Atom-molecule Reactions Research Articles

Product State-Resolved Reactive Scattering Studies of the H + Cl2 (v0 = 1-3, j0 = 0) → HCl + Cl Reaction by the Time-Dependent Wave Packet Method.

The typical hydrogen atom plus halogen molecule reaction H + Cl2 → HCl + Cl has implications across many fields. In this paper, product state-resolved quantum dynamics calculations for the vibrationally excited reaction H + Cl2 (v0 = 1-3, j0 = 0) → HCl + Cl were conducted using the time-dependent wave packet method on a newly developed accurate potential energy surface. Numerical results indicate that the initial vibrational excitation of Cl2 does enhance the reactivity for this early barrier reaction, although less than the enhancement of the translational energy. The calculated product vibrational state-resolved integral cross sections and rate constants reveal that the product vibrational state distribution and the initial vibrational state of Cl2 are highly correlated. The thermal rate constant in the temperature range from 100 to 1000 K was given and is found to be in reasonable agreement with the experimental measurements.

Cross-sections for O2 + N system using the QCT method

Collision Induced Exchange cross-sections for the Zeldovich reaction N2+O⇌NO+N for high energy atom-molecular collisions are calculated using the Quasi-Classical Trajectory method. The dynamical study of O2+N collision is conducted using the 2A′ and 4A′ NO2 PESs reported Sayos et al. Cross-sections for the reverse exchange reaction involving NO + O collisions are also calculated and reported. In addition to these exchange reactions, Collision Induced Dissociation cross-sections for dissociation of molecules are also published. Overall, a cross-section database for four important atom-molecule reactions of the air chemistry is provided as supplementary material with this paper.

Variation of potential energy surface height and bound state depth induced by laser phase along the reaction path in atom-molecule reactions: Application to Li + CH4 → LiH + CH3

Laser atom-molecule reaction interaction through polarizability and dipole moment contribution leads to potential energy surface barrier reshaping and bound states along the reaction path. The polarizability is maximum in the transition state. We will show here by using gauge representation (electric field gauge) for wave length λ = 20.6 μm, intensity I = 1 × 1012 W/cm2, I = 5 × 1012 W/cm2, I = 1 × 1013 W/cm2, I = 3 × 1013 W/cm2, that we can create laser induced potential energy surface barrier reshaping in the transition state region (–1–0.5 a. u.). We illustrate such effects for the LiH + CH3 ↔ Li + CH4 reaction with a barrier using ab-initio methods for calculating the reaction path, polarizability and dipole moment contribution of the atom-molecule reaction.

Adiabatic channel capture theory applied to cold atom–molecule reactions: Li + CaH LiH + Ca at 1K

We use quantum and classical adiabatic capture theories to study the chemical reaction Li + CaH LiH + Ca. Using a recently developed ab initio potential energy surface, which provides an accurate representation of long-range interactions in the entrance reaction channel, we calculate the adiabatic channel potentials by diagonalizing the Li-CaH Hamiltonian as a function of the atom-molecule separation. The resulting adiabatic channel potentials are used to calculate both the classical and quantum capture probabilities as a function of collision energy, as well as the temperature dependencies of the partial and total reaction rates. The calculated reaction rate agrees well with the measured value at 1 K (V Singh et al 2012 Phys. Rev. Lett. 108 203201), suggesting that the title reaction proceeds without an activation barrier. The calculated classical adiabatic capture rate agrees well with the quantum result in the multiple-partial-wave regime of relevance to the experiment. Significant differences are found only in the ultracold limit ( mK), demonstrating that adiabatic capture theories can predict the reaction rates with nearly quantitative accuracy in the multiple-partial-wave regime.

Laser Control of Ion/Atom–Molecule Interaction: Application to (X + CH4, X = Li+ and Li) Reactions

Abstract Laser atom/ion–molecule reaction interaction through dipole moments leads to potential energy surface crossings in ion molecule (Li+ + CH4) and barrier reshaping in atom–molecule reaction (Li + CH4). We will show here by using gauge representation (electric field gauge) that we can create laser-induced surface crossing in the region (4 au) where the dipole moment changes sign for the LiH + CH3+ Li+ + CH4 reaction and reshape the barrier height (increase/decrease the height) by changing the laser phase for Li + CH4 → LiH + CH3 where dipole moment changes sign in the transition state region.

Dynamical and stereodynamical studies of alkaline-earth atom–molecule reactions

Alkaline-earth atom molecule reactions have been extensively investigated since the early days of reaction dynamics. Though these reactions have many similarities to those of corresponding alkali atoms, as one would expect from their low ionization potential, there are important differences due to the presence of the two valence electrons in the alkaline-earth atom. This feature confers a distinct dynamical or stereodynamical behaviour whose basic description and analysis is the main objective of this article. The adopted approach is phenomenological based on the huge body of data revealed by modern crossed-beam and laser pump and probe techniques with no intention to be comprehensive but rather to put into context both authors’ work. The paper starts with a resumé of the experimental approaches followed by a visit to full-collision studies under gas cell, beam–gas and crossed-beam arrangements. Detailed analysis and discussion is given of the M + H2 (M = Ca, Mg) → MH + H reaction family whose dynamics is discussed using their Potential Energy Surfaces. In these studies care was taken to discuss the most relevant features of the molecular reaction dynamics with emphasis on the energy selectivity and on the stereodynamical (vectorial) nature of the reaction. To this end the available data are discussed in light of topological features of the underlying potential energy surface favouring either insertion or abstraction mechanisms. Particular attention is paid to the discussion of alkaline-earth atom reactions with halogen containing molecules. In these reactions the presence of two valence electrons leads to reaction channels, as chemiluminescent or chemi-ionization reactions, which are specifically associated with the transfer of the more energetic (inner harpooning) electron, whose dynamics or stereodynamics pattern differs from those of reaction channels associated to the transfer of the first, less energetic electron as, for example, in ion-pair reactions. A full section of the paper is dedicated to photo-induced intracluster reactions and, in particular, to the transition state dynamics of the prototype Ba···FCH3 + hν → BaF + CH3 reaction investigated in great detail by both nanosecond and femtosecond photo–ion and photo–electron pump and probe techniques. For this system the combination of these ultrahigh resolution techniques with isotopic labelling and ab initio calculations allowed the deciphering of its ultrafast reaction mechanism. Finally, the paper ends with some concluding remarks based on the observed propensity rules or general trends found in these alkaline-earth atom reactions.

Termolecular kinetics for the Mu+CO+M recombination reaction: A unique test of quantum rate theory

The room-temperature termolecular rate constants, k0, for the Mu + CO + M<==>MuCO + M (M = He, N2, Ar) recombination reaction have been measured by the muSR technique, and are reported for moderator gas pressures of up to approximately 200 bar (densities less, similar 0.4 x 10(22) molec cm(-3)). The experimental relaxation rates reveal an unusual signature, in being dominated by the electron spin-rotation interaction in the MuCO radical that is formed in the addition step. In N2 moderator, k0 = 1.2+/-0.1 x 10(-34) cm(6) s(-1), only about 30% higher than found in Ar or He. The experimental results are compared with theoretical calculations carried out on the Werner-Keller-Schinke (WKS) surface [Keller et al., J. Chem. Phys. 105, 4983 (1996)], within the framework of the isolated resonance model (IRM). The positions and lifetimes of resonance states are obtained by solving the complex Hamiltonian for the nonrotating MuCO system, using an L2 method, with an absorbing potential in the asymptotic region. Accurate values of the vibrational bound and resonance states of MuCO reveal unprecedented isotope effects in comparisons with HCO, due to the remarkable effect of replacing H by the very light Mu atom (m(Mu) approximately (1/9)m(H)). Due to its pronounced zero-point energy shift, there are only two (J = 0) bound states in MuCO. Contributions from nonzero J states to the termolecular rate constants are evaluated through the J-shifting approximation, with rotational constants evaluated at the potential minimum. The value of the important A constant (181 cm(-1)) used in this approximation was supported by accurate J = K = 1 calculations, from which A = 180 cm(-1) was obtained by numerical evaluation. The calculations presented here, with a "weak collision factor" beta c = 0.001, indicative of the very sparse density of MuCO states, give a very good account of both the magnitude and pressure dependence of the experimental rates, but only when the fact that the two initially bound (J = 0) states become resonances for J > 0 is taken into account. This is the first time in IRM calculations of atom-molecule recombination reactions where J not equal to 0 states have proven to be so important, thus providing a truly unique test of quantum rate theory.

Reaction Mechanism of Oxygen Atoms with Cyanoacetylene in the Gas Phase and on Water Ice

Reactions of atoms and molecules on dust grains coated by water ice have been considered to play an important role in the formation of many of the detected interstellar molecules. The catalytic role of water ice in some molecule-molecule reactions has been shown. Yet, there is rather little knowledge on the water-catalyzed atom-molecule reactions. In this paper, we present an evident theoretical clue that a considerably barrier-consumed atom-molecule reaction 3O + HC3N in the gas phase can be effectively catalyzed by water ice to be barrierless. Detailed Gaussian-3 potential energy surface studies show that both the 3O + HC3N and 1O + HC3N reactions in the gas phase can eventually lead to the product 3HCCN + CO with the involvement of singlet-triplet intersystem crossings. Yet the gaseous 3O + HC3N reaction faces a considerable entrance barrier, suggestive of its low likelihood in the low-temperature interstellar space. Fortunately, inclusion of water molecules significantly reduces the entrance barrier to zero. This is caused by the dual role of water molecules in the rate-determining O-addition transition state for 3O + HC3N, i.e., the water molecules act as both a proton acceptor with the CH bond and a proton donor with the 3O atom to cooperatively decrease the reaction barrier. The results can well account for the recent cyanoketene detection in a photoreactivity study of HC3N/O3/Ar mixtures. Therefore, on water ice, the simplest cyanopolyynes, HC3N, can be effectively depleted by oxygen atoms and can generate the elusive cyanomethylene (3HCCN), whose formation mechanism in space was still unclear up to now.

Reflected Shock Tube Studies of High-Temperature Rate Constants for OH + NO2→ HO2+ NO and OH + HO2→ H2O + O2

The motivation for the present study comes from the preceding paper where it is suggested that accepted rate constants for OH + NO2 --> NO + HO2 are high by approximately 2. This conclusion was based on a reevaluation of heats of formation for HO2, OH, NO, and NO2 using the Active Thermochemical Table (ATcT) approach. The present experiments were performed in C2H5I/NO2 mixtures, using the reflected shock tube technique and OH-radical electronic absorption detection (at 308 nm) and using a multipass optical system. Time-dependent profile decays were fitted with a 23-step mechanism, but only OH + NO2, OH + HO2, both HO2 and NO2 dissociations, and the atom molecule reactions, O + NO2 and O + C2H4, contributed to the decay profile. Since all of the reactions except the first two are known with good accuracy, the profiles were fitted by varying only OH + NO2 and OH + HO2. The new ATcT approach was used to evaluate equilibrium constants so that back reactions were accurately taken into account. The combined rate constant from the present work and earlier work by Glaenzer and Troe (GT) is k(OH+NO2) = 2.25 x 10(-11) exp(-3831 K/T) cm3 molecule(-1) s(-1), which is a factor of 2 lower than the extrapolated direct value from Howard but agrees well with NO + HO2 --> OH + NO2 transformed with the updated equilibrium constants. Also, the rate constant for OH + HO2 suitable for combustion modeling applications over the T range (1200-1700 K) is (5 +/- 3) x 10(-11) cm3 molecule(-1) s(-1). Finally, simulating previous experimental results of GT using our updated mechanism, we suggest a constant rate for k(HO2+NO2) = (2.2 +/- 0.7) x 10(-11) cm3 molecule(-1) s(-1) over the T range 1350-1760 K.

The Reactions of O(3P) Atom with Halomethanes: Discharge Flow-Chemiluminescence Imaging Method

The reactions of triplet oxygen atom with halomethanes as a potential fire extinguisher were studied by a discharge flow-chemiluminescence imaging method. The experiments were carried out under second order conditions. The bimolecular atom-molecule reaction rate constants were determined in terms of the initial rate method. The initial concentration of oxygen atom was also determined under second order rate law instead of the pseudo-first order conditions with <TEX>$[O(^3P)]_0{\ll}[sample]_0$</TEX>. The second order conditions were more reliable than pseudo-first order conditions for the determinations of rate constants. The rate constants of the reactions <TEX>$CF_3I\;+\;O(^3P)$</TEX>, <TEX>$CH_3PI\;+\'O(^3P)$</TEX>, and <TEX>$CHBrCl_2\;+\;O(^3P)$</TEX> were determined to be <TEX>$5.0\;{\times}\;10^{-12}$</TEX> , <TEX>$1.1\;×\;0^{-11}$</TEX> , and <TEX>$1.9\;{\times}\;10^{-14}cm^3molecule^{-1}s^{-1}$</TEX>, respectively.

Dynamics by crossed-beam digital imaging: collisional energy effects on the Ba(1P1) + N2O→BaO(A′ 1Π, v′ ≤ 3) + N2 differential reaction cross-section

The dynamics of the excited-state Ba(1P1) + N2O→BaO*(A′ 1Π, ′⩽3) + N2 reaction were investigated by crossed-beam digital imaging at 0.065 eV collision energy. To this end, the spatial (angular) distribution of the long-lived BaO*(A′ 1Π, ′⩽3) product was imaged using a CCD camera. The analysis of the product angular distribution suggests a direct mechanism in which the excited-state BaO*(A′ 1Π, ′⩽3) shows a backward scattering with the total energy of the reaction mostly channelled into product internal energy. A comparison between previous results obtained for the same reaction investigated at 0.155 eV (C. Rinaldi et al., Phys. Chem. Chem. Phys., 2000, 2, 723) and the present data suggests little change in the dynamics except for a modest enhanced forward character of the BaO* angular distribution when the reactant collision energy increases from 0.065 up to 0.155 eV. A comparison of the excited-Ba(1P1) and ground-state Ba(1S0) + N2O reaction translational energy disposal with an impulsive and kinematic model prediction for atom–molecule reactions leads to the conclusion that kinematic factors play an essential role in both reactions. However, the ground-state reaction seems to proceed ia a more direct rebound mechanism with a predominance of collinear transition-state configurations.

The dilution effect of noble gases on the structure and composition of chemically vapour-deposited diamond films

Diamond thin films were prepared by decomposition of methane and hydrogen in a microwave plasma discharge. The dilution effect of noble gases on the deposition of diamond film was investigated by scanning electron microscopy, optical emission spectroscopy, X-ray diffraction and Raman spectroscopy. It was found that the introduction of noble gases changes the morphology of the diamond film from triangular {111} to square {100} facets and leads to fewer crystalline defects. Such results are more pronounced for Ar and Ne but less pronounced for He, Kr and Xe. The noble gases can be regarded as efficient dilution gases in the diamond deposition by inducing additional ion-molecule and excited atom-molecule reactions. As a result, the nucleation and growth of diamond have been enhanced, and the formation of non-diamond carbon phases has been substantially suppressed through the reaction of noble gases in methane and hydrogen plasmas. The results indicate that the diamond films prepared in this way were frequently deposited on a buffer layer composite of crystalline graphite and non-crystalline form of carbon.

Effects of noble gases on diamond deposition from methane-hydrogen microwave plasmas

The deposition of diamond films by microwave plasmas has been studied in gaseous mixtures of methane, hydrogen, and noble gases. Plasma diagnostic results are compared with growth rates and Raman spectra of the films. The noble gases, which influence the degree of excitation or reactant molecules by energy transfer or charge transfer from their excited and ionic states, are active in the deposition process by inducing additional ion-molecule and excited atom-molecule reactions. As a result, enhanced deposition rates have been observed. Small oxygen additions along with the noble gases can suppress the formation of nondiamond carbon phases, leading to an effective way to rapidly deposit diamond films at high methane concentrations while still retaining minimal nondiamond carbon components in the films.

A review of microelectronic film deposition using direct and remote electron-beam-generated plasmas

Soft-vacuum-generated electron beams employed to create a large-area plasma for assisting chemical vapor deposition (CVD) of thin films are reviewed. The electron-beam plasma is used directly, where electron-impact dissociation of feedstock gases plays a dominant role, and indirectly in a downstream afterglow, where electron-impact dissociation of feedstock reactants plays no role. Photodissociation and metastable atom-molecule reactions dominate in the downstream afterglow. The transmitted beam spatial-intensity profiles are quantified from initial generation at a slotted line-shaped cold cathode through acceleration in the cathode sheath and propagation in the ambient gas. The vacuum ultraviolet (VUV) output spectrum and VUV generation efficiency from electron-beam-excited plasmas are measured. The properties of films deposited by direct electron-beam-generated plasma-assisted CVD and downstream afterglow CVD are reviewed and compared to conventional plasma-assisted CVD films. >

Decay of hydrogen(deuterium) atoms in solid hydrogen at 4.2 K: rate constant for tunneling reaction hydrogen (deuterium, hydrogen deuteride) + hydrogen (deuterium) atom

Decay of H (or D) atoms at 4.2 K, produced by {gamma}-radiolysis of solid hydrogen, has been studied by ESR spectroscopy. The decay is caused by quantum mechanical tunneling. The decay rate of H atoms in H{sub 2} depends upon the initial concentration of the H atoms, and their decay is represented by second-order kinetics. D atoms decay very slowly in the D{sub 2} solid and disappear by reaction with HD, which exists as an impurity. In the HD solid, D atoms decay fast, while H atoms increase complementarily. Since the decay of these atoms is associated with hydrogen atom-molecule tunneling reactions the rate constants for the reactions are obtained from the decay rates. The rate constants for the tunneling reactions H{sub 2} + H {yields} H + H{sub 2}, D{sub 2} + D {yields} D + D{sub 2}, and HD + D {yields} H + D{sub 2} were 1.8 {times} 10, 1.8 {times} 10{sup {minus}3}, and 1.9 {times} 10{sup {minus}3} cm{sup 3} mol{sup {minus}1} s{sup {minus}1}, respectively, at 4.2 K. Room light and desk light promote remarkably the decay rate of H atoms in the H{sub 2} solid and slightly the decay rate of D atoms in the D{submore » 2} solid. The decay of D atoms in the HD solid is not, however, affected by the light illumination.« less

Hybrid approximate solution—Approximate potential approach to the solution of coupled equations for atom-molecule reactive scattering

The generalized log-derivative method is applied to the coupled equations problem for collinear atom-molecule reactions formulated in terms of natural collision coordinates. A comparison with the “ R -matrix propagation” method is made. The comparison is based on numerical tests performed on the H+ H 2 reaction. Some tests were made also on the asymmetric F + H 2 system. In consequence of these tests two hybrid versions of the RXN1D program were developed. They are demonstrated to be about two times faster than the original version.

Properties of gas-phase ions. Information to be obtained from photoelectron spectroscopy of unstable molecules

The information to be obtained from the study of unstable molecules in the vapour phase by photoelectron spectroscopy is reviewed, using examples drawn from high-temperature pyrolysis experiments and rapid atom-molecule reaction. An example is given of a chemi-ionization reaction studied by electron spectroscopy, and the information to be obtained from such studies is outlined.

Rotational state dependence of the reactivity of oriented symmetric top molecules

Extending the angle-dependent line-of-centers model originally developed to treat the orientation dependence of atom-molecule reaction cross sections, it is possible to predict the JKM dependence of the reactivity of rotationally state-selected, oriented symmetric top molecules in collision with spherically symmetric co-reagents. In a certain well-defined limit the cross section depends only upon the characteristic quantum number combination KM/( J 2+ J), the average orientation of the symmetric top.

Energy-conserving trajectories in bond-angle-bond-distance coordinate systems

Trajectory calculations for atom-molecule reactions which define the potential energy in terms of a bond angle and (adjacent) bond distance coordinate system will show discontinuities due to the necessity of switching surfaces during a trajectory. We show how to remove these discontinuities for the F + H 2 → HF + H surface, where a rotated Morse curvecubic spline potential function is used.

Dynamics of centrifugal barrier complexes close to orbiting

Trajectories close to orbiting have been studied for central and various non-central potentials in atom—molecule reactions. The complex formation of K + NaCl is considered as a prototype process. These trajectories move close to the peak of the centrifugal barriers and correspond to the trapped trajectories found by Pollak and Pechukas in bimolecular direct reaction systems. An iteration procedure with rapid convergence for finding trajectories close to orbiting has been developed. It is found that the stability of the trajectory is not strongly affected by the form of the potential and the rotational energy of the diatomic molecule. The atom—diatomic molecule distance can vary up to 10% during a complete molecular rotation. Angular momenta and rotational energies show cyclic variations at low but not at high molecular energies due to Coriolis effects. The relation of bimolecular reactions to periodic orbits is discussed.