Radiative lifetimes of NO A [sup 2]Σ[sup +](v[sup ʹ]=0,1,2) and the electronic transition moment of the A [sup 2]Σ[sup +]−X [sup 2]Π system

mm-wave Rydberg-Rydberg transitions gauge intermolecular coupling in a molecular ultracold plasma

Out-of-equilibrium, strong correlation in a many-body system can trigger emergent properties that act to constrain the natural dissipation of energy and matter. Signs of such self-organization appear in the avalanche, bifurcation, and quench of a state-selected Rydberg gas of nitric oxide to form an ultracold, strongly correlated ultracold plasma. Work reported here focuses on initial stages of avalanche and quench, and uses the mm-wave spectroscopy of an embedded quantum probe to characterize the intermolecular interaction dynamics associated with the evolution to plasma. Double-resonance excitation prepares a Rydberg gas of nitric oxide composed of a single selected state of principal quantum number, n0. Penning ionization, followed by an avalanche of electron-Rydberg collisions, forms a plasma of NO+ ions and weakly bound electrons, in which a residual population of n0 Rydberg molecules evolves to a state of high orbital angular momentum, l. Predissociation depletes the plasma of low- l molecules. Relaxation ceases and n0l(2) molecules with l {greater than or equal to} 4 persist for very long times. At short times, varying excitation spectra of mm-wave Rydberg-Rydberg transitions mark the rate of electron-collisional l-mixing. Deep depletion resonances that persist for long times signal energy redistribution in the basis of central-field Rydberg states. The widths and asymmetries of Fano lineshapes witness the degree to which coupling in the arrested bath i) broadens the allowed transition and ii) mixes the local network of levels in the ensemble.

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.

Temperature perturbation related to the invisible ink vibrationally excited nitric oxide monitoring (VENOM) technique: a simulation study

The limits of applicability of the invisible ink variant of the vibrationally excited nitric oxide monitoring (VENOM) technique for three distinct flow fields is reported in this work. This technique involves the generation of a grid of vibrationally excited NO (X,Π2) by exciting the NO A-X electronic transition at 226 nm, which subsequently relaxes via fluorescence and collisional quenching to produce vibrationally excited NO (X,Π2). This grid is then probed by two laser sheets tuned to distinct rotational states. The resulting images allow for the simultaneous measurement of temperature and velocity. The flow fields presented in this work provide a range of NO concentrations, vibrational lifetimes, pressures, temperatures, and collisional quenching, which explore the applicability of the invisible ink variant to a wide range of conditions. We have modelled the initial NO, O₂, and N₂ vibrational and rotational energy distribution resulting from the combination of fluorescence and quenching of electronically excited NO. The subsequent rethermalization of the sample, in particular the long-time vibrational relaxation, has been modelled using a forced harmonic oscillator model. The time-dependent temperature perturbation due to the invisible ink technique is evaluated for two distinct timescales: a short-timescale temperature rise resulting from collisional quenching and rotational/translational thermalization and a long-timescale temperature rise caused by vibrational thermalization. Under low pressures where fluorescence dominates quenching, there is minimal temperature perturbation of the flow field on the timescale of a VENOM measurement, and the short-timescale temperature perturbation only becomes significant at high NO seed concentrations. The predicted signal-to-noise ratio of the invisible ink method is unaffected for low-pressure, low-temperature flow fields. However, preserving signal-to-noise ratio for a high-temperature, high-pressure flow field could prove challenging due to the impact of quenching and self-absorption. Overall, we find that the invisible ink method is predicted to be a viable laser-based diagnostic for velocimetry and thermometry over a wide range of experimental conditions.

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.

Calibration-free self-absorption model for measuring nitric oxide concentration in a pulsed corona discharge

The effect of self-absorption on emission intensity distributions can be used for species concentration measurements. A calculation model is developed based on the Beer-Lambert law to quantify this effect. And then, a calibration-free measurement method is proposed on the basis of this model by establishing the relationship between gas concentration and absorption strength. The effect of collision parameters and rotational temperature on the method is also discussed. The proposed method is verified by investigating the nitric oxide emission bands (A²Σ⁺→X²∏) that are generated by a pulsed corona discharge at various gas concentrations. Experiment results coincide well with the expectations, thus confirming the precision and accuracy of the proposed measurement method.

Vibrationally excited NO tagging by NO(A²∑⁺) fluorescence and quenching for simultaneous velocimetry and thermometry in gaseous flows

We present measurements demonstrating simultaneous determination of velocity and temperature using a variant of the Vibrationally Excited Nitric Oxide Monitoring (VENOM) technique that does not employ NO2. The variant is based on tagging by electronic excitation of NO in the A²∑(1/2)⁺ (v'=0)←X2Π1/2(v''=0) band and subsequent formation of vibrationally excited NO(X2Π) by spontaneous emission and collisional quenching. Sequential planar laser-induced fluorescence imaging of the nascent NO(X2Π, v''=1) was used to obtain spatially resolved average streamwise velocity and rotational/translational temperature. The temperature determination using this approach extends the applicability of the VENOM technique to low-density, high-speed flows, where slow thermalization of the tagged molecules represents a limiting factor.

Spectroscopic observation of neutral carbon during photodissociation of explosive-related compounds in the vapor phase

We perform time-resolved laser-induced fluorescence measurements of mononitrotoluenes (MNTs) and dinitrotoluenes (DNTs) in nitrogen and air. We observe the multipeak emission spectrum of NO and find that the emission peak intensity in the 247-248 nm range is stronger than expected compared to the other NO emission peak intensities. This increased emission intensity is believed to be due to neutral carbon [C(I)], which has a strong emission peak at 247.85 nm. By comparing the ratios of integrated emission peak intensities with those expected from the Franck-Condon factors for NO, we are able to identify samples that exhibit C(I) emission. We show that the DNTs exhibit C(I) emission for gate delays of 1500 ns and beyond, while the MNTs exhibit C(I) emission for gate delays of only up to about 500 ns. Carbon deposits in the analysis chamber confirm the presence of C. Ambient NO in air enhances the observed NO+C(I) signal from MNTs and DNTs.

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).

Nitric-oxide planar laser-induced fluorescence at 10 kHz in a seeded flow, a plasma discharge, and a flame

This study demonstrates high-repetition-rate planar laser-induced fluorescence (PLIF) imaging of both cold (~300 K) and hot (~2400 K) nitric oxide (NO) at a framing rate of 10 kHz. The laser system is composed of a frequency-doubled dye laser pumped by the third harmonic of a 10 kHz Nd:YAG laser to generate continuously pulsed laser radiation at 226 nm for excitation of NO. The laser-induced fluorescence signal is detected using a high-frame rate, intensified CMOS camera, yielding a continuous cinematographic propagation of the NO plume where data acquisition duration is limited only by camera memory. The pulse energy of the beam is ~20 μJ with a spectral width ~0.15 cm(-1), though energies as high as 40 μJ were generated. Hot NO is generated by passing air through a DC transient-arc plasma torch that dissociates air. The plasma torch is also used to ignite and sustain a CH(4)/air premixed flame. Cold NO is imaged from a 1% NO flow (buffered by nitrogen). The estimated signal-to-noise ratio (SNR) for the cold seeded flow and air plasma exceeds 50 with expected NO concentrations of 6000-8000 parts per million (ppm, volume basis). Images show distinct, high-contrast boundaries. The plasma-assisted flame images have an SNR of less than 10 for concentrations reaching 1000 ppm. For many combustion applications, the pulse energy is insufficient for PLIF measurements. However, the equipment and strategies herein could be applied to high-frequency line imaging of NO at concentrations of 10-100 ppm. Generation of 226 nm radiation was also performed using sum-frequency mixing of the 532 nm pumped dye laser and 355 nm Nd:YAG third harmonic but was limited in energy to 14 μJ. Frequency tripling a 532 nm pumped dye laser produced 226 nm radiation at energies comparable to the 355 nm pumping scheme.

Concentration measurement of NO using self-absorption spectroscopy of the γ band system in a pulsed corona discharge

Nitric oxide (NO) concentrations were measured using the γ band system spectrum based on the strong self-absorption effect of NO in pulsed corona discharges. The radiative transitional intensities of the NO γ band were simulated based on the theory of molecular spectroscopy. The intensities of some bands, especially γ(0,0) and γ(1,0), are weakened by the self-absorption. The correlations between the spectral self-absorption intensities and NO concentration were validated using a modified Beer-Lambert law with a combined factor K relating the branching ratio and the NO concentration, and a nonlinear index α that is applicable to the broadband system. Optical emissive spectra in pulsed corona discharges in NO and N2/He mixtures were used to evaluate the two parameters for various conditions. Good agreement between the experimental and theoretical results verifies the self-absorption behavior seen in the UV spectra of the NO γ bands.

Communication: direct angle-resolved measurements of collision dynamics with electronically excited molecules: NO(A2Σ+) + Ar

We report direct doubly differential (quantum state and angle-resolved) scattering measurements involving short-lived electronically excited molecules using crossed molecular beams. In our experiment, supersonic beams of nitric oxide and argon atoms collide at 90°. In the crossing region, NO molecules are excited to the A(2)Σ(+)state by a pulsed nanosecond laser, undergo rotationally inelastic collisions with Ar atoms, and are then detected 400 ns later (approximately twice the radiative lifetime of the A(2)Σ(+)state) by 1 + 1(') multiphoton ionization via the E(2)Σ(+) state. The velocity distributions of the scattered molecules are recorded using velocity-mapped ion imaging. The resulting images provide a direct measurement of the state-to-state differential scattering cross sections. These results demonstrate that sufficient scattering events occur during the short lifetimes typical of molecular excited states (∼200 ns, in this case) to allow spectroscopically detected quantum-state-resolved measurements of products of excited-state collisions.