Sulfur and chlorine-based molecules are intriguing to study given their applicability to Venus’s atmosphere which is difficult to investigate given the optically dense clouds covering the planet and the existence of a mystery near UV absorber. Furthermore, sulfur and halogen-containing molecules are involved in the chemistry of Earth’s atmosphere, especially that which occurs within volcanic plumes. We will report on our investigation of the stability and photochemistry of the intermediate chemical reaction species, involving Cl, S, and O such as ClSO, ClSO2, and ClSO3. We will discuss how the chemistry of these new species impact our current understanding of the ozone depletion through coupling of chlorine and sulfur species in the stratosphere and enhanced our understanding of the sulfur chemistry in Venus's atmosphere.

Electro-optic sampling using few-optical-cycle probing pulses offers a novel technique for ultrasensitive molecular spectroscopy with high dynamic range and broadband frequency access. By combining this method with dual-frequency-comb spectroscopy with a new class of ultrafast lasers, high-resolution (∆ν ≈ 10 MHz/0.0003 cm⁻¹with spectral interleaving) spectroscopic measurements have been performed in in the mid-IR to THz frequency range (6.6–200 μm), with an instantaneous spectral coverage exceeding an octave. As a driving source, we use a pair of mutually coherent combs from Kerr-lens mode-locked solid-state Cr:ZnS (2.35 μm) lasers. One of the combs is frequency downconverted via intrapulse difference frequency generation to produce a longwave “sensing” comb, while the second comb is frequency doubled to produce a near-IR “probe” comb for electro-optic sampling (EOS). The low intensity and phase noise of our dual-comb system allow for capturing a large amount of spectral information (200 000 comb-mode-resolved spectral lines spaced by 80 MHz) in the mid-IR portion of the spectrum at a video rate of 69 Hz, with the signal-to-noise ratio limited by the shot noise of the near-IR EOS balanced detection system.

The combination of broadband frequency combs with intrapulse difference frequency generation and electro-optic sampling using few-optical-cycle probing pulses offers a novel technique for ultrasensitive high resolution molecular spectroscopy over an exceptionally broad mid-IR to THz frequency range (6.6–200 μm). In this talk, we discuss first spectroscopic applications of this powerful technique, presenting preliminary data and analysis of high S/N Doppler limited molecular spectra in the mid IR frequency range for several species relevant to terrestrial and exoplanetary atmospheres. Specifically, we observe and identify c-type rovibrational bands of isoprene corresponding to out-of-plane methylenic bending in the 800-1200 cm⁻¹region, a/b/c-type methyl rocking rovibrational bands of dimethylsulfide (900-1100 cm⁻¹), as well as multiple hybrid a/b-type rovibrational bands in ethanol due to asymmetric CO/CC stretching and COH bending vibrations near 1000 cm-1. Of special interest, the THz frequency comb capabilities from intrapulse difference frequency generation also permit pure rotation-tunneling spectra to be observed, which can be used to selectively detect trace conformers with a non-vanishing dipole moment. Progress toward using this novel capability to further characterize large amplitude C-C-C-C dynamics of room temperature gauche- and cis-1,3 butadiene conformers in the presence of excess trans-1,3 butadiene will be discussed.

To further enhance the line position and intensity accuracy of the recently published Ames-296K IR line list for nitrous oxide (N₂O) and its isotopologues,Huang, Schwenke, and Lee Mol. Phys. (2023) e2232892. new refinement of the potential energy surface has been conducted to achieve a σ_rms value of less than 0.01 cm⁻¹for the line positions of the transitions in HITRAN2020. By utilizing a CCSD(T)/aug-cc-pV(T/Q/5)Z dipole moment surface with optimal fitting accuracy up to 10,000 cm⁻¹, we have recalculated the N₂O IR line lists for all 12 isotopologues of ^14/15N and ^16/17/18O. Consequently, our agreement with the highly accurate intensity measured by the NIST group for the 50⁰0 and 42⁰0 bandsAdkins, Long, Fleisher, and Hodges, JQSRT (2021) 262, 107527as also been improved to 1.6-2.1% when compared to the previously published Ames-296K line list. Moreover, the rovibrational energy levels of the six most abundant isotopologues have been matched to and replaced by accurate levels derived from experimental data or Effective Hamiltonian models. In this presentation, we will outline the construction of the new line list and focus on the accuracy improvements and the replacement of energy levels. We also will discuss the impacting factors and remaining issues beyond 10,000 cm⁻¹. Huang, Schwenke, and Lee Mol. Phys. (2023) e2232892.a Adkins, Long, Fleisher, and Hodges, JQSRT (2021) 262, 107527h

We present an update of our double resonance (DR) investigation of the CH₄ spectroscopic levels in the 3ν₃ spectral region, using a continuous wave optical parametric oscillator to pump lines in the ν₃ fundamental band ( 3000 cm⁻¹) and a cavity-enhanced frequency comb to probe the induced absorption in the region of the ν₃ → 3ν₃ bands, covering (5550-6100 cm⁻¹). Compared to results previously reported,¹, there have been several improvements in the instrumentation and the data analysis that will be discussed. We have extended the spectral coverage of the spectrometer, allowing the detection of lower energy members of the P = 6 polyad of interacting vibrational states (down to 8550 cm⁻¹). Two theoretical predictions of the spectra, TheoReTS² and ExoMol,³ have been updated and we find they both are now in significantly better agreement with our observed transitions. In addition to the Doppler-free 3-level DR transitions, we observe many Doppler-broadened, pump-induced absorption lines. We have assigned many of these to 4-level DR transitions that involve rotational state changes in the ν₃ fundamental vibrational level. Using combination-difference searches and the improved TheoReTS predictions, we have made confident assignments of over 100 of these transitions. 1. V. Silva de Oliveira, I. Silander, L. Rutkowski, G. Soboń, O. Axner, K. K. Lehmann and A. Foltynowicz, Nature Communications 15 (1), 161 (2024). 2. M. Rey, Journal of Chemical Physics 156, 224103, 224103 (2022). 3. S. N. Yurchenko, A. Owens, K. Kefala, and J. Tennyson, Monthly Notices of the Royal Astronomical Society 528, 3719 (2024).

The reactive Criegee intermediate, ClCHOO, has been lab generated and examined using both spectroscopic and theoretical methods. Specifically, syn- and anti- ClCHOO conformers have been discerned, and their electronic absorption spectra was investigated. Electronic spectroscopy of ClCHOO was achieved through a UV-vis depletion method in which the ground state depletion is detected as a reduced photoionization signal (10.5 eV) using time-of-flight mass spectrometry (TOF-MS) on the parent mass channel of the Criegee intermediate. Experimental results were compared with electronic structure calculations and absorption simulations that utilize a nuclear ensemble method. The combined findings from theory and experiment reveal a strong π* ← π transition within the solar irradiance spectral range. The theoretical calculations also indicate the presence of a bright Rydberg transition, adding complexity to the understanding of ClCHOO's photochemical behavior. Halogen bonding has clear effects on the electronic structure, and the π donating nature of chlorine shifts their electronic spectra. Comparison with the simplest and well-studied Criegee intermediate, CH₂OO, gives a framework in which to understand how electron donating substituents and conformation play a crucial role in the photochemistry of this potentially atmospherically relevant species.

We propose a new two-step strategy for computing ro-vibrational energy levels and wavefunctions of a triatomic molecule and apply it to CO₂. A two-step method uses a basis whose functions are products of K− dependent "vibrational" functions and symmetric top functions. K is the quantum number for the molecule-fixed z component of the angular momentum. For a linear molecule, a two-step method is efficient because the Hamiltonian used to compute the basis functions includes the largest coupling term. The most important distinguishing feature of the two-step we propose is that it uses an associated Legendre basis and quadrature rather than a K− dependent discrete variable representation. This reduces the cost of the calculation and simplifies the method. We present new ro-vibrational energy levels with J up to 100 for CO₂, on the most accurate available potential energy surface which is known as AMES-2 PES. We have converged most levels up to 20000 cm⁻¹ to 0.0001 cm⁻¹

In bimolecular collisions between open-shell radicals, such as nitric oxide (NO) and progressively larger alkanes, the relative impact configurations may lead to both reactive and nonreactive outcomes. In this talk, we will discuss the isomer-specific scattering mechanisms between NO and alkanes with growing chain length by characterizing the spectroscopy and dynamics of large NO-alkane complexes formed along the collision potential energy surface. Infrared action spectroscopy paired with velocity map imaging was used to identify the spatial orientations and energy-exchange pathways of large NO-alkane complex isomers. We will highlight the similarities and key differences along the potential energy surfaces that distinguish the decay mechanisms to product formation.

Understanding ultrafast photochemical processes requires sensitivity to electronic, vibronic, and vibrational motions occurring across a large range of time and frequency. Localized reporting on critical changes to photo-excited coherences can be useful to characterize ultrafast timescales of dynamics such as crossings through conical intersections and non-adiabatic phenomena. Two-Dimensional Electronic-Vibrational spectroscopy (2D EV) is a coherent multidimensional spectroscopy capable of directly resolving vibronic correlations in excited states. In this work, we discuss the extension of 2D EV to include two-quantum (2Q) coherence pathways to monitor ultrafast photochemical dynamics explicitly through vibronic coherences. We theoretically propose this technique and simulate the expected spectra using a phenomenological model, the results of which motivate future experimental implementations of this 2Q 2D EV technique.