Adiabatic Channel Research Articles

Weak- and strong-field Stark energy levels of symmetric top dipolar molecules

Perturbation expansions of the energy levels of symmetric top dipolar molecules in weak and strong electric fields (field strength F) are extended up to terms proportional to F3 (for weak fields) and to F−1 (for strong fields). For small quantum numbers, suitable analytical interpolations between the weak- and strong-field expansions are obtained from matrix diagonalization results. For large quantum numbers, where this approach becomes inconvenient, quasiclassical techniques provide these interpolations. The expressions are of use not only in spectroscopy but also in reaction kinetics, e.g., in adiabatic channel treatments of reactions on attractive potentials with cos θ anisotropies.

The rate of ion-linear dipole capture at extremely low temperatures estimated with the use of the statistical adiabatic channel model

The rate constant for ion—linear dipole capture is estimated over a range of very small energies using the microcanonical version of the statistical adiabatic channel model (SACM). The energy dependence is found to be very diverse and not following any simple pattern. The dimensionless parameters y and ξ affect the issue considerably. Depending on these parameters, the function rate constant as a function of energy may increase either monotonously or stepwise, it may attain a plateau value at some energy and next, having surpassed some threshold point, may further increase monotonously at a rate that is affected by the magnitude of ξ and y.

Laser-induced temperature jump measurements in the kinetics of association and dissociation of the nitrogen oxide (N2O3) + M = nitrogen dioxide + nitrous oxide + M system

Kinetic measurements have been made on the gas-phase reaction N2O4 + M .dblharw. 2NO2 + M by applying the temp.-jump relaxation method. Equil. mixts. contg. NO2, N2O4, a small quantity of an IR absorber, and an excess of a third-body bath gas, have been subjected to a short CO2 laser pulse. The induced temp. jump, of the order of 1 K, displaces the equil. towards NO2 formation. After the fast heating of the mixt., the time dependence of the NO2 and N2O4 concns. has been monitored at 420 and 250 nm, resp. For small perturbations, the relaxation to the new equil. concns. at the higher temp. follows first-order kinetics, and the thermal rate const. can be deduced from the measured relaxation rate const. Measurements have been made at 255 K at 0.3-200 bars He, which corresponds to the falloff range for this reaction. The high- and low-pressure limiting rate consts., krec,... = (7.0 +- 0.7) * 1011 cm3 mol-1 s-1 and krec,0/[He] = (2.1 +- 0.2) * 1014 cm3 mol-1 s-1, resp., extrapolated from the exptl. data at 255 K, were in agreement with those calcd. with a simplified statistic adiabatic channel model. The temp. dependence of the high- and low-pressure recombination rate const., detd. at 255-273 K, is given by the relations krec,... = (2.2 +- 0.2) * 106 * T(2.3+-0.2) cm3 mol-1 s-1 and krec,0/[He] = (7.5 +- 0.8) * 1035 * T(-9.0+-0.9) cm3 mol-1 s-1. The corresponding high- and low-pressure dissocn. rate consts. are kdiss,... = (2.8 +- 0.3) * 1013 * T(1.3+-0.2) exp{-(6790 +- 700)/T} s-1 and kdiss,0/[He] = (9.6 +- 0.9) * 1042T(-10.0+-1.0) * exp{-(6790 +- 700)/T} s-1, resp.The kinetics of the N2O3 + M = NO + NO2 + M reaction was studied over a wide range of pressure by a laser-induced temp. jump relaxation method. Equil. mixts. at 225 K contg. nitrogen dioxide, dinitrogen trioxide, dinitrogen tetroxide, nitric oxide, a small quantity of SiF4 as an IR absorber, and an excess of Ar as a 3rd-body gas collider are rapidly heated by a short pulse of CO2 laser radiation. The induced temp. jump, at 0.5-3 K, displaces the equil. toward NO2 and NO formation. The rate of recombination reaction to the new equil. concns. at higher temp. is monitored by observing spectroscopically the time dependence of the N2O3 concn. in the UV. For small perturbations, simple mathematics relates the measured chem. relaxation time const. to the thermal rate consts. of the system. Exptl. results (obtained in the 0.5-200 bar pressure range of Ar corresponding to the 3rd-order and falloff region of the reaction) were fitted by falloff curves established accordingly to a method developed by Troe. High- and low-pressure limiting rate consts. for the NO + NO2 recombination at 227 K are extrapolated from the exptl. data. These values lie within a factor of 2 of those that may be predicted using a simplified statistic adiabatic channel (SAC) model. The temp. dependence of the recombination rate const., measured exptl. at 227-260 K is reported as well as the calcd. kinetics of dissocn. for N2O3.

Evidence of adiabatic channels in the singlet photoisomerization of cis-1,2-diarylethenes: a fluorimetric study

The deactivation of the first excited singlet state (S 1) of some cis isomers of 1,2-diarylethenes was studied by fluorimetric and photochemical measurements. When at least one of the aryl groups is a large polycyclic group of low S 1 energy (as in the case of pyrenyl and anthryl derivatives) the minimum at 90° (typical of stilbene) in the potential energy curve as a function of the twisting coordinate may disappear. This leads to an unusual fluorescence emission at room temperature and favours the adiabatic 1cis * → 1trans * isomerization mechanism instead of the well-known diabatic pathway, which implies internal conversion to the ground state from the 1perp * configuration.

A SACM study of the cross-combination ratio of rate constants

An analysis of the cross-combination ratio of the rate constants ϕ in terms of the statistical adiabatic channel model allows to factorize ϕ into two contributions: one due to the motion along the reaction coordinates and another due to the reaction transitional modes. ϕ for the CH3/CCl3, CH3/C2H3, CH3/C3H5, CH3/C2H5 and C2H5/C3H5 radical pairs were calculated.

Partial cross sections in the predissociation of quasibound states of NeHF. An adiabatic channel interpretation

We have computed partial cross sections for the internal rotational predissociation of certain quasibound states of the NeHF van der Waals complex, using the time-dependent close-coupled wave packet method. These cross sections show strong selectivity in the orbital angular momentum of the outgoing fragments. We provide an explanation of these propensities based on an adiabatic channel interpretation, and discuss the observable physical consequences of the partial cross sections we calculate.

Absolute rate constants of R+NO(+M)→RNO(+M) reactions. II. Measurements for R=C 2F 5, i-C 3F 7, n-C 4F 9, and t-C 4F 9 at T = 295 K

Absolute rate constants of the reactions C 2F 5+NO(+M)→C 2F 5NO(+M) (4), i-C 3F 7+NO(+M)→ i-C 3F 7NO(+M) (5), n-C 4F 9+NO(+M)→ n=C 4F 9NO(+M) (6) and t-C 4F 9+NO)(+M)→ t-C 4F 9NO(+M) (7) for M=He over the pressure range 10–500 Torr have been measured at T=295 K by means of time-resolved intractivity absorption of He-Ne laser radiation combined with excimer laser flash photolysis. The fall-off curves k([M]) have been constructed using a procedure based on Troe's formalism. The limiting high-pressure rate constants obtained are k 4,∞ = (1.8 ± 0.2) × 10 −11 cm 3/s, k 5,∞ = (1.7±0.2) × 10 −11 cm 3/s, k 6,∞ = (1.6±0.2) × 10 −11 cm 3/s and k 7,∞ = (1.25±0.20) × 10 −11 cm 3/s. A statistical adiabatic channel model (SACM) calculation of k ∞ for the R+NO→RNO reaction has been performed for several radicals R. Experimental k ∞ values are described by SACM with 0.50<α/β<0.62.

Statistical modeling of ion–molecule electrostatic capture

A statistical model is presented which enables rapid and accurate calculation of capture rate coefficients for the interaction of ions with neutrals possessing any combination of dipole, quadrupole, and isotropic or anisotropic induced-dipole moments. Rate coefficients at very low temperatures are calculated via the state-resolved statistical adiabatic channel model (SACM). At higher temperatures, a classical state-counting technique is utilized which may be derived either from microcanonical variational transition state theory or from the high-temperature limit of the quantized SACM approach. The predicted rate coefficients are generally accurate to within 10% in comparison with classical trajectory calculations.

Temperature-jump measurements on the kinetics of association and dissociation in weakly bound systems: N2O4+M=NO2+NO2+M

Kinetic measurements have been made on the gas-phase reaction N2O4+M=2NO2+M by applying the temperature-jump relaxation method. Equilibrium mixtures containing nitrogen dioxide, dinitrogen tetroxide, a small quantity of an IR absorber, and an excess of a third-body bath gas, have been subjected to a short CO2 laser pulse. The induced temperature jump, of the order of 1 K, displaces the equilibrium towards NO2 formation. After the fast heating of the mixture, the time dependence of the NO2 and N2O4 concentrations has been monitored at 420 and 250 nm, respectively. For small perturbations, the relaxation to the new equilibrium concentrations at the higher temperature follows first-order kinetics, and the thermal rate constant can be deduced from the measured relaxation rate constant. Measurements have been made at 255 K in the pressure range between 0.3 and 200 bars He, which corresponds to the falloff range for this reaction. The high- and low-pressure limiting rate constants, krec,∞=(7.0±0.7)×1011 cm3 mol−1 s−1 and krec,0/[He]=(2.1±0.2)×1014 cm3 mol−1 s−1, respectively, extrapolated from the experimental data at 255 K, are in agreement with those calculated with a simplified statistic adiabatic channel model. The temperature dependence of the high- and low-pressure recombination rate constant, determined between 255 and 273 K, is given by the relations krec,∞=(2.2±0.2)×106×T(2.3±0.2) cm3 mol−1 s−1 and krec,0/[He]=(7.5±0.8)×1035×T(−9.0±0.9) cm3 mol−1 s−1. The corresponding high- and low-pressure dissociation rate constants are kdiss,∞= (2.8±0.3)×1013×T(1.3±0.2) exp{−(6790±700)/T} s−1 and kdiss,0/[He]=(9.6±0.9)×1042T(−10.0±1.0) ×exp{−(6790±700)/T} s−1, respectively.

Nonadiabatic effects in the statistical adiabatic channel model: The atom+diatom case

Nonadiabatic coupling between exit adiabatic channel states is considered for an atom+diatom collision complex. Nonadiabatic transition probabilities are estimated on the basis of a sequential two-state correlation diagram which qualitatively reproduces adiabatic potential barriers arising from an anisotropic interaction in a system ‘‘point charge–linear dipole rotor.’’ The implication of nonadiabatic interaction in changing statistical adiabatic channel predictions for product distribution functions are briefly discussed.

Integral cross sections for the reaction N+2+X2→N2X++X(X=H,D): A low energy crossed-beam experiment

Using a high-resolution crossed-beam apparatus, we have measured the integral cross section for the title reactions in the collision energy range from 0.025 to about 2 eV. Structure in the energy dependence of the cross section has been observed for both reactions. However, features for H2 and D2 appear at different collision energies. This has been attributed to the successive opening, as collision energy increases, of new adiabatic reactive channels via different vibronic states of the intermediate charge transfer complex N2+X2+(v) (X=H,D). Calculations of model potential energy surfaces show that the reactants’ surface does not cross the vibronic ground state. Therefore reactions proceed only via excited vibronic states resulting, in agreement with data from previous experiments, in vibrationally excited products.

Quasigeostrophic Vertical Motions Diagnosed from Along- and Cross-isentrope Components of the Q Vector

Abstract In a recent paper on the kinematics of frontogenesis, Keyser et al. conjectured that partitioning the Q vector into along- and cross-isentrope components yields vertical-motion patterns that are respectively cellular and banded—the former on the scale of the baroclinic disturbance and the latter on the scale of the embedded frontal zones. This conjecture is examined diagnostically through solution of the quasigeostrophic omega equation, using the output from a nearly adiabatic and frictionless f-plane primitive equation channel model of the evolution of a baroclinic disturbance to finite amplitude. The results of the present study support the proposed conjecture, suggesting the following interpretation of the characteristic comma structure of the vertical-motion field in midlatitude baroclinic disturbances: The comma shape arises from the modification or distortion of a wave-scale dipole pattern by frontal-scale asymmetries. The dipole is associated with the along-isentrope component of the Q vec...

Rate coefficient for the reaction H+O2→OH+O: Results at high temperatures, 2000 to 5300 K

The tunable-laser flash-absorption technique has been used to study the high-temperature behavior of the reaction H+O2→OH+O by monitoring the absorption of the hydroxyl radical. Sensitivity analysis of a detailed reaction mechanism shows that for fuel rich mixtures only two reactions are sensitive when hydroxyl is monitored: H2+M→2H+M and H+O2→OH+O. Rate coefficients for these reactions have been determined by least-squares analysis of measured absorption profiles. For the rate of dissociation of H2 in krypton we obtain k1(T)=(8.86±0.88)×10−10 exp[−48321/T(K)] cm3 s−1 from 3450 to 5300 K. For the H+O2 reaction we combined our results with previous low temperature measurements and recommend k2(T)=(1.62±0.12)×10−10 exp[−(7474±122)/T(K)] cm3 s−1 from 960 to 5300 K. The uncertainties are at the 95% confidence level. Measured cross sections for rotational and vibrational energy transfer in O2 and OH have been used to show that relaxation effects do not influence the results. We compare our results to recent trajectory calculations. In addition, we calculate the rate of the reverse reaction, OH+O→H+O2, and compare it to trajectory and statistical adiabatic channel calculations. Finally, we point out that the first excited surface of the hydroperoxyl radical, 2A′, which correlates with H(2S)+O2(1Δg), may be needed to explain very high temperature results.

Angular momentum and rotational energy relaxation in N 2-N 2 collisions calculated from coherent and stimulated Raman spectroscopy data

Rotational relaxation fitting and scaling laws, previously determined from coherent Raman spectroscopy of nitrogen, allow us to study the energy τ E and angular momentum τ J relaxation times. Consistent results with available experimental data are obtained and the high-temperature behaviour is discussed. To get a more detailed analysis of the collisional relaxation processes, new exact analytical expressions of the basic strong collision (SC) model for the Raman Q-branch are established for τ E and τ J . For the first time, quantum and quasiclassical descriptions of collisional narrowing of the Q-branch are thoroughly compared for the whole region of density variation manifest in the spectral transformation from the fully resolved isotropic Raman line up to the completely collapsed one. On this basis it is stated with confidence that: (1) scaling laws of quantum origin with adiabatic corrections are capable to excellently describe the resolved spectral structure and fail for the smoothed line under high densities; (2) contrary, the nonadiabatic quasiclassical model is in evident disagreement with state-to-state rates but suggests a qualitatively good prediction for the Q-branch isothermic narrowing. In our opinion these drastic features may indicate a change of the collisional regime and, in particular, an effective suppression of adiabatic channels of collisional relaxation in dense media.

Demonstration of moleculelike normal modes of Coulombic three-body systems in hyperspherical coordinates.

The adiabatic channel wave functions of Coulombic three-body systems are investigated in mass-weighted hyperspherical coordinates. We consider the ABA Coulombic systems, where two of the particles are identical, and examine the density distribution functions at fixed hyperradii R for different systems as the mass ratio \ensuremath{\lambda}=${\mathit{m}}_{\mathit{A}}$/${\mathit{m}}_{\mathit{B}}$ varies from the atomic limit (\ensuremath{\lambda}\ensuremath{\rightarrow}0, as in ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$) to the molecular limit (\ensuremath{\lambda}\ensuremath{\rightarrow}\ensuremath{\infty}, as in ${\mathrm{H}}_{2}^{+}$). The bonding and antibonding as well as the rovibrational characters of the three-body systems are illustrated by these density plots.

Adiabatic channel potential curves for two linear dipole rotors. I. Classification of states and numerical calculations for identical rotors

Adiabatic channel potential curves for a system of two linear dipole rotors are discussed. A general classification of states is given and a numerical procedure for calculating eigenvalues as a function of interrotor distance is formulated, both in a limited and extended basis set. A system of identical (but distinguishable) rotors is treated explicitly. Unexpectedly, the adiabatic potential curves show narrow avoided crossings which suggests the possibility of constructing diabatic channel potential curves. The validity of the adiabatic assumption for the relative motion of the dipoles is discussed.

Effect of orbital-rotational coupling on the rate of ion-dipole capture at extremely low temperatures

The rate of the ion-dipole capture at very low temperatures or energies has been examined using the Statistical Adiabatic Channel model.

Influence of the essential features of the potential energy surface on the temperature dependence of the H+CH3→CH4 reaction

Microcanonical statistical adiabatic channel model calculations show that the temperature dependence of the limiting high pressure rate coefficients of the title reaction becomes less positive (or more negative) when the attractiveness of the radial or the attenuation of anisotropic parts of the potential energy surface increases.

Infrared diode laser study of the 248 nm photodissociation of CH3I

The vibrational distribution of CH3 radicals produced by the 248 nm photodissociation of CH3I has been investigated by infrared diode laser kinetic spectroscopy. The ν2 bands of CH3, v2=1←0 up to 4←3, were observed as functions of time for the sample pressure ranging from 9 to 100 mTorr. The observed Doppler line shape showed a clear correlation between the transition dipole moment and the recoil velocity (μ−v) in the photodissociation, and the observed linewidth indicated that CH3 fragments were predominantly generated through the I(2P1/2) adiabatic dissociation channel. The relative population in each vibrational level was determined to be 0.66:0.26:0.08:0.004 for v=0, 1, 2, and 3, in sharp contrast to the inverted vibrational distribution reported previously. The present result suggests that the geometry of the CH3 moiety in the precursor CH3I is relaxed gradually to its equilibrium planar structure before the cleavage of C–I bond takes place.

Long-range, nonadiabatic effects in statistical adiabatic channel models: Dynamic orientation of diatomic fragments formed in the decomposition of long-lived triatomic complexes

Using the example of the decomposition of a long-lived triatomic complex into an atom and a diatom, it is shown that nonadiabatic interactions between adiabatic channel states at large interfragment distances strongly reduce the dynamic rotational orientation of the diatom. The degree of orientation is expressed in terms of a number of open adiabatic channels and ‘‘slipping factors.’’ These factors are estimated for an anisotropic potential following an inverse power law.