We present the design and construction of a simple and low-cost waveguide chirped pulse Fourier transform microwave (CP-FTMW) spectrometer suitable for gas-phase rotational spectroscopy experiments in undergraduate physical chemistry labs as well as graduate level research. The spectrometer operates with modest bandwidth, using phased locked loop (PLL) microwave sources and a direct digital synthesis (DDS) chirp source, making it an affordable for undergraduate labs. The performance of the instrument is benchmarked by acquiring the pure rotational spectrum of the J = 1 - 0 transition OCS and its isotopologues from 11-12.5 GHz.

A chirped-pulse Fourier transform spectrometer operating from 110 - 170 GHz was constructed. The design of this spectrometer is directly adapted from that of the 260 - 295 GHz chirped-pulse spectrometer built by Steber and co-workers at the University of Virginia¹. In this instrument, an arbitrary waveform generator (AWG) produces a chirped pulse which is frequency shifted to a range between 9.2 and 14.1 GHz and then multiplied by a factor of 12 via an active multiplier chain to a range between 110 and 170 GHz. As in the Pate lab design, the AWG also serves as a local oscillator (LO) source; this LO is multiplied and used to downconvert the molecular emission, allowing it to be collected by a 40 GS/s digitizer. Benchmark measurements were taken for methanol at room temperature, and details of the instrument's performance will be discussed.
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¹A.L. Steber, B.J. Harris, J.L. Neill, and B.H. Pate, J. Mol. Spectrosc., 280, 3 (2012)

Chirped-pulse Fourier transform microwave spectroscopy (CP-FTMW) has proven to be a powerful tool for broadband spectral surveys in the cm-wave band. In conjunction with a non-specific production source, such as an electrical discharge, new, unexpected molecules can be detected by their rotational spectra provided that they can be disentangled from other species that may be present. As an example, we have recently used a CP-FTMW spectrometer operating in the 8-18 GHz band to detect and identify two new silicon nitrides, HSiNSi and H₃SiNSi, in a discharge of dilute silane and nitrogen, although neither species had been the subject of prior experimental or theoretical study. However, of the  ∼ 100 lines that are observed in this plasma, only  ∼ 20 have been assigned to known species. To further investigate unassigned lines in CP-FTMW spectra, we take advantage of the higher sensitivity of a traditional cavity FTMW spectrometer to rapidly perform follow-up assays in an approach we call ßpectral taxonomy." Lines are classified according to whether their intensities are significantly altered by, for instance, turning off the discharge, applying a magnetic field, or removing a precursor gas; lines that show the same behavior for all tests may arise from a common carrier. After taxonometric classification, lines within each group are exhaustively tested with double resonance methods in an attempt to establish linkages which would identify lines arising from a shared quantum state and give clues as to the structure of the carrier. Using newly-designed control software for our cavity spectrometer, this entire procedure can be performed with minimal human intervention.

We have built a new molecular beam source that implements 20 K Neon buffer gas cooling for the study of the spectra of small molecules. In particular, laser ablation of BaF₂ pellets has been optimized to produce a molecular beam of BaF with a number density more than 100 times greater than what we have previously obtained from a typical Smalley-type photoablation supersonic beam source. Moreover, the forward beam velocity of  150 m/s in our apparatus represents an approximate 10-fold reduction, improving spectroscopic resolution from 500 kHz to better than 50 kHz at 100 GHz in a chirped-pulse millimeter-wave experiment in which resolution is limited by Doppler broadening. Novel improvements in our buffer gas source and advantages for CPmmW spectroscopy studies will be discussed. We thank David Patterson, John Barry, John Doyle, and David DeMille for help in the design of our source.

Rydberg-Rydberg transitions of Ca atoms are directly observed by chirped-pulse millimeter-wave spectroscopy, which is a form of broadband, high-resolution, free induction decay-detected (FID) spectroscopy with accurate relative intensities. A new setup, a 20 K Neon buffer gas cooled molecular beam system, has been constructed and tested in our lab. The number density of our target molecules, BaF, is increased by a factor of > 100 relative to a Smalley-type laser ablation supersonic beam source. In addition, the laboratory frame velocity is decreased by factor 10, which improves our spectroscopic resolution to better than 50 kHz FWHM at 100 GHz. The improved molecular beam source opens the door to an extension of the CPmmW spectroscopy from atomic Rydberg states to molecular Rydberg states. I expect to present preliminary data from "pure electronic" spectra of BaF Rydberg molecules. We expect to produce 10⁸ state-selected core-nonpenetrating Rydberg molecules in a single pulse of a laser-laser-mm-wave triple resonance excitation sequence.

The usual simplified view of Rydberg states of molecules as consisting of a single Rydberg electron loosely bound to a molecular ion core in a well-defined rotation-vibration state suggests an attractive possibility for state-selective production of molecular ions. A Rydberg electron excited above the energy of the ground state of the ion core will spontaneously autoionize, leaving behind a molecular ion. The autoionizing states are of strongly mixed character due to the ubiquitous nonadiabatic interactions between Rydberg series associated with different states of the ion core. Using our complete Multichannel Quantum Defect Theory (MQDT) fit model for CaF, we have predicted the locations and strengths of special autoionizing resonances that decay into a single rotation-vibration state of a molecular ion. Few molecules are as well characterized as CaF, nor as elegantly simple. We additionally describe the use of core nonpenetrating states as a general method to produce an ensemble of molecular ions in a single, selectable quantum state.

Core nonpenetrating Rydberg states of molecules are a neglected state of matter. They could have a variety of uses, notably state-selective production of molecular ions. Due to the l(l+1)/r² centrifugal barrier that prevents the Rydberg electron in high-l states from penetrating inside of the ion core, the electron is essentially uncoupled from the ion core, and the system becomes atom-like with long lifetimes, an älmost good" l quantum number, and electronic transitions that follow ∆J⁺=0 and ∆v⁺=0 propensity rules. However, in most molecules access to these states, via a sequence of ∆l=+1 transitions from low-n*, low-l states, is blocked by the necessity to traverse the "zone of death," in which nonradiative decay mechanisms are prohibitively fast. We exploit Chirped Pulse millimeter-Wave (CPmmW) spectroscopy to efficiently excite Ca atoms and BaF molecules to core nonpenetrating states in the absence of nonradiative decay mechanisms. A universal method for preparing core nonpenetrating Rydberg states of molecules, which combines CPmmW spectroscopy with STImulated Raman Adiabatic Passage (STIRAP), will be discussed.

The pure rotational spectrum of the open-shelled diatomic PbI is presented. The deep averaged broadband spectrum has allowed for characterization of several rotational spectra belonging to various lead isotopologues, as well as multiple vibrationally excited states of PbI. Being an opened-shelled diatomic, Λ-type doubling was observed. Analysis of the diatomic’s spectral constants and potential energy surface will be described. These results will also be compared to early results from SnI.

The rotational spectrum of allyl bromide was recorded from 8.7 to 26.5 GHz at -20 ^°C with a waveguide chirped-pulse Fourier transform microwave spectrometer. The rotational spectrum of allyl bromide has been previously studied by Niide and coworkers. ^1,2 However, previous assignments of this spectrum only extended to J = 12 and K_a = 1. Newly acquired data from our spectrometer has allowed us to extend the previous work to higher values of J and K_a, leading to significant improvements in the distortion constants in particular. Comparisons between the spectra and conformational preferences of the allyl halides will also be discussed.
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¹Y. Niide, M, Takano,T. Satoh, and Y. Sasada J. Mol. Spectrosc., 63, 108(1976) ²Niide, Yuzuru, J. Sci. Hiroshima Univ., Ser. A, 48, 1(1984)

The rotational spectrum of 3-methoxypropylamine was collected from 8.0 to 18.5 GHz with the Coker College chirped-pulse FTMW molecular beam spectrometer. Ab initio predictions using the B3LYP-D3 dispersion-corrected density functional gave high quality starting geometries, enabling us to quickly assign the spectrum of the lowest energy conformer, which has a g'gt configuration (moving from the amine end to the methoxy end of the molecule). Attempts were also made to collect the spectrum of this molecule in the room-temperature waveguide instrument at New College, but these attempts were unsuccessful as the molecule rapidly reacts with the copper walls of the waveguide to produce ammonia.

Microwave spectra for a family of three halogenated cyclic alkenes, C_nF_2n−4Cl₂; n = 4, 5, and 6, have been observed and analyzed. Rotational constants, quartic centrifugal distortion constants, and nuclear electric quadrupole coupling constants for a total of 9 isotopologues have been reported. These molecules are near oblate tops containing two quadrupolar nuclei. Data was first obtained via a chirp pulse Fourier transform microwave (FTMW) spectrometer, and then further analyzed with a Balle-Flygare type FTMW spectrometer to determine the chlorine nuclear electric quadrupole coupling constants. The spectroscopic parameters for all 9 species will be presented and compared.

The pure rotational spectra of 2-chloro-3-fluoropyridine and 2-chloro-6-fluoropyridine were measured on a chirped-pulsed Fourier transform microwave (CP-FTMW) spectrometer in the 8 – 18.5 GHz frequency range. The spectra were analyzed to find the rotational constants of the molecules for both the ³⁵Cl and the ³⁷ Cl isotopologues. The measured rotational transitions exhibit hyperfine splitting, from which the nuclear quadrupole coupling constants have been assigned. The rotational constants and the nuclear quadrupole coupling constants have been compared to ab initio calculations performed using the Gaussian 03W software package.