Influence of collision energy on the N „ 2 D ... ¿ O 2 \ O „ 3 P ... ¿ NO reaction dynamics : A quasiclassical trajectory study involving four potential energy surfaces

Influence of collision energy on the N „D...¿O2\O„P...¿NO reaction dynamics: A quasiclassical trajectory study involving four potential energy surfaces Irene Miquel, Jordi Hernando, R. Sayós, and Miguel González Departament de Quı ́mica Fı́sica i Centre Especial de Recerca en Quı ́mica Teo `rica, Universitat de Barcelona, i Parc Cientific de Barcelona, C/Martı ́ i Franquès, 1, 08028 Barcelona, Spain


I. INTRODUCTION
The relevance of the study of the nitrogen atom in its first excited electronic state ͓N( 2 D)͔ is due to the presence of this species in the earth-high atmosphere. Actually, its deactivation with ground-state molecular oxygen ͓O 2 (X 3 ⌺ g Ϫ )͔ could control the production of NO in the thermosphere and be an efficient source of highly vibrationally excited NO in the atmospheric region of 70-120 km. 1 Moreover, N( 2 D) plays an important role in photolysis systems, discharges in plasmas and chemical reactions.
The reaction between N( 2 D) and O 2 can take place through two reactive channels, 2,3 Due to its interest, several experimental studies were devoted to the measurement of the global rate constant ͓including both reactive channels and N( 2 D) physical electronic quenching͔ of the N( 2 D)ϩO 2 system. 4 -14 The recommended value of the global rate constant at room temperature ͑298 K͒ is 5.2ϫ10 Ϫ12 cm 3 molecule Ϫ1 s Ϫ1 . 15 In addition, some experimental works determined the temperature dependence of the rate constant. 16,17 However, none of these measurements provided any information about the branching ratio of the two reactive channels of the N( 2 D)ϩO 2 system.
Concerning the dynamic properties of the process, only one experimental work was reported, 18 where the vibrational distribution for the NO product arising from the N( 2 D)ϩO 2 reaction at 100 K was measured. The vibrational distribution obtained is inverted and peaked at vЈϭ7.
From a theoretical point of view, a wealth of work was recently carried out in our group to investigate the N( 2 D) ϩO 2 system. In Ref. 1 we showed that the rate constant for reaction ͑2͒ is several decades higher than the rate constant for reaction ͑1͒, so that the contribution of the latter process to the reactivity of the N( 2 D)ϩO 2 system can be assumed to be negligible. This is due to the fact that the only potential energy surface ͑PES͒ that correlates reactants and products of reaction ͑1͒ has a very high energy barrier. 1 On the other hand, reaction ͑2͒ can take place through six PESs (2 2 AЈ, 3 2 AЈ, 1 2 AЉ, 2 2 AЉ, 3 4 AЈ, and 3 4 AЉ), although only four of them (2 2 AЈ, 3 2 AЈ, 1 2 AЉ, and 2 2 AЉ) present reaction barriers that are small enough to allow reactivity at low and moderate temperatures. 1 The contribution of the highly energetic 3 4 AЈ and 3 4 AЉ PESs can be ruled out.
In several recent works 1,19-23 we performed extensive ab initio studies on the most important PESs involved in reaction ͑2͒, using the complete active space self-consistent field/ second-order perturbation theory on a CASSCF wave function ͑CASSCF/CASPT2͒ method. Afterwards, analytical functions describing satisfactorily the ab initio calculations of these PESs were derived and used to calculate the rate constant of reaction ͑2͒ over a wide range of temperatures employing the quasiclassical trajectory ͑QCT͒ and variational transition state theory ͑VTST͒ methods. A good agreement with the experimental values was obtained. The QCT vibrational distribution of NO was also computed for reaction ͑2͒ at 100 K and it was found to be more excited than the experimental one.
Other authors 24  on some of the most important PESs for reaction ͑2͒ as well. They also carried out kinetic calculations on reaction ͑2͒ and computed the NO product vibrational distribution at 500 K by means of the QCT method. However, a thorough investigation of the dynamics of the title reaction has not been done yet.
The main purpose of this work is to employ the analytical representations of the PESs previously developed in our group to study theoretically the dynamics of reaction ͑2͒ by means of the QCT method, paying particular attention to the investigation of collision energy (E T ) on reactivity. The work is organized as follows: Section II deals with the PESs used and the computational method. Section III presents the results obtained and the discussion. The conclusions are given in Sec. IV.

II. COMPUTATIONAL METHOD
The four PESs used in this study were recently obtained in our group [19][20][21][22][23] by means of an extensive ab initio study based on the CASSCF ͑Refs. 25, 26͒ ͑17,12͒ calculations, where the standard correlation-consistent cc-pVTZ basis set of Dunning and co-workers 27 was employed. The treatment of the dynamic correlation was performed by means of the CASPT2 ͑Ref. 28͒ method using the G2 correction to the Fock matrix.
A many-body expansion 29 was used to obtain suitable analytical representations of the 2 2 AЈ, 19 1 2 AЉ, 20 3 2 AЈ, 21 and 2 2 AЉ ͑Ref. 22͒ PESs of the title reaction. In all cases the minimum energy reaction path ͑MEP͒ involves an abstraction mechanism through a saddle point of bent geometry. The classical energy barriers associated to the saddle points are 0.19, 0.21, 4.60, and 7.51 kcal mol Ϫ1 for the 2 2 AЈ, 1 2 AЉ, 3 2 AЈ, and 2 2 AЉ PESs, respectively. The corresponding NOO angles at the saddle points are 120.5°, 109.3°, 113.1°, and 115.6°, respectively. In the case of the 1 2 AЉ PES an insertion mechanism evolving through a saddle point of C 2v geometry was also found. This saddle point has an ONO angle of 30.8°and a classical energy barrier of 1.43 kcal mol Ϫ1 .
To study the dynamics of reaction ͑2͒ on the four selected PESs, the QCT method 30,31 as implemented in the TRIQCT program 32 was employed, as we did in previous works from our group ͑see, e.g., Refs. 20, 33-37͒. Although the principles of this method were established long time ago, it can still be considered as the most useful theoretical tool to study the dynamics of chemical reactions. The accuracy of the numerical integration of Hamilton's differential equations was verified by checking the conservation of the total energy and total angular momentum for every trajectory, and performing back-integrations on some batches of trajectories. The integration step size chosen (5ϫ10 Ϫ17 s) was found to fulfill these conservation requirements for all calculated trajectories. The trajectories were started at an initial distance of 8 Å between the N( 2 D) atom and the center of mass of the O 2 molecule, thus ensuring that the interaction energy could be neglected with respect to the available energy of reactants. To study the reaction dynamics, the collision energy was fixed at different values ͑from 0.0388 eV to 1.00 eV͒, the vibrational level of the O 2 molecule was set at vϭ0 ͑99% of the molecules are placed in this level at 300 K͒, and the O 2 rotational distribution was sampled from the Maxwell-Boltzmann distribution at Tϭ300 K. The final quantized vibrational distributions ͓i.e., P(vЈ)] were obtained from vibrational radial action angle variables. 30 Four collision energies were considered in particular detail in the calculations: E T ϭ0.0388 ͑the average collision energy at 300 K͒, 0.25, 0.50, and 1.00 eV. Typically, for the computation of the cross section, batches of 15 000 trajectories were sampled for each collision energy and PES, while the calculation of product state distributions and two-vector properties for each E T value required batches of 150 000 trajectories for each PES. The standard error ͑one standard deviation͒ was always kept below 5%. Figure 1 shows the dependence of the cross section ͑͒ with E T for each one of the PESs studied. To derive the values depicted in Fig. 1, the cross sections calculated by the QCT method on each PES were multiplied by 1/15 to take into account all the potential energy surfaces that correlate with reactants (2 2 AЈ, 3 2 AЈ, 4 2 AЈ, 1 2 AЉ, 2 2 AЉ, 3 4 AЈ, 4 4 AЈ, 5 4 AЈ, 3 4 AЉ, and 4 4 AЉ). Figure 1 also shows the cross section of reaction ͑2͒ obtained as the sum of the contributions for the four main PESs involved in the reaction (2 2 AЈ, 3 2 AЈ, 1 2 AЉ, and 2 2 AЉ).

A. Scalar properties
The contribution of the 3 2 AЈ and 2 2 AЉ PESs to the cross section of reaction ͑2͒ is negligible for E T values lower than 0.25 and 0.4 eV, respectively, due to their energy barriers. The threshold energies found for these two PESs are consistent with their energy barriers. As they present much lower energy barriers, the 2 2 AЈ and 1 2 AЉ PESs are reactive even for E T ϭ0.0388 eV. For all PESs increases with E T until it reaches a plateau at sufficiently high energies, as expected for reactions with energy barrier along the MEP. The raise of with E T is due to the increase of the reaction probability ͑P͒ with collision energy in all the PESs. With respect to the value of the maximum impact parameter (b max ), it only increases slightly with E T for the 3 2 AЈ and 2 2 AЉ PESs, whereas for the more reactive 2 2 AЈ and 1 2 AЉ PESs b max decreases significantly from 0.0388 to 0.25 eV and it remains constant for higher energies.
The opacity function ͓reaction probability, P(b), vs the initial impact parameter ͑b͔͒ presents a similar shape in all the PESs ͑Fig. 2͒. P(b) is essentially constant between b ϭ0 and some intermediate value of b, and after this it decreases progressively and reaches a value of zero at b ϭb max . When the value of the collision energy is close to the threshold energy of the PES, that is to say at 0.0388 eV for the 2 2 AЈ and 1 2 AЉ PESs, 0.25 eV for the 3 2 AЈ PES and 0.5 eV for the 2 2 AЉ PES, reactivity is more important at low values of b.
In Table I we show the average fractions of energy in for the different PESs and the global reaction. For the overall reaction, the majority of the available energy (E av ) is channeled into NO vibrational energy and relative translational while a smaller fraction of E av is channeled into NO rotation (͗ f R Ј ͘ϭ0.21). As the collision energy increases, the average fraction of vibrational energy in products diminishes, which is accompanied by an increase in final translational energy .00 eV). The average fraction of NO rotational energy essentially remains constant at all collision energies. This behavior is consistent with the large exoergicity of the PESs, which leads to the existence of early saddle points. A similar trend is also observed for the 2 2 AЈ, 3 2 AЈ, and 2 2 AЉ PESs, while in the case of the 1 2 AЉ found. This fact is due to the occurrence of different microscopic reaction mechanisms for the PESs, which will be further discussed in Sec. III C. Changes in the energy distribution of NO with E T are also evident from the evolution of the average NO vibrational level (͗vЈ͘) ͑Table II͒ and NO vibrational distribution ͑Fig. 3͒. For the 2 2 AЈ, 3 2 AЈ, and 2 2 AЉ PESs unimodal and inverted NO vibrational distributions are found for all E T values explored. For these PESs the higher the collision energy is, the lower the peak of the distribution becomes. This trend is also observed for the global reaction. On the other hand, the vibrational distribution of the 1 2 AЉ PES presents a bimodal feature that results from the overlap of an inverted distribution similar to that found for the other PESs and a less excited distribution peaked at low vЈ values. In this case, a slight increase of the average NO vibrational level with E T is found. This behavior arises from the existence of two microscopic reaction mechanisms for the 1 2 AЉ PES ͑cf. in Sec. III C͒. Hence, the vibrational distributions for the global reaction at E T ϭ0.0388 and 0.25 eV are bimodal as well.

B. Two-vector correlations
The analysis of the angular momentum transformation when the system evolves from reactants to products was per-     formed at all collision energies studied. Except for E T ϭ0.0388 eV, the total angular momentum of the system ͑J͒ can be assumed to be equal to the reactants orbital angular momentum ͑l͒, as the modulus of l is much larger than that of the O 2 rotational angular momentum ͑j͒. In these cases, we observe a clear partition of the orbital angular momentum ͑l͒ between the final orbital (lЈ) and NO rotational (jЈ) angular momenta (l→lЈϩjЈ). The partition of l between lЈ and jЈ changes for the different PESs and collision energies. For example, at medium and high collision energies (E T ϭ0.25, 0.5, and 1.0 eV͒ and for the 2 2 AЈ PES, a clear equipartition exists, i.e., l/2→lЈ and l/2→jЈ. In other cases, the angular momentum transformation is less evident and the ͉jЈ͉/͉l͉ and ͉lЈ͉/͉l͉ distributions are very broad. To obtain a more complete description of the stereodynamics of the title reaction, we also examined the two-vector angular distributions kkЈ, kjЈ, kЈjЈ, and lЈjЈ, where k (kЈ) is the initial ͑final͒ relative velocity vector. These properties are expressed in terms of the solid angle differential cross section ͓d/d⍀, differential cross section ͑DCS͒ hereafter͔. Figure 4 shows the global kkЈ distribution at different collision energies, as well as the contribution of the different PESs to the total DCS. At low E T values the overall kkЈ angular distribution is predominantly backward ͓͗kkЈ͘ ϭ99.9°and the forward/backward ( f /b) scattering ratio is equal to 0.63͔ ͑Table III͒. When E T increases the backward scattering diminishes and the kkЈ distribution evolves to an essentially sideways distribution. Thus, at high collision energies (E T ϭ1.00 eV) ͗kkЈ͘ϭ90.8°and f /bϭ1.00. The loss of backward feature with E T for the overall reaction arises from the particular behavior of each one of the PESs investigated.
In general, the 2 2 AЈ, 3 2 AЈ, and 2 2 AЉ PESs present a clear backward distribution for all collision energies explored. However, this behavior becomes less noticeable as  ͗kkЈ͘ and ͗lЈjЈ͘ stand, respectively, for the average kkЈ and lЈjЈ angles values derived from the DCS(kkЈ) and DCS(lЈjЈ) distributions; f/b is the ratio of products recoiled in the forward (kkЈϽ90°) and backward (kkЈ Ͼ90°) hemispheres; p/ap is the ratio of products formed within the parallel (lЈjЈϽ90°) and antiparallel (lЈjЈϾ90°) lЈjЈ orientation regions.

͑3͒
In Eq. ͑3͒ (b) is a function of the impact parameter and kk Ј (b,E T ) is the scattering function that gives the kkЈ angle for fixed values of b and E T . From this expression it comes out that more forward scattering is expected for a given value of b as E T increases. Moreover, changes in the opacity function with E T may also influence the scattering of products because a correlation between b and the kkЈ angle is expected. Thus, we found that the higher b is, the lower the kkЈ angle becomes for the 2 2 AЈ, 3 2 AЈ, and 2 2 AЉ PESs. Therefore, the larger contribution to reactivity of high b values as E T rises ͑see Fig. 2͒ also favors the loss of backward feature of the DCS(kkЈ) distribution when collision energy increases.
On the other hand, the DCS(kkЈ) distribution for the 1 2 AЉ PES is not backward, but it presents a sideways feature for E T ϭ0.0388 eV that transforms to a forward feature as collision energy increases. Actually, ͗kkЈ͘ϭ93.5 and 70.4°a nd the f /b ratio is equal to 0.83 and 2.83 for E T ϭ0.0388 and 1.0 eV, respectively. This behavior is due to the occurrence of two reaction mechanisms for the 1 2 AЉ PES. The larger forward feature of the DCS(kkЈ) distributions corresponding to the two mechanisms as collision energy increases ͑due to the same reasons outlined above for the other PESs͒ and their different contribution to reactivity depending on E T explain the behavior of the global DCS(kkЈ) function for the 1 2 AЉ PES. This will be further discussed in Sec. III C.
The kjЈ DCS distributions for the global reaction and the four different PESs studied are symmetric around 90°͑as they must be͒, where they exhibit a maximum ͑Fig. 5͒. The global DCS(kjЈ) becomes narrower as collision energy increases, which means that the tendency to a perpendicular kjЈ arrangement is more noticeable with E T . This trend can also be inferred from the analysis of the product rotational alignment parameter ͓A 0 (2) ϵ͗3 cos 2 Ϫ1͘, with being the kjЈ angle in this case͔, whose limiting values correspond to a fully perpendicular ͓A 0 (2) ϭϪ1͔ or parallel ͓A 0 (2) ϭϩ2͔ kjЈ alignment. The value of this parameter for the overall reaction is equal to Ϫ0.26, Ϫ0.28, Ϫ0.32, and Ϫ0.38 for E T ϭ0.0388, 0.25, 0.50, and 1.0 eV, respectively. Thus, the higher the collision energy is, the closer A 0 (2) is to the perpendicular limiting value. This situation is also observed for the 2 2 AЈ, 3 2 AЈ, and 2 2 AЉ PESs, while the occurrence of two reaction mechanisms for the 1 2 AЉ PES favors a less perpendicular kjЈ alignment trend as E T increases. For E T ϭ1.00 eV, A 0 (2) ϭϪ0.48, Ϫ0.50, Ϫ0.56, and Ϫ0.14 for the 2 2 AЈ, 3 2 AЈ, 2 2 AЉ, and 1 2 AЉ PESs, respectively.
With regard to the lЈjЈ DCS, the lЈ and jЈ vectors present a predominant antiparallel orientation at low collision energies, and when collision energy increases, this distribution tends to lose preference for the antiparallel orientation ͑Fig. 6͒. This behavior is found for both the global reaction and all the PESs investigated. For the global reaction ͗lЈjЈ͘ ϭ154.4°and the parallel/antiparallel (p/ap) ratio is equal to 0.016 at E T ϭ0.0388 eV, while ͗lЈjЈ͘ϭ102.1°and p/ap ϭ0.62 at E T ϭ1.00 eV. Nevertheless, the antiparallel orientation in general dominates within the range of E T values explored.
The kЈjЈ DCS for the global reaction and all the PESs studied corresponds to a symmetric distribution, as it must be ͑Fig. 7͒. For E T ϭ0.0388 eV, the overall DCS(kЈjЈ) presents a maximum around 90°and preferential perpendicular alignment ͓A 0 (2) ϭϪ0.81͔. As collision energy increases, the peak at 90°disappears and two new maxima at around 40°and 140°are observed. This results in a loss of kЈjЈ perpendicular alignment with E T ͓A 0 (2) ϭϪ0.26, Ϫ0.11, and 0.07 for E T ϭ0.25, 0.5, and 1.0 eV, respectively͔. This situation correlates with the behavior of the DCS(lЈjЈ) because antiparallel lЈjЈ orientation leads to perpendicular kЈjЈ alignment. As mentioned above, the antiparallel lЈjЈ orientation diminishes with E T and so does the perpendicular kЈjЈ alignment. Such a behavior is specially accentuated for the 2 2 AЈ, 3 2 AЈ, and 2 2 AЉ PESs, which results in pronounced minima at 90°in the DCS(kЈjЈ) for high E T values. On the other hand, the loss of antiparallel lЈjЈ orientation as the collision energy increases is less clear for the 1 2 AЉ PES, which explains that the DCS(kЈjЈ) for high collision energies still preserves the peak at 90°, although it broadens with E T .

C. Reaction mode
We investigated the microscopic reaction mechanisms ͑reaction modes͒ taking place on the PESs studied. On the 2 2 AЈ, 3 2 AЈ, and 2 2 AЉ PESs all reactive trajectories analyzed evolve through an abstraction mechanism of direct type via a saddle point of bent geometry. In fact, the dynamic properties described for these three PESs in the previous sections are similar to the ones expected for a rebound-type reaction mode. Namely, reactivity is favored for low b values yielding backward scattering of products.
However, the 1 2 AЉ PES presents an additional mode of reaction ͑insertion͒. As we have already explained, this PES has two saddle points with low enough energy barriers to allow reaction to occur at low temperatures. The first saddle point has C s symmetry and connects reactants ͓N( 2 D) ϩO 2 ͔ and products ͓O( 3 P)ϩNO͔ of reaction ͑2͒ through an abstraction pathway, in a similar way as in the 2 2 AЈ, 3 2 AЈ, and 2 2 AЉ PESs. The second saddle point, however, presents C 2v symmetry and connects reactants with a NO 2 minimum. From this minimum, after distortion of the C 2v symmetry, it is possible to reach products without surmounting any barrier. Therefore, the saddle point of C 2v symmetry gives rise to an insertion mechanism, in which trajectories are influenced by the presence of the NO 2 minimum that is located 23.62 kcal mol Ϫ1 below products. Reactive trajectories on the 1 2 AЉ PES could evolve through any of these two mechanisms, then leading to different dynamic properties. Figure 8 shows a schematic representation of two ex-amples of both types of reactive trajectories ͑abstraction and insertion͒. For the reactive trajectories corresponding to the insertion mechanism, there is a point where the N-O1 and N-O2 distances have small and similar values. Then the geometry of the system is close to that of the NO 2 minimum and, consequently, the value of the potential energy is below the energy of products. Despite the exploration of this minimum, we did not observe the formation of collision complexes for this kind of trajectories. This is usually explained on the basis of the small depth of the minimum with respect to products, the high energy content of the system when it reaches this region of the PES and the low number of degrees of freedom involved. All these characteristics prevent the formation of collision complexes. With regard to the abstraction mechanism, there is an oxygen atom ͑O1 in Fig. 8͒ that remains at a long distance ͑у2.0 Å͒ from the attacking nitrogen atom during the trajectory, while the potential energy is always kept higher than the energy of products. Moreover, at the closest approach distance between the three atoms, the trajectories evolving through abstraction present a NOO angle of about 120°͑corresponding to the angle of the abstraction saddle point͒, while the trajectories evolving through insertion show a much smaller NOO angle, corresponding to the insertion saddle point. This indicates that the trajectories exhibiting an abstraction mechanism do not explore the region of the PES where the NO 2 minimum is located. Table IV shows the percentage of reactive trajectories that evolve through each type of microscopic mechanism on the 1 2 AЉ PES as a function of collision energy. At low collision energies ͑0.0388 eV͒ the number of reactive trajectories occurring via insertion is only about 18% of the total ones. This fact is due to the different values of the energy barriers for both mechanisms. The saddle point of C s symmetry that allows the abstraction reaction mode to take place presents an energetic requirement of 0.21 kcal mol Ϫ1 , whereas in the case of the saddle point of C 2v symmetry associated to the insertion mode the energy barrier is 1.43 kcal mol Ϫ1 . The contribution of the insertion mechanism increases with E T , the insertion and abstraction mechanisms having similar yields at E T ϭ0.5 and 1.0 eV. The cross section, average fraction of energies, NO vibrational distribution and DCS(kkЈ) for the 1 2 AЉ PES, as a function of the reaction mode, are given in Fig. 9 for some selected conditions. Concerning the scalar properties, we observed some differences in both mechanisms. While abstraction yields excited NO vibrational distributions similar to the ones obtained for the 2 2 AЈ, 3 2 AЈ, and 2 2 AЉ PESs, the insertion mechanism results in less excited distributions peaked at low vЈ ͑at E T ϭ0.5 eV, v max Ј ϭ1) ͓Fig. 9͑c͔͒. This might be due to the presence of the NO 2 minimum in the insertion pathway, which may allow for a more efficient equipartition of the available energy among translational, vibrational, and rotational energy. The different behavior of both mechanisms leads to a bimodal NO vibrational distribution for the 1 2 AЉ PES. This bimodal behavior is smoothed out at high collision energies, as the inverted vibrational distribution arising from the abstraction mechanism is less excited.
The average fractions of energy in products are also different for both mechanisms ͓Fig. 9͑b͔͒. On the one hand, we observed that the average fractions of vibrational, rotational and translational energies are rather constant with collision energy for both mechanisms. As refers to ͗ f R Ј ͘ there are no important differences between the two mechanisms. Instead, ͗ f V Ј ͘ and ͗ f T Ј͘ present different features for insertion and abstraction. ͗ f V Ј ͘ is higher for abstraction than for insertion ͑in consistency with what we have observed before: abstraction yields more excited vibrational distributions than insertion͒. Moreover, reactive trajectories that evolve through the insertion mechanism present higher values of ͗ f T Ј͘.
Regarding to the two-vector properties, the most important difference between the two mechanisms refers to the kkЈ angular distribution. The insertion mechanism yields in all cases a clearly forward scattering of products, whereas the abstraction mechanism is backward at low collision energy (E T ϭ0.0388 eV) and becomes sideways at medium and high energies, as it occurs for the kkЈ distributions of the 2 2 AЈ, 3 2 AЈ, and 2 2 AЉ PESs. For the lЈjЈ, kjЈ, and kЈjЈ angular distributions, there are no important differences between both mechanisms.

IV. SUMMARY AND CONCLUSIONS
A study of the collision energy effects on the dynamics of the N( 2 D)ϩO 2 →O( 3 P)ϩNO reaction was performed using the QCT method on the main four analytical potential energy surfaces recently derived by us, and neglecting nonadiabatic couplings between them. These PESs correspond to the main surfaces of the system. Scalar and two-vector properties of the reaction were analyzed in terms of the collision energy, paying particular attention to the following E T values: 0.0388, 0.25, 0.50, and 1.00 eV.
The cross section of the reaction increases with E T , due to the increase of the reaction probability in the four PESs studied. The available energy is mainly channeled into vibrational and translational energy of products. The vibrational distribution of NO is inverted, though it becomes less excited as E T increases.
Concerning the two-vector correlations ͑expressed as DCSs͒, the kkЈ angular distribution is backward at low collision energy and at higher E T values it becomes sideways. In addition, a predominant antiparallel orientation of the lЈ and jЈ vectors is observed at low collision energies, though this orientation preference is smoothed out when E T increases.
Finally, we investigated the microscopic reaction mechanism occurring on the four PESs considered. For the 2 2 AЈ, 3 2 AЈ, and 2 2 AЉ PESs only an abstraction mechanism is observed. However, for the 1 2 AЉ PES both an abstraction mechanism and an insertion mechanism can occur leading to different dynamic properties. Nevertheless, both mechanisms are of direct type.
Further work on this reaction studying the influence of the internal excitation of reactants on the dynamics is in progress in our group.  9. ͑a͒ Cross section ͑-͒ for the 1 2 AЉ PES and for the two reaction mechanisms of this PES: ͑᭺͒ abstraction and ͑ᮀ͒ insertion; ͑b͒ average fractions of energy in the 1 2 AЉ PES for the two mechanisms: ͑--᭝--͒ translation, ͑--᭞--͒ rotation, and ͑--छ--͒ vibration for insertion, and ͑-᭝-͒ translation, ͑-᭞-͒ rotation, and ͑-छ-͒ vibration for abstraction; ͑c͒ vibrational distribution of NO ͑-͒ for the 1 2 AЉ PES and for the two reaction mechanisms of this PES: ͑᭺͒ abstraction and ͑ᮀ͒ insertion; ͑d͒ kkЈ angular distribution ͑-͒ for the 1 2 AЉ PES and for the two reaction mechanisms: ͑᭺͒ abstraction and ͑ᮀ͒ insertion.