This grant from the Organic Dynamics Program supports the continuing work of Dr. Joshua L. Goodman of the University of Rochester. This work will improve our basic understanding of the fundamental processes by which chemical reactions occur. The experiments that will be carried out by Dr. Goodman will utilize time-resolved photoacoustic calorimetry, which allows measurement of microscopic volume changes of a system following photoexcitation. The magnitude of these volume changes can be related to reaction enthalpies and to volume changes involving reactive intermediates. The investigation will focus on processes involving ion radicals and carbenes. A new two-laser system will be developed in which the first laser pulse is used to generate the reactive intermediate and a second laser pulse is employed to photoexcite the intermediate.