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%.

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).

Laser initiated reactions in N2O clusters studied by time-sliced ion velocity imaging technique

Laser initiated reactions in N2O clusters were studied by a time-sliced velocity imaging technique. The N2O clusters, (N2O)n, generated by supersonic expansion were irradiated by an ultraviolet laser around 204 nm to convert reactant pairs, O((1)D2)-(N2O)n-1. The NO molecules formed from these reactant pairs were ionized by the same laser pulse and their velocity distribution was determined by the time-sliced velocity imaging technique. At low nozzle pressure, lower than 1.5 atm, the speed distribution in the frame moving with the clusters consists of two components. These components were ascribed to the products appeared in the backward and forward directions in the center-of-mass frame, respectively. The former consists of the vibrational ground state and the latter consists of highly vibrational excited states. At higher nozzle pressure, a single broad speed distribution became dominant for the product NO. The pressure and laser power dependences suggested that this component is attributed to the product formed in the clusters larger than dimer, (N2O)n (n ≥ 3).

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).

O((1)D) + N(2)O reaction: NO vibrational and rotational distributions

The O((1)D) + N(2)O → 2NO(X (2)Π) reaction has been studied in a molecular beam experiment in which O(3) and N(2)O were coexpanded. The precursor O((1)D) was prepared by O(3) photodissociation at 266 nm, and the NO(X (2)Π) molecules born from the reaction as the O((1)D) recoiled out of the beam were detected by 1+1 REMPI over the 220-246 nm probe laser wavelength range. The resulting spectrum was simulated to extract rotational and vibrational distributions of the NO(X (2)Π) molecules. The product rotational distribution is found to be characterized by a constant rotational temperature of ≈4500 K for all observed bands, v = 0-9. An inverted vibrational distribution is observed. A consistent explanation of this and previous experimental results is possible if there are two channels for the reaction, one producing a nearly statistical vibrational distribution for low O((1)D)-N(2)O relative velocity collisions and a second producing the inverted distribution observed here for high relative velocity collisions. The former might correspond to an insertion/complex-formation reaction, while the latter might correspond to a stripping reaction. Velocity relaxation of the O((1)D) is argued to compete strongly with reaction in most bulb studies, so that these studies see predominantly the nearly statistical distribution. In contrast, the beam experiments do not detect the part of the vibrational distribution produced in low relative velocity reactions because the O((1)D) is not relaxed from its initial velocity before it either reacts or leaves the beam.

Vibrational relaxation of NO (v = 1-16) with NO, N2O, NO2, He and Ar studied by time-resolved Fourier transform infrared emission

Rates of vibrational quenching of NO (v = 1-16) in collisions with a series of quenching species NO, NO2, N2O, He and Ar have been measured at 295 K. NO (v) was formed both by the O(1D) + N2O reaction and the 193 nm photolysis of NO(2), and time-resolved FTIR emission was used to follow the behaviour of the vibrationally excited species. The trends in quenching rate constants can be explained in terms of V-T transfer, V-V transfer and by the effects of competing processes. He and Ar show trends expected from SSH theory, but with relaxation rates that are considerably higher than those expected from previous studies with closed shell molecules, and the influence of non-adiabatic pathways in the relaxation of the NO 2Pi state is discussed. Relaxation with NO2 shows the influence of resonant energy transfer to the nu3 mode, with rate constants peaking at v = 10. For N2O, relaxation rates show essentially a linear increase with v. A linear increase is expected for the change of the transition moment with v for the harmonic oscillator approximation, and when this is taken into account the "reduced probabilities" (defined as P/v, where P is probability of a gas kinetic collision changing the vibrational level from v to v-1) are approximately independent of the energy lost in the NO molecule. The influence of complex formation far from resonance is invoked in both this and for quenching of low vibrational levels by NO2. Finally, self-quenching shows rates which initially decrease with increasing v, but then show a marked increase, with a minimum value at v = 9. Both V-V and V-T processes are believed to occur. Where previously published data are available, general agreement is observed in this study.

Crossed molecular beam studies on the reaction dynamics of O(1D)+N2O

The reaction of oxygen atom in its first singlet excited state with nitrous oxide was investigated under the crossed molecular beam condition. This reaction has two major product channels, NO+NO and N2+O2. The product translational energy distributions and angular distributions of both channels were determined. Using oxygen-18 isotope labeled O(1D) reactant, the newly formed NO can be distinguished from the remaining NO that was contained in the reactant N2O. Both channels have asymmetric and forward-biased angular distributions, suggesting that there is no long-lived collision complex with lifetime longer than its rotational period. The translational energy release of the N2+O2 channel (fT = 0.57) is much higher than that of the NO+NO channel (fT = 0.31). The product energy partitioning into translational, rotational, and vibrational degrees of freedom is discussed to learn more about the reaction mechanism. The branching ratio between the two product channels was estimated. The 46N2O product of the isotope exchange channel, 18O+44N2O-->16O+46N2O, was below the detection limit and therefore, the upper limit of its yield was estimated to be 0.8%.

Quasiclassical trajectory study of O(1D) + N2O --> NO + NO: classification of reaction paths and vibrational distribution

Quasiclassical trajectory calculations for the planar reaction of O(1D) + N2O --> NO + NO are performed on a newly constructed ab initio potential energy surface. In spite of the reduced dimension approximation, the agreement between the computational and experimental results is largely satisfactory, especially on the similar amount of excitation of the two kinds of NO products found by Akagi et al. [J. Chem. Phys. 111, 115 (1999)]. Analyzing the initial condition dependence of the trajectories, we find that the trajectories of this reaction can be classified into four reaction paths, which correspond to respective areas in the space of initial condition. In one of the four paths, a long-lived stable complex is formed in the course of reaction, whereas the other three paths have direct mechanism. Contradictory to conventional understanding of the chemical reaction dynamics, the direct paths show more efficient energy exchange between the NO stretching modes than that with a long-lived intermediate. This indicates that the vibrational mode coupling along the short-lived paths is considerably stronger than expected.

The reaction products of the 193 nm photolysis of vinyl bromide and vinyl chloride studied by time-resolved Fourier transform infrared emission spectroscopy

Time-resolved Fourier transform infrared (TRFTIR) emission spectroscopy has been used to study the 193 nm photolysis of vinyl bromide (C(2)H(3)Br) and vinyl chloride (C(2)H(3)Cl). Time-resolved IR emission was analysed to obtain nascent vibrational state populations of two primary photolysis products: HBr (v = 1-7) and HCl (v = 1-6). In both cases the nascent vibrational state populations monotonically decrease with increasing v and are in excellent agreement with previously published data. Time-resolved populations were analysed to yield rate constants for vibrational relaxation of HBr (v = 1-3) and HCl (v = 1-4) by parent vinyl bromide and vinyl chloride, respectively. In both cases the rate constants were found to increase with increasing vibrational quantum number, in agreement with a single quantum de-excitation via vibrational to vibrational energy transfer. Butadiene (C(4)H(6)) was identified as a secondary product of the photolysis of both vinyl halides, and shown to be formed from the reaction of parent vinyl halide with the vinyl radical. The presence of a buffer gas was found to produce a strong emission feature centred at 2,200 cm(-1), the intensity of which was dependent on the pressure of the buffer gas used, and whose kinetics are indicative of a secondary reaction product. We propose that this emission is from the vibrational progression of the electronic transition A(0, v, 1) --> X(0, v, 2) in the secondary reaction product C(2)H, whose formation route is favoured by the presence of buffer gas.

Exit interaction effect on nascent product state distribution of O(1D)+N2O-->NO+NO

We have determined the rotational state distributions of NO(v'=0,1,2) products produced from the reaction O(1D)+N2O. This is the first full characterization of the product rotational distribution of this reaction. The main part of each rotational distribution (up to j' approximately 80) has rotational temperature approximately 20,000 K and all these distributions are quite near to those predicted by the phase space theory (PST). This observation and previously reported vibrational distribution indicate that the most part of the energy partitioning of the reaction products is at least apparently statistical although the intermediate of this reaction is not so stable as to ensure the long lifetime. On the other hand, the distributions in the high rotational levels (j'=80-100) are found to decrease more sharply as j' increases than the PST predictions. The origin of the observed decrease of the distribution is discussed with quasiclassical trajectory (QCT) calculations on a five-dimensional ab initio potential energy surface (PES). The observed near-statistical distribution and the sharp decrease in the high-j' levels are well reproduced by a "half-collision" QCT calculation, where statistical distribution at the reaction intermediate is assumed. This agreement shows the rotation-translation interaction in the exit region has an effect of yielding small high-j' populations. However, a little bias of the calculated distribution toward lower rotational excitation than the observed one indicates that the combination of the statistical intermediate and the exit interaction on the current PES does not completely describe the real system. It is suggested that the reaction intermediate is generated with the distribution which is close to statistical but a little biased toward yielding high-j' products, and that the interaction in the exit region of the PES results in the sharp decrease in the high-j' levels.