Photoacoustic spectroscopy of weakly absorbing media using nanosecond laser pulses

The anomalous behavior of the expansion coefficient of water near 4 °C was studied in a thin photoacoustic cell. We show that the thermal expansion of water vanishes at temperatures lower than 3 ºC when the optical path is shortened below 0.1 mm. We explain this behavior in terms of less favorable hydrogen bonding near the surface of water, which becomes relevant when the fraction of molecules near the surface contributes appreciable to the observed photoacoustic signal. The photoacoustic spectra of water and oleic acid in a thin photoacoustic cell matches the optical spectra from 720 nm to 2200 nm. The high sensitivity of the front-face photoacoustic cell allows for spectroscopy of weakly absorbing media but surface effects may have to be taken into account.