Collisional energy transfer in the decomposition of 2-methyloxetane and 3-methyloxetane, I. Gas/gas collisions

The pressure dependence of the unimolecular rate constants for the thermal decomposition of 2-methyloxetane and 3-methyloxetane has been studied. The average energy transferred downward in gas-gas collision was determined by the application of RRKM theory and a stepladder model of energy transfer.

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2- 3-. , - , .

     

Collisional energy transfer in the two-channel decomposition of 1,1,2,2-tetrafluorocyclobutane and 1-methyl-2,2,3,3-tetrafluorocyclobutane, II. Gas/wall collisions

The efficiency of gas/wall vibrational energy transfer has been studied over the temperature range 800–1100 K by the variable encounter method. The average energies transferred per deactiviting collisions with the wall were determined at 800 K to be 3200 cm^–1 and 2900 cm^–1 for the 1,1,2,2-tetrafluorocyclobutane (TFCB) and 1-methyl-2,2,3,3-tetrafluorocyclobutane (MTFCB) molecules, respectively. This energy increased strongly with decreasing temperature. A comparison is made of with previous results for related molecules.     

Collisional energy transfer in the thermal decomposition of oxetane

The first order rate coefficient for the thermal decomposition of oxetane and oxetane-d₂ has been investigated at two temperatures as a function of pressure. Gas phase collisional relaxation results are obtained by using RRKM theory and various energy tranfer probability models.

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-d₂ . , RRKM .

     

Collisional energy transfer in the two-channel decomposition of 1,1,2,2-tetrafluorocyclobutane and 1-methyl-2,2,3,3-tetrafluorocyclobutane, I. Gas/gas collisons

The two-channel thermal decomposition of 1,1,2,2-tetrafluorocyclobutane (TFCB) and 1-methyl-2,2,3,3-tetrafluorocyclobutane (MTFCB) have been studied in the temperature range of 730–805 K at pressures varied from 1.1 Pa up to 4.6 kPa. In the pressure independent range, Arrhenius expressions were obtained for TFCB decomposition into 2 CH₂CF₂ (k₁) and C₂H₄+C₂F₄ (k₂), respectively. The same kinetic equations were determined for the decomposition of MTFCB into C₃H₄F₂+C₂H₂F₂ (k₃) and C₃H₆+C₂F₄(k₄). From the study of the pressure dependence of the homogeneous decomposition rates, the average downward energy transfer values of 1800±200 cm^–1 and 1600±200 cm^–1 were obtained for the TFCB and MTFCB molecules, respectively.     

Temperature dependent energy transfer in Ar-O3 collisions

The energy transfer between argon atoms and ozone complexes O3*, excited in the region of the dissociation threshold, is calculated for fixed temperatures (100 K< or =T < or =2500 K) using classical trajectories. The internal energy of ozone is resolved in terms of vibrational and rotational energies. For all temperatures, energy flows from O3* to Ar. The vibrational energy transfer, relative to k(B)T, is very small below 500 K, but gradually increases towards high temperatures. The relative rotational energy transfer, on the other hand, monotonously decreases with T; around 1100 K it falls below the relative vibrational energy transfer. Thermally averaged cross sections for vibrational and rotational energy transfers are also calculated. The implications for the stabilization of ozone complexes in the energy transfer model are discussed.     

Collisional energy transfer and fragmentation of size selected CO2 clusters

Carbon dioxide clusters are generated in a supersonic molecular beam and size selected by scattering from a He beam. By analyzing the measured time-of-flight spectra as a function of the deflection angle, differential energy loss spectra for (CO₂)₂ — He are obtained which show a rotational rainbow structure with a maximal energy transfer of ΔE/E=0.4. This result is compatible with the slipped parallel structure of dimer but not with theT-shaped geometry. The scattering analysis is also used to derive information about the pressure dependence of cluster formation and the fragmentation by electron impact ionisation. The latter process leads preferably to the monomer product ion CO ₂ ⁺ with a small but finite probability for other ionic channels.     

Gas/gas and gas/wall average energy transfer from very low-pressure pyrolysis

It is shown that data obtained using very low-pressure pyrolysis (VLPP) on the pressure and temperature dependence of unimolecular rate coefficients of reactants with several reaction channels yield average energies transferred in gas/gas and gas/wall collisions (the wall being seasoned quartz at 800–1200 K). The downward average energy transferred, «ΔEå, for chlorocyclobutane/ethylene collisions is found to be 1600 cm^?1 at 970 K; «ΔEå for chlorocyclobutane/wall collisions varies from 5000 cm^?1 (wall efficiency β_w = 0.8) at 930 K to 3500 cm^?1 (β_w = 0.4) at 1150 K; similar values are found from published data on cycloheptatriene and cyclopropane-d₂. This indicates that the assumption of unit wall efficiency usually used in fitting VLPP experiments to RRKM theory needs revision.     

Rovibrational energy transfer in ortho-H2+para-H2 collisions

We present the results of a full-dimensional quantum mechanical study of the rovibrational energy transfer in the collision between ortho-H2 and para-H2 in the energy range of 0.1-1.0 eV. The multiconfiguration time-dependent Hartree algorithm has been used to propagate the wave packets on the global potential energy surface by Boothroyd et al. [J. Chem. Phys. 116, 666 (2002)] and on a modification of this surface where the short range anisotropy is reduced. State-to-state attributes such as probabilities or integral cross sections are obtained using the formalism of Tannor and Weeks [J. Chem. Phys. 98, 3884 (1993)] by Fourier transforming the correlation functions. The effect of initial rotation of the diatoms on the inelastic and de-excitation processes is investigated.     

Collisional energy transfer involving molecules in 1II electronic states: fully quantum study of collisions of Li2(B1IIu) with He and Ne

Infinite-order-sudden (IOS), coupled-states (CS) and close-coupled (CC) calculations for collisions of Li₂ (B¹II_u) with He and Ne are reported, based on a representation of the potential energy surfaces introduced by Poppe. We explore the range of validity of the CS and IOS approximations and analyse the quantum interference effects in the integral cross sections. For both homonuclear and heteronuclear molecules in ¹II electronic states we discuss, within the IOS approximation, when asymmetries will exist in the cross sections for upward (J → J + ΔJ), as compared to downward (J → J - ΔJ) transitions. In addition, also within the IOS approximation we show that the J → J + ΔJ across sections will not be invariant with respect to the A-doublet level of the initial state. The CC cross sections are compared with previous and current experimental results. Good agreement is found for the magnitude of both the integral cross sections and the cross section asymmetries. The present study as well as previous experimental investigations show that the asymmetry pattern appears to be extremely sensitive to the interaction potential.     

Collisional transfer of rovibrational energy from quantum calculations. II. The case of LiH with He

The collisional exchange of energy between He atoms and a strongly polar diatomic, the LiH target, has been studied theoretically over a range of relative energies that had been previously sampled by experiments with molecular beams. The relative importance of the rotational and/or vibrational channels is examined by studying in detail the effect of the interaction via a model potential coupling parameter. The different behavior exhibited by differential cross sections (total and partial inelastic) is also analyzed in terms of the strength and shape of the interaction anisotropy.     

Near-resonant vibration-to-vibration energy transfer in the NO+-N2 collisions

First principles model calculations of the vibration-to-vibration (VV) energy transfer (ET) processes NO(+)(nu=1)+N(2)(nu=n-1)-->NO(+)(nu=0)+N(2)(nu=n)+(28.64n-14.67) cm(-1) and NO(+)(nu=n)+N(2)(nu=0)-->NO(+)(nu=n-1)+N(2)(nu=1)+(32.52(n-1)+13.97) cm(-1) for n=1-3 in the 300-1000 K temperature range are performed. The VV ET probability is computed for three mechanisms: (1) The charge on NO(+) acting on the average polarizability of N(2) induces a dipole moment in N(2) which then interacts with the permanent dipole moment of NO(+) to mediate the energy transfer. (2) The charge on NO(+) acting on the anisotropic polarizability of N(2) induces a dipole moment in N(2) which then interacts with the permanent dipole moment of NO(+) to mediate the energy transfer. (3) The dipole moment of NO(+) interacts with the quadrupole moment of N(2) to mediate the energy transfer. Because the probability amplitudes of the second and third mechanisms add coherently the ET probability for these two mechanisms is given as a single number. The probability of energy transfer per collision is in the 5 x 10(-3) range. The results of this calculation are compared with the available experimental data. This calculation should help quantify the role of NO(+) in the energy budget of the upper atmosphere.     

Collisional energy transfer in tin by diatomics and inert gases

Collisional energy transfer between laser-excited atomic tin and both inert-gas and diatomic collision partners has been studied Relative populations of collisionally populated tin levels were determined For certain levels, diatomics transferred nearly two orders of magnitude more population than inert gases; for other levels there was nearly equal population transfer by both groups of partners.     

Dynamics of energy transfer in peptide-surface collisions

Classical trajectory simulations are performed to study energy transfer in collisions of protonated triglycine (Gly)(3) and pentaglycine (Gly)(5) ions with n-hexyl thiolate self-assembled monolayer (SAM) and diamond [111] surfaces, for a collision energy E(i) in the range of 10-110 eV and a collision angle of 45 degrees. Energy transfer to the peptide ions' internal degrees of freedom is more efficient for collision with the diamond surface; i.e., 20% transfer to peptide vibration/rotation at E(i) = 30 eV. For collision with diamond, the majority of E(i) remains in peptide translation, while the majority of the energy transfer is to surface vibrations for collision with the softer SAM surface. The energy-transfer efficiencies are very similar for (Gly)(3) and (Gly)(5). Constraining various modes of (Gly)(3) shows that the peptide torsional modes absorb approximately 80% of the energy transfer to the peptide's internal modes. The energy-transfer efficiencies depend on E(i). These simulations are compared with recent experiments of peptide SID and simulations of energy transfer in Cr(CO)(6)(+) collisions with the SAM and diamond surfaces.     

Energy transfer between polyatomic molecules. 1. Gateway modes, energy transfer quantities and energy transfer probability density functions in benzene-benzene and Ar-benzene collisions

We report collisional energy transfer, CET, quantities for polyatomic-polyatomic collisions and use excited benzene collisions with cold benzene bath, B-B, as our sample system and compare our results with the CET of excited benzene with Ar bath. We find that the gateway mode for both systems is the out-of-plane modes and that in B-B CET, vibration to vibration, V-V, is the dominant channel. Rotations play a mechanistic role in the CET but the net rotational energy transfer is small compared to V-V. The shape of the down side of the energy transfer probability density function, P(E,E'), is convex for B-B collisions and it becomes less so as the temperature increases. In Ar-B collisions, P(E,E') is concave and it becomes less so as the temperature decreases. We report average vibrational, rotational, and translational energy transferred, , as function of temperature for various initial conditions.     

Charge transfer in S2++H collisions at intermediate energy

Partial cross-sections for the charge transfer process of S²⁺ ions in collision with atomic hydrogen at impact energies up to 8 keV have been calculated by means of a semi-classical method using ab initio potential energy curves and couplings. The results are in relatively good agreement with experiment and improve significantly previous Landau-Zener calculations.     

Collisional energy transfer with statistical energy exchange: an analytical solution in the statistical limit

Collisional energy transfer between highly vibrationally excited molecules and bath gas is considered with a statistical kernel, describing energy exchange in complex-forming collisions. Knowledge of the bilinear formula for the Laguerre polynomials offers a means for determining eigenvalues and eigenfunctions of the kernel. An exact solution to master equation for the conditional probability is given as an expansion in terms of these eigenfunctions. The bulk averages of internal energy moments and energy transfer moments are calculated analytically.     

Surface-gas energy transfer in the oxetane decomposition system

The method called the variable encounter method was applied in the oxetane decomposition system for the study of vibrational energy transfer between gas molecules and the surface. The average probability of reaction per collision was derived from the experimental data and compared with theoretical calculations based on various energy transfer probability models. The Gaussian model fits the data well. The average down stepsize was found to be 3100 cm^–1 at 750 K and it decreased to 2200 cm^–1 at 1100 K.

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. , . . 3100 cm^–1 750 2200 cm^–1 1100 .

     

Vibrational energy transfer in N2-N2 collisions: a new semiclassical study

The vibrational energy relaxation in collisions between N2 molecules in the low- and medium-lying vibrationally excited levels was revisited using the semiclassical coupled-state method and the use of two different potential-energy surfaces having the same short-range potential recently determined from ab initio calculations but with different long-range interactions. Compared to the data reported in the classical work by Billing and Fisher [Chem. Phys. 43, 395 (1979)], the newly calculated vibration-to-translation rate constant K(1,0 / 0,0) is in much better agreement with the available experimental data over a large temperature interval, from T = 200 K up to T = 6000 K. Nevertheless, as far as the vibration-to-translation exchanges are concerned, the lower-temperature regime remains quite critical in that the new rate constants do not completely account for the rate constant curvature suggested by the experiments for temperatures lower than T = 500 K. The dependence of the state-selected vibration-to-vibration rate constants, K(v,v-delta v / 0,1), both upon the vibrational quantum number v and the gas temperature are calculated. The substantial deviations from previously found behaviors could have major consequences for the vibrational kinetic modeling of N2-containing gas mixtures.     

The role of orbiting collisions in vibrational energy transfer

Using a simple model of molecular collisions under a spherically symmetric interaction, it is shown that orbiting collisions can make very large contributions to the inelastic cross sections of non-resonant processes. Calculations for the system HX + CO₂(001) → HX(υ=1) + CO₂(000), where X = F, Cl, I show good agreement with experimental results.     

Modified statistical theory of energy transfer in ion-molecule collisions

The modified statistical theory developed previously for potentials appropriate to interactions in neutral-neutral collisions, is now extended to more strongly attractive potentials involved in ion-neutral collisions. The model system is the collisional deactivation of C₅H₉⁺ by a variety of both polar and non-polar neutral molecules. A 12 - 6 - 4 potential is used for ion interaction with non-polar neutrals, and a 12 - 6 - 4 - 2 potential, as modified by Su and Bowers to take into account the rotational energy of the neutral, for interaction with polar neutrals. Calculated is (ΔE), the average energy lost by the ion in a collision, and compared with experiment. For C₅H₉⁺-CH₄ collisions, the calculated (ΔE) agrees with experiment within 5%. Predictions of the theory, namely that (ΔE) should increase with excitation energy and should decrease with the size of the excited reactant, are found to be in fair agreement with the somewhat ambiguous experimental evidence.