CRAHCN-O: A Consistent Reduced Atmospheric Hybrid Chemical Network Oxygen Extension for Hydrogen Cyanide and Formaldehyde Chemistry in CO2-, N2-, H2O-, CH4-, and H2-Dominated Atmospheres

Hydrogen cyanide (HCN) and formaldehyde (H₂CO) are key precursors to biomolecules such as nucleobases and amino acids in planetary atmospheres. However, many reactions which produce and destroy these species in atmospheres containing CO₂ and H₂O are still missing from the literature. We use a quantum chemistry approach to find these missing reactions and calculate their rate coefficients using canonical variational transition state theory and Rice-Ramsperger-Kassel-Marcus/master equation theory at the BHandHLYP/aug-cc-pVDZ level of theory. We calculate the rate coefficients for 126 total reactions and validate our calculations by comparing with experimental data in the 39% of available cases. Our calculated rate coefficients are most frequently within a factor of 2 of experimental values and generally always within an order of magnitude of these values. We discover 45 previously unknown reactions and identify 6 from this list that are most likely to dominate H₂CO and HCN production and destruction in planetary atmospheres. We highlight ¹O + CH₃ → H₂CO + H as a new key source and H₂CO + ¹O → HCO + OH as a new key sink, for H₂CO in upper planetary atmospheres. In this effort, we develop an oxygen extension to our consistent reduced atmospheric hybrid chemical network (CRAHCN-O), building off our previously developed network for HCN production in N₂-, CH₄-, and H₂-dominated atmospheres (CRAHCN). This extension can be used to simulate both HCN and H₂CO production in atmospheres dominated by any of CO₂, N₂, H₂O, CH₄, and H₂.

Reinvestigation of the Rydberg W1Π(ν = 1) level of 12C16O, 13C16O, and 12C18O through rotationally dependent photodissociation branching ratio measurements

A recent high resolution photoabsorption study revealed that the Rydberg W¹Π(ν = 1) level of carbon monoxide (CO) is perturbed by the valence E″¹Π(ν = 0) level, and the predissociation linewidth shows drastic variation at the crossing point due to the interference effect [Heays et al., J. Chem. Phys. 141(14), 144311 (2014)]. Here, we reinvestigate the Rydberg W¹Π(ν = 1) level for the three CO isotopologues, ¹²C¹⁶O, ¹³C¹⁶O, and ¹²C¹⁸O, by measuring the rotationally dependent photodissociation branching ratios. The C⁺ ion photofragment spectra obtained here reproduce the recent high resolution photoabsorption spectra very well, including the presence of the valence E″¹Π(ν = 0) level. The photodissociation branching ratios into the spin-forbidden channel C(¹D) + O(³P) show sudden increases at the crossing point between the W¹Π(ν = 1) and E″¹Π(ν = 0) levels, which is in perfect accordance with the drastic variation of the linewidth observed in the recent spectroscopic study. Further analysis reveals that the partial predissociation rate into the lowest channel C(³P) + O(³P) shows a much more prominent decrease at the crossing point, which is caused by the interference effect between the W¹Π(ν = 1) and E″¹Π(ν = 0) levels, than that into the spin-forbidden channel C(¹D) + O(³P), and this is the reason of the sudden increase as observed in the photodissociation branching ratio measurements. We hope that the current experimental investigation will stimulate further theoretical studies, which could thoroughly address all the experimental observations in a quantitative way.

Strong and selective isotope effect in the vacuum ultraviolet photodissociation branching ratios of carbon monoxide

Rare isotope (¹³C, ¹⁷O and ¹⁸O) substitutions can substantially change absorption line positions, oscillator strengths and photodissociation rates of carbon monoxide (CO) in the vacuum ultraviolet (VUV) region, which has been well accounted for in recent photochemical models for understanding the large isotopic fractionation effects that are apparent in carbon and oxygen in the solar system and molecular clouds. Here, we demonstrate a strong isotope effect associated with the VUV photodissociation of CO by measuring the branching ratios of ¹²C¹⁶O and ¹³C¹⁶O in the Rydberg 4p(2), 5p(0) and 5s(0) complex region. The measurements show that the quantum yields of electronically excited C atoms in the photodissociation of ¹³C¹⁶O are dramatically different from those of ¹²C¹⁶O, revealing strong isotope effect. This isotope effect strongly depends on specific quantum states of CO being excited, which implies that such effect must be considered in the photochemical models on a state by state basis.

Rotational dependence of the branching ratios and fragment angular distributions for the photodissociation of 12C16O in the Rydberg 4p(2) and 5p(0) complex region (92.84-93.37 nm)

¹²C¹⁶O shows complicated absorption structures in the wavelength range of 92.84-93.37 nm caused by the mutual interactions among the Rydberg 4p(2) and 5p(0) complexes, and several diffuse valence states. Here, we systematically measured the branching ratios and fragment angular distributions for the photodissociation of ¹²C¹⁶O in the wavelength range using our mini-time-slice velocity-map imaging (mini-TSVMI) setup and a tunable vacuum ultraviolet (VUV) laser radiation source generated by the two-photon resonance-enhanced four-wave mixing scheme. Various patterns of rotational dependence for the photodissociation branching ratios have been observed for different vibronic states, and they are found to be consistent with previous spectroscopic investigations, revealing the complicated coupling schemes and predissociation dynamics in this region. Irregular angular distributions of the photofragments have been observed, especially in the Rydberg 5p(0) complex region. This has been attributed to the simultaneous excitation of multiple states with different symmetries in this region. The newly observed underlying continuum in the Rydberg 5p(0) complex region has been directly detected in the present study, and it is found to be of ¹Σ⁺ symmetry and dissociates predominantly into the C(³P) + O(³P) channel. This study generally confirms the results of previous spectroscopic studies from a different perspective, and adds new knowledge for understanding the complicated predissociation dynamics of ¹²C¹⁶O in the titled wavelength range.

Branching Ratios of the N(2D03/2) and N(2D05/2) Spin-Orbit States Produced in the State-Selected Photodissociation of N2 Determined Using Time-Sliced Velocity-Mapped-Imaging Photoionization Mass Spectrometry (TS-VMI-PI-MS)

Branching ratios for N(²D⁰_3/2) and N(²D⁰_5/2) produced by predissociation of state selected excited nitrogen molecules in the vacuum ultraviolet region have been measured for the first time. The quantum numbers of the excited nitrogen molecule are defined by selective excitation of the nitrogen molecule in the Franck-Condon region from the ground electronic, ¹Σ_g⁺, vibrational, v″, and rotational, J″ state to an excited E_u', v', J' state with a tunable vacuum ultraviolet, VUV₁, laser. The neutral atoms produced by predissociation from this excited state are then selectively ionized with a second tunable VUV₂ laser. Measurement of the relative populations of these two atoms formed in their spin-orbit states defines the quantum states for the atomic products. This means that the wave functions of the initial state and knowledge of the relative yields define all the experimental parameters for this series of unimolecular reactions. The ions formed by VUV₂ are mass analyzed with a time-of-flight mass spectrometer and detected with a time slice velocity ion imaging mass spectrometer. In this manner, we can determine the recoil velocity associated with the predissociation process. Two different techniques are used to determine the spin-orbit ratios, namely, resonant VUV photoionization (RVUV-PI) spectroscopy and total kinetic energy release (TKER) spectroscopy determined from the image produced when the atoms are selectively ionized by VUV₂ in the interaction region. The TKER spectra obtained from the lines at 110 296.25 and 110 304.96 cm^-1 that couple to a newly discovered autoionization line at 129 529.4255 ± 0.0015 cm^-1 prove that the lines observed in this region originate from the N(²D⁰_3/2) and N(²D⁰_5/2) atoms. Two other lines in this region at 110 286.20 and 110 299.89 cm^-1 originate from the nitrogen N(⁴S⁰_3/2) that is photoionized in a 1+ 1 VUV-UV resonant multiphoton ionization process. The spin-orbit branching ratios have been evaluated for valence and Rydberg electronic excited states from 104 129.4 to 118 772.1 cm^-1, and it shows that they are independent of the rotational and vibrational quantum numbers. They are not appreciably affected by the symmetry properties of the wave function in the Franck-Condon region of the excited states. In the energy region below 117 153.8 cm^-1 the pathways at long internuclear distances appear to determine [N(²D⁰_3/2)]/[N(²D⁰_5/2)] branching ratios of ∼0.38, ∼0.62, and ∼1.04. At higher energies, TKER and RVUV-PI spectroscopy have been used to show that the average fraction of the N(²D⁰_3/2) and N(²D⁰_5/2) atoms produced in the spin-allowed channels that produce two N(²D⁰_J) is 0.85 versus 0.15 for spin-forbidden channels. The importance and need for this information for comparison with theory and applications in astrochemistry are briefly discussed.

Branching Ratio Measurements of the Predissociation of 12C16O by Time-Slice Velocity-Map Ion Imaging in the Energy Region from 106 250 to 107 800 cm-1

Photodissociation of CO is a fundamental chemical mechanism for mass-independent oxygen isotope fractionation in the early Solar System. Branching ratios of photodissociation channels for individual bands quantitatively yield the trapping efficiencies of atomic oxygen resulting into oxides. We measured the branching ratios for the spin-forbidden and spin-allowed photodissociation channels of ¹²C¹⁶O in the vacuum ultraviolet (VUV) photon energy region from 106 250 to 107 800 cm^-1 using the VUV laser time-slice velocity-map imaging photoion technique. The excitations to four ¹Π bands and three ¹Σ⁺ bands of ¹²C¹⁶O were identified and investigated. The branching ratios for the product channels C(³P) + O(³P), C(¹D) + O(³P), and C(³P) + O(¹D) of these predissociative states strongly depend on the electronic and vibrational states of CO being excited. By plotting the branching ratio of the spin-forbidden dissociation channels versus the excitation energy from 102 500 to 110 500 cm^-1 that has been measured so far, the global pattern of the ¹Π-³Π interaction that plays a key role in the predissociation of CO is revealed and discussed.