The marriage of chirped pulse millimeter-wave spectroscopy with a buffer gas cooled molecular beam source has yielded an increase in spectral velocity (number of resolution elements per unit time) of a factor of one million! But it gets even better. Essential information is encoded not just in the frequencies of the transitions, but also in the relative intensities and especially phases of the transitions. Transitions between Rydberg states of atoms and molecules are an ideal test ground for techniques that fully exploit these newly accessible observables.

The 10⁶ increase in spectral velocity allowed by the combination of chirped pulse millimeter-wave spectroscopy with a buffer gas cooled molecular beam source qualitatively expands the classes of possible experiments. As an example, it allows for investigation of cooperative radiation effects (such as superradiance and subradiance) in large samples of atoms in Rydberg states in a single shot. However, these same effects can present obstacles to the application of the full increase in spectral velocity to high-resolution spectroscopic experiments through both frequency (chirps, broadenings, and shifts) and intensity effects.

The spectroscopy of Rydberg states of NO has a long history [1], stimulating both experimental and theoretical advances in our understanding of Rydberg structure and dynamics. The closed-shell ion-core (¹Σ⁺) and small NO⁺ dipole moment result in regular patterns of Rydberg series in the Hund’s case (d) limit, which are well-described by long-range electrostatic models (e.g., [2]). We will present preliminary data on the core-nonpenetrating Rydberg states of NO (orbital angular momentum, l ≥ 3) collected by chirped-pulse millimeter-wave (CPmmW) spectroscopy. Our technique directly detects electronic free induction decay (FID) between Rydberg states with ∆n* ≈ 1 in the region of n*  ∼  40-50, providing a large quantity (12 GHz bandwidth in a single shot) of high quality (resolution  ∼  350 kHz) spectra. Transitions between high-l, core-nonpenetrating Rydberg states act as reporters on the subtle details of the ion-core electric structure. [1] Huber KP. Die Rydberg-Serien im Absorptions-spektrum des NO-Molek·· \textuls. Helv. Phys. Acta 3, 929 (1961). [2] Biernacki DT, Colson SD, Eyler EE. Rotationally resolved double resonance spectra of NO Rydberg states near the first ionization limit. J. Chem. Phys. 88, 2099 (1988).

A BrightSpec chirped-pulsed Fourier transform millimeter-wave spectrometer operating in the 75-110 GHz spectral region has been installed at Argonne National Laboratory. The instrument has been tailored for chemical reaction dynamics and kinetics studies, and the arrangement allows for easy alternation between a room temperature flow cell and a supersonic expansion. The molecular beam is equipped with a pyrolysis nozzle for monitoring reaction products. Benchmark measurements in the flow cell will be presented along with early pyrolysis observations.

C₂S and C₃S are common interstellar species, and have relatively simple reaction chemistries. For these reasons, they frequently serve as probes of chemical evolution and physical conditions in rich astronomical sources. Because their rotational lines are often conspicuous there, detection of C₂S and C₃S in vibrationally-excited states might provide additional insight into formation pathways and excitation conditions. However, knowledge of the vibrational satellite transitions of both species is incomplete. Here, we report laboratory measurements of rotational spectra of vibrationally-excited C₂S and C₃S obtained from two microwave spectral taxonomy studies, in which CS₂ alone or in combination with a hydrocarbon precursor (acetylene or diacetylene), were produced using an electrical discharge. For C₃S, these studies, in combination with high-level quantum chemical calculations, greatly extend previous microwave measurements, while for C₂S, satellite transitions from several vibrational states have been observed for the first time. On the basis of precise laboratory rest frequencies, renewed searches for these transitions can be undertaken with confidence in publicly-available astronomical line surveys.

Chirped - Pulse Fourier-transform microwave spectroscopy in uniform supersonic flows (Chirped- Pulse/Uniform Flow: CPUF) has been applied to study the photodissociation of two atmospherically relevant N containing heterocyclic compounds; pyridine and isoxazole. Products were detected using rotational spectroscopy. HC3N, HCN were observed for pyridine and CH3CN, HCO and HCN were observed for isoxazole and we report the first detection of HNC for both of the systems. Key points in potential energy surface were explored and compared with the experimental observations. Branching ratios were calculated for all the possible channels and will be presented.

Chirped-Pulse Fourier-transform microwave spectroscopy in uniform supersonic flows (CPUF) has been applied to study the reaction of Cl atoms with propyne. The approach utilizes broad-band microwave spectroscopy to extract structural information with MHz resolution and near universal detection, in conjunction with a Laval flow system, which offers thermalized conditions at low temperatures and high number densities. Our previous studies have exploited this approach to obtain multichannel product branching fractions in a number of polyatomic systems, with isomer and often vibrational level specificity. This report highlights an additional capability of the CPUF technique: here, the state-specific reactant depletion is directly monitored on a microsecond timescale. In doing so, a clear dependence on the rotational quantum number K in the rate of the reaction between Cl atoms and propyne is revealed. Future prospects for the technique will be discussed.

r0pt Figure At the present time, in the typical broadband, time domain microwave spectroscopy experiment each free induction decay (FID) collected is on the order of 10⁶ data points in length with a sampling rate on the order of 10⁻¹² seconds per point. Traditionally, the FID is processed using a fast Fourier transform algorithm (FFT) with the resulting power spectrum used in ensuing spectral analyses. For use with the FFT algorithm we have implemented some pre- and post-processing techniques to improve the signal quality. These techniques include the use of Lissajous plots to ensure phase stability in signal addition, novel windowing functions, and also automated broadband phase corrections which allow the absorption spectrum to be used as a more highly resolved version of the traditional power spectrum (see figure). We have also implemented alternatives to the FFT algorithm for time domain signal processing including Hankel singular valued decomposition, a maximum entropy method, and wavelet transformations. Although these techniques are unlikely to be used in place of a fast Fourier transform we will demonstrate how each of these techniques may be used to augment the traditional FFT algorithm in regards to spectral analysis.

We present a performance comparison between chirped pulse millimeter wave spectrometers operating over 75-110, 260-290, and 520-580 GHz. For molecules at room temperature, the line strength has an approximately ν³ dependence until the peak of the Boltzmann distribution (typically in the submillimeter) is reached. However, we find competitive performance for 75-110 GHz spectrometers-with an average sensitivity drop of approximately 3-5 in equal measurement time, compared to a 260-290 GHz instrument with the same excitation power and measurement cell length. The narrower linewidth and lower line density at lower frequency, moreover, increase the usable dynamic range at 75-110 GHz by a factor of approximately 3-10 before reaching the confusion limit, giving better performance for extracting weak lines in a strong forest. This talk will discuss the reasons for and implications of these differences in performance for applications of chirped pulse millimeter wave spectroscopy.

A series of hydrogen sulphide-metal iodide complexes (H₂S-MI, M=Cu, Ag and Au) have been measured via chirped pulse Fourier transform microwave spectroscopy between 7.5-18 GHz. The complexes were generated in a supersonic expansion via laser ablation of the metal and decomposition of CF₃I. Experimental structures were obtained by least squares fitting of structural parameters to the rotational constants of deuterium and metal (⁶³Cu / ⁶⁵Cu and ¹⁰⁷Ag / ¹⁰⁹Ag) isotopologues. Interestingly K₋₁=1 transitions were observed in the spectra containing D₂S, these were not observed in previous studies of similar molecules (H₂S-MCl). This allowed for the determination of an extra rotational constant and, consequently, extra structural information could be obtained. The structures are compared to high level coupled cluster theory calculations.