Kelly M. Hotopp

Impact of molecular conformation on barriers to internal methyl rotation: the rotational spectrum of m-methylbenzaldehyde.

The ground state spectrum of m-methylbenzaldehyde (m-MBA) was measured with a chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer. The methyl rotor on m-MBA introduces an internal rotation barrier, which leads to splitting of the torsional energy level degeneracy into A and E states. Ab initio calculations predict a low torsional barrier for both the O-cis and O-trans conformers, resulting in a large doublet splitting up to several gigahertz in the frequency spectrum. The rotational constants, distortion terms, and V(3) values for both species have been determined from the ground state rotational spectrum using the BELGI-C(s) fitting program. There are significant differences in the torsional potential for the O-cis and O-trans m-MBA conformers. Molecular orbitals and resonance structures for each conformer are analyzed to understand the difference in torsional barrier height as well as the irregular shape of the O-trans torsional potential.

Two-dimensional chirped-pulse Fourier transform microwave spectroscopy.

Two-dimensional (2D) correlation techniques are developed for chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy. The broadband nature of the spectrometer coupled with fast digital electronics permits the generation of arbitrary pulse sequences and simultaneous detection of the 8-18 GHz region of the microwave spectrum. This significantly increases the number of rotation transitions that can be simultaneously probed, as well as the bandwidth in both frequency dimensions. We theoretically and experimentally evaluate coherence transfer of three- and four-level systems to relate the method with previous studies. We then extend the principles of single-quantum and autocorrelation to incorporate broadband excitation and detection. Global connectivity of the rotational energy level structure is demonstrated through the transfer of multiple coherences in a single 2D experiment. Additionally, open-system effects are observed from irradiating many-level systems. Quadrature detection in the indirectly measured frequency dimension and phase cycling are also adapted for 2D CP-FTMW spectroscopy.

Microwave Chemical Sensing at Room Temperature Using an Overmoded Waveguide Design

Microwave spectrometers have unique advantages in the ability to determine high-resolution features that are specific to a given chemical. Very sharp lines which correspond to quantum states of the chemical allow for unique identification of the chemical. Recent advances have shown the possibility of room-temperature microwave spectroscopy analysis in which the data are collected in a short amount of time using broadband chirp pulse Fourier transform microwave (CP-FTMW) spectroscopy. In this paper, we explore the design of reduced-size spectrometers focusing on the reduction of the microwave analysis cell, where the chemical is analyzed at room temperature. Through optimization of the features of the test cell, it is shown that a much smaller analysis cell can be utilized. In combination with the established trends towards system on chip high-frequency devices, this technique demonstrates the possibility of a nonlaboratory-based implementation of a high-resolution sensor.

Microwave chemical sensing using an 3–18 GHz wideband overmoded coaxial cable

A room-temperature chirped pulse Fourier transform microwave spectrometer was recently demonstrated using a WRD750 double ridge waveguide. The operation frequency range, 7.5 GHz to 18 GHz, is limited by the waveguide, and the effective sensing area is confined within the small region between the two ridges. This paper shows a new overmoded coaxial cable spectrometer design utilizing a Hamming function tapered transmission line method that provides smooth tapering to increase the spectrometers bandwidth by 43%. The successful spectra measurement in this paper proves the application of using the overmoded coaxial cable as a chemical sensor.