The theory of pulsed Fourier transform microwave spectroscopy carried out in a Fabry–Perot cavity: Static gas

A semiclassical theory has been developed to describe pulsed Fourier transform microwave spectroscopy carried out in a Fabry–Perot cavity. A density matrix formalism is used to study the interaction of a two-level quantum system with a classical standing wave electric field, appropriate for the Fabry–Perot cavity. Equations describing the polarization of, and subsequent emission of radiation by arbitrary distributions of molecules in the cavity are derived. The specific problem of a static Maxwell–Boltzmann gas is studied in detail, both theoretically and experimentally. The static gas line shape in the power-broadened limit is described by an ordinary Doppler and pressure broadened envelope. Sensitivities of the ordinary waveguide cell and Fabry–Perot cavity pulsed Fourier transform spectrometers using static gas samples are compared.