r0pt Figure Silicon tetrafluoride (SiF₄) should be a normal trace component of volcanic gases. However, a better knowledge of spectroscopic parameters is needed for this molecule in order to derive accurate concentrations. As explained last year, we undertook an extensive high-resolution study of its infrared absorption bands, for the there isotopologues in natural abundance: ²⁸SiF₄ (92.23 %), ²⁹SiF₄ (4.67 %) and ³⁰SiF₄ (3.10 %). We present here an update of this study. It includes a new global fit with consistent parameter sets for the ground and excited states (the Figure on the right presents the ν₄ bending fundamental region). In particular, all existing rotational line data have been included. The 2ν₄ band of ²⁸SiF₄ could also be analyzed in detail. A first rough estimates of the dipole moment derivative for the ν₃ band has been performed, leading to to an integrated band intensity which is consistent with literature values, around 680 km/mol. The isotopic dependance of band centers and Coriolis parameters has been studied, thanks to the formula presented in talk P4363.

New particle formation is a process in which particles form from trace vapors in the atmosphere and grow into climatically relevant clusters in a complex and still poorly understood process. Clusters of amines and sulfuric acid are known to yield NPF rates similar to those in the ambient atmosphere and provide a useful window into the surface structural motifs present in these clusters. Identifying the complete structure of clusters of a climatically relevant size is extremely difficult. It is excessively computationally expensive, and validating computed structures is made more difficult by a lack of experimental data containing explicit structural information. A core-shell type of structure has been proposed to explain, among other puzzles, the synergistic effects of ammonia and alkylamines on formation and growth rates, with the bulky amines with lower hydrogen bonding numbers tending to partition to the surface of the cluster and a core with fully hydrogen-bonded ammonium ions. We seek to elucidate the structural motifs of several smaller, experimentally accessible cationic clusters of amines and sulfuric acid that represent surface structural motifs of larger clusters using cryogenic ion vibrational predissociation spectroscopy. By mapping structural features to specific vibrational bands we can create a library of spectral markers through which we can identify surface structural motifs of larger clusters, allowing us to more easily identify the climatically relevant surface groups and better predict the behavior of the clusters. Additionally, work is underway to investigate the role water plays in the growth of these structures and how a variety of structural features influence the likely binding sites for water and the water uptake properties of these clusters.

The ~B - ~X laser-induced fluorescence (LIF) and dispersed fluorescence (DF) spectra of the atmospherically important β-monofluoro ethoxy (MFEO), β,β-difluoro ethoxy (DFEO), and β,β,β-trifluoro ethoxy (TFEO) radicals were recorded with vibronic resolution under jet-cooled conditions. To simulate the spectra, Franck-Condon factors were obtained from quantum chemical computations carried out at the CAM-B3LYP/6-311++G(d,p) level of theory. The simulations reproduce well both the LIF and DF spectra. Both conformers (G and T) of MFEO and one (G) of the two conformers of DFEO contribute to the LIF spectrum. A comparison between the experimental and calculated spectra confirms the expected long-range field effects of the CH_XF_3−X group on electronic transition energies and bond strengths, especially in the excited electronic (~B) state. Although TFEO has only one conformer, its LIF spectrum is highly congested, which is attributed to the interaction between CO stretch and the -CF₃ internal rotation.

The weakly-bound CO₂–(N₂)₂ and CO₂-Ar₂ trimers have been studied in the carbon dioxide ν₃ asymmetric stretch region ( ∼ 2350 cm⁻¹). The van der Waals complexes are generated in a supersonic slit-jet apparatus and probed using an optical parametric oscillator. The interaction of N₂, the most abundant molecule in the Earth's atmosphere, and CO₂ is relevant from the stand point of the overlap of frequencies of the stretching modes of CO₂ with the Earth’s emission and its effect on greenhouse. Here, we have observed the fundamental for the CO₂–(N₂)₂ trimer. It is composed of c-type transitions establishing that the trimer has C_2v point group symmetry with the CO₂ monomer in the ac inertial plane and parallel to the c-axis and the equivalent equatorial N₂ monomers in the ab-plane with molecular axes passing through the center of mass of CO₂ and making an angle of 64^°. Theoretical calculations were performed in support of our observations. Several minima on the PES were found. For the most stable isomer, the vibrational corrections to the equilibrium rotational constants were obtained. The rotational parameters at CCSD(T*)-F12c level of theory gives results in very good agreement with those obtained from the observed vibrational fundamental. Using a dilute mixture of CO₂ and Ar in He, we observed a weak b-type combination band involving an intermolecular mode around 2380 cm⁻¹. This band is assigned to CO₂-Ar₂ trimer which also has C_2v point group symmetry and a structure similar to CO₂–(N₂)₂ trimer. As such, four of the five combination bands are infrared active. From these only one has b-type transitions which uniquely identifies the intermolecular mode as CO₂ bend with an observed frequency of 32.24 cm⁻¹.

Isoprene (C₅H₈) is an important molecular target in trace gas sensing due to its presence in Earth's atmosphere and human breath. Isoprene is the most abundant of the so-called biogenic volatile organic compounds (BVOCs) emitted naturally into the atmosphere by plants, and so plays an important role in the chemistry of the troposphere. Isoprene is also one of the most abundant hydrocarbons present in human breath, and there is interest in measuring isoprene in breath as a way to perform noninvasive monitoring of patients. In both of these settings, isoprene is present as a trace gas at a concentration of parts per billion (ppb), making its detection quite challenging. We have used a quantum cascade laser-based infrared spectrometer to perform sensitive spectroscopy of isoprene down to the ppb level. We have used the strong Q-branch of the ν₂₆ band of isoprene near 992 cm⁻¹to monitor its concentration and achieved a minimum noise-equivalent concentration for our spectrometer of 3 ppb at an optimal averaging time of 25 s. We have also demonstrated the potential real-world applications of our approach by directly measuring the isoprene concentration in a breath sample from a volunteer.

We used two previously described [1,2] cavity ring-down spectroscopy systems to accurately measure line intensities in the following three ¹²C¹⁶O₂ rovibrational bands near 1.6 μm: (30012) ← (00001), (30013) ← (00001), and (30014) ← (00001). These bands are commonly used in remote sensing applications, including the Total Carbon Column Observing Network (TCCON) [3]. We estimate relative combined standard uncertainties for these band intensities of less than 0.1% and obtain percent-level deviations in the measured intensities relative to those in the literature and several spectroscopic databases. However, we find 0.1% level agreement with the (30013) and (30014) band intensities given in the HITRAN 2016 [4] database, which were calculated using ab initio dipole moment surfaces. Incorporation of the resulting line intensities into TCCON retrievals leads to significantly reduced biases in the (30012) and (30013) bands. These results indicate that refinements of spectroscopic databases are required to meet increasingly stringent remote sensing uncertainty targets. [1] Lin, H. et. al. J. Quant. Spectrosc. Radiat. Transfer, 161, 11-20. [2] Truong, G. W. et. al. (2013) Nat. Photonics, 7(7), 532-534. [3] Wunch, D. et. al. Philos. Trans. Royal Soc. A, 369(1943), 2087-211 [4] Gordon, I. E., et al. (2017), J. Quant. Spectrosc. Radiat. Transfer, 203, 3-69.

The reaction of ozone with isoprene, one of the most abundant volatile organic compounds emitted into the atmosphere, produces three distinct carbonyl oxide species (R₁R₂COO) known as Criegee intermediates: formaldehyde oxide (CH₂OO), methyl vinyl ketone oxide (MVK-oxide), and methacrolein oxide (MACR-oxide). Unimolecular decay of MVK-oxide is predicted to be the major source of hydroxyl radicals (OH) derived from isoprene ozonolysis in the atmosphere. Previously, infrared (IR) action spectroscopy of syn-MVK-oxide was shown to yield OH radical products following unimolecular decay via a 1,4-H transfer processJ. Am. Chem. Soc. 2018, 140, 34, 10866-10880 Here, we investigate the IR action spectroscopy and unimolecular decay of selectively deuterated MVK-oxide (d₃-MVK-oxide, (CH₂=CH)(CD₃)COO) with deuterium atom transfer as the rate limiting step. The IR action spectrum of syn-d₃-MVK-oxide is recorded in the CH stretch overtone (2ν_CH) region with detection of OD products by laser-induced fluorescence. The temporal profile arising from unimolecular decay to OD products is also obtained. The experimental results are compared with the calculated IR absorption spectrum and the unimolecular decay rates predicted by Rice–Ramsperger–Kassel–Marcus (RRKM) theory. The IR action spectrum shows two distinctive features in the 6100-6200 cm⁻¹region and two broad features from 5950 to 6050 cm⁻¹. The d₃-MVK-oxide IR action spectrum exhibits notable changes compared to MVK-oxide, as the overtone and combination transitions involving CD stretch shift to lower frequency, resulting in fewer transitions in this region. The unimolecular decay rate for MVK-oxide is predicted to be 100 times slower upon deuteration. Experimentally, a significantly slower appearance of OD products from d₃-MVK-oxide is found compared to OH products from MVK-oxide, indicating that tunneling plays a very important role in 1,4-H/D transfer. The similarities in the IR action spectra indicate that the syn conformers make the main contribution to the observed spectra.

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Footnotes:

J. Am. Chem. Soc. 2018, 140, 34, 10866-10880.

The Great Oxygenation Event, the introduction of O₂ into the Earth’s atmosphere approximately 2.5 billion years ago, is a critical stage in the development of life on Earth. The exact timing of this event is thought to be correlated with the disappearance of sulfur isotope anomalies, called "Sulfur Mass Independent Fractionation" (S-MIF), in the rock record. However, the mechanism for the generation of S-MIF in a reducing atmosphere is still unknown. This talk explores the B-X system of S₂ where the short-lifetime B state is extensively perturbed by a long-lifetime B” state. We employ a master equation model that calculates rotationally and electronically inelastic collisional transfer rates between the B and B” states. For weakly perturbed B/B” level crossings (matrix element less than 1 cm⁻¹), these collisional transfer processes can generate significant isotope effects, where one isotopologue has a larger enhancement of excited state population than another. We discuss the effects of mass-dependent vibrational level shifts and nuclear permutation symmetry on this isotopic fractionation, and propose a possible mechanism for the S-MIF pattern observed in the rock record.