Ultracold collisions and reactions of vibrationally excited OH radicals with oxygen atoms

We report a quantum dynamics study of O + OH (v = 1, j = 0) collisions on its ground electronic state, employing two different potential energy surfaces: the DIMKP surface by Kendrick and Pack, and the XXZLG surface by Xu et al. A time-independent quantum mechanical method based on hyperspherical coordinates has been adopted for the dynamics calculations. Energy-dependent probabilities and rate coefficients are computed for the elastic, inelastic, and reactive channels over the collision energy range E(coll) = 10(-10)-0.35 eV, for J = 0 total angular momentum. Initial state-selected reaction rate coefficients are also calculated from the J = 0 reaction probabilities by applying a J-shifting approximation, for temperatures in the range T = 10(-6)-700 K. Our results show that the dynamics of the collisional process and its outcome are strongly influenced by long-range forces, and chemical reactivity is found to be sensitive to the choice of the potential energy surface. For O + OH (v = 1, j = 0) collisions at low temperatures, vibrational relaxation of OH competes with reactive scattering. Since long-range interactions can facilitate vibrational relaxation processes, we find that the DIMKP potential (which explicitly includes van der Waals dispersion terms) favours vibrational relaxation over chemical reaction at low temperatures. On the DIMKP potential in the ultracold regime, the reaction rate coefficient for O + OH (v = 1, j = 0) is found to be a factor of thirteen lower than that for O + OH (v = 0, j = 0). This significantly high reactivity of OH (v = 0, j = 0), compared to that of OH (v = 1, j = 0), is attributed to enhancement caused by the presence of a HO(2) quasibound state (scattering resonance) with energy near the O + OH (v = 0, j = 0) dissociation threshold. In contrast, the XXZLG potential does not contain explicit van der Waals terms, being just an extrapolation by a nearly constant function at large O-OH distances. Therefore, long-range potential couplings are absent in calculations using the XXZLG surface, which does not induce vibrational relaxation as efficiently as the DIMKP potential. The XXZLG potential leads to a slightly higher reactivity (a factor of 1.4 higher) for O + OH (v = 1, j = 0) compared to that for O + OH (v = 0, j = 0) at ultracold temperatures. Overall, both potential surfaces yield comparable values of reaction rate coefficients at low temperatures for the O + OH (v = 1, j = 0) reaction.     

The reaction of nitric oxide with vibrationally excited ozone

The reaction between nitric oxide and vibrationally excited ozone was studied in a fast flow reactor by monitoring the visible emission from electronically excited NO₂¹. The antisymmetric mode (ν₃) of O₃ was excited with a Q-switched 9.6 μm CO₂ laser, and a laser-induced signal was detected, with a rise rate constant of (4.0 ± 0.5) × 10¹¹ cm³/mole sec and a decay rate constant of (1.1 ± 0.1) × 10¹¹ cm³/mole sec for an NO-rich mixture. The latter was unaffected by addition of large amounts of He or Ar, indicating that the signal was not a thermal effect. Most of the measurements were made at 350°K; however, the He and Ar dilution results suggest that the enhanced reaction rate is not very sensitive to temperature. In order to explain the observed rise times, it was necessary to postulate an intermediate step prior to the chemical reaction. A model which is consistent with our data has energy transferred from ν₃ to ν₂ (the bending mode) at a rate of (2.9 ± 0.5) × 10¹¹ cm³/mole sec for NO and a rate of (1.1 ± 0.2) × 10¹¹ cm³/mole sec for He. According to this model, the rate constant for the reaction of NO with O₃ (ν₂= 1) producing vibrationally excited ground state NO₂²,

$\text{NO + O}{}_3{}^2\text{(010)}\text{3}\text{NO}{}_2{}^2\text{+ O}{}_2$

is (1.5 ± 0.2) × 10¹¹ cm³/mole sec, and the relative rate for the reaction of O₃ (ν₂ = 1) and O₃(ν₂ = 0) with NO was estimated to be $\text{k}{}_3\text{(1)}\text{k}{}_3\text{(0)}\text{≈ 22}$.     

Vibrational relaxation of highly vibrationally excited O3 in collisions with OH

We report a theoretical study of highly excited O3 in collisions with vibrationally cold OH. Special emphasis is placed on initial vibrational energies of O3 between 9 and 21 kcal mol(-1). All calculations have employed the quasiclassical trajectory method and the realistic double many-body expansion potential energy surface for HO 4((2)A). Many aspects of the title relaxation process are presented. The results indicate that it may not be ignorable in studying the stratospheric ozone budget.     

The photodissociation of vibrationally excited ozone in the upper atmosphere

The photodissociation of vibrationally excited O₃ in the sunlit mesosphere is investigated. Dissociation rate coefficients for specific vibrationally excited states of O₃ are calculated for the conditions of an overhead sun. Possible vibrational enhancements of the O(¹D) and O₂(a¹Δ_g) production rates are assessed. It is shown that such enhancements should make only minor contributions to the daytime production of these species in the mesosphere and lower thermosphere.     

Reactivity of vibrationally excited methane molecules in reactions with chlorine atoms

At 148–298 K, the rate constant for the reaction of methane molecules excited into bending vibration with atomic chlorine does not exceed by more than 30 times the corresponding constant for methane in thermal equilibrium. Consequently, at low temperatures and thermal equilibrium the reaction of methane with atomic chlorine proceeds through the vibrational ground state of methane.

---

, 148–298°K , , 30 , . , .

     

Equilibration of vibrationally excited OH in atomic and diatomic bath gases

In this work, a computational model of state-to-state energy flow in gas ensembles is used to investigate collisional relaxation of excited OH, present as a minor species in various bath gases. Rovibrational quantum state populations are computed for each component species in ensembles consisting of 8000 molecules undergoing cycles of binary collisions. Results are presented as quantum state populations and as (approximate) modal temperatures for each species after each collision cycle. Equilibration of OH is slow with Ar as the partner but much faster when N(2) and/or O(2) forms the bath gas. This accelerated thermalization is shown to be the result of near-resonant vibration-vibration transfer, with vibrational de-excitation in OH matched in energy by excitation in bath molecules. Successive near-resonant events result in an energy cascade. Such processes are highly dependent on molecule pair and on initial OH vibrational state. OH rotational temperatures initially increase, but at equilibration, they are lower than those of other modes. Possible reasons for this observation in molecules such as OH are suggested. There are indications of an order of precedent in the equilibration process, with vibrations taking priority over rotations, and potential explanations for this phenomenon are discussed.     

Near-resonant energy transfer from highly vibrationally excited OH to N2

The probability per collision P(T) of near-resonant vibration-to-vibration energy transfer (ET) of one quantum of vibrational energy from vibrational levels nu=8 and nu=9 of OH to N(2)(nu=0), OH(nu)+N(2)(0)-->OH(nu-1)+N(2)(1), is calculated in the 100-350 K temperature range. These processes represent important steps in a model that explains the enhanced 4.3 microm emission from CO(2) in the nocturnal mesosphere. The calculated energy transfer is mediated by weak long-range dipole-quadrupole interaction. The results of this calculation are very sensitive to the strength of the two transition moments. Because of the long range of the intermolecular potential, the resonance function, a measure of energy that can be efficiently exchanged between translation and vibration-rotation degrees of freedom, is rather narrow. A narrow resonance function coupled with the large rotational constant of OH is shown to render the results of the calculation very sensitive to the rotational distribution, or the rotational temperature if one exists, of this molecule. The calculations are carried out in the first and second orders of perturbation theory with the latter shown to give ET probabilities that are an order of magnitude larger than the former. The reasons for the difference in magnitude and temperature dependence of the first- and second-order calculations are discussed. The results of the calculations are compared with room temperature measurements as well as with an earlier calculation. Our calculated results are in good agreement with the room temperature measurements for the transfer of vibrational energy for the exothermic OH(nu=9) ET process but are about an order lower than the room temperature measurements for the exothermic OH(nu=8) ET process. The cause of this discrepancy is explored. This calculation does not give the large values of the rate coefficients needed by the model that explains the enhanced 4.3 microm emission from CO(2) in the nocturnal mesosphere.     

Experimental study on the reactions of vibrationally excited 1,1-difluorocyclopropane

Vibrationally excited 1,1-difluorocyclopropane, generated by addition of singlet methylene to 1,1-difluoroethene, reacts according to four reaction channels by isomerization as well as by elimination of difluoromethylene. Identification of the products leads to a critical view on an early study dealing with this reaction system. The pressure dependence of the reaction is investigated. The apparent rate-constants of the reaction paths were determined.     

Deactivation of vibrationally excited ozone by O(3P) atoms

Using laser-induced fluorescence of ozone (to measure the rate of disappearance of O₃²) and NO₂ titration (to determine O atom concentrations), we have determined bimolecular rate constants for the deactivation by O(³P atoms) of ozone in excited stretching and bending modes. These experiments do not distinguish between deactivation by (a) the exchange of vibrational and translational energy or (b) the chemical reaction O₃ + O → 2O₂. If the non-reactive pathway (a) is assumed to dominate, then O(³P) is 150 times more effective than O₂ in deactivating O²₃. If chemical reaction (b) is dominant, the bimolecular rate constant for O²₃ + O(³P) is larger by a factor of 150–1500 than that for ground-state ozone.     

On the theory of the reaction rate of vibrationally excited CO molecules with OH radicals

The dependence of the rate of the reaction CO+OH-->H+CO2 on the CO-vibrational excitation is treated here theoretically. Both the Rice-Ramsperger-Kassel-Marcus (RRKM) rate constant kRRKM and a nonstatistical modification knon [W.-C. Chen and R. A. Marcus, J. Chem. Phys. 123, 094307 (2005).] are used in the analysis. The experimentally measured rate constant shows an apparent (large error bars) decrease with increasing CO-vibrational temperature Tv over the range of Tv's studied, 298-1800 K. Both kRRKM(Tv) and knon(Tv) show the same trend over the Tv-range studied, but the knon(Tv) vs Tv plot shows a larger effect. The various trends can be understood in simple terms. The calculated rate constant kv decreases with increasing CO vibrational quantum number v, on going from v=0 to v=1, by factors of 1.5 and 3 in the RRKM and nonstatistical calculations, respectively. It then increases when v is increased further. These results can be regarded as a prediction when v state-selected rate constants become available.     

Dynamics study of the OH + O3 atmospheric reaction with both reactants vibrationally excited

The dynamics of the title five-atom atmospheric reaction is studied by the quasiclassical trajectory method for vibrational states of OH over the range 2 < or = v < or = 9 and initial vibrational energies of O3 between 9 and 21 kcal mol-1 using a previously reported double many-body expansion potential energy surface for HO4(2A). The results show that the reaction is controlled by both capture- and barrier-type mechanisms, with the rate constants depending strongly on the reactants' internal energy content. Also suggested from the magnitude of the calculated rate coefficients is that the title processes may not be ignorable when studying the stratospheric ozone budget.     

The production and subsequent relaxation of vibrationally excited OH in the reaction of atomic oxygen with HBr

A fast discharge flow apparatus equipped for EPR detection of radicals has been used to investigate the reaction O + HBr → OH + Br. At 295°K, measurements showed that more than 97% of all OH produced in this reaction was formed initially in its first vibrationally excited state. Rate constants for physical deactivation of OH(v = 1) by O(³P), Br(²P_3/2), H₂O, and HBr were measured as (1.45 ± 0.25) × 10^?10, (6.4 ± 2.4) × 10^?11, (1.35 ± 0.50) × 10^?11, and < 10^?12 cm³/molec·sec, respectively.     

Solution phase isomerization of vibrationally excited singlet nitrenes to vibrationally excited 1,2-didehydroazepine

Photolysis of phenyl and o-biphenylyl azide (at 270 nm) releases vibrationally excited singlet nitrene which isomerizes to the corresponding hot 1,2-didehydroazepine at a rate competitive with thermal relaxation. Using ultrafast vibrational spectroscopy we observe the formation of vibrationally excited 1,2-4,6-azacycloheptatetraene (1,2-didehydroazepine) in picoseconds following photolysis of phenyl azide in chloroform and o-biphenylyl azide in acetonitrile at ambient temperature.     

Production of a beam of highly vibrationally excited CO using perturbations

An intense molecular beam of CO (X(1)Σ(+)) in high vibrational states (v = 17, 18) was produced by a new approach that we call PUMP - PUMP - PERTURB and DUMP. The basic idea is to access high vibrational states of CO e(3)Σ(-) via a two-photon doubly resonant transition that is perturbed by the A(1)Π state. DUMP -ing from this mixed (predominantly triplet) state allows access to high vibrational levels of CO (X(1)Σ(+)). The success of the approach, which avoids the use of vacuum UV radiation in any of the excitation steps, is proven by laser induced fluorescence and resonance enhanced multi-photon ionization spectroscopy.     

Diffusion of vibrationally excited molecules

A treatment is presented for the effect of intermolecular vibrational energy transfer on the diffusion coefficient of vibrationally excited molecules. An analytic treatment based on random walk statistics and a Monte Carlo type calculation have been performed. Both methods yield very similar results which correlate well with existing experimental studies. A hard sphere collision model is treated extensively with comparisons made to other internmolecular potentials. The results support the involvement of long range energy transfer in V → V interactions. The effect of temeprature on the diffusion coefficient of vibrationally excited molecules is calculated, with applications to the CO^*₂CO₂ system.     

Dynamics of bimolecular reactions of vibrationally highly excited molecules: quasiclassical trajectory studies

Excitation functions from quasiclassical trajectory calculations on the H + H2O --> OH + H2, H + HF --> F + H2, and H + H'F --> H' + HF reactions indicate a different behavior at low and high vibrational excitation of the breaking bond. When the reactant tri- or diatomic molecule is in vibrational ground state or in a low vibrationally excited state, all these reactions are activated; i.e., there is a nonzero threshold energy below which there is no reaction. In contrast, at high-stretch excited-states capture-type behavior is observed; i.e., with decreasing translational energy the reactive cross-section diverges. The latter induces extreme vibrational enhancement of the thermal rate consistent with the experiments. The results indicate that the speed-up observed at high vibrational excitation is beyond the applicability of Polanyi's rules in their common form; instead, it can be interpreted in terms of an attractive potential acting on the attacking H atom when it approaches the reactant with a stretched X-H bond.     

Relaxation of highly vibrationally excited molecules

The rate constant for V-V relaxation of diatomic homonuclear molecules is determined from collisions of unexcited molecules with molecules near the dissociation threshold. It is shown that a quasi-resonant transition through several levels dominates in this process so that the energy difference between the initial and final states of the excited molecule is approximately equal to the transition energy from the zero level to the first one. The relaxation kinetics of excited molecules is studied. Absorption of IR radiation by polyatomic molecules is discussed taking into account collisions. A criterion for the negligibility of energy loss is obtained, and the dissociation rate of molecules under the action of IR laser irradiation found. The computational results are compared with experimental data. A self-consistent procedure is formulated for a gas irradiated by a quasi-continuously operating IR laser, in order to determine simultaneously the dissociation rate, dissipation energy flux and temperature. The existence of an optimum radiation region for dissociation is found.     

The decomposition of vibrationally excited molecules

Possible decomposition routes for vibrationally excited hydrocarbons are critically considered using a simple “symmetry” based model. It is shown that C---C bond rupture is characterised by a lower activation energy than the molecular cleavage of hydrogen, although the latter reaction is less endothermic. Other possible decomposition reactions of excited species are also considered.     

Bimolecular reactions of vibrationally excited molecules. Roaming atom mechanism at low kinetic energies

Quasiclassical trajectory calculations have been performed for the H + H'X(v) → X + HH' abstraction and H + H'X(v) → XH + H' (X = Cl, F) exchange reactions of the vibrationally excited diatomic reactant at a wide collision energy range extending to ultracold temperatures. Vibrational excitation of the reactant increases the abstraction cross sections significantly. If the vibrational excitation is larger than the height of the potential barrier for reaction, the reactive cross sections diverge at very low collision energies, similarly to capture reactions. The divergence is quenched by rotational excitation but returns if the reactant rotates fast. The thermal rate coefficients for vibrationally excited reactants are very large, approach or exceed the gas kinetic limit because of the capture-type divergence at low collision energies. The Arrhenius activation energies assume small negative values at and below room temperature, if the vibrational quantum number is larger than 1 for HCl and larger than 3 for HF. The exchange reaction also exhibits capture-type divergence, but the rate coefficients are larger. Comparisons are presented between classical and quantum mechanical results at low collision energies. At low collision energies the importance of the exchange reaction is enhanced by a roaming atom mechanism, namely, collisions leading to H atom exchange but bypassing the exchange barrier. Such collisions probably have a large role under ultracold conditions. The calculations indicate that for roaming to occur, long-range attractive interaction and small relative kinetic energy in the chemical reaction at the first encounter are necessary, which ensures that the partners can not leave the attractive well. Large orbital angular momentum of the primary products (equivalent to large rotational excitation in a unimolecular reaction) is favorable for roaming.     

Relaxation of vibrationally excited HCl in solid Xe

Rates for vibrational relaxation of HCl(ν = 1.2) in solid xenon at 40 and 146 K are reported and are compared to the rate of relaxation of HCl(ν = 1) in liquid xenon near the freezing point. Upon freezing, the rate of relaxation of HCl(ν = 1) is found to decrease significantly and emission from HCl(ν = 2), absent in the liquid phase, is detected. Both of these effects are attributed to a significant decrease in mobility of HCl molecules in the solid phase as compared to the liquid phase. At both 40 and 146 K, the ratio of relaxation rates for HCl(ν = 2) to HCl(ν = 1) is found to deviate significantly from the harmonic oscillator prediction of 2:1. The rate of relaxation for HCl(ν = 1) by xenon is found to be similar in both liquid solution at 200 K and in the solid at 146 K.