Temperature dependence of the probabilities of VV energy exchange between H2 and DCl

The temperature dependence of the removal of the vibrational energy of H₂ by DCl in H₂(1) + DCl(0) has been investigated over the range of 300–3000 K. The energy transfer probability of H₂(1) + DCl(0) → H₂(0) + DCl(1), where the vibrational energy of H₂(1) is removed by both the vibrational and rotational motions of DCl(0), is found to be strongly temperature dependent and increases with temperature closely following the relation log P α T^1/3. Over the temperature range it changes by two orders of magnitude. The probability of the near-resonant process H₂ (1) + DCl(O) → H₂(0) + DCl(2) is very close to that of the former at 300 K, but it increases only slightly as the temperature is raised to 3000 K. The sum of the probabilities of these two processes at 300 K is 3.4 × 10^?5, which agrees with the experimental value of 3.95 × 10^?5.     

The QCT calculation of the rate constants for the N(4S)+O2(X3Σ g ? ) →NO(X2Π)+O(3P) reaction

A quasi-classical trajectory (QCT) calculation with the fourth-order explicit symplectic algorithm for the N(⁴S) + O₂(X³Σ_g⁻) → NO(X²Π) + O(³P) reaction has been performed by employing the ground and first-excited potential energy surfaces (PESs). Since the translational temperature considered is up to 5000 K, the larger relative translational energy and the higher vibrational and rotational level of O₂ molecule have been taken into account. The affect of the relative translational energy, the vibrational and rotational level of O₂ molecule in the reaction cross-sections of the ground and first-excited PESs has been discussed in a extensive range. And we exhibit the dependence of microscopic rate constants on the vibrational and rotational level of O₂ molecule at T = 4000 K. The thermal rate constants at the translational temperature betweem 300 and 5000 K have been evaluated and the corresponding Arrhenius curve has been fitted for reaction (1). It is found by comparison that the thermal rate constants determined in this work have a better agreement with the experimental data and provide a more valid theoretical reference.     

Dissociative excitation of water by metastable argon

The reaction Ar(²P_2,0) + H₂O → Ar + H + OH(A²Σ⁺)was studied in crossed molecular beams by observing the luminescence from OH(A²Σ⁺). No significant dependence of the spectrum on collision energy was found over the 22–52 meV region. Spectral simulation was used to obtain the OH(A) vibrational distribution and rotational temperature, assuming a Boltzmann rotational distribution. Since predissociation is known to strongly affect the rovibrational distribution, the individual rotational state lifetimes were included in the simulation program and were used to obtain the average vibrational state lifetimes. Excellent agreement with experiment was obtained for vibrational population ratios N₀/N₁/N₂ of 1.00/ 0.40/0.013 and a rotational temperature of 4000 K. Correction for the different average vibrational lifetimes gave formation rate ratios P₀/P₁/P₂ of 1.00/0.49/0.25. The differences between these results and those from flowing afterglow studies on the same system are discussed. Three reaction mechanisms are considered, and the vibrational prior distributions are calculated from a simple density-of-states model. Only fair agreement with experiment is obtained. The best agreement for the mechanisms giving OH(A) in two 2-body dissociation steps is obtained by assuming 1.0 eV of internal energy remains in the second step. The OH(A) vibrational population distribution of the present work is similar to that found in the photolysis of H₂O at 122 nm, where there is 1.10 eV of excess internal energy.     

Velocity Mapping Studies of Vibrational Energy Disposal Following Methyl Iodide Photodissociation

In this paper previous results are compared for two different types of velocity mapping studies which probe vibrational energy disposal following the A-band photodissociation of methyl iodide, CH₃I + hv → CH₃ (v) + 1(²P_3/2), 1*(²P_1/2). Full three-dimensional state-specific speed and angular distributions of the nascent fragments have been recorded for the photoelectrons, iodine atoms, and methyl radicals, using state- and mass-selective (2+1) resonance-enhanced multi-photon ionization (REMPI)/time-of-flight spectrometry. Two sources of information on the vibrational energy disposal are available from velocity mapping: (a) the photoelectron images, which give information on the initial stages of vibrational excitation in electronically excited CH₃I, and (b) methyl radical images, which indicate the final energy disposal channels. Even though the two signals are believed to probe very different time-scales of the dissociation process, good agreement between the two is found for the vibrational energy disposal trends. Several trends found in the data for methyl iodide photodissociation indicate that readjustment of the ab initio multi-dimensional potential energy surfaces calculated for this molecule appears to be needed.     

Semiclassical calculation of energy transfer in polyatomic molecules. VIII. Theory for atom + non-linear triatom

The theory for collision of an atom with a non-linear triatomic molecule is presented and the He + H₂O system considered as an example. It is shown that both anharmonic and Coriolis coupling are important for the energy transfer. Seventeen rate constants for vibrational transitions in H₂O are calculated in the temperature range 300–2000 K.     

The effect of the intermolecular potential formulation on the state‐selected energy exchange rate coefficients in N2–N2 collisions

The rate coefficients for N₂–N₂ collision‐induced vibrational energy exchange (important for the enhancement of several modern innovative technologies) have been computed over a wide range of temperature. Potential energy surfaces based on different formulations of the intramolecular and intermolecular components of the interaction have been used to compute quasiclassically and semiclassically some vibrational to vibrational energy transfer rate coefficients. Related outcomes have been rationalized in terms of state‐to‐state probabilities and cross sections for quasi‐resonant transitions and deexcitations from the first excited vibrational level (for which experimental information are available). On this ground, it has been possible to spot critical differences on the vibrational energy exchange mechanisms supported by the different surfaces (mainly by their intermolecular components) in the low collision energy regime, though still effective for temperatures as high as 10,000 K. It was found, in particular, that the most recently proposed intermolecular potential becomes the most effective in promoting vibrational energy exchange near threshold temperatures and has a behavior opposite to the previously proposed one when varying the coupling of vibration with the other degrees of freedom. © 2014 Wiley Periodicals, Inc.     

The born approximation as a simple diagnostic method for direct molecular collisions with applications to Cl + HI and F + H2 reactions

Born approximation computations are presented and discussed for the Cl + HI → I + HCl and F + H₂ → H + HF reactions and their isotopic analogues. Most aspects of the role of reagent energy or the energy disposal in the products previously deduced from experiment or trajectory computations can be accounted for the Born approximation. The procedure used here neglects the interaction between non-bonded atoms. It does thereby provide a very simple computational scheme which requires as input only the spectroscopic constants of the reactants and products. In addition it offers simple qualitative interpretations of the trends in the results. The overall satisfactory agreement between the present results and past studies lends credibility to the basic propensity rule provided by the Born approximation: The most probable transitions are those that minimize the momentum transfer to the nuclei. The principle is discussed with special reference to exothermic (E′_T ? E_T) and endothermic transitions.The computations for Cl + HI indicate a decline of the reaction cross section with increasing kinetic energy and a strong enhancement by HI rotational energy. The surprisal analysis confirms the absence of vibrational population inversion for endothermic transitions. For the F + H₂ (and isotopic variants) reactions, the product-rotational state distribution extends nearly to the energy cut-off. The vibrational state distribution is somewhat different for para- and normal H₂ and, in general, the collision outcome is very sensitive to the initial rotational state of H₂ particularly at low translational energies. The HF/DF branching ratio is F + HD collisions is increasing with increase of the HD rotational state. The vibrational surprisal is essentially isotopically invariant.     

Collisional energy transfer in the reactions I + NO + M → INO + M and I + NO2 + M → INO2 + M

The low-pressure recombination rate constants of the reactions I + NO + M → INO + M (with 14 different M) and I + NO₂ + M → INO₂ + M (with 26 different M) have been measured at 330°K by laser flash photolysis. The collision efficiencies β_c are analyzed and compared with other thermal activation systems. Whereas β_c increases in one reaction with an increasing number of atoms in M, practically no such effect is found when, for the same M, different reactions with varying complexities of the reacting molecules are considered.     

Quasiclassical trajectory simulations of OH(v) + NO2 --> HONO2* --> OH(v') + NO2: capture and vibrational deactivation rate constants

Quasiclassical trajectory calculations are used to investigate the dynamics of the OH(v) + NO(2) --> HONO(2) --> OH(v') + NO(2) recombination/dissociation reaction on an analytic potential energy surface (PES) that gives good agreement with the known structure and vibrational frequencies of nitric acid. The calculated recombination rate constants depend only weakly on temperature and on the initial vibrational energy level of OH(v). The magnitude of the recombination rate constant is sensitive to the potential function describing the newly formed bond and to the switching functions in the PES that attenuate inter-mode interactions at long range. The lifetime of the nascent excited HONO(2) depends strongly not only on its internal energy but also on the identity of the initial state, in disagreement with statistical theory. This disagreement is probably due to the effects of slow intramolecular vibrational energy redistribution (IVR) from the initially excited OH stretching mode. The vibrational energy distribution of product OH(v') radicals is different from statistical distributions, a result consistent with the effects of slow IVR. Nonetheless, the trajectory results predict that vibrational deactivation of OH(v) via the HONO(2) transient complex is approximately 90% efficient, almost independent of initial OH(v) vibrational level, in qualitative agreement with recent experiments. Tests are also carried out using the HONO(2) PES, but assuming the weaker O-O bond strength found in HOONO (peroxynitrous acid). In this case, the predicted vibrational deactivation efficiencies are significantly lower and depend strongly on the initial vibrational state of OH(v), in disagreement with experiments. This disagreement suggests that the actual HOONO PES may contain more inter-mode coupling than found in the present model PES, which is based on HONO(2). For nitric acid, the measured vibrational deactivation rate constant is a useful proxy for the recombination rate, but IVR randomization of energy is not complete, suggesting that the efficacy of the proxy method must be evaluated on a case-by-case basis.     

Laser-induced fluorescence detection of CH(X2Π) produced from the reaction of Ar(3p2,0) at thermal energy

CH (X ²Π) radicals have been observed from the reaction Ar(³P₂,₀) + CH₄→ CH (X) + H₂ + H + Ar by using a flowing afterglow apparatus coupled with laser-induced fluorescence. It has been found that only 3.5% of the available energy is convened into the vibrational energy of CH(X).     

A response-function approach to the direct calculation of the transition-energy in a multiple-cluster expansion formalism

Approximate limiting collision induced dissociation (CID) probabilities are calculated as a function of the vibrational energy and temperature for HCl, HF and O₂, infinitely diluted in an inert gas, by assuming that only the molecules with an energy that is within kT below the threshold dissociate and do so with unit probability. Rotational energy is explicitly included in defining the threshold for dissociation. At the lowest temperatures of our calculations (2500 K for HCl and HF, 3500 K for O₂) the agreement between the calculated CID probabilities and those obtained by us earlier in fitting the experimental dissociation rates is quite remarkable for HCl and HF and satisfactory for O₂ at all vibrational energies. It is therefore argued that the ladder climbing model and the weak bias model for diatomic dissociation are not different in principle if rotational energy is properly accounted for.     

Can the pentazole anion (N5−) be isolated and/or trapped in metal complexes?

The 1,3-dipolar cycloreversion pathway of the pentazole anion (N₅^?) to the azide anion (N₃^?) plus dinitrogen (N₂) has been investigated using ab initio methods. At the MP4SDQ/6–31 + G* level of theory plus zero-point energy contributions, the pentazole anion is predicted to lie at 31 kcal mol^?1 above the N₃^? + N₂ system but the energy barrier for decomposition is 22 kcal mol^?1. This indicates that the pentazole anion could be isolated in an inert matrix at low temperature. Comparison between extended Hückel calculations on the (N₅)M(CO)₃ and (C₅H₅)M(CO)₃ complexes (with M = Fe²⁺, Mn⁺ and Cr) suggests that the N₅^? complexes would be formed if the fragments could be brought together. Predicted vibrational frequencies of the N₅^? anion are also reported.     

Pressure Dependence of Rate Coefficients for Formation and Dissociation of Pentachlorodisilane and Related Chemical Activation Reactions

Four entrance channels to the pentachlorodisilane (Si₂HCl₅) potential energy well, namely, SiCl₂ + SiHCl₃, SiCl₄ + SiHCl, Cl₃SiSiCl + HCl, and SiCl₃ + SiHCl₂, were analyzed in detail through transition state theory, Rice–Ramsperger–Kassel–Marcus (RRKM) theory, and solution of the multichannel master equation. The stationary points in the potential energy surface were optimized, and their vibrational frequencies and rotational constants calculated at the (U)B3LYP/6–31+G(d,p) level of theory; the (U)CCSD(T)/aug‐cc‐pVTZ level was then used for accurate estimation of activation energies. The pressure and temperature dependence of the rate coefficients of the channels related to Si₂HCl₅ stabilization/dissociation was determined along a wide range of conditions, for the first time. All channels showed strong pressure dependence in the four cases, at least at low‐to‐moderate pressure conditions. Each entrance channel leads to the formation of different products under different conditions, and the mechanism was analyzed in detail. The results indicated that at atmospheric pressure the reactions are in the falloff region, and therefore do not support the adoption of high‐pressure limit rate coefficients in reaction models designed for simulation of systems at atmospheric or subatmospheric pressure conditions.     

The dynamics of Cl− + H2 reactive collisions

A multicoincidence analysis of the crossed beam Cl^? + H₂ system in the 5.6–12 eV energy range has shown the existence of four different product channels: reaction (R), reactive detachment (RD), simple detachment (SD) and dissociative detachment (DD). For the whole energy range both R and RD channels give rise to HCl molecules at a unique and common center-of-mass scattering angle whereas the vibrational excitation probability of HCl obeys completely different rules for each channel: v ≤3 in channel R and equal probability in all possible vibrational levels in RD. A Thomas-type collision model joined to curve crossing with an intermediate autodetaching HCl^? state accounts well for all of the experimental findings.     

Cascades of energy among the vibrational levels of diatomic molecules trapped in a rare gas matrix: With application to 12C16O(ν) in Ar

Energy stored in vibrational level ν = 1 of several individual dipolar diatomic molecules AB which are trapped in a rare gas matrix M is automatically accumulated in a higher level ν > 1 of a single molecule AB. This remarkable cascade of energy upwards competes with a cascade of energy downwards. the radiative decay. The interplay of both cascades, first observed by Dubost and Charneau, is explained a simple model. The model incorporates three processes into a master equation for the relative populations P_ν(t) of levels ν: (a) migration of single quanta by resonance energy transfer, AB(1) + AB(0) ? AB(0) + AB(1); (b) phonon assisted excitation of upper levels, AB(1) + AB(ν) → AB(0) + AB(ν+1); and (c) radiative decay, AB(ν) → AB(ν-1). The model assumes that there is only one isotopic species AB which has a small but nonzero vibrational anharmonicity, that the temperature is low, T → 0 K, the concentration ratio ?_M/?_AB is large and that, initially, at time t = 0, a small fraction p₁ of molecules AB is excited to level ν = 1. The master equation has only two parameters, the radiative lifetime t_rad and k  2/[?_AB?₁k(1,1 → 0,2)t_rad], where k(1,1 → 0,2) is the reference rate constant of process (b). The master equation is solved in closed form for the Pν(t). For t_rad = 14 ms and k = 0.2, very satisfactory qualitative agreement is found for the theoretical P_ν(t) and the experimental time evolution of the relative population of vibrational levels of ¹²C¹⁶O in an argon matrix, for ?_M/?_AB = 2000 at T = 9 K. In agreement with experimental results it is concluded that the risetime of the fluorescence signals decreases whereas population inversion increases for decreasing values of ?_M/?AB. At long times, t > t_rad, any population inversion should disappear.     

Competition between dissociation and exchange processes in a collinear A + BC collision. I. Exact quantum results

Exact quantum results for collision-induced dissociation on a reactive surface are presented. A modified LEPS potential-energy surface modeling the H + HD → H₂ + D system has been used. HD and H₂ bearing respectively 7 and 6 bound states. This system has been chosen because it displays significant reactive scattering for total energies above the dissociation threshold. Calculations have been performed using the time-dependent wavepacket method for two initial vibrational quantum numbers of the HD molecule (v = 0, 2). For each vibrational quantum number, two wavepackets with overlapping energy distributions have been run, covering a total energy range up to more than three times the dissociation energy. Comparison with previous collision-induced dissociation calculations shows that the dissociation is greatly enhanced by the presence of concomitant reactive scattering. A vibrational enhancement effect is also observed above the dissociation threshold; for higher energies the system exhibits a pronounced vibrational inhibition effect.     

Collaborative control of branching ratio in the O + HO2 → OH + O2 reaction via vibrational and rotational excitation

To understand the effect of different vibrational and rotational modes of reactant to enhance the reactivity of the O + HO₂ → OH + O₂ reaction, we revisited this important atmospheric reaction. We report here a quasi-classical trajectory (QCT) study of the reaction dynamics on a recently developed full-dimensional potential energy surface (PES). Our previous work has indicated that this reaction has two pathways, the H abstraction (HA) channel and the O abstraction (OA) channel, which lead to totally different product energy distribution. In this work, we identified that the vibrational excitation of the OH stretching (v₁) mode of HO₂ is the switch of the HA channel at low collision energy; meanwhile, the rotational excitation can also greatly change the branching ratio of the two pathways. With the excitation of v₁ mode, the original negligible HA channel controlled by the tight transition state becomes quite important. This work presents an approach to control the branching ratio via collaboration between vibrational and rotational excitation and will enrich the knowledge of the O + HO₂ reaction in atmospheric chemistry and physics.     

Chemiluminescence studies of vibrational-to-vibrational energy transfer: MF3 + HF

The vibrational-to-vibrational energy transfer process, MF³ + HF (ν = 0) → MF + HF³ (ν ? 9) is studied by means of a “triple beam” experiment. Vibrationally excited MF³ molecules are created at the intersection of crude crossed beams of M (M = Na, Mg) and F₂. The metal fluorides thus formed then cross an HF beam, where energy transfer occurs. This is observed by measuring the overtone emission from HF. Upper bounds on the reaction cross sections for M ÷ F₂ are measured to be 135 ± 20 A² for M = Na and 80 ± 15 A² for M = Mg, and laser induced fluorescence is used to determine the vibration energy distribution of MgF, which peaks at 2.6 eV. The chemiluminescence signal from the overtone emission indicates a large vibrational interconversion cross section, which is estimated to be ? 30 A².     

Fully converged sudden rotation total integral cross sections for 4He + H2 (ν = 0, j = 0) → 4He + H2 (ν′ = 1, j′)

Total integral cross sections for ⁴He + H₂ (ν = 0, j = 0) → ⁴He + H₂ (ν′ = 1, j′ = 0, 2) have been calculated in the total energy range 1.2 to 5.5 eV, according to a quantal sudden approximation for the H₂ rotational degrees of freedom and a close coupling expansion of the vibrational degree of freedom. Convergence of the above cross sections is investigated by employing four vibration basis sets in the close coupling calculations, i.e., ν = 0,1, ν = 0,1, 2, ν = 0, 1, 2, 3 and ν = 0, 1, 2, 3, 4. Between 4.2 and 5.5 eV calculations were done with three vibration basis sets; ν = 0.–4, ν = 0–5, and ν = 0–6. It is found that at least four vibrational basis functions are needed to converge (to within 5–10%) these cross sections in the above energy range. Comparison of breathing sphere calculations and summed sudden rotation results shows good agreement for the (weakly anisotropic) Mies-Krauss potential. However, as expected the former results underestimate the vibrational 0 → 1 total integral cross sections.     

Quantum-mechanical calculations on pressure and temperature dependence of three-body recombination reactions: application to ozone formation rates

A quantum-mechanical model is designed for the calculation of termolecular association reaction rate coefficients in the low-pressure fall-off regime. The dynamics is set up within the energy transfer mechanism and the kinetic scheme is the steady-state approximation. We applied this model to the formation of ozone O + O2 + M --> O3 + M for M = Ar, making use of semiquantitative potential energy surfaces. The stabilization process is treated by means of the vibrational close-coupling infinite order sudden scattering theory. Major approximations include the neglect of the O3 vibrational bending mode and rovibrational couplings. We calculated individual isotope-specific rate constants and rate constant ratios over the temperature range 10-1000 K and the pressure fall-off region 10(-7)-10(2) bar. The present results show a qualitative and semiquantitative agreement with available experiments, particularly in the temperature region of atmospheric interest.