Background signal in a photoacoustic system

In order to investigate the sources of the background signal in a photoacou stic system the wavelength dependence of background signal has been measure d in At and C2H2 gas, respectively. When At was used as sound carrier, the background photoacoustic signal arises from optical heating of the reflecti ng mirrors, that is the mirror noise. When polyatomic gas, such as C2H2, wa s filled in the photoacoustic cell, the background signal is due to photoac oustic Raman effect in addition to the noise produced at the reflecting mir rors. In near-infrared and visible region. the photoacoustic Raman signal i s of the same order of magnitude as mirror noise.