On the AILES beamline of the SOLEIL synchrotron the HEROES consortium is currently developing new spectrometers based on heterodyne mixing of the THz synchrotron radiation with dedicated laser sources. We report here the first results on one of these spectrometers that aims at exploiting the discrete nature of coherent synchrotron radiation (CSR) in the 100-1000 GHz region, revealed a few years ago by our teamTammaro, S., Pirali, O., Roy, P., Lampin, J.F., Ducournau, G., Cuisset, A., Hindle, F., Mouret, G. "High density terahertz frequency comb produced by coherent synchrotron radiation" Nature Communications., 6: art. 7733. (2015) to perform dual-comb THz spectroscopy. CSR generated by the so called low-α mode of the SOLEIL machine produces a relatively intense, offset-free, high density frequency-comb in the THz range (THz-FC). We will present the details of our preliminary experimental set-up mixing the THz-FC from SOLEIL with an optical comb from Menlo C-fiber femtosecond laser. Pure rotation absorption transitions of acetonitrile in the frequency domain (covering the 100-500 GHz range) as well as time-domain free induction decays (FIDs) were observed allowing to establish the performances of this new instrument. Tammaro, S., Pirali, O., Roy, P., Lampin, J.F., Ducournau, G., Cuisset, A., Hindle, F., Mouret, G. "High density terahertz frequency comb produced by coherent synchrotron radiation" Nature Communications., 6: art. 7733. (2015),

Cavity enhancement of molecular absorption through laser action has made intracavity laser spectroscopy (ILS) a powerful tool for the detection of trace species and weakly absorbing molecules. The effective pathlength for ILS measurements is proportional to the speed of light, leading to a high degree of sensitivity for spectroscopic measurements, and effective pathlengths of up to 70,000 km have been demonstrated with this technique. Fourier-transform spectroscopy (FTS) is a powerful technique for detection of spectroscopic signals, benefitting both from Fellgett’s advantage and inherent calibration derived from the interference zero crossings of a single frequency light source. The traditional dispersive ILS system at the University of Missouri – St. Louis has been integrated with a Bruker IFS-125M FTS spectrometer. The two time-based techniques are synchronized using a National Instruments field-programmable gate array (FPGA). The maximum instrumental resolution for the combined technique is improved by nearly an order of magnitude, from 0.02 cm⁻¹resolution for 2 m monochromator with 9th order diffraction to 0.0035 cm⁻¹with the Bruker FTS. Similarly, the detection bandwidth (7 cm⁻¹per dispersed ILS spectrum) also has improved by an order of magnitude,l0ptFigure enabling the collection of the entire broadband laser profile (50-100 cm⁻¹) in a single measurement. In addition, a 3-fold improvement in absolute wavenumber accuracy is achieved due to the internal calibration of the FTS detection. The improved resolving power of the integrated spectroscopic system enables Doppler-width limited detection of 5d-metal diatomic molecules. These species are of fundamental interest for the quantitation of the relativistic effects that dominate their electronic energy structure, and newly observed transitions of platinum sulfide and tungsten sulfide have already been recorded and analyzed. Details of the instrumentation, integration method, and improved capabilities will be presented.

We report progress on the construction and development of a cryogenic buffer gas beam (CBGB) instrument for studying the IR spectroscopy of atomic and molecular clusters. Clusters are produced via laser ablation of a solid target inside the CBGB cell, and the nascent clusters are quickly cooled to the buffer gas temperature before exiting the CBGB cell. Upon exiting the cell, the clusters are probed with  ∼ 5 μm tunable light from a quantum cascade laser (QCL). Light from signal and reference beams each hit a detector, and the difference signal is collected using an autobalanced subtractor circuit to achieve shot-noise limited measurements. Recent developments in the construction of the instrument will be presented.

Complex organic molecules 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 region are essential for identifying molecules in space. Double resonance techniques can help to solve these challenges. We present first millimeter-wave−millimeter-wave double resonance (DR) spectra to unambiguously assign new pure rotational lines of propanal (C₂H₅CHO). As already shown in chirped pulse Fourier transform microwave (CP-FTMW) experimentsD. Schmitz et al., J. Phys. Chem. Lett. 6 (2015) 1493 the 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 technique (pulse modulation of pump and frequency modulation of 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 and possible future applications, such as automatization and incarnation of baseline-free DR spectroscopy in chirped pulse experiments.

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Footnotes:

D. Schmitz et al., J. Phys. Chem. Lett. 6 (2015) 1493, S. H. Autler and C. H. Townes, Phys. Rev. 100 (1955) 703a

Over 200 molecules have been identified in the interstellar medium (ISM), roughly a third of which are complex organic molecules, carbon-containing molecules with 6 or more atoms. The chemistry in the non-equilibrium environment of the ISM is in the kinetic, rather than thermodynamic limit, and astrochemical models seeking to reproduce the abundances observed in the interstellar medium (ISM) are often limited by a lack of low-temperature experimental data for relevant reactions. We seek to directly measure rate coefficients at temperatures between 20-90 K. Here we will discuss the development of a new instrument that combines the complementary techniques of laser induced fluorescence (LIF) and chirped pulse Fourier transform microwave (CP-FTMW) spectroscopy with the CRESU method (French acronym for "reaction kinetics in a uniform molecular flow") using pulsed laser photolysis to initiate the desired reaction. The derived rate constants and branching ratios can then be incorporated into chemical models, improving the astronomical community's ability to interpret the wealth of data currently being generated by telescopes like ALMA.

Innovations in ion traps have made it possible to control ion chemistry in real time. Currently, our lab studies microsolvated species and reaction intermediates using a home-built dual cryogenic ion mass spectrometer. CIVS, Cryogenic Ion Vibrational Spectroscopy, produces high-resolution IR spectra using action spectroscopy. The dual trap design allows for ion manipulation and tagging. The first trap, a liquid nitrogen cooled octopole reaction trap, is used for the adding solvent molecules to the analyte or performing reaction chemistry to form unstable intermediates. The second trap, a liquid helium cooled main trap, is held at 10K to promote the adherence of a non-perturbative tag such as D₂. At the current moment, the octopole reaction trap is the sole location for ion manipulation. Thus, we are limited to one chemical reaction or the addition of one solvent molecule. Both intermediate formation and solvation are restricted by the time the ions spend in the reaction trap and the gas pressure. To overcome these limitations, we have developed a mass selective, multi-reaction trap setup via a modular housing design. In addition to addressing the limited number of ion manipulation that can occur, the modular design reduces the cost and increases the adaptability. Preliminary results suggest that a prototype of a dual reaction trapping system can form clusters with different solvents. For example, water clusters were made in the first trap and methanol was clustered in the second trap. Thus, the multi-reaction trap design provides solvent position selectivity for ion manipulation, which is of great advantage for simulating more complex environments. For example, future plans for the multi-reaction trap CIVS instrument include characterizing bulk water using the red-shifted IR peaks of D₂O as a molecular probe.

The E-band (60-90 GHz) is a useful spectral region for studying molecules that are highly reactive. These species are particularly interesting as many products from chemical reactions fall within this range. Chirped pulse spectroscopy allows for these products to be detected at fast timescales compatible with reaction kinetics measurements. To accomplish this, we have constructed a new chirped pulse Fourier transform millimeter wave spectrometer operating within the E-band. The spectrometer uses a 300 mW high power broadband amplifier as well as a low noise receiver protected by a fast switch. The system has been tested with a range of stable molecules both at low pressures and in the presence of rare gas colliders to simulate the experimental environment for reaction kinetics studies. The effects of collisions on spectra have been examined, with particular attention being given to pressure broadening and quenching of the free induction decay. The description of this spectrometer as well as results from pressure broadening tests will be presented and applications towards detecting products of chemical reactions will be discussed.

A camera-based three-dimensional (3D) imaging system with a superb time-of-flight (TOF) resolution and multi-hit capability was recently developed for electron/ion imaging [Lee et al. J. Chem. Phys. 141, 221101 (2014)]. In this work, we report further improvement of the event rate of the system by adopting an event-driven camera, Tpx3Cam, for detecting the 2D positions of electrons, while a high-speed digitizer provides highly accurate (~30 ps) TOF information for each event at a rate approaching 1 Mhits/sec.

Microwave spectroscopy has been looking at molecular structures for years but little has been done on mirror design. More and more interest has been growing for studying larger molecules to aid in the study of species of medical and biomedical interests. Here we will present our findings from experiments conducted on aluminum hemispherical mirrors of varying geometries with the same radius of curvature. These tests were conducted using hornless chirp pulse FTMW. The recently designed hornless chirp FTMW exceeds beyond the limitations of normal chirp pulse techniques, and this study seeks to optimize the collection of electromagnetic radiation for this new lower frequency region. Discussions on these results and recent applications of using 3D Printing to produce cost effective hyperbolic mirrors.

Ultrafast extreme ultraviolet (XUV) absorption spectroscopy can probe the dynamics of excited states in first-row transition metals complexes with sensitivity to the oxidation state, spin state, and ligand field of the metal center. This technique is performed with a tabletop instrument and probes the M-edge transitions from the 3p to the 3d orbital of a metal center. XUV absorption spectroscopy has until now been limited to studying solid-state or gas-phase samples due to the short penetration depth of XUV photons and the high-vacuum conditions. To study solution-phase chemistry with XUV absorption, I used a microfluidic chip to generate free-flowing, submicron thick liquid sheets of nonpolar, XUV transmissive solutions. I have characterized the thickness and stability of dichloroethane liquid sheets under vacuum and have adapted our tabletop high-harmonic instrument to maintain high vacuum during liquid flow. This sample delivery method will be used to study the solution-phase excited state dynamics of iron(III) tetraphenyl porphyrin chloride (FeTPPCl). This will help to understand the impact of a solution-phase on the excited state dynamics at the FeTPPCl metal center and elucidate the ability of FeTPPCl to act as a photocatalyst in solution.

We have investigated the photoelectron properties of argon atom in the photon energy range near photoionization threshold, employing a reaction microscope and a newly-built HHG-based EUV laser system. Photoelectron spectra and angular distribution were obtained through momentum imaging for photoelectrons. Seven satellites were resolved besides the 3s and 3p main lines. The asymmetry parameters β were determined for both the main lines and the satellites. It is found that our measured β for the 3p and 3s main lines are in an excellent agreement with previous results reported by Houlgate et al. [1], Dehmer et al. [2], Adam et al. [3], which demonstrates the reliability of the new system and the data analysis procedure. Furthermore, by analyzing the angular distribution of quasi-degenerate satellites, the branch ratio was obtained, which accord closely with theoretical value. The β parameters measured for the satellites are reported for the first time in this energy range, providing benchmark data for testing different theoretical models. [1] R. Houlgate, J. West, K. Codling, and G. Marr, Journal of Electron Spectroscopy and Related Phenomena 9, 205 (1976). [2] J. L. Dehmer, W. A. Chupka, J. Berkowitz, and W. T. Jivery, Phys. Rev. A 12, 1966 (1975). [3] M. Y. Adam, P. Morin, and G. Wendin, Phys. Rev. A 31, 1426 (1985) [4] B. Hai, et al., submitted to Phys. Rev. A (2020)