REFERENCES FOR LASER-INDUCED OPTOACOUSTIC SPECTROSCOPY USING UV-EXCITATION

In order to extend the use of laser-induced opto-acoustic spectroscopy (LIOAS) to the investigation of the photophysics of molecules which absorb UV radiation, it is necessary to have suitable reference substances which absorb in this wavelength region. In this paper, the use of (2-amino)-benzophenone, (2-hydroxy)-benzophenone, benzophenone quenched by potassium iodide, dihexyloxatrimethinecyanine iodide, dioctadecylmonomethineoxacyanine perchlorate, iron(III)-acetylacetonate, ferrocene and p-(N,N-dimethylamino)-nitroaniline as references for LIOAS experiments is investigated. When the energy of the laser pulse is less than 20-mu-J, the amount of heat, integrated over 250 ns, is (within experimental error) equal to the energy of the laser pulse absorbed by the ground state for all systems, with the exception of a solution of benzophenone and potassium iodide in ethanol. At laser pulse energies higher than 20-mu-J, positive or negative deviations from a linear relationship between the energy of the laser pulse and the amplitude of the photo-acoustic signal are observed. These deviations are due to the occurrence of biphotonic processes and the depletion of the ground state respectively. For the solution of benzophenone and potassium iodide in ethanol the biphotonic processes also occur at pulse energies below 20-mu-J.