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.

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The pure rotational spectra of 41 isotopic species of PbSe and PbTe have been measured in their X 1Sigma+ electronic state with a resonator pulsed-jet Fourier transform microwave spectrometer. The molecules were prepared by laser ablation of suitable target rods and stabilised in supersonic jets of noble gas. Global multi-isotopologue analyses yielded spectroscopic Dunham parameters Y01, Y11, Y21, Y31, Y02, and Y12 for both species, as well as effective Born-Oppenheimer breakdown (BOB) coefficients delta01 for Pb, Se and Te. Unusual large values of the BOB parameters for Pb have been rationalized in terms of finite nuclear size (field shift) effect. A direct fit of the same data sets to an appropriate radial Hamiltonian yielded analytic potential energy functions and BOB radial functions for the X 1Sigma+ electronic state of both PbSe and PbTe. Additionally, the magnetic hyperfine interactions produced by the uneven mass number A nuclei 207Pb, 77Se, 123Te, and 125Te were observed, yielding first determinations of the corresponding nuclear spin-rotation coupling constants.

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