Session V26: The Chemical Physics of Molecules in Space III

Live

Photodissociation is a major destruction pathway for small molecules in a number of astrophysical environments, such as photon-dominated regions, the diffuse interstellar medium, and protoplanetary disks. Accurate wavelength-dependent photodissociation cross sections, including both direct photodissociation and predissociation, are a critical input for astrochemical models. For many stable diatomic molecules (e.g., CO), reliable laboratory measurements have been made over wide wavelength ranges. However, measuring absolute cross sections for reactive molecules such as CS and C₂, both of which are abundant in space, is a major challenge. In both of these molecules, predissociation plays a major role in the vacuum ultraviolet region (100-300 nm), yet few experimental or theoretical studies have targeted electronic states in this energy range. We have performed ab initio calculations using the CASSCF/MRCI+Q method to characterize excited electronic states of CS and C₂ relevant for predissociation. For CS, we focus especially on the B ¹Σ⁺ and C ¹Σ⁺ Rydberg states, both of which are accessible via allowed transitions from the ground X ¹Σ⁺ state (B-X at 154 nm, and C-X at 140 nm). Using a coupled channel model with a diabatic treatment of several ¹Σ⁺, ³Π, and ³Σ^- states, we compute new ab initio photodissociation cross sections that result in a threefold higher photodissociation rate under the standard interstellar radiation field compared with the cross sections available in the Leiden photodissociation database. For C₂, we target the Herzberg F ¹Π_u - X ¹Σ_g⁺ band at 132 nm, which has been observed in Hubble spectra, deriving ab initio spectroscopic constants and oscillator strengths. Ongoing progress for treating the predissociation process as well as new experimental measurements will be discussed.   

Live

Interactions between complex neutral molecules and ions play an important role in the chemical network of the interstellar medium and during combustion of hydrocarbons. To advance our understanding of the underlying fundamental physical and chemical processes, dedicated reaction experiments under controlled conditions are required. Unfortunately, such measurements at cold temperatures are very challenging and thus rare. We use tools borrowed from the cold atom community to measure such reactions under cold and well-controlled conditions. Here we study the influence of molecular structure in bi-molecular interactions. Specifically, we focus on the bi-molecular reaction between the acetylene cation, C₂H₂⁺, and the two isomers of C₃H₄ - propyne and allene. Measurements show a strong dependence on the structural configuration of the neutral reactant on the outcome of the reaction. We were able to identify multiple reactions pathways and measure their reaction rates and branching ratios. Together with high-level quantum-chemical calculations we can understand the distinct reaction mechanisms of the two isomers: For allene long-range charge exchange is favored, while an intermediate reaction complex is formed for propyne.   

Live

The UV photodissociation of the SO₂ C-state was investigated as a spectroscopic target for probing exoplanet atmospheres with potential to support life. Photodissociation was initiated with tunable, pulsed UV light with λ=220-212 nm and nascent SO photoproducts were detected using high-resolution transient IR absorption spectroscopy. Doppler profiles, rotational distributions, and energy partitioning were measured for individual SO spin-rotation states. Doppler profiles established the initial vibrational energy in the SO₂ X-state, yielding the C-state energy and total product energy. The product energy partitioning favors translation over rotation by a factor of four, indicating the equilibrium C-state bent geometry must become more linear prior to dissociation. Multireference ab initio calculations performed as a function of SO₂ geometry show that the dissociation barrier height is energetically accessible for the experimental energies only if the transition state has a linear geometry and includes singlet and triplet coupling to the C-state. The experimental results and ab initio calculations show that SO₂ dissociates with a linear geometry via coupling to a repulsive triplet near its dissociation threshold.   

Live

Silicon monoxide (SiO) rovibrational photodissociation cross sections have been computed using ab initio derived potential curves and electric transition dipole moment functions adjusted to match asymptotic experimental constraints where available. Photodissociation through transitions from nearly the full range of bound levels of the X ¹∑⁺ ground electronic state to excited electronic states of SiO is considered. State-resolved cross sections including predissociative channels have been computed for absorption of photons with wavelengths ranging from 500 Å to threshold. Photodissociation of SiO molecules in local thermal equilibrium at temperatures ranging from 1,000 to 10,000 K is explored. Cross sections are applied to compute photodissociation rates and rate fits for the molecule in the standard interstellar and black-body radiation fields.   

Live

The canonical computational approach of quantum electrodynamics (QED) is not practical for bound states involving multiple correlating electrons. As a result, newer forms^1-3 of QED have been developed to tackle few-body atomic problems and are now being applied to the molecular domain. I will discuss a low-energy reconfiguration of QED especially suitable for molecules, namely non-relativistic QED (NRQED)³, applicable to few-electron, low-Z states. In particular, I will focus on work I am doing to establish the utility of this method for increasingly relativistic bound states, by examining the convergence of computed isoelectronic ionization sequences to experiment. This includes calculation of leading-order relativistic and radiative corrections, as well as consideration of computational subtleties including the Bethe logarithm, finite nuclear mass/size effects, singular expectation values, and α⁶ order corrections.⁴ In doing so, future applications of NRQED to small molecule astrochemistry are suggested, focusing on transitions which may be calculated with accuracies comparable to spectroscopic uncertainties.


1. Shabaev, Phys. Rep., 356, 119
2. Lindgren, Can. J. Phys., 83, 183
3. Caswell, Phys. Lett. B, 167, 437
4. Patkos, Phys. Rev. A, 100, 042510   

Live

As a mysterious existence in the interstellar medium, H₅⁺ has been proposed as the intermediate of the proton transfer reaction, H₃⁺ + H₂ → H₅⁺ → H₂ + H₃⁺. Although products appear identical to reactants, they can possess different nuclear quantum states due to extensive proton scrambling in H₅⁺ driven by large-amplitude motions (LAMs). These LAMs bring challenges to nuclear quantum mechanics as the conventional rigid rotor-harmonic oscillator (RRHO) approximation fails. Herein we introduced diffusion Monte Carlo (DMC) and its extensions to account for the non-rigid and anharmonic properties of H₅⁺. We evaluated the energetics and wave functions for selected nuclear energy levels along proton transfer reaction paths and analyzed the roles of LAMs. We discovered strong couplings among intramolecular modes of H₅⁺, including those between proton hops and dissociations, and between internal and overall rotations. These results managed to decipher and predict the exotic behaviors of H₅⁺ in its rovibrational spectroscopy and reaction dynamics.   

Live

Hydrogen abstraction of molecular hydrogen by carbocations is an important process in the chemical evolution of the interstellar medium. It contributes to the growth of hydrocarbon complexity in molecular clouds and could be responsible for the unexpectedly high abundances of species such as C₂H in photo-dissociation regions. Following the trend of ion-neutral reactions, hydrogen abstraction often occurs barrierlessly if the reaction is exothermic. A notable exception is the reaction H₂ + C₃H₂⁺ → H + C₃H₃⁺, which has a small barrier and proceeds slowly at low temperatures. This reaction was investigated in a cryogenic ion trap, where trapped C₃H₂⁺ ions were exposed to H₂ and subjected to mid-infrared laser radiation. Upon laser excitation of the ν₇ C-H asymmetric stretching mode of c-C₃H₂⁺, the reactivity was greatly reduced. Quantum chemical calculations provided insight into this unusual behavior, revealing a critical pre-reaction complex. In addition, this phenomenon enabled high-resolution vibrational action spectroscopy of c-C₃H₂⁺.   

Live

Ions have been detected in many astrochemical environments but a region of focus is the ionosphere of Titan, where the CASSINI mass spectrometer detected many organic ions, triggering a rich gaseous chemistry [1]. We have measured reaction cross sections and branching ratios for the reaction of H₂CNH⁺ and HCNH₂⁺ ions that have been identified as potential contributors to the m/z 29 peak on Titan. Reacting neutrals are CH₄ as well as unsaturated hydrocarbons and nitriles. Data were collected using a tandem mass RF guide where ions were generated via VUV photoionization at the DESIRS beamline of the SOLEIL synchrotron [2].
We demonstrate that the two isomers show diverse and differing chemistries: in addition to charge transfer, proton-transfer and H-abstraction reactions, they undergo a range of more complex adduct reactions leading to the formation of C-C and C-N bonds.
[1] Vuitton et al., Icarus, 324, 120 (2019)
[2] D. Ascenzi et al. , Frontiers in Chemistry, 7, 537 (2019)   

Live

Many sulfuric compounds are known to be present in the Venus atmosphere. Such species may play an important role in the energy budget of the planet, the formation and destruction of other species, especially when these species reacts between them or with photon. Recently, OSSO was identified as a near-UV absorber in the Venusian atmosphere. This latter is formed from the association of two SO molecules and its destruction leads to SO dimers. In this talk, we will discuss the behavior of the low lying singlet and triplet electronic states and the possible species that may be produced after photon absorption. Using ab-initio and molecular dynamic methods, we investigate the photo-production of S₂, O₂, SO, S₂O and SO₂ from OSSO isomers. Non-adiabatic molecular dynamics methods provide important insight into the importance of photodissociation channels in dictating chemistry in the Venus atmosphere.