Abstract
Photoacoustics is an excellent method to obtain enthalpy and volume changes of photochemical reactions. It is particularly useful for photobiological or other reactions in water where the thermal and inherent volume changes can be separated by measurement at the temperature of maximum density. However, its time range is limited to less than ∼10 μs because the amplitude of the observed pressure wave is proportional to the rate of volume change. We describe a closed pressure cell which perfectly complements the usual photoacoustic methodology. It has a rise time of 20 μs and a thermal decay time of several seconds. Its single pulse sensitivity [signal/noise (S/N)=1] is 10 pL or 15 μJ absorbed energy in water. The assembly and calibration of the instrument is described along with its verification by measurement of the enthalpy and volume change of the photodissociation of CO-myoglobin. The apparatus is eminently suited for the study of proton pumping in bacteriorhodopsin and oxygen formation in photosynthesis.
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