Photoelectron spectra from some autoionizing states of O2near the ionization threshold

SO2 Photodissociation at 193 nm Directly Forms S(3P) + O2(3Σg-): Implications for the Archean Atmosphere on Earth

It is well-documented that photodissociation of SO₂ at λ = 193 nm produces O(³P_j) + SO X(³Σ^-). We provide experimental evidence of a new product channel from one-photon absorption producing S(³P_j) + O₂ X(³Σ_g^-) in 2-4% yield. We probe the reactant and all products with time-resolved photoelectron photoion coincidence spectroscopy. High-level ab initio calculations suggest that the new product channel can only occur on the ground-state potential energy surface through internal conversion from the excited state, followed by isomerization to a transient SOO intermediate. Classical trajectories on the ground-state potential energy surface with random initial conditions qualitatively reproduce the experimental yields. This unexpected photodissociation pathway may help reconcile discrepancies in sulfur mass-independent fractionation mechanisms in Earth's geologic history, which shape our understanding of the Archean atmosphere and the Great Oxygenation Event in Earth's evolution.

Initial Steps of Aromatic Ring Formation in a Laminar Premixed Fuel-Rich Cyclopentene Flame†

A fuel-rich, nonsooting, premixed laminar cyclopentene flame (phi = 2.0) at 37.6 Torr (50 mbar) is investigated by flame-sampling photoionization molecular-beam mass spectrometry utilizing vacuum-ultraviolet synchrotron radiation. Mole fractions as a function of distance from the burner are measured for 49 intermediates with ion masses ranging from 2 (H2) to 106 (C8H10), providing a broad database for flame modeling studies. The isomeric composition is resolved for most species, and the identification of several C4Hx, C7H6, and C7H8 isomers is discussed in detail. The presence of C5H5CCH/C5H4CCH2 and cycloheptatriene is revealed by comparisons between flame-sampled photoionization efficiency data and theoretical simulations, based on calculated ionization energies and Franck-Condon factors. This insight suggests a new potential molecular- weight growth mechanism that is characterized by C5-C7 ring enlargement reactions.

Identification of C5HxIsomers in Fuel-Rich Flames by Photoionization Mass Spectrometry and Electronic Structure Calculations

The isomeric composition of C(5)H(x) (x = 2-6, 8) flame species is analyzed for rich flames fueled by allene, propyne, cyclopentene, or benzene. Different isomers are identified by their known ionization energies and/or by comparison of the observed photoionization efficiencies with theoretical simulations based on calculated ionization energies and Franck-Condon factors. The experiments combine flame-sampling molecular-beam mass spectrometry with photoionization by tunable vacuum-UV synchrotron radiation. The theoretical simulations employ the rovibrational properties obtained with B3LYP/6-311++G(d,p) density functional theory and electronic energies obtained from QCISD(T) electronic structure calculations extrapolated to the complete basis set limit. For C(5)H(3), the comparison reveals the presence of both the H(2)CCCCCH (i-C(5)H(3)) and the HCCCHCCH (n-C(5)H(3)) isomer. The simulations also suggest a modest amount of cyclo-CCHCHCCH-, which is consistent with a minor signal for C(5)H(2) that is apparently due to cyclo-CCHCCCH-. For C(5)H(4), contributions from the CH(2)CCCCH(2) (1,2,3,4-pentatetraene), CH(2)CCHCCH, and CH(3)CCCCH (1,3-pentadiyne) isomers are evident, as is some contribution from CHCCH(2)CCH (1,4-pentadiyne) in the cyclopentene and benzene flames. Signal at m/z = 65 originates mainly from the cyclopentadienyl radical. For C(5)H(6), contributions from cyclopentadiene, CH(3)CCCHCH(2), CH(3)CHCHCCH, and CH(2)CHCH(2)CCH are observed. No signal is observed for C(5)H(7) species. Cyclopentene, CH(2)CHCHCHCH(3) (1,3-pentadiene), CH(3)CCCH(2)CH(3) (2-pentyne), and CH(2)CHCH(2)CHCH(2) (1,4-pentadiene) contribute to the signal at m/z = 68. Newly derived ionization energies for i- and n-C(5)H(3) (8.20 +/- 0.05 and 8.31 +/- 0.05 eV, respectively), CH(2)CCHCCH (9.22 +/- 0.05 eV), and CH(2)CHCH(2)CCH (9.95 +/- 0.05 eV) are within the error bars of the QCISD(T) calculations. The combustion chemistry of the observed C(5)H(x) intermediates and the impact on flame chemistry models are discussed.

Identification and Chemistry of C4H3 and C4H5 Isomers in Fuel-Rich Flames

Quantitative identification of isomers of hydrocarbon radicals in flames is critical to understanding soot formation. Isomers of C4H3 and C4H5 in flames fueled by allene, propyne, cyclopentene, or benzene are identified by comparison of the observed photoionization efficiencies with theoretical simulations based on calculated ionization energies and Franck-Condon factors. The experiments combine molecular-beam mass spectrometry (MBMS) with photoionization by tunable vacuum-ultraviolet synchrotron radiation. The theoretical simulations employ the rovibrational properties obtained with B3LYP/6-311++G(d,p) density functional theory and electronic energies obtained from QCISD(T) ab initio calculations extrapolated to the complete basis set limit. For C4H3, the comparisons reveal the presence of the resonantly stabilized CH2CCCH isomer (i-C4H3). For C4H5, contributions from the CH2CHCCH2 (i-C4H5) and some combination of the CH3CCCH2 and CH3CHCCH isomers are evident. Quantitative concentration estimates for these species are made for allene, cyclopentene, and benzene flames. Because of low Franck-Condon factors, sensitivity to n-isomers of both C4H3 and C4H5 is limited. Adiabatic ionization energies, as obtained from fits of the theoretical predictions to the experimental photoionization efficiency curves, are within the error bars of the QCISD(T) calculations. For i-C4H3 and i-C4H5, these fitted adiabatic ionization energies are (8.06 +/- 0.05) eV and (7.60 +/- 0.05) eV, respectively. The good agreement between the fitted and theoretical ionization thresholds suggests that the corresponding theoretically predicted radical heats of formation (119.1, 76.3, 78.7, and 79.1 kcal/mol at 0 K for i-C4H3, i-C4H5, CH3CCCH2, and CH3CHCCH, respectively) are also quite accurate.

Physics of molecular ion formation

A review of the basic processes in molecular photoionization is made in the light of the classical as well as recent new theoretical models and illustrated on O(2) and N(2)O.