Radical species play an important role in various chemical processes spanning atmospheric chemistry (e.g. ozone formation), bio-medical science, and combustion. These highly reactive chemicals usually occur at very low concentration levels, and are difficult to quantify in experiments¹. Generally, laser-based techniques rely on careful selection of the target transition to minimize spectral interference and achieve high selectivity. In case of complex gas mixtures (such as air) a possibility of spectral interference always exists. Since Faraday rotation spectroscopy (FRS) is sensitive only to paramagnetic species (radicals), it can simultaneously provide ultra-high sensitivity and selectivity. In this talk an overview of novel designs of FRS instrumentation as well as applications of FRS sensing will be provided. Examples will be given for FRS systems that routinely operate at the fundamental limits of optical detection, cavity-enhanced FRS detection schemes for sensitivity enhancement towards sub-pptv detection limits², and high-accuracy FRS spectrometers designed specifically for ratiometry of nitrogen isotopes (¹⁴N, ¹⁵N)³. Prospects for the FRS technology to monitor important atmospheric molecules such as HOx radicals (atmospheric "cleansing" agents) will be discussed. References: 1. Wennberg et al., Äircraft-borne, laser-induced fluorescence instrument for the in situ detection of hydroxyl and hydroperoxyl radicals," Rev. Sci. Instrum. 65, 1858-1876 (1994). 2. Westberg et al., Öptical feedback cavity-enhanced Faraday rotation spectroscopy for oxygen detection," in CES2015(Boulder, CO, 2015). 3. Zhang, "Nitric Oxide Isotopic Analyzer Based on a Compact Dual-Modulation Faraday Rotation Spectrometer," Sensors 15, 25992 (2015).

The relationship between the molecular structure and rotational frequencies makes rotational spectroscopy highly structural specific and an ideal tool for complex mixture analysis. The modern developments in broadband microwave techniques have immensely reduced the data acquisition time, while creating a need for high speed data analysis procedures. A new microwave-microwave double resonance method will be introduced, to perform single conformer/isomer microwave spectroscopy in complex chemical mixtures. The method combines the selective excitation schemes possible in chirped pulse microwave spectroscopy with multi-resonance effects observed upon sweeping in the rapid adiabatic passage regime, enabling perturbations to be induced in the intensities of most of the transitions ascribable to a single molecular constituent (e.g. a conformational isomer) in a mixture. Details of the method, experimental implementation and future challenges will be discussed.

There are many circumstances in modern microwave spectroscopy where the observed spectra contain contributions from many distinct sub-populations, creating a complicated spectrum with interleaved transitions due to its components making spectral assignment challenging. A new method, exploiting multi resonance effects with broadband CP-FTMW was developed and implemented to differentiate the structural isomers: (E)- and (Z)-phenylvinylnitrile. This method will output an exclusive set of isomer-specific transitions reducing the spectral assignment time. Details of the method implementation and structural analysis of the two-isomer mixture will be discussed. The application of the method to other circumstances where selective modulation of the transitions due to a single set of connected transitions is vital for complex spectral assignment, will also be considered.

Perturbations of the bare O2- and O4- electronic structure arising from VOC (VOC = hexane, isoprene, benzene and benzene.D6) interactions are investigated using anion photoelectron imaging at 2.33 and 3.49 eV photon energies. Trends observed from comparing features in the spectra include VOC-identity-dependent electron affinities of the VOC complexes relative to the bare oxide clusters, due to enhance stability in the anion complex relative to the neutral. Autodetachment is observed in all O4-.VOC spectra and only isoprene with O2-. In addition, the intensities of transitions to states correlated with the singlet states of O2 neutral via detachment from the O2-.VOC anion complexes show dramatic VOC-identity variations. Most notably, benzene as a complex partner significantly enhances these transitions relative to O2- and O2-.hexane. A less significant enhancement is also observed in the O2-.isoprene complex. This enhancement may be due to the presence of low-lying triplet states in the complex partners.

We present temperature dependent cross sections of propene (C₃H₆; CH₂-CH-CH₃, propylene), which was detected in the stratosphere of Titan.C. A. Nixon, et al., Astrophys. J. Lett., 776, L14 (2013).or this study, a series of high-resolution (0.0022 cm⁻¹) spectra of pure and N₂-mixture samples were recorded at 150 – 296 K in the 650 – 1530 cm⁻¹(6.5 – 15.3 μm) at the Jet Propulsion Laboratory using a Fourier-transform spectrometer and a custom-designed cold cellA.W. Mantz, K. Sung, et al. 65th Symposium on Molecular Spectroscopy, Columbus, OH, 2010.K. Sung, A.W. Mantz, et al., J. Mol. Spectrosc. 262, 122 – 134 (2010).. The observed spectral features cover the strongest band (ν₁₉) with its outstanding Q-branch peak at 912 cm⁻¹and three other strong bands: ν₁₈, ν₁₆ and ν₇ at 990, 1442, and 1459 cm⁻¹, respectively. In addition, we have generated a HITRAN-format empirical ‘pseudoline list' consisting of line positions, intensities, and effective lower state energies, which were determined by fitting all the observed propene spectra simultaneously. A newly derived partition function was used in the analysis. The results are compared with early work from relatively warm temperatures (278 – 323 K).Research described in this talk was performed at the Jet Propulsion Laboratory, California Institute of Technology, Connecticut College, and NASA Langley Research Center under contracts and cooperative agreements with the National Aeronautics and Space Administration.html:<hr /><h3>Footnotes: C. A. Nixon, et al., Astrophys. J. Lett., 776, L14 (2013).F A.W. Mantz, K. Sung, et al. 65th Symposium on Molecular Spectroscopy, Columbus, OH, 2010.

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Research described in this talk was performed at the Jet Propulsion Laboratory, California Institute of Technology, Connecticut College, and NASA Langley Research Center under contracts and cooperative agreements with the National Aeronautics and Space Administration.

The atmosphere of Saturn’s moon Titan is replete with hydrocarbons and nitriles, but knowledge of the formation and sink processes as well as the identities of molecules on the large end of photochemical models of the atmosphere is very limited. 3-phenyl-2-propynenitrile (Ph-C ≡ C-C ≡ N) is of potential importance in this atmosphere because it is a likely product of photochemical reaction between cyanoacetylene and benzene, bringing together two of the key functional groups in Titan’s atmosphere in a single molecule. We present the UV spectrum of this molecule in the gas phase, under jet-cooled conditions, using 2-color resonant two-photon ionization. The spectrum was recorded from 292-208nm, taking advantage of the wide tunability of a BBO-based OPO as the excitation source. On its long wavelength end, the spectrum has sharp transitions arising from a ΠΠ* transition characteristic of a phenyl derivative, while deeper into the UV the spectrum is broadened in a manner reminiscent of cyanoacteylene.

The phenomenon of collisional transfer of intensity due to line mixing has an increasing importance for atmospheric monitoring. From a theoretical point of view, all relevant information about the collisional processes is contained in the relaxation matrix where the diagonal elements give half-widths and shifts, and the off-diagonal elements correspond to line interferences. For simple systems such as those consisting of diatom-atom or diatom-diatom, accurate fully quantum calculations based on interaction potentials are feasible. However, fully quantum calculations become unrealistic for more complex systems. On the other hand, the semi-classical Robert-Bonamy formalism, which has been widely used to calculate half-widths and shifts for decades, fails in calculating the off-diagonal matrix elements resulting from applying the isolated line approximation. As a result, in order to simulate atmospheric spectra where the effects from line mixing are important, semi-empirical fitting or scaling laws such as the energy corrected sudden (ECS) and the infinite order sudden (IOS) models are commonly used. Recently, we have found that in developing this semi-classical line shape theory, to rely on the isolated line approximation is not necessary. By eliminating this unjustified assumption, and accurately evaluating matrix elements of the exponential operators, we have developed a more capable formalism that enables one not only to reduce uncertainties for calculated half-widths and shifts, but also to calculate the whole relaxation matrix. This implies that we can address the line mixing with the semi-classical theory based on interaction potentials between molecular absorber and molecular perturber. We have applied this formalism for Raman and infrared spectra of linear and asymmetric-top molecules. Recently, the method has been extended into symmetric-tops with inverse symmetry such as the NH₃ molecule. Our calculated half-widths of NH₃ lines in the ν₁ and the pure rotational bands match measurements very well. Then, the model has been applied to the calculation of the shape of the Q branch and of some R manifolds, for which an obvious signature of line mixing effects has been experimentally demonstrated. Comparisons with measurements show that the present formalism leads to an accurate prediction of the available experimental lineshapes.

r0pt Figure A new accurate global potential energy surface for the ground electronic state of ozone [J. Chem. Phys. 139, 201103 (2013)] was published fairly recently. The topography near dissociation differs significantly from previous surfaces, without spurious submerged reefs and corresponding van der Waals wells. This has enabled significantly improved descriptions of scattering processes, capturing the negative temperature dependence and large kinetic isotope effects in exchange reaction rates. The exchange reactivity was found to depend on the character of near-threshold resonances and their overlap with reactant and product wavefunctions, which in turn are sensitive to the potential. Here we present global “three-well” calculations of all bound vibrational states of three isotopic combinations of ozone for J = 0 and J = 1 with a focus on the character and density of highly excited states. The calculations were done using a parallel symmetry-adapted Lanczos method with the RTR code, enabling the use of as many as 64.8 million basis functions. Tunneling splittings and the pseudorotation isomerization path will be discussed.

Cooling and trapping of polar molecules has attracted attention at cold and ultracold temperatures. Extended study of molecular inelastic collision processes of polar interstellar species with proton finds an important astrophysical application to model interstellar medium. Present study includes computation of rate coefficient for molecular rotational quenching process in proton collision with isovalent CO and CS molecules using quantum dynamical close-coupling calculations. Full dimensional ab initio potential energy surfaces have been computed for the ground state for both the systems using internally contracted multireference configuration interaction method and basis sets. Quantum scattering calculations for rotational quenching of isovalent species are studied in the rigid-rotor approximation with CX (X=O, S) bond length fixed at an experimental equilibrium value of 2.138 and 2.900 a.u., respectively. Asymptotic potentials are computed using the dipole and quadrupole moments, and the dipole polarizability components. The resulting long-range potentials with the short-range ab initio interaction potentials have been fitted to study the anisotropy of the rigid-rotor surface using the multipolar expansion coefficients. Rotational quenching cross-section and corresponding rates from j=4 level of CX to lower j′ levels have been obtained and found to obey Wigner’s threshold law at ultra cold temperatures.

Using the generalized model for photodetachment of electrons from mixed-character molecular orbitals, we gain insight into the nature of the HOMO of HO₂⁻ by treating it as a coherent superpostion of one p- and one d-type atomic orbital. Fitting the pd model function to the ab initio calculated HOMO of HO₂⁻ yields a fractional d-character, γ_p, of 0.979. The modeled curve of the anisotropy parameter, β, as a function of electron kinetic energy for a pd-type mixed character orbital is matched to the experimental data.

In this talk we present an update on the HITRAN Application Programming Interface (HAPI) Kochanov RV, Gordon IE, et al. Submitted to JQSRT HighRus Special Issue 2016.Kochanov RV, Hill C, et al. ISMS 2015. http://hdl.handle.net/2142/79241. HAPI is a free Python library providing a flexible set of tools to work with the most up-to-date spectroscopic data provided by HITRANonline (www.hitran.org) Rothman LS, Gordon IE, et al. JQSRT 2013;130:4–50.Hill C, Gordon IE, et al. Accepted to JQSRT HighRus Special Issue 2016.. HAPI gives access to the spectroscopic parameters which are continuously being added to HITRANonline. For instance, these include non-Voigt profile parameters Wcislo P, Gordon IE, et al. Accepted to JQSRT HighRus Special Issue 2016. foreign broadenings and shifts Wilzewski JS, Gordon IE, et al. JQSRT 2016;168:193–206. and line mixing. HAPI enables more accurate spectra calculations for the spectroscopic and astrophysical applications requiring the detailed modeling of the broadener. HAPI implements an expert algorithm for the line profile selection for a single-layer radiative transfer calculation, and can be extended by custom line profiles and algorithms of their calculations, partition sums, instrumental functions, and temperature and pressure dependences. Possible HAPI applications include spectroscopic data validation and analysis Kochanov RV, Gordon IE, et al. Clim Past 2015;11:1097–105.s well as radiative-transfer calculations, experiment verification and spectroscopic code benchmarking.

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Kochanov RV, Gordon IE, et al. Submitted to JQSRT HighRus Special Issue 2016.

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Rothman LS, Gordon IE, et al. JQSRT 2013;130:4–50.

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Wcislo P, Gordon IE, et al. Accepted to JQSRT HighRus Special Issue 2016., Wilzewski JS, Gordon IE, et al. JQSRT 2016;168:193–206., Kochanov RV, Gordon IE, et al. Clim Past 2015;11:1097–105.a