A reaction vessel for holding a sample for a heat-exchanging chemical process has two opposing major faces and a plurality of contiguous minor faces joining the major faces to each other. The major and minor faces form an enclosed chamber having a triangular-shaped bottom portion. The ratio of the thermal conductance of the major faces to that of the minor faces is at least 2:1, and the minor faces forming the triangular-shaped bottom portion of the chamber are optically transmissive. The vessel also has a port for introducing a sample into the chamber and a cap for sealing the chamber.