Technological innovation is a key driver in the field of molecular spectroscopy, and despite recent leaps forward with chirped-pulse spectroscopy, multiplication chains, dual-comb techniques, and mid-infrared lasers, there remains still a sizeable spectral gap, untouched by these innovations, from around 1 to 10 THz. In this terahertz gap it is still difficult to produce high-resolution broadband spectra. The HEROES project aims to fill the gap, by combining a new generation of far-IR molecular laser with synchrotron radiation.Lampin, J.-F. et al. Opt. Lett. (2019) 44, 20Paiges, A. et al. APL Photonics (2016) 1, 031302 The HEROES spectrometer uses heterodyne mixing of a far-IR laser, pumped by a quantum cascade laser, with broadband far-IR synchrotron radiation, within a hot electron bolometer to produce spectra from 1 to 5.5 THz with 5 GHz instantaneous bandwidth and 80 kHz resolution. This resolution is 375 times finer than the ultimate resolution available with the Bruker Fourier-transform infrared (FTIR) spectrometer installed on the AILES beamline at SOLEIL (30 MHz). This allows for far more precise determinations of transition energies, particularly important in the field of astrochemistry. Recent improvements to the HEROES spectrometer will be presented, along with a demonstration of its ability to resolve complicated spectral features, such as transitions in the rotational-torsional spectrum of methanol and its deuterated isotopologues. Further improvements to the technique will also be proposed.

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Lampin, J.-F. et al. Opt. Lett. (2019) 44, 20

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The Rydberg atom spectroscopy, exploited for SI-traceable electrometry, allowed to measure weak microwave electric fields at the μV/cm level. The Doppler-free spectroscopy methods allowed recently significant improvements of the resolution and precision of the measurements with cold trapped HD⁺ molecular ions. The ab-initio molecular ion theory provided accurate predictions of the HD⁺ energy levels and their shifts in external fields. This contribution proposes a new method to characterize a THz electric field that is off-resonantly coupled to the HD⁺ energy levels based on the comparison of the measurements of the lighshifts induced on a two-photon rovibrational transition with the theoretical ab-initio predictions. Precisely, a THz-wave slightly detuned to the Zeeman subcomponents of the (v,L)=(0,0)-(0,1) transition may be characterized by measuring the lightshift of a Zeeman subcomponent of the (v,L)=(0,0)-(2,0) two-photon transition with potential Hz-level uncertainty limited by the molecular ion quantum projection noise. This method allows detecting a THz electric field at the μV/m level from a THz-wave optimally detuned to a hyperfine transition of HD⁺. An algorithm is proposed to retrieve the amplitudes and the phases of a THz electric field in a Cartesian reference frame from six lightshift measurements using two orientations and three values of the static magnetic field in the ion trap. The Cartesian components of the electric field of a THz-wave, with an intensity of 1 W/m², circularly polarized, and detuned to a hyperfine transition of HD⁺, may be characterized with mV/m-level uncertainties for the amplitudes, and 0.1 rad-level for the phases.

The Terahertz (THz) region of the electromagnetic spectrum between the infrared to the microwave domains (0.1 – 10 THz) is often referred to as the “THz gap”. Nevertheless, it has become widely accessible over the last decades by different sources, particularly with multiplication chains. In this context, the HEROES consortium has developed a heterodyne spectrometer based on a far-infrared (FIR) molecular laser pumped by a mid-infrared (MIR) quantum cascade laser (QCL) to address this spectral domain Lampin, J.-F. et al. Opt. Lett. (2019) 44, 20,Pagies, A. et al. APL Photonics (2016) 1, 031302 We present here a laboratory setup mixing on a hot electron bolometer (HEB) the FIR emission from the QCL-pumped molecular laser with a blackbody radiation transmitted by an single-path cell filled with a few μbar of the gaz under investigation. This setup enables to perform high resolution spectroscopy of rotational transitions of small molecules between 1 THz and 3.37 THz with a 5 GHz bandwidth and 80 kHz resolution. We will demonstrate these performances through absorption spectra of methanol at 1 THz and 2 THz along with emission spectra against a cold bakground of methanol and CD₃OD.

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Lampin, J.-F. et al. Opt. Lett. (2019) 44, 20 Pagies, A. et al. APL Photonics (2016) 1, 031302.

The development of Cavity-Enhanced Techniques and Cavity Ring Down Spectroscopy has allowed many sensitive infrared measurements to be undertaken. We have constructed a THz resonator with a high finesse to form the basis of a new instrument for sensitive cavity enhanced THz measurements. The resonator uses a low loss oversized corrugated waveguide and high reflectivity photonic mirrors. An effective path length of one kilometer is obtained for a device that is 50 cm in length. A significant sensitivity improvement has been evaluated at 620 GHz by the measurement of minority isotopologues of OCSFrancis Hindle, Robin Bocquet, Anastasiia Pienkina, Arnaud Cuisset, and Gaël Mouret, Terahertz gas phase spectroscopy using a high finesse Fabry-Pérot cavity Optica (2019) vol 6, 1449-1454 DOI: 10.1364/OPTICA.6.001449 The sensitivity achieved has allowed several centrifugal distortion-induced rotational lines of CF₄ to be recorded and its tetrahedral splitting to be resolved Cuisset A, Hindle F, Mouret G, Bocquet R, Bruckhuisen J, Decker J, Pienkina A, Bray C, Fertein E, Boudon V. Terahertz Rotational Spectroscopy of Greenhouse Gases Using Long Interaction Path-Lengths Applied Sciences (2021) vol 11(3) p 1229. https://doi.org/10.3390/app11031229

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Francis Hindle, Robin Bocquet, Anastasiia Pienkina, Arnaud Cuisset, and Gaël Mouret, Terahertz gas phase spectroscopy using a high finesse Fabry-Pérot cavity Optica (2019) vol 6, 1449-1454 DOI: 10.1364/OPTICA.6.001449. Cuisset A, Hindle F, Mouret G, Bocquet R, Bruckhuisen J, Decker J, Pienkina A, Bray C, Fertein E, Boudon V. Terahertz Rotational Spectroscopy of Greenhouse Gases Using Long Interaction Path-Lengths Applied Sciences (2021) vol 11(3) p 1229. https://doi.org/10.3390/app11031229.

Detection of radicals in the interstellar medium, such as hydroxymethyl (CH₂OH) and methoxy (CH₃O), is a highly interesting tool for better understanding the formation of commonly observed complex organic molecules such as glycolaldehyde, ethanol, ethylene glycol, and dimethyl ether. In this context, improving the predictions of astronomical lines with a well-defined model, based on laboratory measurements, becomes decisive. In an attempt to enrich models of CH₂OH [1] and CH₃O with new frequencies, we have re-investigated their pure rotational spectrum in the millimeter-wave domain. Both radicals were produced at room temperature by fluorine abstraction of hydrogene from methanol. We will report the technical improvements that have been made to increase both the sensitivity and the signal-to-noise ratio of our experimental set-up [1]. We suceeded to increase synthesis yield of both radicals by multiplying fluorine injections and we also further improved our optical set-up, now using a double passage of the beam in the cell. Finally, we wrapped an induction coil around the cell to create a magnetic field, in addition to the usual frequency modulation, allowing us to operate using double modulation detection scheme thus making us sensitive to species affected by Zeeman splitting. Both signals (single and double modulation) are recovered separately and displayed simultaneously on the measurement software. A strength of the double modulation scheme is that only lines arising from open-shell molecules, such as CH₂OH and CH₃O, are visible over a flat baseline (no residual Fabry-Perot interference fringes). These improvements significantly increase our sensitivity to short-lifetime species and grant a powerful tool for distinguishing radicals from stable molecules.

[1] O. Chitarra et al., Astronomy and Astrophysics (2020) 644, A 123

Optically-pumped molecular THz lasers are powerful and versatile sources. We have demonstrated a continuous wave molecular laser emission based on ammonia () optically pumped by a quantum cascade laser at about 1 THz A. Pagies, G. Ducournau, and J.-F. Lampin, “Low-threshold terahertz molecular laser optically pumped by a quantum cascade laser,” APL Photonics, vol. 1, 031302, 2016. The purpose of this work is to investigate laser transitions at higher frequencies. This molecule is pumped by the pump laser from the ground state to the v₂ = 1 vibrational state. Within the excited state, two types of THz transitions can occur in this molecule: pure inversion transitions and rotation-inversion transitions. Inversion motion is a particular quantum phenomenon of the molecule that splits the energy levels: the N atom is able to tunnel through the potential barrier formed by the three H triangle. The inversion splitting is about 1 THz in the v₂ = 1 state but rotation-inversion transitions can be used to generate higher frequencies. The investigation of molecular laser shows a lack of available databases of potentially lasing transitions for users to exploit. We propose a molecule dependent figure of merit which enables to discriminate potentially lasing transitions by their lasing potential. We report here its use to predict lasing lines of ammonia. The demonstration of its pertinence is made by observing 32 lasing lines of and 5 lines of up to 5.5 THz. This work is supported by the ANR project HEROES (ANR-16-CE30-0020), the “Photonics for Society” CPER and the Interreg project TERAFOOD.

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A. Pagies, G. Ducournau, and J.-F. Lampin, “Low-threshold terahertz molecular laser optically pumped by a quantum cascade laser,” APL Photonics, vol. 1, 031302, 2016..

Building off the previous works of Schnell, Patterson, and Pate, a new microwave three-wave mixing (M3WM) spectrometer was constructed and tested at the Missouri University of Science and Technology. This new instrument consists of a four-horn design, allowing traditional CP-FTMW experiments to be performed in addition to M3WM experiments. Within this presentation, the design, construction, and demonstration of the instrument's capabilities using carvone and preliminary work on 2-bromo-1,1,1,2-tetrafluoroethane will be discussed. Preliminary work on chirality detection and dipole forbidden transition analysis utilizing M3WM techniques will also briefly be presented.

The introduction by Pate and coworkersBrown, G. G.; Dian, B. C.; Douglass, K. O.; Geyer, S. M.; Shipman, S. T.; Pate, B. H. A Broadband Fourier Transform Microwave Spectrometer Based on Chirped Pulse Excitation. Rev. Sci. Instrum. 2008, 79 (5), 053103. https://doi.org/10.1063/1.2919120.f broadband capabilities in rotational spectrometers revolutionized the field and widened the scope of applications of rotational spectroscopy.Park, G. B.; Field, R. W. Perspective: The First Ten Years of Broadband Chirped Pulse Fourier Transform Microwave Spectroscopy. J. Chem. Phys. 2016, 144 (20), 200901. https://doi.org/10.1063/1.4952762.ince its introduction, many different experimental setups have been reported depending on the end application. In most cases, the bandwidth is restricted to that of commercially available electronic components (mainly high-power amplifiers). The most common setups operate, thus, in limited bands of 2-8, 8-18, 18-26GHz, etc., and the collection in different bands entails the recording of the spectrum several times with the consequent increase in time and sample consumption. Here we present a new experimental approach that allows for the collection of two of those otherwise separate bands in a single measurement, which reduces the time and sample consumption by half. We use two dual-polarization broadband horn antennae to simultaneously polarize the molecular sample in the 2-8 GHz (horizontal polarization) and 8-12 GHz (vertical polarization). The molecular emission is then combined and directly collected on a fast oscilloscope as in a traditional broadband experiment. We will show the performance of this setup compared to separated measurements, which allows for a faster collection of the whole frequency range without compromising the sensitivity. This approach only requires minimal modifications to the current spectrometers.

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Brown, G. G.; Dian, B. C.; Douglass, K. O.; Geyer, S. M.; Shipman, S. T.; Pate, B. H. A Broadband Fourier Transform Microwave Spectrometer Based on Chirped Pulse Excitation. Rev. Sci. Instrum. 2008, 79 (5), 053103. https://doi.org/10.1063/1.2919120.o Park, G. B.; Field, R. W. Perspective: The First Ten Years of Broadband Chirped Pulse Fourier Transform Microwave Spectroscopy. J. Chem. Phys. 2016, 144 (20), 200901. https://doi.org/10.1063/1.4952762.S

The CPUF (Chirped Pulse in Uniform supersonic Flow) technique is a combination of the CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme, or Reaction Kinetics in Uniform Supersonic Flow) technique to provide a very low temperature (down to 5 K) gas-phase collisional environment with chirped pulse Fourier transform mm-wave/microwave (CPFTMW) spectroscopic detection. The goal of the current CRESUCHIRP project is to determine branching ratios of astrochemically relevant bimolecular reactions at low temperatures using the CPUF technique. Two new CPFTMW spectrometers have been built and characterized in the Ka band (26.5-40 GHz) and the E-band (60-90 GHz). While reaction products have been successfully detected at temperatures down to 10 K in continuous CRESU flows in Rennes, the sensitivity of the CPFTMW technique is reduced by collisional broadening in the relatively high pressure CRESU flows. Reducing the collisional frequency rate is critical, and one of the methods that have been developed to achieve this aim is molecular beam sampling of the uniform flow via a skimmer to create an expansion into a much higher vacuum environment where both the temperature and pressure of the gas will be lowered, creating favorable conditions for the detection of the molecular signal. The latest results from these experiments will be presented.

At the 74th ISMS, our group showed that it is possible to detect microwave radiation in a CP-FTMW experiment that did not copropagate in the direction of incident radiation. In that talk, it was shown that this could be achieved with the broadcast antenna by the implementation of a circulator, switch, and low noise amplifier in order to also detect off of the same antenna. Coincidentally, the circulator that was utilized attenuated the input power below a level where this detection would be advantageous to the overall signal-to-noise ratio (SNR) of the collected free induction decays (FIDs). However, this discovery provided both the preliminary data and the impetus for a new hypothesis to be tested: Could FIDs be detected at other points not in the direct linear path of the incident microwaves? To address this, we utilized our new microwave three-wave mixing (M3WM) instrument based upon the four-antennae design of Pate in order to utilize the orthogonality of the setup for the purposes of CP-FTMW FID collection. The design of the new four-antennae MAD-CP-FTMW at Missouri S&T will be presented along with the exciting results of detection in traditional, 180 degree, and quadrature angles. Additional testing, done to mitigate the concern of reflection voiced in the original presentation, will also be discussed. Additional information concerning this spectrometer's design and functionality, as well as further details related to data processing, will be discussed in presentations given by Nicole Moon, and Christian Swanson, respectively.

While previous iterations of laser ablation designs have been attempted at Missouri S&T, none had the capability to adjust the location of the nozzle in relation to the horn antenna in situ without breaking vacuum. This design serves two purposes. The first is to optimize signal strength in real time and the second is to make improvements on spatial and mass efficiency. The source was designed as a prototype for current and future CP-FTMW instruments primarily concerned with studying metal-containing molecules. Aspects of the design as well as the implementation and viability of the source in one of the CP-FTMW instruments will be discussed.

To improve sensitivity of absorption spectrometers a source modulation with lock-in detection is usually employed. In the Kharkiv millimeter wave spectrometer we employ DDS based square-wave frequency modulation which is characterized by a high certainty of modulation parameters. Commonly applied approaches developed for a sine-wave frequency modulation (see for example [1]) are not able to fully account for all lineshape modifications due to the square-wave frequency modulation. That is why we have developed a new expression which allows us to take into account lineshape modification due to square-wave frequency modulation as well as well-known problem of lineshape distortions caused by presence of standing wave in an absorbing cell. Our approach reproduce well observed line profiles and as the result provides some improvement of measurement accuracy. In the talk the details of the new approach will be discussed. [1] L. Dore, J. Mol. Spectrosc. 221, pp.93-98, 2003.

Complex organic molecules (COMs) show regularly very rich spectra, sometimes at a line density close to the confusion limit. The plethora of lines often originates from the presence of different conformers and/or low-lying vibrational states. The analysis of spectra may considerably be further complicated by vibration-rotation or other interactions. However, accurate spectroscopic predictions in the millimeter-wave (MMW) region are essential for identifying molecules in space. Double-resonance techniques can help to solve these challenges. We present first MMW−MMW double-resonance (DR) spectra to unambiguously assign weak and perturbed rotational lines of propanal (C₂H₅CHO). As already shown in chirped pulse Fourier transform microwave (CP-FTMW)D. Schmitz et al., J. Phys. Chem. Lett. 6 (2015) 1493nd in MW−MMW DR experiments,K. M. Roenitz et al., J. Phys. Chem. A. 122 (2018) 6321he Autler-Townes splittingS. H. Autler and C. H. Townes, Phys. Rev. 100 (1955) 703llows for distinguishing between regressive or progressive energy level schemes. The Autler-Townes splitting is clearly visible in our 2D spectra. Furthermore, implementation of a double-modulation double-resonance (DM-DR) technique, pulse modulation of the pump and frequency modulation of the probe source, allows for confusion- and baseline-free spectra containing only the line(s) of interest. We discuss details of the observed Autler-Townes splitting, benefits of simplified spectra, and possible future applications, such as automatization.

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D. Schmitz et al., J. Phys. Chem. Lett. 6 (2015) 1493a K. M. Roenitz et al., J. Phys. Chem. A. 122 (2018) 6321t S. H. Autler and C. H. Townes, Phys. Rev. 100 (1955) 703a

This presentation describes the application of a miniature millimeter-wave rotational spectrometer to the detection of laser-ablation products. The laboratory experiments we present are forerunners to possible future field- or space deployments. We focus on NaCl and KCl salts, which are interesting targets at the icy moons of the outer solar system. The laser-coupled spectrometer incorporates a collisional-cooling, pulsed carrier gas beam. We leverage the small size of the spectrometer cavity to probe the ablation product signal along the beam length. We find that the volatilized species are widely dispersed in the carrier-gas. Additionally, we present centimeter-wave rotational and mass spectroscopic measurements to help characterize the laser ablation process.