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Starter Kit Instruction Manual Maintenance 54

9.1.3 Photoacoustic detection

In the photoaco us tic detection, the optical beam is

periodically modulated in amplitude before illumi-

nating the cell containing the absorbing chemical.

The expansion generated by the periodic heating

of the chemical creates an ac oustic wave which is

detected by a microphone.

The two very important advantages of photoa-

coustic detection are:

• a signal is detected only in the presence of

absorption from the molecule;

• no mid-IR detectors a re needed.

For these reasons, photoaco us tic detection has

the potential of being cheap and very sensitive. How-

ever, ultimate sensitivity is us ually limited by the

optical power of the source.

QC Laser Chemical Cell

Microphone

Detection of the

acoustic wave at w

a.c.modulation of

the laser amplitude

Fig.53: Photoacoustic technique

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Quantum Instruments Cascade Laser Starter Kit Manual do Utilizador Página 55