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 .

     

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 decomposition of 2-methyloxetane and 3-methyloxetane, II. Gas/wall collisions

The variable encounter method has been applied for the study of gas/wall vibrational energy transfer in the thermal decomposition of 2-methyloxetane and 3-methyloxetane. The average probability of reaction per collision was deduced from the data and was compared with stochastic calculations based on gaussian and exponential energy transfer probability models. The gaussian model fits the experimental data best. Using this model, the average down step energies were 2600 and 2690 cm^–1 for 2-methyloxetane and 3-methyloxetane, respectively, at 750 K, and they decreased with increasing temperature.

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/ 2- 3-. , . . , 2600 2690 ^–1 750 K 2- 3-, . .

     

Collisional energy transfer to methane by octupole coupliing

A linear molecule or one electron atom interacting with a tetrahedral molecule is considered. Formulae are presented for various rotationally averaged E → R and V → R first order transition probabilities arising from dipole-octupole (R^?5) and quadrupole-octupole (R^?6) coupling. Relatively large amounts of rotational energy can be transferred in first order (ΔJ ? 3). With CH₄ as the octupolar partner, energy transfer upto 350 cm^?1 at room temperature is shown to proceed very efficiently. For the 5²P_{$\text{3}\text{2}$} → 5²P_{$\text{1}\text{2}$} transition in Rb, σ_qu is calculated to be ? 28 A² in quite good agreement with experiment and using an independent value of the octupole moment of CH₄. Energy transfer above 350 cm^?1 becomes rapidly less efficient. Among V → R transfer processes in the 500–700 cm^?1 range, the long range mechanism is almost certainly not the dominant one in the relaxation of CO₂⁺ (010) by CH₄ (σ_calc ≈ 10^?3 σ_obs) but will be important in the relaxation of SO₂.     

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.     

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.     

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 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.     

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.     

Activation energy for thermal decomposition of nitric oxide

Variation of the reaction mechanism for homogeneous thermal decomposition of NO into N₂ and O₂ in the temperature range between 1000 and 4000 K is studied. The decomposition always proceeds through an atomic chain mechanism initiated by formation of oxygen atom. However the step of the oxygen atom initiation depends on the reaction condition, i.e., collision between two NO molecules at low conversions (when P_O2/P_NO ratio≪ ≪ 1) and collision between NO and O₂ and/or unimolecular decomposition of O₂ at high conversions (after substantial O₂ has been accumulated from the reaction). In this study, apparent activation energy (E_app) of the decomposition reaction has been theoretically determined on the basis of our proposed mechanisms. The E_app thus determined varies widely (from 254 to 401 kJ mol⁻¹) with the accepted step of initiation. This variation can account for the variations among experimental activation energies for the decomposition reaction in the literature. © 1996 John Wiley & Sons, Inc.     

The unimolecular thermal decomposition of oxetane and its methyl derivatives: An Ab initio and RRKM calculations

Quantum mechanical and Rice-Ramsperger-Kassel-Marcus (RRKM) calculations are carried out to study the thermal unimolecular decomposition of oxetane (1), 2-methyloxetane (2), and 2,2-dimethyloxetane (3) at the MPW1PW91/6-311 + G** level of theory. The results of the calculations reveal that decomposition reaction of compounds 1?C3 yields formaldehyde and the corresponding substituted olefin. The predicted high-pressure-limit rate constants for the decomposition compounds 1?C3 are represented as 6.61 × 10¹³exp(?32472/T), 9.33 × 10¹³exp(?29873/T), and 4.79 × 10¹³exp(?27055/T) s^?1, respectively. The fall-off pressures for the decomposition of compounds 1?C3 are found to be 9.42 × 10^?2, 3.67 × 10^?3, and 7.26 × 10^?4 mm Hg, respectively. As the fall-off pressure of the decomposition process of compounds 1?C3 are in the following order: P _1/2(1) > P _1/2(2) > P _1/2(3); therefore the decomposition rates are as follow: rate(1) < rate(2) < (3).     

The study of decomposition reactions through the exchange of thermal energy

In many decomposition reactions, the reaction velocity can be described as a product of two functions: a temperature dependent part K(T) and the kinetic function f(1 – α), where T designates the temperature and α the fraction of reactant that has decomposed. The physical interpretation of these functions is discussed for both solid and homogeneous systems. A method is described by which f(1 – α) and K(T) can be determined from kinetic data. The mechanism of decomposition can subsequently be identified which should be consistent with the derived kinetic parameters. The method has been applied to analyze the kinetics of the thermal decomposition of nitromethane. © 1994 John Wiley & Sons, Inc.     

On the non-isothermal determination of activation energy for thermal decomposition reactions

Attention is called to the frequently encountered linear sector in the isothermal course of the topochemical processes which is used in the kinetic treatment of the differential curves of thermal CoOOH decomposition. The effect of sample weight and heating rate on the values of the activation energy of this process under polythermal conditions has been investigated. Most of the values obtained are in good agreement among themselves as well as with those found isothermally. Existing differences have been explained satisfactorily.     

Collisional transfer of population and orientation in NaK

Collisional satellite lines with |ΔJ| ≤ 58 have been identified in recent polarization spectroscopy V-type optical-optical double resonance (OODR) excitation spectra of the Rb(2) molecule [H. Salami et al., Phys. Rev. A 80, 022515 (2009)]. Observation of these satellite lines clearly requires a transfer of population from the rotational level directly excited by the pump laser to a neighboring level in a collision of the molecule with an atomic perturber. However to be observed in polarization spectroscopy, the collision must also partially preserve the angular momentum orientation, which is at least somewhat surprising given the extremely large values of ΔJ that were observed. In the present work, we used the two-step OODR fluorescence and polarization spectroscopy techniques to obtain quantitative information on the transfer of population and orientation in rotationally inelastic collisions of the NaK molecules prepared in the 2(A)(1)Σ(+)(v' = 16, J' = 30) rovibrational level with argon and potassium perturbers. A rate equation model was used to study the intensities of these satellite lines as a function of argon pressure and heat pipe oven temperature, in order to separate the collisional effects of argon and potassium atoms. Using a fit of this rate equation model to the data, we found that collisions of NaK molecules with potassium atoms are more likely to transfer population and destroy orientation than collisions with argon atoms. Collisions with argon atoms show a strong propensity for population transfer with ΔJ = even. Conversely, collisions with potassium atoms do not show this ΔJ = even propensity, but do show a propensity for ΔJ = positive compared to ΔJ = negative, for this particular initial state. The density matrix equations of motion have also been solved numerically in order to test the approximations used in the rate equation model and to calculate fluorescence and polarization spectroscopy line shapes. In addition, we have measured rate coefficients for broadening of NaK 3(1)Π ← 2(A)(1)Σ(+)spectral lines due to collisions with argon and potassium atoms. Additional broadening, due to velocity changes occurring in rotationally inelastic collisions, has also been observed.     

Collisional excitation transfer between lithium isotopes

Excitation transfer between the 3S _1/2 states of the lithium isotopes⁶Li and⁷Li is measured in a thermionic diode. The 3S level is excited by off-resonant two-photon transitions with a single mode ew laser. The relative densities of the directly excited and collisionally populated levels are probed by further laser excitation to the 12P levels. An energy transfer cross section of 585 Ų±160 Ų is found at the experimental temperature of about 850 K. A simple semiclassical calculation yields a cross section of 450 Ų.     

Collisional energy transfer in the intermediate states used for optical-optical double resonance excitation of ion-pair states in I2

The optical-optical double resonance spectra of I(2) and I(2)-Xe mixtures at room temperature reported in the literature using a fixed-wavelength, broad band pump laser have now been recorded using a tuneable, narrow band source. We show that during the time of the overlapped laser pulses ( approximately 10 ns) and with 10-20 Torr of Xe there is widespread collisional energy transfer in the intermediate state and that this phenomenon offers an alternative explanation for the broad bands in the excitation spectrum, assigned to XeI(2) complexes by the authors of the earlier study (M. E. Akopyan, I. Y. Novikova, S. A. Poretsky and A. M. Pravilov, Chem. Phys., 2005, 310, 287). Dispersed emission bands, previously attributed to direct fluorescence from the ion-pair state(s) of the complexes, are re-assigned to emission from ion-pair states of the parent I(2) that are populated by collisional energy transfer out of the initially excited state.     

Collisional relaxation of transient vibrational energy distributions in a thermal unimolecular system. The variable encounter method

A method has been developed, called the Variable Encounter Method, for the study of the relaxation of an initial vibrationally cold ensemble of molecules into a vibrationally hot distribution by a known and variable number of successive collisions with a hot wall. The theory of the experiment is presented. The system studied was the isomerization of 1,1-cyclopropane-d₂ with a fused quartz wall temperature of 800 K to 1175 K, and average number of collisions from 2.3 to 22.3. Various modified gaussian and exponential models of energy transfer were found to give agreement with the data. The average down-step size was found to decline from ≤ 3500 cm^?1 at the lowest temperature to ≈ 2500 cm^? at the highest on the basis of a gaussian model. A mathematical analysis of the relation between mean first passage times and incubation times is given. Incubation times increase from ≈ 7 to ≈ 12 collisions with increasing temperature. Transient population distributions and the sequential reaction probabilities as a function of collision number are calculated.     

A theoretical approach to the role of transfer reactions in the thermal decomposition of polymers

The relative importance of intermolecular and intramolecular transfers is investigated in the thermal degradation of polymers, in particular polystyrene.It is indicated that any mechanism for the pyrolysis behaviour of this polymer must include both modes of transfer. The model is offered as a diagnostic tool for mechanistic investigations of the thermal decompositions of polymers.     

Collisional energy dependence of peptide ion fragmentation

The energy dependence of fragmentation in a collision cell was measured for 2400 protonated peptide ions derived from the digestion of 24 proteins. The collision voltage at which the sum of the fragment ion abundances was equal to the remaining parent ion (V _1/2) was the principal measure of fragmentation effectiveness. Each class of peptides was characterized by a linear relation between V _1/2 and m/z whose slope depended on the peptide class and, with little adjustment, intersected the origin. Peptide ions where the number of protons is no greater than the number of arginine residues show the greatest slope, V _1/2/(m/z)=0.0472 (all slopes in units of V Da⁻¹ e). For peptides where the number of protons is greater than the number of arginines, but not greater than the total number of basic residues, the slope decreases to 0.0414 for singly charged ions, 0.0382 for doubly charged, 0.0346 for triply charged, and 0.0308 for more highly charged ions. With one mobile proton, the slope is about 0.029 for singly and doubly charged ions and slightly lower for more highly charged ions. With two or more mobile protons the slope is 0.0207. By removing m/z dependence, the deviation of V _1/2 from a line provides a relative measure of the ease of fragmentation of an ion in each class. This information can guide the selection of optimal conditions for tandem mass spectrometry studies in collision cells for selected peptide ions as well as aid in comparing the reactivity of ions differing in m/z and charge state.     

Potentials of boiling heat transfer in advanced thermal energy systems

Journal of Thermal Analysis and Calorimetry - Boiling process is a highly efficient mechanism of heat transfer, which has an important role in industrial and domestic sectors. In this process, a...