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

Abstract

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.

Precise dipole moments and quadrupole coupling constants of the cis and trans conformers of 3-aminophenol: determination of the absolute conformation

“…The experimental setup of the Hannover COBRA-FTMW-spectrometer is described in detail elsewhere [14,15]. In brief, 3-aminophenol (purity ≥ 98 %) was purchased from Sigma-Aldrich and used without further purification.…”

“…Stark shift measurements were performed with the Coaxially Aligned Electrodes for Stark-effect Applied in Resonators (CAESAR) setup [15]. This setup provides a homogeneous electric field over the entire volume, from which molecules are effectively contributing to the emission signal.…”

“…Often sufficiently homogeneous electric fields are generated by applying a voltage difference using two planeparallel electrodes [7]. For the large volumes sampled in FTMW spectrometers this is difficult and, therefore, in the Hannover COBRA-FTMW experiment a different approach is taken: Within the CAESAR setup, the two concave mirrors of the spectrometer resonator are used as the primary capacitor electrodes and electrode rings between them are used to create a homogeneous field over a very large fraction of the resonator volume [15]. Therefore, all detected molecules experience practically the same field strength, even for the large modes at frequencies below 3 GHz.…”

Electric Dipole Moments of Acetone and of Acetic Acid Measured in Supersonic Expansion

“…The resulting loss in electric field uniformity is such that the expanding molecules are subjected to an electric field that is on average only 95% of that given by the standard ∆V /d relation for infinite parallel plate electrodes, separated by distance d and charged to potential difference ∆V . This was realised relatively early on [13,14], and resulted in varying instrumental solutions, including the use of guard wires and resistors for the perpendicular FTMW configuration [15], and guard rings [16] for the parallel configuration. Our solution [11] uses small guard electrodes attached to the sides of the parallel plates, that focus the electric field to close to the ∆V /d condition and lead to considerable improvement in field uniformity.…”

Six-fold-symmetry internal rotation in toluenes: the low barrier challenge of 2,6- and 3,5-difluorotoluene

“…The principal advantage in the COBRA arrangement is the longer transit time of the molecular jet, which can be exploited for Stark effect experiments if, rather than using external electrode plates, the spherical reflectors forming the resonator are utilized as high-voltage electrodes (coaxially aligned electrodes for Stark effect applied in resonators, CAESAR 23 ). In the experiment the direction of the electric Stark field is perpendicular to the polarized microwave field, leading exclusively to the DM J = AE1 transitions.…”