquids and gases are contained
in a sample holder that has windows at Brewster's angle to minimize losses
due to reflection.
Small absorptions are detected by monitoring the output of the laser
beam with a detector.
Since the laser light is relatively intense, the main requirement for the
detector is that it not be destroyed by the laser beam.