Two-photon photoacoustic spectroscopy of liquid solutions under degenerate and non-degenerate laser excitation

A new apparatus to measure the two-photon (2P) absorption of a dilute liquid solution is designed using tunable laser excitation with an optical parametric oscillator and photoacoustic (PA) detection. Two-photon absorption of non- or less-fluorescent solute molecules in dilute liquid solutions was investigated. When PA signal intensity was plotted versus half of an excitation laser wavelength, a degenerate 2P PA spectrum of nitrobenzene showed a shape similar to that of the absorption spectrum, as expected from Laporte’s selection rule, with a slight red shift. The non-degenerate 2P PA spectrum of diphenylacetylene was compared with the degenerate 2P PA spectrum and good matching was observed. Results demonstrate that this method is suitable for observing 2P absorption for weakly fluorescent substances.