Molecular dynamics of some X2Y3 systems

A complete vibrational analysis has been carried out for ¹⁰B₂O₃, ¹¹B₂O₃, and sulfur dicyanide and a set of molecular constants, i.e. kinetic constants, potential constants, compliance constants, vibrational mean amplitudes, Coriolis coupling constants and centrifugal distortion constants, is reported. The values of the centrifugal distortion constants of S(CN)₂, are in good agreement with the observed values, which bears out the significance of the method of kinetic constants adopted in this work.     

Vibrational energy transfer in H2 + He

A semiclassical approach is developed to study vibrational energy transfer in H₂ + He by use of the a priori interaction potential including all nonzero impact parameter collisions. The calculated values of the rate coefficient are found to be in excellent agreement with experimental data which are available in the temperature ranges 60–450 K and 1350-3000 K. The temperature dependence is shown to seriously deviate from the Landau-Teller prediction below 1000 K. The calculation was carried out over the temperature range of 30 to 10000 K.     

Differential energy-loss spectra for CH4 + Ar collisions

In a crossed molecular-beam experiment differential time-of-flight spectra have been measured for CH₄ + Ar collisions at E = 93.1 meV. Ale energy-loss spectra, which show a remarkable transfer of rotational energy, are compared with coupled-states calculations based on a model potential previously derived.     

Quantum mechanical investigations of the collinear collisions F + H2 and F + D2 using the wavepacket approach

Fur a study of the quantum dynamics of the collinear collisions F + H₂ and F + D₂ a wavepacket approach is applied using three different semi-empirical potential functions due to Muckerman. The calculations give details of the time behaviour of the system and, by means of a careful energy analysis, information on the partitioning of the total energy into translational and vibrational parts as well as on mean populations of product vibrational states. Pronounced quantum effects are observed.     

Energy transfer in reactive and nonreactive collisions of Na with Na2

Stimulated by the experimental finding of vibrationally and rotationally cold dimers in supersonic nozzle molecular beams of sodium, we have studied energy transfer in collisions of Na with Na₂ over a wide range of initial relative translation energies E and impact parameters b by a classical mechanical trajectory method. The vibrational and rotational energies were initialized using Boltzmann distributions characterized by temperatures T_vib = 150 K, T_rot = 50 K. We find that for large values of E the energy transfer in reactive collisions increases with b while it decreases with b for the nonreactive collisions. For low values of E, energy transfer is a decreasing function of b for both reactive and nonreactive encounters. Both the reactive and nonreactive mechanisms are very efficient in effecting transfer, between 40–70% of the initial relative translational energy is converted into internal energy of the diatom, leading to the conclusion that the reverse collisions would result in the rapid relaxation observed in experiment.     

Semi-classical study of the molecular dissociation: H2 + He

Accurate vibrational wave functions and a state-dependent model interaction potential were used in the study, within the framework of a semi-classical theory, of the vibrational excitation and dissociation of the hydrogen molecule in collinear collisions with the helium atom. A molecule initially in the excited state is shown to be very efficient in energy transfer and twice more likely to be further excited than to be de-excited. The change in the population distribution among the vibrational states at the first few collisions was analyzed. It is shown that the population of the first vibrational excited state ψ₁ reaches its maximum after the very first collision and that of ψ₂ after the second. It is also found that at a sufficiently high collision energy, ψ₅ is the most efficient state in dissociation at the second collision while ψ₆ contributes most at the third collision.     

Vibrational deactivation of CO2(0001) by ethane

The deactivation of CO₂(00⁰1) by ethane in the temperature range 300–600 K has been studied using a laser induced fluorescence technique. The energy transfer cross section decreased from 0.23 Ų at 300 K to 0.16 Ų at 600 K. The magnitude of the cross section is consistent with the expectation that near resonant V-V energy transfer processes are responsible for the energy transfer between CO₂(00⁰1) and ethane during collisions even though the observed temperature dependence of the energy transfer cross section does not follow that predicted by the existing theories.     

Brillouin scattering in liquid mixtures: CCl4/CHCl3

The Brillouin light scattering spectra of mixtures of liquid CCl₄ and CHCl₃ have been obtained. The resulting relaxation rates and the relaxing energy reservoir were studied as a function of the mole fraction over the entire concentration range. The energy exchange between the pertinent degrees of freedom are discussed in terms of the relaxation rates of homomolecular and heteromolecular collisions. We came to the conclusion that the resonance energy transfer between vibrational levels play a significant role in determining the vibration—translation energy migration observed by Brillouin scattering'     

Trajectory studies of energy transfer: CH3NC with He,Xe, H2 and N2

A trajectory program was used to simulate the collisions of CH₃NC with He, Xe, H₂ and N₂. Calculated energy transfer is in accord with experiment. The pattern of CH₃NC vibrational mode energization is found to be noticeably non-random. The approximate sampling methods used in thermal unimolecular trajectory studies produce a more uniform state distribution.     

Relaxation in a Na/Na2 nozzle expansion

Atom—molecule collisions in a Na/Na₂ nozzle expansion are investigated. The atoms are marked near the nozzle exit by preparing them alternatively into the 3²S and 3²P state. The effect of collisions between these atoms and the molecules is studied far downstream the beam. We find that the internal energy distribution of the molecules depends on the internal energy of the atoms. The specific change of the internal energy distribution gives information on the energy relaxation in the nozzle expansion.     

Conformational analysis of X3PNP(O)X2 and (X3PNPX3)+ for X = H,F, Cl,CH3 by the pcilo method

Conformational energy maps have been calculated, using the PCILO method, for X₃PNP(O)X₂ and (X₃PNPX₃)⁺ for X = H, F, Cl, CH₃ as a function of the PNP angle. In H₃PNP(O)H₂ the global energy minimum corresponds to the eclipsed conformation of the H₃P and P(O)H₂ fragments for all PNP angles, while in Cl₃PNP(O)Cl₂, the global minimum always has Cl₃P and P(O)C1₂ staggered: the global minimum in F₃PNP(O)F₂ corresponds to eclipsed F₃P and P(O)F₂ fragments at low PNP angles and staggered fragments at high PNP angles: in (CH₃))₃PNPO(CH₃)₂ the global minimum conformation is very sensitive to ∠ PNP. Subordinate energy minima occur for all X₃PNP(O)X₂, species: in particular, there are two local conformational minima for Cl₃PNP(O)Cl₂ at the optimum value of ∠ PNP, and the relative energies of the three stable conformations are in good agreement with those derivable from the ³¹P NMR spectrum of this compound. In (X₃PNPX₃)⁺ the global minimum, usually the sole minimum on the conformational energy surface, is always close to the eclipsed conformation: free rotation of the X₃P groups relative to one another is approached in each (X₃PNPX₃)⁺ ion as ∠PNP approaches 180°. The conformations of the transition states for the equilibria between energy minima are reported with their relative energies, for X₃PNP(O)X₂ (X = H, F. Cl, CH₃) and for (Cl₃PNPCl₃)⁺     

A crossed-beam study of low energy Ar + H2O collisions: Charge transfer and chemical reaction

Dynamics of Ar⁺ + H₂O collisions, charge transfer (H₂O⁺ formation) and chemical reaction (ArH⁺ formation), was investigated in crossed-beam experiments in the collision energy range 1–3 eV. The charge transfer occurs both by a simple-electron-jump mechanism and in impulsive intimate collisions. The distributions of relative translational energy of the products show that, most probably, the collisions are only slightly superelastic (by about 0.1 eV), and thus the product H₂O⁺ is highly internally excited. Formation of a small amount of H₂O⁺(B²B₂) in very inelastic collisions was observed. The chemical reaction is a direct process with characteristics of the spectator stripping mechanism.     

Low energy crossed beam study of the endothermic charge transfer reaction H+2(Ar,H2)Ar+

A crossed beam study of the title reaction if reported, from 0.45 to 7.8 eV. The reaction is predominantly translationally endothermic. At the lowest energy, there is evidence for two reaction paths: a long-range electron transfer and an intimate collision with electron transfer. Branching ratios for the competitive proton transfer reaction are presented.     

Triatom-triatom collisions: Fixed angle close-coupling study of vibrational excitation in the 12CO2+13CO2 system

A close-coupling approach to the calculation of quantal vibrational transition probabilities for the fixed angle scattering of a linear triatomic molecule with another linear triatomic molecule is described. The method is applied to the ¹²CO₂+¹³C0₂ collisional system. For a calculated inelastic transition probability to have an appreciable magnitude, it is found that the amount of energy transferred in a transition must be very small and just one quantum of energy must be exchanged between either the symmetric stretch or the asymmetric stretch vibrational modes of ¹²C0₂ and ¹³CO₂. For collisional energies away from threshold, the probabilities for transitions involving the symmetric stretch ¹²CO₂ and ¹³CO₂ modes are insensitive to long range multipole terms in the potential energy surface, while the probabilities for energy exchange between the asymmetric stretch modes are considerably diminished when the long range terms are removed from the potential energy surface. A brief discussion is presented on the possibilities of extending the technique to the calculation of vibrational excitation cross sections for three-dimensional triato—triatom collisions.     

Further studies of the N2+ + N2 → N4+ association reaction

Data are presented which strongly suggest that stabilisation of the excited intermediate (N₄⁺)* complex in the reaction (1) N₂⁺ + 2N₂ (rate coefficient k₁) occurs via N₂ switching whereas for (2) N₂⁺ + N₂ + He (rate coefficient k₂) it occurs via superelastic He collisions. This explains the differing temperature variations of k₁ and k₂ previously obtained for these reactions. Drift tube data are also presented which show how k₁ varies with N₂⁺/N₂ centre-of-mass energy as compared with thermal energy.     

Excitation and decay of the C2Σ+u state of N+2 following collisions of He+ ions with N2 isotopes

A quantitative analysis is made of the N⁺₂ “2nd negative” emission (“2N”: C²Σ⁺_u → X²Σ⁺_g) produced by the impact of 500 eV to 25 keV He⁺ beams on ¹⁴N₂, ¹⁴N¹⁵N and ¹⁵N₂. Above about 5 keV, the relative 2N emission rates from the various vibrational levels of the C state are the same as those observed for ? 2 keV Ne⁺, or > / 90 eV electron-impact. These limiting distributions are compared to those predicted for a Franck-Condon excitation of the C state, modified by configuration interaction. The weakening in 2N emission at the vibrational levels ν′ > / 3 is ascribed to spontaneous C-state predissociation. The data fully confirm recent reports that this predissociation extends over a wide range of ν′ and that it is subject to a strong isotope effect. The ratios of the rates of C-state predissociation to 2N emission are obtained for the levels ν′ = 3 to 8 of each nitrogen isotope. By means of these data it is shown that near-resonant charge transfer dominates the distribution of vibrational excitation probabilities only at energies below about 10 eV. A comparison is made of absolute cross-sections for C-state emission with those for N⁺ and N⁺₂ production in He⁺/¹⁴N₂ collisions at energies between 5.5 eV and 25 keV.     

Resolution of angular rotational rainbows in Na2 + Ne,Ar collisions

State-to-state dfferential cross sections for rotational excitation in Na₂-Ne collisions have been measured and the characteristics of such processes involving repulsive surfaces demonstrated. The rotational rainbow maxima shift to larger scattering angles and broaden as the angular momentum transfer increases. Good agreement with IOS calculations is found. Results for Na₂-Ar are included.     

Dynamic Renner effect in collisions of C+ with H2

The dynamic Renner effect is shown to permit formation of CH⁺₂ in its first excited (²B₁) state from low energy collisions of C⁺ + H₂. The consequences for C⁺ + H₂. The consequences for C⁺ + H₂ radiative association are discussed.     

Chemiluminescent ion-molecule reactions in the system C+ + H2

The optical emission resulting from collisions between C⁺ ions and H₂ gas was measured in the energy range 2 to 20 eV_c.m.. The observed spectrum consists mainly of the CH⁺ A ¹Π → X ¹Σ⁺ band system; CH⁺ (A ^fΠ) is shown to be formed in the chemiluminescent reactio: C⁺(²P⁰) + H₂ → CH⁺(A ¹Π) + H(²S). The energy dependence of the emission cross section was measured. The occurrence of this reaction is discussed in terms of a electronic state correlation diagram for the system.