A clue of the photoacoustic spectroscopy stays
in the fact, that low-energy (i.e. long wavelength, i.e. infrared excitations)
have longer lifetimes.
For this reason, the decay from upper levels undergoes through collisions with other molecules, rather than through
a direct radiative de-excitation.
Collisions with gas mean pressure waves, i.e. sound.
This is the way:
1) infrared excitation (a well selected IR laser line)
2) chopped with an acoustic frequency
3) and when a sound of this frequency is detected,
it means that this particular IR line is absorped.
And, as the IR laser lines are numerous
(from CO2 lasers) and complex molecules have numerous vibration frequences, it is easy to make a
fingerprint of even quite complex chemical