RECENT ADVANCES IN PHOTOACOUSTIC CALORIMETRY - THEORETICAL BASIS AND IMPROVEMENTS IN EXPERIMENTAL-DESIGN

Recent developments in time-resolved photoacoustic calorimetry (PAC) are discussed. An equation is derived relating the amplitude of the maximum photoacoustic signal to parameters of a photoacoustic cell, to physical properties of the solvents, and to the energy deposited as heat. The equation is validated by a series of scaling experiments: it correctly correlates the dependence of the PAC signal on cell thickness, on energy deposition, and on solvent properties. The dominant sources of background signal have been determined and the background signal reduced substantially relative to previous work. It is now possible to measure energies and lifetimes of transients in solutions with optical densities as low as approximately 5 x 10(-4). Data are presented for the energy of 2-cyclopentenone triplet, a transient for which interpretable PAC measurements were previously not possible. Its triplet energy is 73.1+/-1.1 kcal/mole. With reduced background and a faster digital storage oscilloscope, a quite short transient lifetime, 7.53+/-0.2 ns, and a triplet energy of 51.0+/-2.1 kcal/mol. have been measured for 1,1-bis-(p-bromophenyl) ethene. The lifetime agrees very well with that determined by kinetic absorption spectroscopy and the triplet energy agrees with expectation based on the unsubstituted 1,1-diphenylethene.