碰撞能量对反应Sr＋HF转动取向的影响

在推广LEPS势能面上,用经典轨线方法,研究了反应碰撞能量对反应Sr＋HF的转动取向的影响.计算结果与产物轨道角动量模型进行比较.计算结果表明,随着碰撞能量的增加,产物转动取向越强烈.     

Collision energy dependence of the HD(nu' = 2) product rotational distribution of the H + D2 reaction in the range 1.30-1.89 eV

An experimental and theoretical investigation of the collision energy dependence of the HD(nu' = 2,j') rotational product state distribution for the H + D2 reaction in the collision energy range of Ecol = 1.30-1.89 eV has been carried out. Theoretical results based on time-dependent and time-independent quantum mechanical methods agree nearly perfectly with each other, and the agreement with the experiment is good at low collision energies and very good at high collision energies. This behavior is in marked contrast to a previous report on the HD(nu' = 3,j') product state rotational distribution [Pomerantz et al., J. Chem. Phys. 120, 3244 (2004)] where a systematic difference between experiment and theory was observed, especially at the highest collision energies. The reason for this different behavior is not yet understood. In addition, this study employs Doppler-free spectroscopy to resolve an ambiguity in the E, F-X resonantly enhanced multiphoton ionization transition originating from the HD(nu' = 2,j' = 1) state, which is found to be caused by an accidental blending with the transition coming from the HD(nu' = 1,j' = 14) state.     

Microcanonical statistical study of ortho-para conversion in the reaction H3+ + H2-->(H5+)*-->H3+ + H2 at very low energies

The ortho-para conversion of H(3) (+) and H(2) in the reaction H(3) (+)+H(2)-->(H(5) (+))(*)-->H(3) (+)+H(2) in interstellar space is possible by scrambling the five protons via (H(5) (+))(*) complex formation. The product distribution of the ortho-para conversion reaction can be given by ratios of cumulative reaction probabilities (CRP) calculated by microcanonical statistical theory with conservation of energy, motional angular momentum, nuclear spin, and parity. A statistical method to calculate the state-to-state reaction probabilities for given initial nuclear spin species, rotational states, and collision energies is developed using a simple semiclassical approximation of tunneling and above-barrier reflection. A new calculation method of branching ratios for given total nuclear spins and scrambling mechanisms is also developed. The anisotropic long-range electrostatic interaction potential of H(2) in the Coulomb field of H(3) (+) is taken into account using the first-order perturbation theory in forming the complex. The CRPs and the product distribution of the ortho-para conversion reaction at very low energies with reactants in their ground vibronic and lowest rotational states for given initial nuclear spin species are presented as a function of collision energy assuming complete proton scrambling or incomplete proton scrambling. The authors show that the product distribution at very low energies (or very low temperatures) differs substantially from the high energy (or high temperature) limit branching ratios.     

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.     

Role of angular momentum conservation in unimolecular translational energy release: validity of the orbiting transition state theory

The translational kinetic energy release distribution (KERD) for the halogen loss reaction of the bromobenzene and iodobenzene cations has been reinvestigated on the microsecond time scale. Two necessary conditions of validity of the orbiting transition state theory (OTST) for the calculation of kinetic energy release distributions (KERDs) have been formulated. One of them examines the central ion-induced dipole potential approximation. As a second criterion, an adiabatic parameter is derived. The lower the released translational energy and the total angular momentum, the larger the reduced mass, the rotational constant of the molecular fragment, and the polarizability of the released atom, the more valid is the OTST. Only the low-energy dissociation of the iodobenzene ion (E approximately 0.45 eV, where E is the internal energy above the reaction threshold) is found to fulfill the criteria of validity of the OTST. The constraints that act on the dissociation dynamics have been studied by the maximum entropy method. Calculations of entropy deficiencies (which measure the deviation from a microcanonical distribution) show that the pair of fragments does not sample the whole of the phase space that is compatible with the mere specification of the internal energy. The major constraint that results from conservation of angular momentum is related to a reduction of the dimensionality of the dynamics of the translational motion to a two-dimensional space. A second and minor constraint that affects the KERD leads to a suppression of small translational releases, i.e., accounts for threshold behavior. At high internal energies, the effects of curvature of the reaction path and of angular momentum conservation are intricately intermeddled and it is not possible to specify the share of each effect.     

Vibrationa-rotational structure in the angular distribution and intensity of photoelectrons from diatomic molecules. II. H2

Total intensity and angular distribution of photoelectrons ejected from H₂ are calculated for individual vibrational-rotational transitions accompanied by photoionization The relevant transition moment is evaluated in the two-center spheroidal coordinates with varying internuclear distance. The calculations for 584 Å and 736 Å lines are compared with the observed spectra. The theoretical result clearly shows the dependence of the angular distribution on the initial and final vibrational states and the dominance of the rotational transition with ΔJ = 0 over that with |Δ| = 2.     

High Resolution Crossed Beams Scattering Study of the F＋HD→DF＋H Reaction

Crossed beams scattering study was carried out on the F＋HD→DF＋H reaction using high- resolution H-atom Rydberg tagging time-of-flight technique. Vibrational state-resolved differential cross sections were measured, with partial rotational state resolution, at eight collision energies in the range of 2.51-5.60 kJ/mol. Experimental results indicated that the product angular distributions are predominantly backward scattered. As the collision energy increases, the backward scattered peak becomes broader gradually. Dependence of product vibration branching ratios on the collision energy was also determined. The experimental results show that the DF products are highly inverted in the vibrational state distribution and the DF （v＇=3） product is the most populated state. Furthermore, the DF （v＇=l） product has also been observed at collision energy above 3.97 kJ/mol.     

Influence of collision energy and reagent rotation on the cross sections and product polarizations of the reaction F + HCl

Quasiclassical trajectory calculations have been carried out for the F+HCl reaction in three dimensions on a recent DHSN PES of the ground 1(2)A' electronic state [M. P. Deskevich, M. Y. Hayes, K. Takahashi, R. T. Skodje, and D. J. Nesbitt, J. Chem. Phys. 124, 224303 (2006)]. The effects of the collision energy and the reagent initial rotational excitation on the cross sections and product polarization are studied for the v = 0 and j ≤ 10 states of HCl over a wide collision energy range. It has been found that either the collision energy or the HCl rotational excitation increase remarkably reaction cross sections. The QCT-calculated integral cross sections are in good agreement with previous QM results. A detailed study on product polarization for the title reaction is also performed. The calculated results show that the product rotational angular momentum j' is not only aligned, but also oriented along the direction perpendicular to the scattering plane. The orientation of the HF product rotational angular momentum vector j' depends very sensitively on the collision energy and also affected by the reagent rotation. The theoretical findings and especially the roles of the collision energy and initial rotational momentum on the product polarization are discussed and reasonably explained by the HLH mass combination, the property of the PES, as well as the reactive mechanism.     

Rotational quantum changes accompanying vibrational relaxation in the 1Au state of glyoxal

Changes of the rotational quantum state occurring simultaneously (in one single collision step) with the deactivation of the torsional vibration ν₇ have been measured in glyoxal ¹A_u. Selective excitation of rotational levels in the vibronic level 7¹ was accomplished by means of a tunable dye laser. A near thermal distribution of rotational levels in the O⁰ state was attained as the result of a single collision. Intramolecular vibration-to-rotation transfer was not found. Similarly, a nearly thermalized distribution of rotational levels in the O⁰ state has been observed as the result of one single collision after selective excitation of a restricted number of rotational levels in the vibronic 8¹ state in the presence of different collision partners.     

Accurate quantum mechanical calculations of differential and integral cross sections and rate constant for the O+OH reaction using an ab initio potential energy surface

The authors report accurate quantum mechanical studies of the O+OH reaction on the improved Xu-Xie-Zhang-Lin-Guo potential energy surface. The differential cross section was obtained at several energies near the reaction threshold using a time-independent method. The dominant forward and backward peaks in the angular distribution are consistent with a complex-forming mechanism, which is also confirmed by the extensive rotational excitation in the O2 product. However, the asymmetry of these peaks suggests a significant nonstatistical component. The initial state (upsilon i=0, j i=0) specified integral cross section, which was calculated up to 1.15 eV of collision energy using the Chebyshev wave packet method, shows no energy threshold and decreases with the increasing collision energy, consistent with the barrierless nature of the reaction. The resulting rate constant exhibits a negative temperature dependence for T>100 K and decays as the temperature is lowered, in qualitative agreement with available experimental data.     

The dynamics of reaction of Cl atoms with tetramethylsilane

Rotational state distributions and state-selected CM-frame angular distributions were measured for HCl (v' = 0, j') products from the reaction of Cl-atoms with tetramethylsilane (TMS) under single collision conditions at a collision energy, E(coll), of 8.2 +/- 2.0 kcal mol(-1). The internal excitation of these products was very low with only 2% of the total energy available partitioned into HCl rotation. A transition state with a quasi-linear C-H-Cl moiety structure was computed and used to explain this finding. A backward peaking differential cross section was also reported together with a product translational energy (T') distribution with a maximum at T' approximately E(coll). This scattering behaviour is accounted for by reactions proceeding through a tight transition state on a highly skewed potential energy surface, which favours collisions at low impact parameters with a strong kinematic constraint on the internal excitation of the products. The large Arrhenius pre-exponential factor previously reported for this reaction is reconciled with the tight differential scattering observed in our study by considering the large size of the TMS molecule.     

Phase space analysis of formaldehyde dissociation branching and comparison with quasiclassical trajectory calculations

We investigate the dependence of the branching ratio of formaldehyde dissociation to molecular and radical products on the total energy and angular momentum and the HCO rotational state distributions by using a combination of transition state/Rice-Ramsperger-Kassel-Marcus theory and phase space theory. Comparisons are made with recent quasiclassical trajectory (QCT) calculations [Farnum, J. D.; Zhang, X.; Bowman, J. M. J. Chem. Phys. 2007, 126, 134305]. The combined phase-space analysis is in semiquantitative agreement with the QCT results for the rotational distributions of HCO but is only in qualitative agreement for the branching ratio. Nevertheless, that level of agreement serves to provide insight into the QCT results, which showed suppression of the radical channel with increasing total angular momentum for a fixed total energy.     

An analytic hindered rotor model for calculating microcanonical variational unimolecular rate constants from reaction path properties

A model is proposed for performing microcanonical variational transition state theory calculations which incorporates ideas from vibrator and flexible variational transition state models. Vibrational frequencies, moments of inertia, and potential energy for the variational transition state are found by reaction path following as for the vibrator model. However, the transitional modes are treated as hindered rotors using an analytic potential and an analytic density of states, which are fit to barriers for hindered rotation determined from reaction path following. The model proposed here differs from the flexible transition state model in that the density of states for the transitional modes is analytic and transitional modes and external rotational angular momenta are uncoupled. For the H + CH₃ ? CH₄ system, rate constants calculated with this new model are only 6–23% smaller on average from those of the flexible transition state model for values of total angular momentum which correspond to average rotational temperatures of 0–2000 K. Harmonic frequencies calculated for the transitional modes from the hindered rotor Hamiltonian are in good agreement with the exact values found by a reaction path analysis. © 1994 John Wiley & Sons, Inc.     

Energy transfer of highly vibrationally excited azulene. III. Collisions between azulene and argon

The energy transfer dynamics between highly vibrationally excited azulene molecules (37 582 cm(-1) internal energy) and Ar atoms in a series of collision energies (200, 492, 747, and 983 cm(-1)) was studied using a crossed-beam apparatus along with time-sliced velocity map ion imaging techniques. The angular resolved collisional energy-transfer probability distribution functions were measured directly from the scattering results of highly vibrationally excited azulene. Direct T-VR energy transfer was found to be quite efficient. In some instances, nearly all of the translational energy is transferred to vibrational/rotational energy. On the other hand, only a small fraction of vibrational energy is converted to translational energy (V-T). Significant amount of energy transfer from vibration to translation was observed at large collision energies in backward and sideway directions. The ratios of total cross sections between T-VR and V-T increases as collision energy increases. Formation of azulene-argon complexes during the collision was observed at low enough collision energies. The complexes make only minor contributions to the measured translational to vibrational/rotational (T-VR) energy transfer.     

Monte Carlo simulation of O(1D) + H2 and O(1D) + HCl — rotational excitation of product OH radicals

Experimental studies with molecular beam and LIF techniques have independently shown that the reaction O(¹D) + H₂ → OH + H passes through a long-lived complex and gives products with small translational and large rotational excitation. We have previously published a statistical algorithm, based on ordinary RRKM theory with angular momentum restrictions included, which was designed to simulate molecular beam experiments. It has now been modified and applied to simulate the experimental rotational OH distributions from O(¹D)+H₂, measured by Luntz et al. The present study also includes simulation of similar results by Luntz for O(¹D) + HCI → OH + Cl. The purely statistical algorithm successfully simulates the apparently non-statistical experimental rotational distributions. For these reactions the total angular momentum conservation. which is applied at the transition state, proves to be decisive for the product energy distributions.     

Monte Carlo simulation of RRKM unimolecular decomposition in molecular beam experiments. V. product ox angular and energy distributions from O(3P)+X2 (X = Br,I)

Statistical simulation was applied to the unimolecular decomposition of the collision complexes formed in the crossed beam experiments on O(³P) + Br₂ by Fernie et al. and O(³P) + I₂ by Durkin et al. The simulation procedure used the fundamentals of RRKM theory and included exact angular momentum conservation. The impact parameter distributions were varied to obtain the best fits. Good agreement with experimental laboratory angular distributions measured with O atoms seeded in both He and Ne was found for impact parameter distributions which were peaked at quite small values, in most cases between 2 and 3 Å. Product OX molecules were found to be rotationally excited and inverted with a mean rotational energy close to twice the value expected without angular momentum restrictions. The differences found between the calculated and the experimental angular distributions do not support any assumptions about osculating or short-lived complexes. The normal exoergicity ΔD₀ of 27 kJ/mol for the O + I₂ reaction agrees well with the experiments by Dunkin et al.     

Elastic scattering dynamics of translationally and rotationally aligned molecular fragments via Doppler-resolved laser spetroscopy. OH(X Π3/2) + Ar/He

The collision dynamics of the OH + Ar/He systems have been studied following production of the translationally and rotationally aligned radical species via polarized photolysis of H₂O₂ at a wavelength of 248 nm. Elastically scattered OH molecules were probed via polarized Fourier-transform Doppler spectroscopy. The manner in which the speed distribution and laboratory speed averaged translational anisotropy of the OH photofragments change with time is well reproduced by a hard sphere collision model. In addition, the degradation of the laboratory speed averaged v-j correlation by collision with argon is best reproduced if the direction of the OH rotational angular momentum vector is conserved throughout an elastic collision encounter.     

Differential Cross Sections of F+HD→DF+H Reaction at Collision Energies from 3.03 meV to 17.97 meV

The prototypical reaction of F+HD→DF+H was investigated at collision energies from 3.03 meV to 17.97 meV using a crossed molecular beam apparatus with multichannel Rydberg tagging time-of-flight detection. Significant contributions from both the Born-Oppenheimer (BO) forbidden reaction F^*(²P_1/2)+HD→DF+H and the BO-allowed reaction F(²P_3/2)+HD→DF+H were observed. In the backward scattering direction, the contribution from the BO-forbidden reaction F^*(²P_1/2)+HD was found to be considerably greater than the BO-allowed reaction F(²P_3/2)+HD, indicating the non-adiabatic effects play an important role in the dynamics of the title reaction at low collision energies. Collision-energy dependence of differential cross sections (DCSs) in the backward scattering direction was found to be monotonously decreased as the collision energy decreases, which does not support the existence of resonance states in this energy range. DCSs of both BO-allowed and BO-forbidden reactions were measured at seven collision energies from 3.03 meV to 17.97 meV. It is quite unexpected that the angular distribution gradually shifts from backward to sideway as the collision energy decreases from 17.97 meV to 3.03 meV, suggesting some unknown mechanisms may exist at low collision energies.