Molecular Beam Experiments for the Physical Chemistry Laboratory

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New experiments using a molecular beam to explore the kinetic theory of gases and the concept of temperature will be developed for the physical chemistry lab. A relatively simple apparatus, capable of producing both effusive and supersonic beams, will be constructed for the project. The principal experiment will use time of flight methods to directly verify the Maxwell-Boltzmann distribution of molecular speeds in a thermal beam. By examining a series of gases and using computer aided fitting of their data, students will be able to verify the dependence of the speed distribution on molecular mass and temperature. As an extension of this experiment, the operating conditions will be modified so that the beam becomes supersonic. This leads to a nonthermal speed distribution in which the beam molecules are not properly characterized by a temperature. Observation of this distribution will lead students to consider the physical meaning of temperature. The project provides a demonstration of important classical ideas in physical chemistry while at the same time exposing students to modern instrumentation and techniques.