Coaxially aligned electrodes for Stark-effect applied in resonators using a supersonic jet Fourier transform microwave spectrometer

In this article, we present a setup for Stark-effect measurements using a Fourier transform-microwave (FTMW) spectrometer with coaxially oriented beam-resonator arrangement (COBRA). The open Fabry–Pérot-type arrangement of spherical mirrors provides the possibility to mount the reflectors electrically insulated. Since no electrical surface currents of the resonator are inhibited, the propagation of the microwave field remains unaffected. If the microwave antennas and the valve are mounted at the same mirror, the other reflector can be set to a static high voltage potential. Together with a number of coaxially arranged circular electrodes a homogeneous field along the resonator axis can be achieved, allowing accurate Stark-effect measurements with COBRA-FTMW spectrometers. The arrangement is suitable even at low frequencies, where a parallel plate arrangement for Stark-effect measurement would increasingly interfere with the microwave field. The design of the coaxially aligned electrodes for Stark-effect applied in resonators (CAESAR) arrangement has been supported by electrostatic calculations optimizing the geometry. The performance of the Stark-effect arrangement will be demonstrated by spectral examples of OCS and Ar-OCS.