Absorption of sunlight drives atmospheric chemistry. Consequently, photochemical products and quantum yields of atmospherically-relevant species have been investigated, experimentally and theoretically, for more than half a century. Almost all photochemical mechanisms are attributed to reaction on an excited singlet or triplet state of a molecule. Notable exceptions exist, e.g. HCHO produces H₂ + CO on the ground state following excitation in the near-ultraviolet. The S₀ potential energy surface, at energies relevant to photolysis, is complex. Many chemical pathways are available, including formation of radicals and closed shell molecules, and photoisomerisation. In addition, the vibrationally-excited S₀ molecule has sufficient internal energy to open up new “activated” chemical pathways. In this talk, I will show experimental and theoretical evidence of multiple competing pathways on the ground state of selected carbonyl compounds. In addition to the well-known Norrish Types I and II reactions, we have identified the following active pathways:

• Concerted triple fragmentation into CO, H₂ plus an alkene;
• Production of H₂ from all carbonyls;
• Dissociation into ketenoids (ketene, methylketene, dimethylketene, etc);
• Photoisomerisation to enols; and
• Activated reaction with O₂ molecules.

Bulb experiments are used to provide end-products and quantum yields. These are complemented by molecular beam experiments which provide mechanistic information on the primary processes. Theory and modelling include ab initio calculations of critical energies, Master Equation models of kinetics, and box and global transport models of atmospheric chemistry. Our conclusion is that the ground state photochemistry of carbonyls is a rich source of airborne radicals and molecules. These mechanisms are not present in current atmospheric models and their impact on atmospheric chemistry is not well-known.

Formic acid and acetic acid are the most abundant organic acids in both Earth’s atmosphere and in interstellar molecular clouds. In Earth’s atmosphere, formic acid is typically remotely sensed in its strongest vibrational band (ν₆), which has a band centre at 1105 cm⁻¹. While the current HITRAN line list of this band is in good agreement with experimental spectra at resolutions that are relevant to remote sensing, at higher resolution, there are inconsistencies [1]. This was evidenced when comparing the line list with high-resolution FTIR spectra that we acquired at the Canadian Light Source. In an effort to improve this situation, we simulated the spectrum of formic acid using the 114 diagonal and 105 off diagonal parameters that were previously determined in a global fit to the heptad of interacting vibrational states near 9 μm [2]. Unfortunately, we found that residuals in the line positions are greater than the measurement uncertainty at high K_a values. While the relative intensities are in good agreement for most of the lines, we find poor agreement for many of the lines which are strongly perturbed. It seems the choice of coupling terms used in the Hamiltonian could be improved upon [3], and it's our quest to refine the global fit so that an improved line list of formic acid can be generated in this terrestrial window. The latest progress in this endeavor will be presented. [1] I. E. Gordon, L. S. Rothman, R.J. Hargreaves, et al., JQSRT, 277, 107949 (2022). [2] O. I. Baskakov et al., J. Mol. Struct., (1-3), 54 (2006). [3] J. Demaison, M. Herman, and J. Liévin, J. Chem. Phys., 126, 164305 (2007).

In this talk, we report on the design and construction of a continuous-wave mid-infrared optical parametric oscillator (cw mid-IR OPO) for high-resolution infrared spectroscopy. The singly-resonant OPO is pumped by a narrow linewidth ( < 20 kHz), high power (up to 15 W), single-frequency fiber laser at 1064 nm. The idler frequency can be tuned from 2400 – 3400 cm⁻¹ with a high output power ( > 3 W). The signal frequency is locked to a Fabry-Perot cavity, which is stabilized to a polarization-stabilized HeNe laser. To test the performance of the mid-IR OPO, it is applied to saturated absorption spectroscopy of methane. Progress toward applications of the home-built mid-IR OPO to high-resolution infrared spectroscopy of jet-cooled radicals (e.g. cyclopentadienyl) and molecular ions (e.g. CH₅⁺) will be reported.

Methanediol (CH₂(OH)₂), generated from the hydration of formaldehyde (HCHO), has been proposed to be a potential precursor for the formic acid (HCOOH) in the atmosphere. In this work, we report the rotationally resolved infrared absorption spectra of gaseous methanediol in the range of 1016–1084 cm⁻¹. The spectra of gaseous methanediol were recorded with a spectral resolution of 0.0013 cm⁻¹by using a tunable difference frequency generation (DFG) laser system near 9 μm coupled with a multipass absorption cell. The measured spectra were assigned to be the antisymmetric and symmetric OCO stretching modes of trans-CH₂(OH)₂ at 1058.6 and 1026.3 cm⁻¹, respectively. Furthermore, the IR absorption cross-section of CH₂(OH)₂ were evaluated based on direct measurement of CH₂(OH)₂, H₂O and HCHO in the cell using the DFG laser system near 9 μm and a dual comb spectrometer near 3 μm simultaneously.

Ion-pair formation mechanisms in small organic molecules and clusters through UV light absorption have attracted substantial attention due to their prominent role in acid-base chemistry, atmospheric ozone depletion, and acid rain. In particular, the excited state proton transfer (ESPT) within formic acid dimer has long been utilized as a model for describing the double proton transfer mechanism thought to protect biological systems from damaging ultraviolet light absorption. The timescale predicted through various theoretical treatments for the ultrafast ESPT within formic acid dimer spans more than 4 orders of magnitude but have so far eluded experimental measurement. I will present new experimental measurements¹⁻³ on the ultrafast ESPT dynamics for formic acid dimer and its clusters (FA)_n, n < 10. We employ femtosecond time-resolved mass spectrometry to monitor the ESPT mechanism following ultraviolet excitation, which accesses the Rydberg manifold within the Franck-Condon region. The subsequent excited state relaxation through the valence/ion-pair state is monitored through the transient growth of the cation component in the mass spectra. Although we record a clear absence of signal for the ESPT in the dimer, we find the signal for successful ESPT increases logarithmically with cluster size. Ab initio calculations demonstrate similar excitation/relaxation behavior for each cluster, revealing a contact ion pair [(FA)_nH⁺ –COOH^–] forms between two molecules composing the cluster before finally a formate anion (HCOO^–) is dissociated by the probe pulse. The sub–ps timescale for ESPT increases almost linearly with cluster size, requiring  ∼ 72 fs per additional formic acid molecule and ranging from 213 ±51 fs for the trimer to 667 ±116 fs for FA₉. The near-linear trends measured for both rearrangement lifetime and ion pair formation resolve a long-standing scientific debate, showing that although sequential proton transfer is inhibited in the formic acid dimer, the mechanism is enabled through clustering.

Observation of the atomic oxygen fine structure line at 2.06 terahertz from an orbiting limb sounder serves as a compelling platform for the characterization of neutral winds in the lower thermosphere and E-region ionosphere (LTEI;  ∼ 100  140 km). The precision of wind measurements retrieved from this prospective instrument class are directly linked to the rest frequency of this targeted transition as determined from laboratory measurements. This talk will detail recent work at the Jet Propulsion Laboratory to deploy a custom designed transmitter and heterodyne receiver pair optimized for performance at 2.06 THz for use in obtaining high-precision rest frequencies for the oxygen atom ³P₀ - ³P₁ atomic fine structure splitting. Oxygen atom (¹⁶O) is generated by an inductively coupled plasma system with direct absorption measurements obtained by a combined frequency and plasma modulated experimental scheme. Results will be discussed in the context of previous laboratory measurements and the impact on prospective neutral wind measurements in the LTEI.

The remote sensing of the Earth's atmosphere is based on the data obtained by the ground-based and satellite-borne spectroscopic instruments. In turn, the calibration of these instruments and the interpretation of the recorded spectra requires the knowledge of the collisional perturbations in the spectra of atmospheric molecules. In recent years much effort was put into studying the collision-induced effects in the O₂ b¹Σ⁺_g(v=0) ← X³Σ⁻_g(v=0) transition (the A-band), due to its wide applicability in the instrument calibration and retrieval of the terrestrial properties. We report the first fully quantum calculation of the collisional line-shape parameters of the P1 P1 A-band line in O₂ perturbed by N₂, the most abundant atmospheric constituent. Since this is an electronic transition, we use two ab initio O₂-N₂ potential energy surfaces to calculate the relevant scattering amplitudes by solving the exact close-coupling equations. We account for the non-zero electronic spin of O₂(X³Σ⁻_g) in the calculations, and we determine the pressure broadening and shift parameters as well as the real and imaginary parts of the complex Dicke parameter. Furthermore, we determine the temperature dependence of the line-shape parameters and we calculate the ab initio speed dependence of the broadening and shift. We compare the theoretical results with the new accurate experimental A-band spectra recorded with the CRDS technique. From the methodological point of view, the calculations reported here are the first ab initio treatment of the electronic transition in a diatomic molecule perturbed by another diatomic molecule.

The oxygen A-band at 760 nm is used to quantify airmass by ground- and space-based remote sensing missions, such as NASA's Orbiting Carbon Observatory-2. Reaching the desired accuracy goals of these missions places stringent demands on spectroscopic parameters, line shapes, and models of collisional effects, including line mixing and collision-induced absorption. Ongoing efforts to improve the precision and accuracy of A-band spectroscopy have involved multi-spectrum analysis of data from high precision laboratory techniques, including cavity ring-down spectroscopy and Fourier transform spectroscopy. While these efforts have produced substantial improvements in A-band spectroscopy, atmospheric retrievals indicate errors remain, particularly in line mixing and collision-induced absorption. Photoacoustic spectroscopy, which has a large dynamic range, high sensitivity, and minimal background, can complement existing laboratory techniques and improve models of collisional effects. We have constructed a photoacoustic spectrometer capable of measuring high resolution, unsaturated spectra of the full A-band for pressures from 50 to 4,000 Torr. However, characterizing energy transfer and other photoacoustic effects to the high precision necessary for determination of line mixing and collision-induced absorption has proven challenging. We will present results from our pressure-dependent measurements and discuss our efforts towards incorporating photoacoustic spectra into multi-spectrum analyses with other laboratory data.

Oxygen spectra are widely used in remote sensing. Serving as a reference for the determination of the vertical column density (VCD), the accuracy of its spectral parameters affects retrievals of greenhouse gases and air pollutants concentrations. The B band, located in the spectral range near 690 nm, is the second most intense atmospheric band of the oxygen molecule. Being relatively weak, it does not become saturated on the atmospheric pathlengths. It is used by such satellite missions as EPIC-DISCOVR, ESA Sentinel 5&5P, GOME-2, and SCIAMACHY. The presented line-shape dataset [1] contains all parameters of the speed-dependent Nelkin-Ghatak profile (SDNGP), including the Dicke narrowing, speed dependence, and temperature dependence. Air-perturbed O₂ B-band spectra were acquired with a cavity ring-down spectrometer (CRDS) assisted by an optical frequency comb (OFC). It is the first parameter set beyond the Voigt profile. Our pressure broadening coefficients differ from the HITRAN values, given for the Voigt profile, by up to 9%, and the pressure shift coefficients differ by up to 30%. Also, previously available line intensities are corrected by 0.2% up to 3.5%. Moreover, we present the first experimental temperature dependencies of pressure broadening and shift parameters. We show that based on the speed-dependence of the collisional broadening measured at a single temperature, it is possible to obtain a reasonable estimation of its temperature dependence using a simple analytical model. The recent parameters can be implemented for the VCD retrievals, leading to a 5-fold improvement in agreement between A- and B-band-based results [2].
1. K. Bielska et al., Spectrochim. Acta A 303, 123185 (2023).
2. P. Edinger, Developing an instrumental setup for atmospheric validation of absorption cross-sections for the O₂ A and B band, master thesis, University of Heidelberg 2023.

At this conference in 2022, we reported new measurements of the 193 nm photodissociation of acrylonitrile using a Time-Resolved Kinetic Chirped-Pulse (TReK-CP) spectrometer Zaleski, D. P.; Harding, L. B.; Klippenstein, S. J.; Ruscic, B.; Prozument, K. J. Phys. Chem. Lett. 2017, 8, 6180.t a time resolution of approximately 1 μs.N. A. Seifert, K. Prozument, ISMS 2022, WH07.hese experiments showed strong evidence for the formation of vibrationally cold, nascent cyanoacetylene forming through a barrier-proximal sequential hydrogen elimination with a significant tunneling component in a 298 K and 1 μbar continuous flow at an approximate rate of 10⁵ s⁻¹. Through vibrational state- and temperature-dependent analysis, we show strong evidence for a slow, hydrogen elimination mechanism from a cyanovinyl radical intermediate. However, due to the low pressure of this initial experiment, it was difficult to fully ascertain the nascent nature of these cold products, since the timescale of the onset of collisions with the reactor wall or residual gas was slightly longer than the length of the spectroscopy experiment. Here, we present new results from a new Summer 2023 measurement campaign on these photodissociation measurements, using a slightly larger ambient pressure of 2.5 μbar and a more precise protocol for ambient heating of the flow cell, with a larger effective temperature range and a more narrow uncertainty of ±2 K. Here, through use of product vibrational state selection and deuterated precursors, we find that these observed cold cyanoacetylene molecules are truly nascent, and the observed time-dependence shows competition between nascent formation and collisional relaxation of hotter products. This competition is state-sensitive, in that higher vibrational states show slower onset of collisions, consistent with a smaller residual kinetic energy. Zaleski, D. P.; Harding, L. B.; Klippenstein, S. J.; Ruscic, B.; Prozument, K. J. Phys. Chem. Lett. 2017, 8, 6180.a N. A. Seifert, K. Prozument, ISMS 2022, WH07.T