Simultaneous vibrational relaxation and thermal dissociation of diatomic molecules

A model of a truncated harmonic oscillator with equidistant levels and one-quantum transitions is applied. The balance equations for O₂ have been solved numerically with 26 levels, with the transition probabilities taken as P _k,k +1k exp (k) P ₁₀, in which k is the number of the level and is an adjustable parameter that takes account of the anharmonicity. A Boltzmann distribution is obtained up to level 19 with =0, but for 0 there are deviations from that distribution in lower levels. Levels with k=1, 2, 3 are populated with relaxation times substantially less than the relaxation time for the vibrational energy. Dissociation depletes levels with k 19.     

Stochastic dissociation of diatomic molecules

The fragmentation of diatomic molecules under a stochastic force is investigated both classically and quantum mechanically, focusing on their dissociation probabilities. It is found that the quantum system is more robust than the classical one in the limit of a large number of kicks. The opposite behavior emerges for a small number of kicks. Quantum and classical dissociation probabilities do not coincide for any parameter combinations of the force. This can be attributed to a scaling property in the classical system which is broken quantum mechanically.     

Thermal dissociation of diatomic molecules

The establishment of equilibrium in the chemical reaction X₂+M 2X+M is examined. All possible transitions between the vibrational levels and the continuous spectrum are considered. Taking account of two relaxation times, an expression is obtained for the concentration of the X atoms and the chemical reaction rate as a function of time. The conditions under which the dependence of the chemical reaction rate on time acquires an exponential nature are determined. An expression free from the requirement that the initial concentrations of X-atoms and X₂ molecules should be close to their equilibrium values is obtained for the rate constant.In conclusion, the authors thank Prof. N. D. Sokolov for discussion of the work.     

On the dissociation of diatomic molecules at metal surfaces

The role of activation barriers in the process of dissociative adsorption of diatomic molecules on metal surfaces is considered in terms of the topology of the diabatic potential energy surfaces of the interacting system. The location of possible barriers on the potential energy surface determines the dependence of the dissociative threshold on either the normal translational or total energy content of the incident molecules.     

The dissociation energy of the new diatomic molecules SiPb and GePb

The diatomic molecules SiPb and GePb were for the first time identified by producing high temperature vapors of the constituent pure elements in a "double-oven-like" molecular-effusion assembly. The partial pressures of the atomic, heteronuclear, and homonuclear gaseous species observed in the vapor, namely, Si, Ge, Pb, SiPb, GePb, Pb2, Gen, and Sin (n=2-3), were mass-spectrometrically measured in the overall temperature ranges 1753-1961 K (Ge-Pb) and 1992-2314 K (Si-Pb). The dissociation energies of the new species were determined by second- and third-law analyses of both the direct dissociation reactions and isomolecular exchange reactions involving homonuclear molecules. The selected values of the dissociation energies at 0 K (D0 degrees) are 165.1+/-7.3 and 141.6+/-6.9 kJ/mol, respectively, for SiPb and GePb, and the corresponding enthalpies of formation (DeltafH0 degrees) are 476.4+/-7.3 and 419.3+/-6.9 kJ/mol. The ionization efficiency curves of the two species were measured, giving the following values for the first ionization energies: 7.0+/-0.2 eV (SiPb) and 7.1+/-0.2 eV (GePb). A computational study of the species SiPb and GePb was also carried out at the CCSD(T) level of theory using the relativistic electron core potential approach. Molecular parameters, adiabatic ionization energies, adiabatic electron affinities, and dissociation energies of the title species were calculated, as well as the enthalpy changes of the exchange reactions involving the other Pb-containing diatomics of group 14. Finally, a comparison between the experimental and theoretical results is presented, and from a semiempirical correlation the unknown dissociation energies of the SiSn and PbC molecules are predicted as 234+/-7 and 185+/-11 kJ/mol, respectively.     

Bifurcations as dissociation mechanism in bichromatically driven diatomic molecules

We discuss the influence of periodic orbits on the dissociation of a model diatomic molecule driven by a strong bichromatic laser fields. Through the stability of periodic orbits, we analyze the dissociation probability when parameters, such as the two amplitudes and the phase lag between the laser fields, are varied. We find that qualitative features of dissociation can be reproduced by considering a small set of short periodic orbits. The good agreement with direct simulations demonstrates the importance of bifurcations of short periodic orbits in the dissociation dynamics of diatomic molecules.     

Probability of excitation of vibrations and dissociation of diatomic molecules

A numerical solution is given for the dynamic problem of collision between an atom and a diatomic molecule for the case of O₂ and Ar within the framework of classical mechanics. The molecule is approximated by the Rydberg-Klein-Riess oscillator. The probabilities of change in the vibrational state and dissociation of the molecule are calculated by the method of varying the initial conditions.The authors are indebted to N. A. Generalov, V. B. Leonas, and A. I. Osipov for useful discussion.     

Diffusion theory of the thermal decomposition of diatomic molecules

A quasi-stationary solution of the Fokker—Planck equation for definite limiting conditions, describing the process of thermal decomposition of diatomic molecules in a light inert gas, has been considered. The rate of thermal dissociation of Morse oscillators has been calculated for a relatively arbitrary intermolecular interaction potential.     

The dissociation and equation of state of dense fluid oxygen at high pressures and high temperatures

The dissociation, pressure, and internal energy of dense fluid oxygen at high temperatures and densities have been calculated from the free-energy functions using the self-consistent fluid variational theory. In this paper, we focused on a mixture of oxygen atoms and molecules, and investigated the phenomenon of pressure dissociation at finite temperature. The single-shock Hugoniot derived from this equation of state agrees well with gas-gun experiments for pressure versus density. The equation of state and dissociation degree are predicted in the ranges of temperature of 5000-16,000 K and density of 0.1-4.5 g/cm(3). These data are formulated in the analytical forms of dissociation degree-density-temperature and pressure-density-temperature equation of state.     

Analysis of lattice thermal conductivity of GaAs at high temperatures

The lattice thermal conductivity of GaAs has been analysed in the entire temperature range 100–800 K in the frame of the Sharma-Dubey-Verma (SDV) model of phonon conductivity, and very good agreement has been found between the calculated and experimental values of the lattice thermal conductivity in the entire temperature range of study. The temperature exponentm(T) for the three-phonon scattering relaxation rate for GaAs has also been calculated in the above temperature range. The separate percentage contributions due to transverse and longitudinal phonons have also been studied.

---

Zusammenfassung Die Gitter-Warmeleitfähigkeit von GaAs wurde im Temperaturbereich zwischen 100 und 800 K im Rahmen des Modells der Phononen-Leitfähigkeit von Sharma-Dubey-Verma (SDV) untersucht und eine sehr gute übereinstimmung zwischen den berechneten und Versuchswerten der Gitter-Wärmeleitfähigkeit im ganzen untersuchten Temperaturbereich gefunden. Der Temperaturexponentm=m(T) für die Drei-Phonenen-Streuungs-Relaxationsgeschwindigkeit bei GaAs wurde im obengenannten Temperaturbereich ebenfalls berechnet. Die durch transversale und longitudinale Phonone verursachten einzelnen prozentualen Beiträge wurden ebenfalls untersucht.

---

Résumé La conductibilité thermique du réseau de GaAs a été analysée dans tout l'intervalle de température allant de 100 à 800 K, dans le cadre du modÊle de conductibilité de phonons de Sharma-Dubey-Verma (SDV). Une trÊs bonne concordance a été observée entre les valeurs calculées et expérimentales de la conductibilité thermique du réseau dans l'intervalle de température étudié. L'exposant de températurem(T) a également été calculé dans ce domaine de température pour la vitesse de relaxation de la diffusion de trois phonons. Les pourcentages individuels des contributions des phonons transversaux et longitudinaux ont également été étudiés.

---

100–800 CaAs -- . . m() CaAs. , .

     

Non-adiabatic unimolecular dissociation of polyatomic molecules. II. The thermal dissociation of nitrous oxide

The theory presented in part I of this series is applied to the non-adiabatic spin-forbidden thermal dissociation N₂O(¹Σ⁺)→N₂(¹Σ⁺_g)+O(³P) as a test case. The molecular model is multidimensional and includes all vibrational modes of the molecule. Specifically considered is the fact that the initial singlet state of N₂O is linear and the final triplet state is bent. The best available data are used for describing the intersection of singlet and triplet potential energy surfaces. Calculated microcanonical rate constants are averaged over Boltzmann distribution of energies and compared with k_co, the high-pressure rate constant deduced from experiment. The agreement between theory and experiment is satisfactory. Analysis of the calculations shows that the driving force for the N₂O dissociation is the flow of energy into the bending vibrations. This is because the bendings have very different equilibrium angles in the initial and final states.     

Vibrational energy storage in high pressure mixtures of diatomic molecules

CO/N₂, CO/Ar/O₂, and CO/N₂/O₂ gas mixtures are optically pumped using a continuous wave CO laser. Carbon monoxide molecules absorb the laser radiation and transfer energy to nitrogen and oxygen by vibration–vibration energy exchange. Infrared emission and spontaneous Raman spectroscopy are used for diagnostics of optically pumped gases. The experiments demonstrate that strong vibrational disequilibrium can be sustained in diatomic gas mixtures at pressures up to 1 atm, with only a few Watts laser power available. At these conditions, measured first level vibrational temperatures of diatomic species are in the range T_V=1900–2300 K for N₂, T_V=2600–3800 K for CO, and T_V=2200–2800 K for O₂. The translational–rotational temperature of the gases does not exceed T=700 K. Line-of-sight averaged CO vibrational level populations up to v=40 are inferred from infrared emission spectra. Vibrational level populations of CO (v=0–8), N₂ (v=0–4), and O₂ (v=0–8) near the axis of the focused CO laser beam are inferred from the Raman spectra of these species. The results demonstrate a possibility of sustaining stable nonequilibrium plasmas in atmospheric pressure air seeded with a few percent of carbon monoxide. The obtained experimental data are compared with modeling calculations that incorporate both major processes of molecular energy transfer and diffusion of vibrationally excited species across the spatially nonuniform excitation region, showing reasonably good agreement.     

Theoretical study of above-threshold dissociation on diatomic molecules by using nonresonant intense laser pulses

The above-threshold dissociation of the ground state of a OH molecule under intense nonresonant laser pulses has been studied using the time-dependent Schr?dinger equation with discrete variable representation. The applied field is assumed as a two-color mixed nonresonant laser pulses which has the nonresonant frequency omega and the overtone 2omega. After modulating the relative phase factor between the omega and 2omega pulse, we extracted a three-photon absorption peak or a five-photon absorption peak in the ATD spectrum.     

Studies on the full vibrational energies and dissociation energies of some heteronuclear diatomic molecules

A parameter-free analytical formula for dissociation energies of diatomic molecules is proposed by Fan and Sun (2009) [20] based on LeRoy and Bernstein's vibrational energy expression near dissociation limit. Using three highest vibrational energies which may be generated by the algebraic method (AM) presented in our previous study and by some other physical methods, the new formula is applied to study the molecular dissociation energies of 10 electronic states of KH, ⁷LiD, ⁷LiH, ⁶LiH, NaK, NaLI and NaRb heteronuclear diatomic molecules which have regular (Morse-like) potentials in this work. The results show that the AM energies and dissociation energies have excellent agreement with experimental values.     

Analysis of lattice thermal conductivities of rare-earth sulphides at high temperatures

The lattice thermal conductivities of rare-earth sulphides have been analyzed at high temperatures in the frame of the two-mode conduction of phonons for the first time by studying the total lattice thermal conductivities of GdS and LaS in the entire temperature range 100–1000 K. The temperature exponents for the three-phonon scattering relaxation rates are reported for the transverse as well as for the longitudinal phonons. The separate percentage contributions due to the transverse and longitudinal phonons towards the total lattice thermal conductivities of the above samples have similarly been studied. The role of the four-phonon processes too has been included in the present investigation.