Rate constants for collisional quenching of NO (A(2)Σ(+), v = 0) by He, Ne, Ar, Kr, and Xe, and infrared emission accompanying rare gas and impurity quenching

Molecular properties and excited state van der Waals potentials in the NO A2Σ+ + O2 XΣg- collision complex

We characterize NO A²Σ⁺ + O₂ X³Σ_g^- van der Waals (vdW) Potential Energy Surface (PES) with RHF/RCCSD(T) and CASSCF/CASPT2 calculations. To do this, we first assess our computational setup to properly represent the individual molecular properties of O₂ X³Σ_g^-, NO X²Π, and NO A²Σ⁺. Specifically, we show that highly augmented basis sets are necessary to properly represent the NO A²Σ⁺ polarizability. Then, we optimize different vdW geometries, and provide BSSE corrected plots of the quartet vdW PES. The surfaces show a confined channel at a distance of approximately 6 Å with a depth of at least 20 cm^-1 that we believe is caused by NO A²Σ⁺ hyper-polarizability. At shorter distances, the channel is connected to a vdW basin centered around the O-N O-O linear geometry with an inter-molecular separation of 4.3 Å, and a depth of 95 cm^-1 at the RCCSD(T) level. A CASPT2 scan along the linear geometry show that this vdW basin exists on both the doublet and quartet excited surfaces. These results infer the existence of a collision complex between NO A²Σ⁺ and O₂ X³Σ_g^-, as predicted by earlier experiments.

Time-resolved observations of vibrationally excited NO X 2Π (v') formed from collisional quenching of NO A 2Σ+ (v = 0) by NO X 2Π: evidence for the participation of the NO a 4Π state

Time-resolved observations have been made of the formation of vibrationally excited NO X ²Π (v') following collisional quenching of NO A ²Σ⁺ (v = 0) by NO X ²Π (v = 0). Two time scales are observed, namely a fast production rate consistent with direct formation from the quenching of the electronically excited NO A state, together with a slow component, the magnitude and rate of formation of which depend upon NO pressure. A reservoir state formed by quenching of NO A ²Σ⁺ (v = 0) is invoked to explain the observations, and the available evidence points to this state being the first electronically excited state of NO, a ⁴Π. The rate constant for quenching of the a ⁴Π state to levels v' = 11-16 by NO is measured as (8.80 ± 1.1) × 10^-11 cm³ molecule^-1 s^-1 at 298 K where the error quoted is two standard deviations, and from measurements of the increased formation of high vibrational levels of NO(X) by the slow process we estimate a lower limit for the fraction of self-quenching collisions of NO A ²Σ⁺ (v = 0) which lead to NO a ⁴Π as 19%.

Characterization of two photon excited fragment spectroscopy (TPEFS) for HNO3 detection in gas-phase kinetic experiments

We have developed and tested two-photon excited fragment spectroscopy (TPEFS) for detecting HNO₃ in pulsed laser photolysis kinetic experiments. Dispersed (220-330 nm) and time-dependent emission at (310 ± 5) nm following the 193 nm excitation of HNO₃ in N₂, air and He was recorded and analysed to characterise the OH(A²Σ) and NO(A²Σ⁺) electronic excited states involved. The limit of detection for HNO₃ using TPEFS was ∼5 × 10⁹ molecule cm^-3 (at 60 torr N₂ and 180 μs integration time). Detection of HNO₃ using the emission at (310 ± 5 nm) was orders of magnitude more sensitive than detection of NO and NO₂, especially in the presence of O₂ which quenches NO(A²Σ⁺) more efficiently than OH(A²Σ). While H₂O₂ (and possibly HO₂) could also be detected by 193 nm TPEFS, the relative sensitivity (compared to HNO₃) was very low. The viability of real-time TPEFS detection of HNO₃ using emission at (310 ± 5) nm was demonstrated by monitoring HNO₃ formation in the reaction of OH + NO₂ and deriving the rate coefficient, k₂. The value of k₂ obtained at 293 K and pressures of 50-200 torr is entirely consistent with that obtained by simultaneously measuring the OH decay and is in very good agreement with the most recent literature values.

Imaging the nonreactive collisional quenching dynamics of NO (A2Σ+) radicals with O2 (X3Σg -)

Nitric oxide (NO) radicals are ubiquitous chemical intermediates present in the atmosphere and in combustion processes, where laser-induced fluorescence is extensively used on the NO (A²Σ⁺ ← X²Π) band to report on fuel-burning properties. However, accurate fluorescence quantum yields and NO concentration measurements are impeded by electronic quenching of NO (A²Σ⁺) to NO (X²Π) with colliding atomic and molecular species. To improve predictive combustion models and develop a molecular-level understanding of NO (A²Σ⁺) quenching, we report the velocity map ion images and product state distributions of NO (X²Π, v″ = 0, J″, F_n, Λ) following nonreactive collisional quenching of NO (A²Σ⁺) with molecular oxygen, O₂ (X³Σ_g ^-). A novel dual-flow pulse valve nozzle is constructed and implemented to carry out the NO (A²Σ⁺) electronic quenching studies and to limit NO₂ formation. The isotropic ion images reveal that the NO-O₂ system evolves through a long-lived NO₃ collision complex prior to formation of products. Furthermore, the corresponding total kinetic energy release distributions support that O₂ collision coproducts are formed primarily in the c¹Σ_u ^- electronic state with NO (X²Π, v″ = 0, J″, F_n, Λ). The product state distributions also indicate that NO (X²Π) is generated with a propensity to occupy the Π(A″) Λ-doublet state, which is consistent with the NO π^* orbital aligned perpendicular to nuclear rotation. The deviations between experimental results and statistical phase space theory simulations illustrate the key role that the conical intersection plays in the quenching dynamics to funnel population to product rovibronic levels.

Predicting acoustic relaxation absorption in gas mixtures for extraction of composition relaxation contributions

The existing molecular relaxation models based on both parallel relaxation theory and series relaxation theory cannot extract the contributions of gas compositions to acoustic relaxation absorption in mixtures. In this paper, we propose an analytical model to predict acoustic relaxation absorption and clarify composition relaxation contributions based on the rate-determining energy transfer processes in molecular relaxation in excitable gases. By combining parallel and series relaxation theory, the proposed model suggests that the vibration-translation process of the lowest vibrational mode in each composition provides the primary deexcitation path of the relaxation energy, and the rate-determining vibration-vibration processes between the lowest mode and others dominate the coupling energy transfer between different modes. Thus, each gas composition contributes directly one single relaxation process to the molecular relaxation in mixture, which can be illustrated by the decomposed acoustic relaxation absorption spectrum of the single relaxation process. The proposed model is validated by simulation results in good agreement with experimental data such as N2, O2, CO2, CH4 and their mixtures.

An FTIR emission study of the products of NO A2Σ+ (v = 0, 1) + O2 collisions

Collisional quenching of NO A²Σ⁺ (v = 0, 1) by O₂ has been studied through the detection of vibrationally excited products by time-resolved Fourier transform infrared emission spectroscopy. Non-reactive quenching of NO A²Σ⁺ (v = 0) produces a vibrational distribution in NO X²Π which has been quantified for v = 2-22, and is found to be bimodal. The results are consistent with two quenching channels. The first forms the ground X³Σ or low-lying a ¹Δ_g electronic state of O₂ with a distribution including high vibrational levels of NO X²Π which is slightly hotter than statistical. Two possibilities are identified for the second channel. The first, with a similar quantum yield to that producing higher vibrational levels, forms a highly electronically excited state, such as O₂ c¹Σ, with low vibrational levels in NO X²Π which are inverted with a distribution resembling that resulting from a sudden or harpoon mechanism. The second is that ground state oxygen is formed with low vibrational energy partitioned into NO X²Π. In addition, vibrationally excited NO₂ is observed, but at intensities which indicate that it is formed in low quantum yield. Quantitatively unobservable processes (defined as those which do not form ground state NO (v ≥ 2)) are found to have a branching ratio of at most 25 ± 5%. The results are compared with those of previous studies and the most consistent interpretation suggests that dissociation of O₂ to form ground state O(³P) atoms and ground vibrational state NO X²Π (v = 0) is the main reactive process rather than NO₂ formation. Qualitatively similar results are seen for the quenching of NO A²Σ⁺ (v = 1).

Rotationally inelastic scattering of NO(A(2)Σ(+)) + Ar: Differential cross sections and rotational angular momentum polarization

We present the implementation of a new crossed-molecular beam, velocity-map ion-imaging apparatus, optimized for collisions of electronically excited molecules. We have applied this apparatus to rotational energy transfer in NO(A(2)Σ(+), v = 0, N = 0, j = 0.5) + Ar collisions, at an average energy of 525 cm(-1). We report differential cross sections for scattering into NO(A(2)Σ(+), v = 0, N' = 3, 5, 6, 7, 8, and 9), together with quantum scattering calculations of the differential cross sections and angle dependent rotational alignment. The differential cross sections show dramatic forward scattered peaks, together with oscillatory behavior at larger scattering angles, while the rotational alignment moments are also found to oscillate as a function of scattering angle. In general, the quantum scattering calculations are found to agree well with experiment, reproducing the forward scattering and oscillatory behavior at larger scattering angles. Analysis of the quantum scattering calculations as a function of total rotational angular momentum indicates that the forward scattering peak originates from the attractive minimum in the potential energy surface at the N-end of the NO. Deviations in the quantum scattering predictions from the experimental results, for scattering at angles greater than 10°, are observed to be more significant for scattering to odd final N'. We suggest that this represents inaccuracies in the potential energy surface, and in particular in its representation of the difference between the N- and O-ends of the molecule, as given by the odd-order Legendre moments of the surface.