Two-photon photoacoustic calorimetry and the absolute measurement of molar absorption coefficients of transient species in solution.

The theory of two-photon photoacoustic calorimetry (PAC) is developed for the case of a homogeneously irradiated volume and it is shown that the laser-intensity dependence of the photoacoustic signals can be used to determine the molar absorption coefficient of transient species. The application of the method is illustrated via the measurement of the absorption coefficients of benzophenone and acetophenone triplets using a front-face PAC cell. The very high sensitivity exhibited by this cell strongly recommends the inclusion of laser-intensity dependence studies in the procedure for measuring heat depositions. Only extrapolation to zero laser intensity can afford reliable enthalpies of formation for very short-lived species.