High temperature quenching cross sections for nitric oxide laser‐induced fluorescence measurements

Excited-state van der Waals potential energy surfaces for the NO A2Σ+ + CO2X1Σg+ collision complex

Excited state van der Waals (vdW) potential energy surfaces (PESs) of the NO A2Σ+ + CO2X1Σg+ system are thoroughly investigated using coupled cluster theory and complete active space perturbation theory to second order (CASPT2). First, it is shown that pair natural orbital coupled cluster singles and doubles with perturbative triples yields comparable accuracy compared to CCSD(T) for molecular properties and vdW-minima at a fraction of computational cost of the latter. Using this method in conjunction with highly diffuse basis sets and counterpoise correction for basis set superposition error, the PESs for different intermolecular orientations are investigated. These show numerous vdW-wells, interconnected for all geometries except one, with a maximum depth of up to 830 cm-1; considerably deeper than those on the ground state surface. Multi-reference effects are investigated with CASPT2 calculations. The long-range vdW-surfaces support recent experimental observations relating to rotational energy transfer due the anisotropy in the potentials.

Theoretical Study of the Photochemical Mechanisms of the Electronic Quenching of NO(A2Σ+) with CH4, CH3OH, and CO2

The electronic quenching of NO(A2Σ+) with molecular partners occurs through complex non-adiabatic dynamics that occurs on multiple coupled potential energy surfaces. Moreover, the propensity for NO(A2Σ+) electronic quenching depends heavily on the strength and nature of the intermolecular interactions between NO(A2Σ+) and the molecular partner. In this paper, we explore the electronic quenching mechanisms of three systems: NO(A2Σ+) + CH4, NO(A2Σ+) + CH3OH, and NO(A2Σ+) + CO2. Using EOM-EA-CCSD calculations, we rationalize the very low electronic quenching cross-section of NO(A2Σ+) + CH4 as well as the outcomes observed in previous NO + CH4 photodissociation studies. Our analysis of NO(A2Σ+) + CH3OH suggests that it will undergo facile electronic quenching mediated by reducing the intermolecular distance and significantly stretching the O-H bond of CH3OH. For NO(A2Σ+) + CO2, intermolecular attractions lead to a series of low-energy ON-OCO conformations in which the CO2 is significantly bent. For both the NO(A2Σ+) + CH3OH and NO(A2Σ+) + CO2 systems, we see evidence of the harpoon mechanism and low-energy conical intersections between NO(A2Σ+) + M and NO(X2Π) + M. Overall, this work provides the first detailed theoretical study on the NO(A2Σ+) + M potential energy surface of each of these systems and will inform future velocity map imaging experiments.

Reactive quenching of NO (A2Σ+) with H2O leads to HONO: a theoretical analysis of the reactive and nonreactive electronic quenching mechanisms

In this study, we develop a mechanistic understanding of the pathways for nonreactive and reactive electronic quenching of NO (A2Σ+) with H2O. In doing so, we identify a photochemical mechanism for HONO production in the upper atmosphere.

Stereodynamic Control of Collision-Induced Nonadiabatic Dynamics of NO (A2Σ+) with H2, N2, and CO: Intermolecular Interactions Drive Collision Outcomes

Intermolecular interactions, stereodynamics, and coupled potential energy surfaces (PESs) all play a significant role in determining the outcomes of molecular collisions. A detailed knowledge of such processes is often essential for a proper interpretation of spectroscopic observations. For example, nitric oxide (NO), an important radical in combustion and atmospheric chemistry, is commonly quantified using laser-induced fluorescence on the A²Σ⁺ ← X²Π transition band. However, the electronic quenching of NO (A²Σ⁺) with other molecular species provides alternative nonradiative pathways that compete with fluorescence. While the cross sections and rate constants of NO (A²Σ⁺) electronic quenching have been experimentally measured for a number of important molecular collision partners, the underlying photochemical mechanisms responsible for the electronic quenching are not well understood. In this paper, we describe the development of high-quality PESs that provide new physical insights into the intermolecular interactions and conical intersections that facilitate the branching between the electronic quenching and scattering of NO (A²Σ⁺) with H₂, N₂, and CO. The PESs are calculated at the EOM-EA-CCSD/d-aug-cc-pVTZ//EOM-EA-CCSD/aug-cc-pVDZ level of theory, an approach that ensures a balanced treatment of the valence and Rydberg electronic states and an accurate description of the open-shell character of NO. Our PESs show that H₂ is incapable of electronically quenching NO (A²Σ⁺) at low collision energies; instead, the two molecules will likely undergo scattering. The PESs of NO (A²Σ⁺) with N₂ and CO are highly anisotropic and demonstrate evidence of electron transfer from NO (A²Σ⁺) into the lowest unoccupied molecular orbital of the collision partner, that is, the harpoon mechanism. In the case of ON + CO, the PES becomes strongly attractive at longer intermolecular distances and funnels population to a conical intersection between NO (A²Σ⁺) + CO and NO (X²Π) + CO. In contrast, for ON + N₂, the conical intersection is preceded by an ∼0.40 eV barrier. Overall, our work shines new light into the impact of coupled PESs on the nonadiabatic dynamics of open-shell systems.

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%.

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.

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).

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

The quenching rates of NO (A(2)Σ(+), v = 0) with He, Ne, Ar, Kr and Xe have been studied at room temperature by measurements of the time dependence of the fluorescence decay following laser excitation. The rates are slow, with upper limits of rate constants determined as between 1.2 and 2.0 × 10(-14) cm(3) molecule(-1) s(-1), considerably lower than those reported before in the literature. Such slow rates can be markedly influenced by impurities such as O2 and H2O which have quenching rate constants close to gas kinetic values. Time resolved Fourier transform infrared emission has been used to observe the products of the quenching processes with the rare gases and with impurities. For He, Ne Ar and Kr there is no difference within experimental error of the populations in NO (X(2)Π v ≥ 2) produced with and without rare gas present, but the low quantum yields of such quenching (of the order of 5% for an atmosphere of rare gas) preclude quantitative information on the quantum states being obtained. For quenching by Xe the collisional formation of electronically excited Xe atoms dominates the emission at early times. Vibrationally excited NO (X(2)Π, v) and products of reactive quenching are observed in the presence of O2 and H2O.

Products of the quenching of NO A 2Σ+ (v = 0) by N2O and CO2

Collisional quenching of NO A (2)Σ(+) (v = 0) by N(2)O and CO(2) has been studied through measurements of vibrationally excited products by time resolved Fourier transform infrared emission. In both cases vibrationally excited NO X (2)Π (v) is seen and quantified in levels v≥ 2 with distributions which are close to statistical. However the quantum yields to produce these levels are markedly different for the two quenchers. For CO(2) such quenching accounts for only ca. 26% of the total: for N(2)O it is ca. 85%. Far more energy is seen in the internal modes of the CO(2) product than those of N(2)O. The results are rationalised in terms of cleavage of the N(2)-O bond being dominant in the latter case, with either a similar O atom production or a specific channel producing almost exclusively NO in low vibrational levels (v = 0,1) for quenching by CO(2). Minor reactive channels yielding NO(2) are seen in both cases, and O((1)D) is observed with low quantum yield in the reaction with N(2)O. The results are discussed in terms of previous models of the quenching processes, and are consistent with the very high yield of NO X (2)Π (v = 0) previously observed by laser induced fluorescence for quenching of NO A (2)Σ(+) (v = 0) by CO(2).

Collisional quenching of NO A 2Sigma+(v' = 0) between 125 and 294 K

We report measurements of the temperature-dependent cross sections for the quenching of fluorescence from the A (2)Sigma(+)(v(')=0) state of NO for temperatures between 125 and 294 K. Thermally averaged cross sections were measured for quenching by NO(X (2)Pi), N(2), O(2), and CO in a cryogenically cooled gas flow cell. Picosecond laser-induced fluorescence was time resolved, and the thermally averaged quenching cross sections were determined from the dependence of the fluorescence decay rate on the quencher-gas pressure. These measurements extend to lower temperature the range of previously published results for NO and O(2) and constitute the first reported measurements of the N(2) and CO cross sections for temperatures below 294 K. Between 125 and 294 K, a negative temperature dependence is observed for quenching by NO, O(2), and CO, implicating collision-complex formation in all three cases. Over the same temperature range, a constant, nonzero cross section is measured for quenching by N(2). Updated empirical models for the temperature dependence of the cross sections between 125 and 4500 K are recommended based on weighted least-squares fits to the current low-temperature results and previously published measurements at higher temperature. The results of over 250 measurements presented here indicate that the collisionless lifetime of NO A (2)Sigma(+)(v(')=0) is approximately 192 ns.

Temperature- and species-dependent quenching of NO A 2Sigma+(v'=0) probed by two-photon laser-induced fluorescence using a picosecond laser

We report improved measurements of the temperature-dependent cross sections for the quenching of fluorescence from the A 2Sigma+(v'=0) state of NO. Cross sections were measured for gas temperatures ranging from 294 to 1300 K for quenching by NO(X (2)Pi), H(2)O, CO(2), O(2), CO, N(2), and C(2)H(2). The A 2Sigma+(v'=0) state was populated via two-photon excitation with a picosecond laser at 454 nm, and the decay rate of the fluorescence originating from A 2Sigma+(v'=0) was measured directly. Thermally averaged quenching cross sections were determined from the dependence of the fluorescence decay rate on the quencher gas pressure. Our measurements are compared to previous measurements and models of the quenching cross sections, and new empirical fits to the data are presented. Our new cross-section data enable predictions in excellent agreement with prior measurements of the fluorescence lifetime in an atmospheric-pressure methane-air diffusion flame. The agreement resolves discrepancies between the lifetime measurements and predictions based on the previous quenching models, primarily through improved models for the quenching by H(2)O, CO(2), and O(2) at temperatures less than 1300 K.

NO Reburn and Formation Chemistry in Methane Diffusion Flames

We have applied time-resolved picosecond linear laser-induced fluorescence to obtain spatially resolved profiles of NO in laminar methane/air and methane/nitric oxide/air counterflow diffusion flames at atmospheric pressure. Temperature profiles have been measured with broadband CARS of nitrogen. Flames at strain rates from 59 s−1 to 269 s−1 were studied to characterize the strain rate dependence of the NO concentration. The work shows that NO concentrations decrease with increasing strain rate. Comparisons have been made with predicted NO levels obtained using two different chemical kinetic mechanisms (Lindstedt and co-workers and GRI-Mech. 3.0). Computed concentrations are shown to be in good agreement with experimental data. The addition of up to 600 ppm NO to the fuel did permit an assessment of differences in the reburn chemistry between the two mechanisms.

Strategies for laser-induced fluorescence detection of nitric oxide in high-pressure flames. III. Comparison of A-X excitation schemes

Laser-induced fluorescence (LIF) has proven a reliable technique for nitric oxide (NO) diagnostics in practical combustion systems. However, a wide variety of different excitation and detection strategies are proposed in the literature without giving clear guidelines of which strategies to use for a particular diagnostic situation. We give a brief review of the high-pressure NO LIF diagnostics literature and compare strategies for exciting selected transitions in the A-X(0, 0), (0, 1), and (0, 2) bands using a different detection bandpass. The strategies are compared in terms of NO LIF signal strength, attenuation of laser and signal light in the hot combustion gases, signal selectivity against LIF interference from O2 and CO2, and temperature and pressure sensitivity of the LIF signal. The discussion is based on spectroscopic measurements in laminar premixed methane-air flames at pressures between 1 and 60 bars and on NO and O2 LIF spectral simulations.

Dinitrobenzene detection by use of one-color laser photolysis and laser-induced fluorescence of vibrationally excited NO

Trace concentrations of 1, 4-dinitrobenzene (DNB) are detected by a combination of laser photolysis and laser-induced fluorescence. A one-color laser is applied to induce DNB photodissociation and for subsequent detection of NO photofragments by excitation and emission through A(v' =0) <-- X(v" = 0 - 2) and A(v' =0) --> X(v" = 0,1) transitions, respectively. The resulting NO rovibrational excitation spectra serve as markers for the presence of DNB. The NO is produced in vibrational ground and excited states with peak height ratios of (v" = 0):(v" = 1):(v" = 2) = 1:0.5:0.13. The limits of detection of DNB mixed with 100 or 500 Torr of air with v" = 2 excitation at 248 nm are 13 and 11 parts in 10(9) by weight, respectively, for a 30-s integration time. The application of this scheme for DNB detection has the advantage that no ambient ground state NO interferes and that the fluorescence is collected at shorter wavelengths than the exciting radiation, precluding background fluorescence.

Measurement of quenching cross sections for laser-induced fluorescence of atomic arsenic

The measurement of collisional quenching cross sections for the (4p)(2) (5s)(1) (4)P(1/2) state of atomic arsenic is reported. The arsenic (4)P(1/2) state was prepared by excitation from the (4p)(3) (4)S(3/2) ground state with 197.2-nm laser radiation. The fluorescence signal from the (4p)(2) (5s)(1) (4)P(1/2) ? (4p)(3) (2)D(1/2) transition was monitored at 249.3 nm. Quenching cross sections were obtained for hydrogen, methane, nitrogen, carbon monoxide, and ethylene.

Temperature measurements in gases by use of planar laser-induced fluorescence imaging of NO

Two techniques based on planar laser-induced fluorescence of NO are applied to the measurement of two-dimensional temperature fields in gaseous flows. In the single-line technique, the NO fluorescence signal, which is in general a function of temperature, pressure, and mole fraction, can be reduced to a function of temperature alone. In this limit, a single measurement of fluorescence can be directly related to temperature. In contrast, in the two-line thermometry technique the ratio of fluorescence signals resulting from excitation of two different rovibronic states is related to the fractional populations in the initial states, which are solely a function of temperature. The one-line method is applied to the study of a laminar heated jet, and the two-line technique is used to measure temperature in a supersonic underexpanded jet. In addition, energy transfer in NO laser-induced fluorescence is analyzed with multilevel rate equation models. Finally, an accurate model is developed for prediction of the temperature dependence of the NO fluorescence signal.