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In this paper, the electronically nonadiabatic Landau-Zener (LZ) mechanism for the vibrational relaxation v = 1 → v = 0 of NO(X2Π) in collisions with Ar(S01) is discussed. It corresponds to nonadiabatic transitions between two crossing vibronic potential energy surfaces (PESs) originating from vibrational states of the collision complex and supported by two coupled electronic PESs. The LZ rate coefficients k10LZ are calculated within the uniform Airy approach in the reaction coordinate approximation with parameters derived from ab initio PESs and an asymptotic estimation of the Franck-Condon factor in the nonadiabatic coupling region. The rate coefficients are close to the experimental rate coefficients available over the range of 900-2500 K, where the electronically adiabatic Landau-Teller (LT) mechanism with the rate coefficients k10LT does not make a noticeable contribution to the total relaxation rate. The ratio k10LZ/k10LT increases with temperature and the LZ and LT mechanisms have comparable rates at about 4000 K.

Intramolecular vibrational energy flow in excited bridged azulene-anthracene compounds is investigated by time-resolved pump-probe laser spectroscopy. The bridges consist of molecular chains and are of the type (CH(2))(m) with m up to 6 as well as (CH(2)OCH(2))(n) (n=1,2) and CH(2)SCH(2). After light absorption into the azulene S(1) band and subsequent fast internal conversion, excited molecules are formed where the vibrational energy is localized at the azulene side. The vibrational energy transfer through the molecular bridge to the anthracene side and, finally, to the surrounding medium is followed by probing the red edge of the azulene S(3) absorption band at 300 nm and/or the anthracene S(1) absorption band at 400 nm. In order to separate the time scales for intramolecular and intermolecular energy transfer, most of the experiments were performed in supercritical xenon where vibrational energy transfer to the bath is comparably slow. The intramolecular equilibration proceeds in two steps. About 15%-20% of the excitation energy leaves the azulene side within a short period of 300 fs. This component accompanies the intramolecular vibrational energy redistribution (IVR) within the azulene chromophore and it is caused by dephasing of normal modes contributing to the initial local excitation of the azulene side and extending over large parts of the molecule. Later, IVR in the whole molecule takes place transferring vibrational energy from the azulene through the bridge to the anthracene side and thereby leading to microcanonical equilibrium. The corresponding time constants tau(IVR) for short bridges increase with the chain length. For longer bridges consisting of more than three elements, however, tau(IVR) is constant at around 4-5 ps. Comparison with molecular dynamics simulations suggests that the coupling of these chains to the two chromophores limits the rate of intramolecular vibrational energy transfer. Inside the bridges the energy transport is essentially ballistic and, therefore, tau(IVR) is independent on the length.

The capture rate coefficients of homonuclear diatomic molecules (H(2) and N(2)) in the rotational state j=1 interacting with ions (Ar+ and He+) are calculated for low collision energies assuming a long-range anisotropic ion-induced dipole and ion-quadrupole interaction. A comparison of accurate quantum rates with quantum and state-specific classical adiabatic channel approximations shows that the former becomes inappropriate in the case when the cross section is dominated by few partial contributions, while the latter performs better. This unexpected result is related to the fact that the classical adiabatic channel approximation artificially simulates the quantum effects of tunneling and overbarrier reflection as well as the Coriolis coupling and it suppresses too high values of the centrifugal barriers predicted by a quantum adiabatic channel approach. For H2(j=1)+Ar+ and N(2)(j=1)+He+ capture, the rate constants at T--0 K are about 3 and 6 times higher than the corresponding values for H2(j=0)+Ar+ and N(2)(j=0)+He+ capture.

We present an efficient method for the direct solution in the time domain of the equations of a novel recently proposed non-Markovian quantum-classical approximation, valid well beyond the applicability limits of both Redfield theory and Fermi’s Golden Rule formula. The method is based on an ab initio molecular dynamics description of the classical bath and is suitable for applications to systems with a fairly large number of quantum levels. A simple model of the breathing sphere in a Lennard-Jones fluid was used to compare the results of the quantum-classical and purely classical treatments of vibrational energy relaxation.

Transitional modes in simple unimolecular bond fission and in the reverse recombination reactions are characterized quantitatively by statistical adiabatic channel (SACM) and classical trajectory (CT) calculations. Energy E- and angular momentum J-specific numbers of open channels (or activated complex states) W(E,J) and capture probabilities w(E,J) are determined for a series of potentials such as ion—dipole, dipole–dipole, and various model valence potentials. SACM and CT treatments are shown to coincide under classical conditions. Adiabatic as well as nonadiabatic dynamics are considered. The dominant importance of angular momentum couplings is elaborated. A sequence of successive approximations, from phase space theory neglecting centrifugal barriers E0(J), via phase space theory accounting for centrifugal barriers E0(J), toward the final result, expressing the effects of the anisotropy of the potentials by specific rigidity factors frigid(E,J), is described. This approach emphasizes the importance to...

The association reaction NH4++NH3(+M)→N2H7+(+M), leading to the proton-bound dimer of ammonia, was studied by the CRESU flow technique over the temperature range 15–170 K, in the bath gases M=He, Ar, and N2, and over the range of bath gas concentrations (0.5–15)×1016 molecule cm−3. The rate coefficients are shown to depend on the temperature, the pressure, and the nature of the bath gas. Theoretical modelling of the reaction involves a combination of ion–molecule capture and unimolecular reaction rate theory. It is shown that the present experiments all correspond to the intermediate falloff regime of the reaction ranging up to the high pressure bimolecular capture limit, whereas earlier experiments between 200 and 350 K were made close to the low pressure termolecular limit. Temperature- and pressure-dependent rate coefficients over wide ranges of conditions are calculated and compared with experimental results from this and earlier work. For the bath gas N2, under most of the applied conditions, the rea...

The formation of collision complexes, as a first step towards reaction, in collisions between two open-electronic shell radicals is treated within an adiabatic channel approach. Adiabatic channel potentials are constructed on the basis of asymptotic electrostatic, induction, dispersion, and exchange interactions, accounting for spin-orbit coupling within the multitude of electronic states arising from the separated reactants. Suitable coupling schemes (such as rotational + electronic) are designed to secure maximum adiabaticity of the channels. The reaction between C((3)P) and OH((2)Π) is treated as a representative example. The results show that the low temperature association rate coefficients in general cannot be represented by results obtained with a single (generally the lowest) potential energy surface of the adduct, asymptotically reaching the lowest fine-structure states of the reactants, and a factor accounting for the thermal population of the latter states. Instead, the influence of non-Born-Oppenheimer couplings within the multitude of electronic states arising during the encounter markedly increases the capture rates. This effect extends up to temperatures of several hundred K.

The effect of rotational excitation on the specific rate constants k(E,J) of the unimolecular decomposion of NO2 has been investigated. Time-resolved pump- and probe experiments with sub-ps time resolution are reported in which NO2 molecules with well-defined rotational and vibrational energy distributions were optically excited near and above the dissociation threshold. The subsequent unimolecular decay of the reacting NO2 molecules was probed by time-resolved laser-induced fluorescence of the disappearing NO2 via excitation to Rydberg states. At constant photolysis wavelength, increasing rotational energy (up to 310 cm−1) was found to leave the overall decay rate nearly unaffected. This observation can be rationalized by assuming a compensation of the angular momentum and energy dependences of the specific rate constants when J and E are changed at the same time. Keeping the total energy E nearly constant and changing J independently, the effect of rotation on the decay rate can be separated and observe...

A comparably simple new analytical expression of the potential energy surface for the HO+O⇔HO2⇔H+O2 reaction system is designed on the basis of previous high precision ab initio calculations along the minimum energy path of the HO2→H+O2 and HO2→HO+O dissociations. Thermal rate constants for the reaction HO+O→H+O2 are determined by extensive classical trajectory calculations. The results depend on the policy to solve the zeropoint energy problem. We show that, with the chosen policy, there are nearly equal amounts of statistical and nonstatistical backdissociations HO+O←HO2 following HO+O→HO2; however, backdissociations become important only at temperatures above about 500 K. Below 500 K, the reaction is completely capture-controlled. Below 300 K, classical trajectory treatments become inadequate, because quantum effects then are so important that only the quantum statistical adiabatic channel model gives reliable results. For the reaction HO+O→H+O2 and the range 300–5000 K, a rate constant of k/10−11 cm3 molecule−1 s−1=0.026(T/1000 K)1.47+1.92(1000 K/T)0.46 is obtained from the trajectory calculations. Converting experimental results for the reaction H+O2→HO+O to the reverse reaction on the basis of the revised enthalpy of formation of OH, agreement between experiment and theory within better than 20% is obtained between 300 and 5000 K.

The potential energy surface of the HO+O⇔HO2⇔H+O2 reaction system is characterized by ab initio calculations. The complex-forming bimolecular reaction is then treated by statistical rate theory, using statistical adiabatic channel and classical trajectory calculations for the HO+O⇔HO2 and HO2⇔H+O2 association/dissociation processes. Specific rate constants k(E,J) of both reactions as well as thermal rate constants are calculated over wide ranges of conditions. Open shell quantum effects are important up to room temperature. The good agreement with experimental results suggests that the ab initio potential is of sufficient accuracy. There is no evidence for non-statistical effects or for a significant contribution from electronically excited states. The comparison with rate data for the H+O2→HO+O reaction, because of the remaining uncertainty in the heat of formation of HO, is somewhat inconclusive. Apart from this problem, the calculated rate constants appear reliable between 0 and 5000 K.

The recombination reactions HO+NO+M⇒HONO+M(1) and HO+NO2+M⇒HNO3+M(2) have been investigated over an extended pressure (1–1000 bar) and temperature (250–400 K) range. HO radicals were generated by laser flash photolysis of suitable precursors and their decays were monitored by saturated laser-induced fluorescence (SLIF) under pseudo-first-order conditions. The measured rate constants were analyzed by constructing falloff curves which provide the high pressure limiting rate constants k∞. In the given temperature range, these rate constants are k1,∞=(3.3±0.5)×10−11×(T/300 K)−(0.3±0.3) and k2,∞=(7.5±2.2)×10−11 cm3 molecule−1 s−1.

The combination of two linear rotors forming linear or nonlinear adducts is treated using standardized valence potentials. Classical trajectory (CT) and statistical adiabatic channel (SACM) calculations are used for the calculation of thermal capture rate constants. At very low temperatures, only SACM applies. At intermediate temperatures SACM and CT approach each other; however, Landau–Zener-type multiple crossings of adiabatic channel potentials introduce local nonadiabaticity which has to be accounted for. The high-temperature transition from globally adiabatic to nonadiabatic (sudden) dynamics is studied by CT. Thermal rigidity factors, accounting for the influence of the anisotropy of the potential on the capture rate constant, are expressed in simple analytical form which facilitates practical applications. The present work complements similar studies on the addition of atoms to linear molecules in standardized valence potentials (part IV of this series).

The collisional deactivation of vibrationally highly excited azulene was studied from gas into compressed liquid phase by pump-and-probe picosecond laser spectroscopy. Collisional deactivation rates were compared with solvatochromic shifts Δν of the azulene S3←S0 absorption band under identical conditions. Employing supercritical fluids at pressures between 0.03 and 4000 bars and temperatures between 298 and 640 K, measurements covering the complete gas–liquid transition were performed. For the energy transfer experiments, azulene with an energy of ∼20000 cm−1 was generated by laser excitation into the S1- and internal conversion to the S0*-ground state. The subsequent loss of vibrational energy was monitored by following the transient absorption at the red wing of the S3←S0 absorption band near 290 nm. Transient signals were converted into energy-time profiles using hot band absorption coefficients from shock wave experiments for calibration and accounting for solvent shifts of the spectra. Under all con...

Thermal rate constants for capture of permanent and induced dipoles by ions are calculated by the statistical adiabatic channel model (SACM) using analytical representations of adiabatic channel eigenvalues. Linear, symmetric, and asymmetric top dipoles with closed and open electronic shells are considered. Analytical representations of the capture rate constants over broad temperature ranges are presented for all systems. Numbers of open channels W(E,J) are also investigated. The present analytical SACM treatment provides an economical, transparent, and accurate approach to the considered type of capture processes. At the same time, the implementation to ion fragmentation processes is elucidated.

The collisional deactivation of vibrationally highly excited azulene was studied from the gas to the compressed liquid phase. Employing supercritical fluids like He, Xe, CO2, and ethane at pressures of 6–4000 bar and temperatures ≥380 K, measurements over the complete gas–liquid transition were performed. Azulene with an energy of 18 000 cm−1 was generated by laser excitation into the S1 and internal conversion to the S0*‐ground state. The subsequent loss of vibrational energy was monitored by transient absorption at the red edge of the S3←S0 absorption band near 290 nm. Transient signals were converted into energy‐time profiles using hot band absorption coefficients from shock wave experiments for calibration and accounting for solvent shifts of the spectra. Under all conditions, the decays were monoexponential. At densities below 1 mol/l, collisional deactivation rates increased linearly with fluid density. Average energies 〈ΔE〉 transferred per collision agreed with data from dilute gas phase experiments. For Xe, CO2, and C2H6, the linear relation between cooling rate and diffusion coefficient scaled collision frequencies ZD turned over to a much weaker dependence at ZD≳0.3 ps−1. Up to collision frequencies of ZD=15 ps−1 this behavior can well be rationalized by a model employing an effective collision frequency related to the finite lifetime of collision complexes.

The addition of HO to HO was studied by saturated laser induced fluorescence at temperatures between 200 and 700 K and at pressures of the bath gas helium up to 100 bar. In combination with earlier measurements at 298 K, a set of falloff curves is constructed for the given temperature range. The limiting high‐pressure rate constant for the reaction HO+HO(+He)→H2O2(+He) follows as k1,∞=(2.6±0.8)×10−11 (T/300 K)0±0.5 cm3 molecule−1 s−1, practically independent of the temperature between 200 and 400 K. At higher temperatures, k1,∞ decreases. These results serve as a reference for statistical adiabatic channel model calculations of the recombination rate.

Thermal rate constants of the complex‐forming bimolecular reaction HO+CO■HOCO→H+CO2 were measured between 90 and 830 K in the bath gas He over the pressure range 1–700 bar. In addition, the vibrational relaxation of HO in collisions with CO was studied between 300 and 800 K. HO was generated by laser photolysis and monitored by saturated laser‐induced fluorescence. The derived second‐order rate coefficients showed a pronounced pressure and complicated non‐Arrhenius temperature dependence. Above 650 K, the disappearance of HO followed a biexponential time law, indicating thermal instability of collisionally stabilized HOCO. By analyzing the corresponding results, an enthalpy of formation of HOCO of ΔHof,0=−(205±10) kJ mol−1 was derived. On the basis of energy‐ and angular‐momentum‐dependent rates of HOCO formation, activated complex properties for the addition reaction HO+CO→HOCO were derived from the limiting high‐pressure rate constants; with the limiting low‐pressure rate constants, activated complex pr...

Rate coefficients for capture of H-2(j = 0,1) by H-2(+) are calculated in perturbed rotor approximation, i.e., at collision energies considerably lower than Bhc (where B denotes the rotational constant of H2). The results are compared with the results from an axially nonadiabatic channel (ANC) approach, the latter providing a very good approximation from the low-temperature Bethe-Wigner to the high temperature Langevin limit. The classical ANC approximation performs satisfactorily at temperatures above 0.1 K. At 0.1 K, the rate coefficient for j = 1 is about 25% higher than that for j = 0 while the latter is close to the Langevin rate coefficient. The Bethe-Wigner limit of the rate coefficient for j = 1 is about twice that for j = 0. The analysis of the relocking of the intrinsic angular momentum of H-2 during the course of the collision illustrates the significance of relocking in capture dynamics in general.

The reaction F+O2+M→FOO+M(1) in the bath gases M=He, Ar, and N2 was studied by time‐resolved uv absorption spectroscopy of the FOO radical at 220 nm. The experiments were performed at total pressures between 1 and 1000 bar and temperatures between 100 and 420 K. The absorption cross section of FOO was determined as σ (300 K)=1.6⋅10−17 cm21 at 220 nm. The F–O2 bond energy was derived by third law analysis of the equilibrium constant K1=9.15⋅10−24⋅T−0.45⋅exp(5990 K/T) cm3 (between 315 and 420 K) for reaction (1), being (49.8±1) kJ mol−1 (at 0 K). Limiting low pressure rate constants for the recombination reaction (1) of k1,0/[He]=3.7⋅10−33⋅(T/300 K)−1.1 cm6 s−1, k1,0/[Ar]=4.4⋅10−33⋅(T/300 K)−1.4 cm6 s−1, and k1,0/[N2]=5.8⋅10−33⋅(T/300 K)−1.7 cm6 s−1 were obtained between 100 and 373 K. The transition to a high pressure plateau of the rate constants was observed in all cases although the transition to diffusion‐controlled reaction may have been superimposed at very high pressures. Correcting for this effect,...

It is shown that the adiabatic channel states of an atom–diatom system with a low‐rank interaction anisotropy (proportional to cos γ and cos2 γ) exhibit a nonlocalized nonadiabatic coupling which persists into the strong coupling region. This feature of adiabatic channel states restricts application of the statistical adiabatic channel model (SACM) for processes of complex decomposition and complex formation to low energies. The change of the representation from adiabatic into the postadiabatic (dynamic) one transforms the coupling to a localized form and makes it possible to find conditions for description of the half‐collision dynamics in terms of uncoupled dynamic states. This result can be regarded as the extension of the statistical adiabatic channel model beyond its formal limits of applicability provided the adiabatic channel potentials are replaced by the postadiabatic (dynamic) potentials. The obtained results are exemplified by calculation of the capture cross section in the approximation of unc...

The competition between C–Br–bond fission and three‐center elimination of molecular bromine (Br2) in highly excited CF2Br2 molecules has been studied under collision‐free conditions. Transient resonantly enhanced multiphoton ionization (REMPI) was used to monitor Br(2P1/2) and Br(2P3/2) formation during and after infrared (IR) multiphoton excitation of CF2Br2; time‐resolved laser‐induced fluorescence (LIF) spectroscopy was employed for the detection of transient CF2 after Br2 elimination. Direct time‐resolved measurements of the sum of afterpulse reaction rates, absolute product yields for the CF2 and Br(2P3/2) channels as well as absorbed energies per excitation pulse were used to characterize parts of the vibrational energy distribution P(E) established after IR multiphoton excitation and to determine rate coefficients and branching ratios for the elimination and dissociation reaction as a function of the average internal energy 〈E〉. The existence of both channels, the dissociation and the elimination c...

Analytical expressions are presented which interpolate between weak field and strong field expansions of rotational energy levels of polar symmetric top molecules in homogeneous electrical fields. Besides Stark spectroscopy, the derived formulas can be used as well for calculating adiabatic channel potential curves in capture processes of species whose interaction is characterized by an anisotropy of the cos θ type.

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.

Biexponential models for collisional energy transfer are formulated representing superimposed weak and strong collisions. Analytical solutions of the master equation for low pressure thermal unimolecular reactions at steady state are derived for such models. Nonequilibrium populations of excited states as well as collision efficiencies βc in the low pressure rate constant differ considerably from the corresponding quantities for simple exponential collision models. The results are analyzed in terms of general expressions of the collision efficiency βc as a function of the average energy transfer quantities 〈ΔE〉 and 〈ΔE2〉.

Time‐resolved resonantly enhanced multiphoton ionization spectroscopy of I(2P3/2) and I(2P1/2) dissociation products of CF3I can be used to characterize populations of highly‐excited CF3I*. This technique has been applied to monitor infrared multiphoton excitation dynamics of CF3I in the presence of collisions. Compared to collision‐free situations, collisions tend to destroy bimodal vibrational energy distributions and to enhance the overall pumping rate. Collisional energy transfer could be monitored for molecules above the dissociation threshold. Collisional relaxation of highly‐excited molecules has been followed and analyzed with respect to average energies transferred per collision.

Transient detection of I(2P3/2) by resonantly enhanced multiphoton ionization (REMPI) was used to monitor in‐pulse and after‐pulse dissociation of CF3I excited by infrared (IR) multiphoton absorption. After‐pulse reaction is characterized by time‐dependent dissociation rate coefficients. The apparent reaction rates increase with increasing laser intensity. These observations are attributed to strong rotational dependencies of the specific dissociation rate constants k(E,J) and to CO2 laser‐intensity‐dependent rotational distributions of the excited CF3I. The corresponding rotational distributions are reconstructed from the observed experimental time profiles of the reaction rates. In addition to the detection of the I(2P3/2) dissociation product from infrared multiphoton excitation, populations of vibrationally highly excited CF3I* were identified via electronic excitation in the visible, subsequent fast dissociation and REMPI detection of the resulting I(2P1/2). At weak IR laser intensities these vibrati...

The reaction Cl+O2+M■ClOO+M was studied by laser flash photolysis in the bath gases M=He, Ar, N2, and O2 over the temperature range 160–300 K and the pressure range 1–1000 bar. UV absorptions of ClOO were monitored, a maximum absorption cross section of σ(248 nm)=3.4×10−17 cm2 was determined. An expression for the equilibrium constantK p =5.3×10− 6 exp(+23.4 kJ mol− 1/R T) bar− 1 was derived between 180 and 300 K, which, by a third law analysis, yields ΔH ○ 0 =−20.2±0,2 kJ mol−1. Limiting low pressure rate coefficients for Cl+O2 recombination of k 0=[He]8.8×10−34(T/300 K)−3.0, k 0=[O2]1.6×10−33(T/300 K)−2.9, k 0=[N2]1.4×10−33(T/300 K)−3.9 cm6 s−1 were obtained over the range 160–260 K, as well as k 0 (160 K)=[Ar]2.2×10−32 cm6 s−1. Rate constants for the reactions Cl+ClOO→Cl2+O2 or 2ClO, ClOO+ClOO→products, and ClOO+Cl2→Cl2O+ClO were also derived. The recombination Cl+O2(+M)→ClOO(+M) at pressures above 10 bar shows a transition to a high pressure plateau and, at pressures above 200 bar, to diffusion control. It is suggested that, like O+O2(+M)→O3(+M), the reaction is governed by a radical complex mechanism.

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.

The ion-ion mutual neutralization reactions Ar{sup +}+SF{sub n}{sup -}{yields}Ar+SF{sub n} (n=6, 5, and 4) have been studied in a flowing afterglow-Langmuir probe (FALP) apparatus at 300 K and 1 Torr of He buffer gas. Electron concentrations and product ion fractions were measured, and neutralization rate constants of 4.0x10{sup -8}, 3.8x10{sup -8}, and 4x10{sup -8} cm{sup 3} s{sup -1} for SF{sub 6}{sup -}, SF{sub 5}{sup -}, and SF{sub 4}{sup -}, respectively, were derived, with uncertainties of {+-}25% ({+-}35% for SF{sub 4}{sup -}). During the neutralization process, excited neutrals are generated that are able to dissociate to neutral fragments. In the case of SF{sub 6}, the formation of SF{sub 5} and SF{sub 4}, and similarly in the case of SF{sub 5}, the formation of SF{sub 4} and SF{sub 3} were observed and quantified. The mechanism of primary and secondary reaction was analyzed in detail, and rate constants for the dissociative electron attachments e{sup -}+SF{sub 5}{yields}F{sup -}+SF{sub 4} (k=3x10{sup -9} cm{sup 3} s{sup -1},{+-}40%) and e{sup -}+SF{sub 3}{yields}F{sup -}+SF{sub 2} (k=2x10{sup -8} cm{sup 3} s{sup -1},+400%,-75%) were also derived. The experimental ion-ion neutralization rate constants were found to be in good agreement with estimates from an optimum two-state double-passage Landau-Zener model. It wasmore » also found that energy partitioning in the neutralization is related to the extent of electronic excitation of Ar generated by the electron transfer processes.« less

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.

The recombination O+O2+M→O3+M in the bath gases M=He, Ar, and N2 was studied over the temperature range 90–370 K and the pressure range 1–1000 bar. The temperature and pressure dependences of the reaction rates show an anomalous behavior which is attributed to superpositions of mechanisms involving energy transfer, complex formation and participation of weakly bound electronically excited O3 states. The results also show an analogy to oxygen isotope enhancements observed in ozone recombination and dissociation. Experiments in compressed liquid N2 were also made showing a transition to diffusion control.

The pressure and temperature dependence of the photoisomerization rate coefficient of trans‐stilbene in the S1 state have been measured in the solvents C2H6, C3H8, C4H10, Xe, Co2, SF6, and CHF3. At constant temperature, the pressure dependences up to 6 kbar can be well represented by the Kramers–Smoluchowski model. The comparison of results in different solvents clearly indicates the importance of reactant–solvent cluster formation modifying the height and imaginary frequency of the barrier. The change of the temperature dependence with pressure points towards a multidimensional barrier of nonseparable character. Multidimensional barrier effects manifest themselves most clearly via the temperature dependence of the rate coefficient in the Kramers–Smoluchowski limit.

Correlation diagrams for adiabatic channels between free atoms and linear molecules (both in nondegenerate electronic states) and the corresponding reaction complexes are constructed. The accurate calculation of adiabatic channel potential curves is discussed. The magnitude of Coriolis coupling terms is illustrated.

The photoisomerization of diphenylbutadiene was studied by picosecond absorption spectroscopy over wide pressure and temperature ranges in liquid and supercritical alkanes, CO2, SF6, and He. The reaction shows typical features of a thermal unimolecular reaction on the S1 potential energy surface. The rate can be expressed by a combination of standard unimolecular rate theory and Kramers–Smoluchowski theory. However, multidimensional behavior manifests itself in the transition to the gas phase low pressure range as well as to the high density Kramers–Smoluchowski range: in the former case, the low pressure limit of a unimolecular reaction of the polyatomic molecule is approached; in the latter case, the effective imaginary barrier frequency shows a marked apparent temperature dependence. The experiments also suggest contributions of reactant–solvent cluster interactions, which modify the barrier height even in nonpolar solvents.

The temperature dependence of the rate coefficients for vibrational relaxation of H2 in neat H2 is interpreted within the semiclassical effective mass approach. Across the temperature range of 80-3000 K, the experimental rate coefficients vary by five orders of magnitude and fall onto a strongly nonlinear Landau-Teller plot. This behavior is explained by the nonclassical nature of the energy release and by a substantial participation of rotation of the colliding partners in inducing the vibrational transition. A single fitting parameter, the optimal reduced mass, permits one to represent the temperature dependence of the rate coefficient within a factor of 2. This parameter is found to be close to that obtained from a simple model suggested by Sewell et al. [J. Chem. Phys. 99, 2567 (1993)].

Experimental results for the rate of the association reaction H3O+ + H2O (+M) --> H5O2(+) (+M) obtained with the Cinetique de Reactions en Ecoulement Supersonique Uniforme flow technique are reported. The reaction was studied in the bath gases M=He and N2, over the temperature range of 23-170 K, and at pressures between 0.16 and 3.1 mbar. At the highest temperatures, the reaction was found to be close to the limiting low-pressure termolecular range, whereas the limiting high-pressure bimolecular range was approached at the lowest temperatures. Whereas the low-pressure rate coefficients can satisfactorily be reproduced by standard unimolecular rate theory, the derived high-pressure rate coefficients in the bath gas He at the lowest temperatures are found to be markedly smaller than given by simple ion-dipole capture theory. This result differs from previous observations on the related reaction NH4(+) + NH3 (+M) --> N2H7(+) (+M). This observation is tentatively attributed to more pronounced contributions of the valence part of the potential-energy surface to the reaction in H5O2(+) than in N2H7(+). Falloff curves of the reaction H3O+ + H2O (+M) --> H5O2(+) (+M) are constructed over wide ranges of conditions and represented in compact analytical form.

The rate coefficients for capture of charged particles by dipolar polarizable symmetric top molecules in the quantum collision regime are calculated within an axially nonadiabatic channel approach. It uses the adiabatic approximation with respect to rotational transitions of the target within first-order charge-dipole interaction and takes into account the gyroscopic effect that decouples the intrinsic angular momentum from the collision axis. The results are valid for a wide range of collision energies (from single-wave capture to the classical limit) and dipole moments (from the Vogt-Wannier and fly-wheel to the adiabatic channel limit).

The kinetics of electron attachment to CF(3) as a function of temperature (300-600 K) and pressure (0.75-2.5 Torr) were studied by variable electron and neutral density attachment mass spectrometry exploiting dissociative electron attachment to CF(3)Br as a radical source. Attachment occurs through competing dissociative (CF(3) + e(-) → CF(2) + F(-)) and non-dissociative channels (CF(3) + e(-) → CF(3)(-)). The rate constant of the dissociative channel increases strongly with temperature, while that of the non-dissociative channel decreases. The rate constant of the non-dissociative channel increases strongly with pressure, while that of the dissociative channel shows little dependence. The total rate constant of electron attachment increases with temperature and with pressure. The system is analyzed by kinetic modeling in terms of statistical theory in order to understand its properties and to extrapolate to conditions beyond those accessible in the experiment.

Following our general approach to Lambda-doubling specificity in the capture of dipolar molecules by ions [M. Auzinsh et al., J. Chem. Phys. 128, 184304 (2008)], we calculate the rate coefficients for the title process in the temperature range 10(-4)

Limiting high pressure rate constants for the recombination reaction H+O(2)--HO(2) are modeled between 0 and 5000 K on an ab initio potential. Quantum capture theory is employed for the temperature range from 0 to about 1 K, while classical trajectory calculations are suitable for covering temperatures above about 200 K. The intermediate temperature range is analyzed by adiabatic channel capture theory. The system is characterized by transition-state switching from outer transition states in the long-range-C(6)R(6) potential to inner transition states in the range of a "shoulder" of the potential. The limiting high pressure rate constants from the trajectory calculations are sufficient for comparison with the experimental data which are available over the range from 300 to 900 K. Specific rate constants k(E,J) for HO(2) dissociation are also given and analyzed with respect to internal consistency with capture cross sections.

The low-energy capture of a dipolar diatomic molecule in an adiabatically isolated electronic state with a good quantum number Hund’s coupling case a by an ion occurs adiabatically with respect to rotational transitions of the diatom. However, the capture dynamics may be nonadiabatic with respect to transitions between the pair of the -doubling states belonging to the same value of the intrinsic angular momentum j. In this work, nonadiabatic transition probabilities are calculated which define the -doubling j-specific capture rate coefficients. It is shown that the transition from linear to quadratic Stark effect in the ion-dipole interaction, which damps the T �1/2 divergence of the capture rate coefficient calculated with vanishing -doubling splitting, occurs in the adiabatic regime with respect to transitions between -doubling adiabatic channel potentials. This allows one to suggest simple analytical expressions for the rate coefficients in the temperature range which covers the region between the sudden and the adiabatic limits with respect to the -doubling states. © 2008 American Institute of Physics. DOI: 10.1063/1.2913519

State-selected rate coefficients for the capture of ground and rotationally excited homonuclear molecules by ions are calculated, for low temperatures, within the adiabatic channel classical (ACCl) approximation, and, for zero temperature, via an approximate calculation of the Bethe limit. In the intermediate temperature range, the accurate quantal rate coefficients are calculated for j = 0 and j = 1 states of hydrogen isotopes (H2, HD, and D2) colliding with hydrogen-containing ions, and simple analytical expressions are suggested to approximate the rate coefficients. For the ground rotational state of diatoms, the accurate quantal rate coefficients are higher compared to their ACCl counterparts, while for the first excited rotational state the reverse is true. The physical significance of quantum effects for low-temperature capture and the applicability of the statistical description of capture are considered. Particular emphasis is given to the role of Coriolis interaction. The relevance of the present capture calculations for rates of ortho-para conversion of H2 in collisions with hydrogen-containing ions at low temperatures is discussed.

The energy dependence of the capture cross section and the temperature dependence of the capture rate constants for inverse power attractive potentials V∝−R−n is considered in the regime where the quantum character of the relative motion of colliding partners is important. For practically interesting cases n=4 and n=6, a simple formula for the cross section is suggested which interpolates between the classical and the quantum Bethe limits. We have shown that the classical approximation for the capture cross section performs well far below the simple estimations of the onset the quantum regime. This seemingly “classical” feature of the cross section and the rate constant is due to the large quantum effects of the waves in transmission through and reflection above the centrifugal potential barriers.

The thermal unimolecular isomerization reactions of cycloheptatriene, and of methyl‐ , ethyl‐ , and isopropyl‐cycloheptatriene, have been studied over the temperature range 900–1400 K in shock waves. Even for these large molecules, falloff behavior has to be taken into account mainly due to weak collision effects. The limiting high pressure rate constants k∞ and the specific rate constants k (E) are derived for use in later photoactivation experiments.

Vibrationally highly excited toluene molecules with 52 000 cm−1 of vibrational energy have been prepared by UV laser excitation of the isomer cycloheptatriene and subsequent isomerization. The collisional loss of energy from the excited toluene molecules has been observed directly by monitoring hot UV absorption spectra. Direct measurements of the average energies 〈ΔE〉 transferred per collision have been made for about 60 different bath gases. For complex bath gases, 〈ΔE〉 values appear to be considerably smaller than those derived from earlier indirect measurements. 〈ΔE〉 was found not (or only slightly) to depend on the excitation energy.

Modified neglect of differential overlap‐Configuration interaction (MNDO‐CI) calculations of the low‐lying electronically excited states and the electronic ground state of stilbene have been performed. The dependences of the potential energy on the angle of rotation around the central ethylene bond, on the length of this bond, and on the twist angle of the two phenyl groups are explored. There is no evidence for a nonadiabatic participation of the doubly excited ‘‘phantom state’’ in the photoisomerization dynamics. Instead, the calculated properties of the singly excited 1 1B state support a mechanism with adiabatic rotation around the central ethylene bond to the perpendicular conformation, followed by internal conversion. There appears to be a small ‘‘CI‐induced’’ energy barrier along this pathway which will be overcome by the combined motion in several coordinates. Therefore, a complicated activated complex structure arises.

Transient UV absorption spectra of CF3I were recorded during the IR multiphoton excitation of this molecule. Short laser pulses (20–50 ns) without longer tails were employed. Pressure effects were analyzed in the 10–100 mTorr range. Unimolecular dissociation rates (in the 106–107 s−1 range) after the pulse were identified unambiguously by time‐resolved low‐pressure observations and collisional quenching experiments. Marked laser intensity effects on the rates are attributed to intensity‐dependent changes of rotational state distributions of the reacting molecules. Excited state distributions have been resolved in direct observations. Bimodal distributions were identified.