In this talk, I will discuss the growing field of entangled-photon spectroscopy as related to molecular systems. An entangled state is represented by a wavefunction, describing a group of particles, which cannot be factored into separate states of individual particles. For photons, these correlations lead to non-classical light-matter interactions, creating new opportunities for spectroscopic techniques. For example, second-order processes with two entangled photons, such as two-photon absorption or sum frequency generation, occur at a linear rate with a cross section that should be close to that of one-photon classical processes. However, early results in the field have produced conflicting measurements as to its magnitude. I will discuss the role of excited state dephasing and time-ordering for entangled photon interactions and their role in producing conflicting interaction cross sections for the same molecule on different experimental setups. I will also briefly remark on other interesting prospects of entangled photons, such as their ability to break the temporal-spectral Fourier transform limit. For instance, few-femtosecond spectroscopy is predicted to be possible with entangled light sources of MHz linewidths.

We introduce a novel and sensitive ns-resolved atomic magnetometer, which is at least three orders of magnitude faster than conventional magnetometers. The magnetic field dependence of hyperfine beating of high-density spin-polarized H atoms, produced from the rapid photodissociation of HCl gas with sub-ns laser pulses, results in a few nT sensitivity for a spin-projection limited sensor with 10 nl measurement volume after 1 ns measurement time. The magnetometer will allow ultrafast continuous B-field measurements in many fields, including spin chemistry, spin physics, and plasma physics.

r0pt Figure Detection of trace amounts of molecules in multi-species mixtures is important for many purposes, ranging from chemical dynamics in complex environments to breath analysis. 2DIR spectroscopy is a powerful tool for studying both molecular structure and dynamics. It can provide better selectivity than linear spectroscopy due to the extra degrees of freedom for distinguishing the components of a complex mixture, but its application to trace-gas detection has been hindered by insufficient sensitivity and resolution of the technique. Cavity-enhancement methods for ultrafast nonlinear spectroscopy using frequency combs have the potential to overcome this problemT. K. Allison. J. Phys. B 50, 044004 (2017). In 2DIR spectroscopy of coupled vibrational modes, it is common to use specific beam polarization angles to suppress parts of the molecular response and simplify structure determinationM. T. Zanni, N. Ge, Y. S. Kim, R. M. Hochstrasser. Proc. Nat. Acad. Sci. 98, 11265 (2001). Here we show how to suppress parts of the rotational response of a single vibrational mode. We provide specific polarization angles that suppress diagonal or antidiagonal peaks, or the pathways with interstate coherences after the second excitation. We present simulations for carbon monoxide that demonstrate the effect of eliminating specific pathways on 2DIR rotationally-resolved spectra.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   

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Footnotes:

T. K. Allison. J. Phys. B 50, 044004 (2017).. M. T. Zanni, N. Ge, Y. S. Kim, R. M. Hochstrasser. Proc. Nat. Acad. Sci. 98, 11265 (2001)..

High resolution coherent 2D and 3D spectroscopies are unusually powerful tools for overcoming problems of spectral congestion because they can 1) automatically sort peaks by rotational and vibrational quantum number, 2) spread out peaks into higher dimensions in order to improve resolution, and 3) limit the number of possible peaks that are produced at higher dimensions through the use of parametric and nonparametric four wave mixing processes. This talk will describe new multidimensional pattern recognition techniques that can be used with high resolution coherent multidimensional spectroscopy.

Of the roughly 210 species detected outside our Solar System, about a quarter of them are radicals. Radicals are sensitive probes of their chemical and physical environments and have been implicated in the formation of complex organic molecules (COMs). Astronomical observations of these species with mm-wave telescopes such as the Atacama Large Millimeter Array depend upon accurate rest frequencies of rotational transitions derived from laboratory spectroscopic measurements. Owing to uncertainties in theoretical rotational constants, centrifugal distortion, and angular momentum coupling, identification of a new radical from direct mm-wave spectroscopy is challenging in the absence of cm-band measurements at low J, requiring extensive frequency searches. We aim to determine mm-wave transition frequencies indirectly through rovibrational spectroscopy, which substantially alleviates the frequency search challenges of pure rotational spectroscopy in the mm-wave region. However, the much greater Doppler broadening at higher optical frequencies has traditionally prevented rovibrational spectroscopy from achieving the  1 MHz accuracy required for astronomical applications. To overcome this, we are combining cavity enhanced frequency modulation spectroscopy with an AC magnetic field generated by a solenoid to achieve a sensitive, radical-selective, and Doppler-free technique that we call NICE-OHZMS (Noise Immune Cavity Enhanced Optical Heterodyne Zeeman Modulation Spectroscopy). In conjunction with absolute frequency calibration afforded by an optical frequency comb, we expect to measure rovibrational transitions of radicals with an accuracy and precision of better than 1 MHz. Here, the status of the instrument and proof-of-concept measurements of the first overtone band of nitric oxide will be discussed.

Super-resolution spectroscopy is essential for the understanding of molecular fine structures, excited state population, distribution function of given energy states, and remote sensing applications. We addressed several key measures for achieving high performances of sprectral resolution and absorbance precision in a mid-infrared modulated laser spectrometer with a distributed feedback interband cascade laser, including linearization of laser scanning, suppression or deduction of optical fringe interferences, and internal calibration for baseline problems and nonlinear response of the photodiode used in the system. The performances of our modified spectrometer were verified with the spectral resolution in the order of 10⁻⁶ cm⁻¹ (~hundreds kHz) and absorbance uncertainty in the order of 10⁻³. We provided a detailed error analysis with the uncertainty model of GUM, i.e. Guide to the expression of uncertainty in measurement. Further, we ensured the traceability of the spectrometer by linking a practical calibration reference standard. We measured the fundamental absorption of several organic molecules, e.g. dimethyl sulfide (CH₃)₂S), methyl mercaptan (CH₃SH) with our modified spectrometer. The recorded spectra were demonstrated to be similar to that in the Pacific Northwest National Laboratory (PNNL) database. These data have been used in our remote sensing applications.

The Far-Infrared beamline at the Canadian Light Source (CLS), provides access to High-Resolution Spectroscopy to researchers across Canada and around the world. The primary, instrument on the beamline is a Bruker IFS125HR Spectrometer capable of better than 0.001 cm⁻¹ resolution. By exploiting the high brightness of the synchrotron source the beamline can achieve signal to noise levels up to 8 times better than that achievable using a thermal source. Equipped with a temperature-controlled, 2-meter multi-pass gas cell with a maximum pathlength of 72 meters, a 1.5-meter glow discharge cell, the Far-Infrared facility is well equipped for high-resolution molecular spectroscopy. A horizontal microscope is available for micro-spectroscopy experiments. This talk will provide details on the available equipment, techniques, and the methods of access at the Far-Infrared as well as some examples of recent research projects on the beamline.

We demonstrate a laser system providing user-designed laser pulses with durations tunable from 10 ns to 1 μs and pulse energies up to 680 mJ at a repetition rate of 25 Hz and at a wavelength of 1064 nm. This radiation is frequency doubled in an LBO crystal yielding pulses of 532 nm radiation with pulse energies of 90 mJ. The frequency-doubled laser is used as a pump source for a dye amplification line seeded by the output of a commercial continuous-wave ring dye laser. This system enables us to generate 10 ns to 120 ns long pulses with a Gaussian temporal profile in the visible range of the electromagnetic spectrum with output energies of up to 5 mJ per pulse. We also generate long pulses of UV radiation (pulse energies up to 500 μJ, pulse length up to 85 ns) by frequency doubling in a BBO crystal.

The properties of the laser system are illustrated in measurements of the Rydberg spectrum of atomic argon in the 3p⁵4s[3/2]₂ ³P₂ and 3p⁵4s′[1/2]₀ ³P₀ metastable states. Metastable argon is generated in a supersonic beam using a dielectric barrier discharge. We have recorded spectra of Rydberg series of Ar below the ²P_3/2 and ²P_1/2 ionization thresholds. We use the long lifetimes of the former series to characterize the bandwith of the laser system and determine the autoionization linewidth and line shapes of the later series for comparison with earlier work.V. L. Sukhorukov, I. D. Petrov, M. Schäfer, F. Merkt, M.-W. Ruf, and H. Hotop, J. Phys. B: At. Mol. Opt. Phys. 45, 092001 (2012) and references therein.html:<hr /><h3>Footnotes: V. L. Sukhorukov, I. D. Petrov, M. Schäfer, F. Merkt, M.-W. Ruf, and H. Hotop, J. Phys. B: At. Mol. Opt. Phys. 45, 092001 (2012) and references therein.

Since their inception, charged-particle imaging techniques have transformed how chemical reactions and dynamics are studied. Ion imaging is a powerful detection method for many as it allows for higher resolution measurements for photoion and photoelectron spectroscopic experiments and photodissociation dynamics. Although there have been many experimental innovations over the last two decades, major advancements in how we analyze these images are few and far between. A general weakness found in ion imaging experiments is low signal-to-noise ratios, which are intrisic to all single particle detection methods. Motivated by the development of convolutional neural networks (CNN) for general imaging applications, we explored the ability of using CNNs for ion imaging reconstruction with the goal to parametrize a neural network to learn the approximate mapping between the ion image projection and its central distribution. We demonstrate the effectiveness of our model to not only reduce noise in the reconstructed image but also produce sharp images. We also compare with standard image reconstruction methods to quantitatively show the improvements by our method on real spectroscopic images. We have also made the model publicly available on Github.

Benzonitrile was recently detected in the interstellar medium and currently remains the largest aromatic molecule identified. Subsequent reactions with benzonitrile and related cations are good candidates to form polycyclic aromatic molecules (PAHs), potentially contributing to the UIB band. Previous investigations into the reactions of the three distonic radical isomers of the benzonitrile cation with ethylene and acetylene have identified that these reactions are barrierless and should proceed within the ultra-low temperatures of the interstellar medium – but what products do these reactions form? Expanding on this previous work, the reactions of the 2-dehydrobenzonitrileH⁺ distonic radical cation + propyne (CHCCH₃) were investigated using a combination of computational and experimental techniques. Experimental investigations, performed within the room temperature linear ion-trap reveal that multiple product channels arise, ranging from: C₃H₄ addition [M + 40]⁺, C₂H₃ loss following C₃H₄ addition [M + 40 - 27]⁺, double C₃H₄ addition [M + 40 + 40]⁺, and H-atom loss following secondary C₃H₄ addition [M + 40 + 40 - 1]⁺. Targeting the double C₃H₄ addition, calculations suggest that two tri-cyclic PAH isomers are energetically favourable products. To assign these putative tri-cyclic PAHs, photodissociation action spectroscopy experiments were performed to structurally characterise these secondary addition products. Exploiting the ion trap, these reaction products can be selectively isolated, stored and then subjected to tunable laser photons. Photodissociation action spectra are recorded by tracking the yield of photoproduct ions as a function of photon energy. Spectra are compared to Franck-Condon simulations and good matches are found with the two tri-cylcic PAH products. These experiments allow us to definitely assign the reaction products, giving us a new measurement to benchmark calculations against.

Innovations in cryogenic ion traps greatly enhance our ability to control ion-neutral chemistry and spectroscopically probe ionic species in real time. Currently, our lab studies microsolvated species and reaction intermediates using a home-built dual cryogenic ion mass spectrometer. The dual trap design allows for controlled ion manipulation and subsequent tagging in preparation for infrared action spectroscopy. The first trap, a liquid nitrogen cooled octupole “reaction trap”, is used for the condensation of solvent molecules to the analyte or to perform temperature-controlled ion-neutral reaction chemistry to access unstable intermediates. The second “tagging” trap is held at 10K to cluster weakly perturbative tags such as D₂, which can serve as messengers for the subsequent action spectroscopy step. Currently, the “reaction trap” is the sole location for ion manipulation which limits us to one chemical reaction or the addition of a single type of solvent molecules. To overcome these limitations, we have developed a mass selective, multi-reaction trap setup via a modular housing design. The modular design also reduces the cost and increases the adaptability. In addition, the ion traps utilize digital ion technology (DIT) to facilitate mass-selection, improve the signal-to-noise and increase the mass range for the study of larger clusters. Preliminary results suggest that a prototype of a dual reaction trapping system can form clusters with two different solvents. For example, water clusters were made in the first trap, mass-selected and methanol was added to the cluster in the second trap. Thus, the multi-reaction trap design provides solvent position selectivity which is of great advantage for simulating more complex environments. Future plans for the multi-reaction trap CIVS instrument include characterizing a water network around an ion by inserting a D₂O as a spectroscopic molecular probe.

Weak emission signals are usually measured and analyzed with one or more sensitive detectors, such as a photomultiplier tube, to obtain sufficient signal from fluorescent samples. Here we demonstrate the measurement of fluorescence from solutions of Rhodamine 6G in ethanol for optical excitation at 532 nm without a photomultiplier, but with a much less sensitive spectrometer (Ocean Optics OCEAN-FX-XR1-ES) and a Software Implementation of a Multi-Channel, Multi-Frequency Lock-in Amplifier (SILIA). SILIA is a software implementation of a multi-channel, multi-frequency Lock-in Amplifier to extract modulated signals from noisy data with arbitrary dimensionality and number of channels. It emulates the functionality of a multi-channel, multi-frequency lock-in amplifier in a post-processing step following data acquisition. Unlike most traditional lock-in amplifiers, SILIA can work with any number of input channels and is especially useful to analyze data with high dimensionality. Since a photomultipler tube was not used, the observed spectrum contains spectral information from scatter photons of the pulsed 532 nm laser light and fluorescent signal of Rhodamine 6G onto the spectrometer. Without SILIA, this pollution renders the fluorescent signal indistinguishable from the background signal. Present the methodology behind SILIA and demonstrate its application for extracting weak signals in spectroscopy. We also discuss more general applications and exhibit a method to automatically estimate error from a lock-in result.

Spectra of complex molecules are dense and complicated, especially if isotopologues, low-lying vibrationally excited states, hyperfine structure or other interactions are present. Analysis of these spectra can be difficult due to line confusion. One approach to accommodate this challenge are Loomis-Wood plots (LWPs), which are a visual aid for displaying series of transitions in a spectrum in order to ease assignments. Programs utilizing LWPs exist already in the literature, e.g. AABS [1], Pgopher [2] and LWW [3]. Here, we present a newly developed software which focuses on being intuitive and user friendly while simultaneously allowing for fast and confident assignments of molecular spectra. The software is called LLWP and is written in Python. The core functionality and selected features are presented on the example of first results of the analysis of isotopically enriched propanone-¹³C₁ (¹³CH₃COCH₃), which was synthesized as its signal at natural abundance only allowed for a very limited analysis [4, 5]. The software and its full documentation are available at ltotheois.github.io/LLWP. [1] Z. Kisiel et al., J. Mol. Spectrosc. 233 (2005) 231 – 243. [2] C. M. Western, J. Quant. Spectrosc. Radiat. Transf. 186 (2017) 221 – 242. [3] W. Lodyga et al., J. Mol. Spectrosc. 243 (2007) 182 – 188. [4] F. J. Lovas and P. Groner, J. Mol. Spectrosc. 236 (2006) 173. [5] M. H. Ordu et al., Astron. Astrophys. 629 (2019) A72.

The spatial resolution of the ps Electric Field Induced Second Harmonic (EFISH) generation has been enhanced by using a non-collinear laser beam phase matching geometry. This approach is similar to the one used in Unstable Resonator – Enhanced Detection Coherent Anti-Stokes Raman Scattering (USED CARS). Briefly, the pump laser beam (1064 nm, pulse duration 150 ps, pulse energy 20-35 mJ) is separated into two coaxial beams before the focusing lens. The two beams, with the combined pulse energy of 10-18 mJ, overlap only near the focal point, generating the Spatially Enhanced EFISH (SEEFISH) signal over a significantly shorter region compared to that in the collinear phase matching. The resulting SEEFISH signal is spatially isolated from the “conventional” EFISH signal and measured by a photomultiplier detector. Blocking of either of the two beams results in a complete suppression of the signal, demonstrating that it is generated by the superposition of the two beams overlapping near the focal point. Measurements of a known Laplacian field generated between two parallel cylinder electrodes in ambient air demonstrated that SEEFISH improves the spatial resolution of the measurements by well over a factor of 2. The spatial resolution is improved further by reducing the focal distance of the lens. As expected, the magnitude of the SEEFISH signal is lower compared to that of the “conventional” EFISH, by a factor of 2-10, depending on the lens focal distance. The estimated spatial resolution of the SEEFISH diagnostic is approximately 1 mm, in the direction of the laser beam. The SEEFISH spatial resolution can be improved further, potentially to  ∼  0.1 mm, by using two independently focused, crossed laser beams. However, this would result in a further reduction of the signal, which may necessitate the use of a fs pump laser. SEEFISH measurements of the electric field distribution in an atmospheric pressure argon plasma jet are underway.

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