Diffusion of vibrationally excited molecules

A treatment is presented for the effect of intermolecular vibrational energy transfer on the diffusion coefficient of vibrationally excited molecules. An analytic treatment based on random walk statistics and a Monte Carlo type calculation have been performed. Both methods yield very similar results which correlate well with existing experimental studies. A hard sphere collision model is treated extensively with comparisons made to other internmolecular potentials. The results support the involvement of long range energy transfer in V → V interactions. The effect of temeprature on the diffusion coefficient of vibrationally excited molecules is calculated, with applications to the CO^*₂CO₂ system.     

Relaxation of highly vibrationally excited molecules

The rate constant for V-V relaxation of diatomic homonuclear molecules is determined from collisions of unexcited molecules with molecules near the dissociation threshold. It is shown that a quasi-resonant transition through several levels dominates in this process so that the energy difference between the initial and final states of the excited molecule is approximately equal to the transition energy from the zero level to the first one. The relaxation kinetics of excited molecules is studied. Absorption of IR radiation by polyatomic molecules is discussed taking into account collisions. A criterion for the negligibility of energy loss is obtained, and the dissociation rate of molecules under the action of IR laser irradiation found. The computational results are compared with experimental data. A self-consistent procedure is formulated for a gas irradiated by a quasi-continuously operating IR laser, in order to determine simultaneously the dissociation rate, dissipation energy flux and temperature. The existence of an optimum radiation region for dissociation is found.     

The decomposition of vibrationally excited molecules

Possible decomposition routes for vibrationally excited hydrocarbons are critically considered using a simple “symmetry” based model. It is shown that C---C bond rupture is characterised by a lower activation energy than the molecular cleavage of hydrogen, although the latter reaction is less endothermic. Other possible decomposition reactions of excited species are also considered.     

Bimolecular reactions of vibrationally excited molecules. Roaming atom mechanism at low kinetic energies

Quasiclassical trajectory calculations have been performed for the H + H'X(v) → X + HH' abstraction and H + H'X(v) → XH + H' (X = Cl, F) exchange reactions of the vibrationally excited diatomic reactant at a wide collision energy range extending to ultracold temperatures. Vibrational excitation of the reactant increases the abstraction cross sections significantly. If the vibrational excitation is larger than the height of the potential barrier for reaction, the reactive cross sections diverge at very low collision energies, similarly to capture reactions. The divergence is quenched by rotational excitation but returns if the reactant rotates fast. The thermal rate coefficients for vibrationally excited reactants are very large, approach or exceed the gas kinetic limit because of the capture-type divergence at low collision energies. The Arrhenius activation energies assume small negative values at and below room temperature, if the vibrational quantum number is larger than 1 for HCl and larger than 3 for HF. The exchange reaction also exhibits capture-type divergence, but the rate coefficients are larger. Comparisons are presented between classical and quantum mechanical results at low collision energies. At low collision energies the importance of the exchange reaction is enhanced by a roaming atom mechanism, namely, collisions leading to H atom exchange but bypassing the exchange barrier. Such collisions probably have a large role under ultracold conditions. The calculations indicate that for roaming to occur, long-range attractive interaction and small relative kinetic energy in the chemical reaction at the first encounter are necessary, which ensures that the partners can not leave the attractive well. Large orbital angular momentum of the primary products (equivalent to large rotational excitation in a unimolecular reaction) is favorable for roaming.     

Temperature dependence of the quasiclassical reactivity for vibrationally excited hydrogen molecules colliding with hydrogen atoms

A discussion of reactants vibrational energy and temperature dependence of reactive rate constants for the hydrogen atom hydrogen molecule reaction is presented for a matrix of values calculated at 0 < v <10 and temperatures in the range from 300 K to 4000 K. A parametrization ofthe results is attempted. A comparison with rate constant values obtained from an approximate quantum treatment is also reported.     

Non-radiative decay rate of vibrationally excited triplet molecules

The non-radiative decay rates of triplet benzene and 2-chloronaphthalene were determined as a function of excitation energy. As the excitation energies were increased, the non-radiative decay rates increased gradually. In the case of 2-chloronaphthalene it increased rapidly for the excitation energies above about 38000 cm^-1.     

Collisional deactivation of highly vibrationally excited hexafluorobenzene molecules

The collisional deactivation of the internal energy of vibrationally highly excited hexafluorobenzene (HFB) molecules was examined by the analysis of ultraviolet absorption spectra of excited HFB molecules produced by excitation with an ArF(193 nm) laser. The decay time profile of the internal energy was calculated from the observed absorption decay profile of the hot molecule using the conversion relation between the absorbance by hot molecules and the internal energy. Thus the average energy 〈ΔE〉 transferred per collision was estimated by two different models; energy-independent and energy-dependent function for the decay of the internal energy. The obtained values of 〈ΔE〉 indicate that the energy-dependent model may give reasonable values for 〈ΔE〉, but as far as the value of 〈ΔE〉 is concerned, the energy-independent model is likely to be applicable to the analysis in this reaction system. The collisional deactivation mechanism of the hot HFB molecule and the heating-up effect observed at shorter wavelengths are discussed on the basis of the conversion curve.     

Supercollisions and energy transfer of highly vibrationally excited molecules

Collisional energy-transfer probability distribution functions of highly vibrationally excited molecules and the existence of supercollisions remain as the outstanding questions in the field of intermolecular energy transfer. In this investigation, collisional interactions between ground state Kr atoms and highly vibrationally excited azulene molecules (4.66 eV internal energy) were examined at a collision energy of 410 cm-1 using a crossed molecular beam apparatus and time-sliced ion imaging techniques. A large amount of energy transfer (1000-5000 cm-1) in the backward direction was observed. We report the experimental measurement for the shape of the energy-transfer probability distribution function along with a direct observation of supercollisions.     

Spontaneous desorption of vibrationally excited molecules physically adsorbed on surfaces

The energy within a vibrationally excited physisorbed molecule often exceeds that needed to break its bond to the surface. Energy transfer from the vibrating chemical bond to the surface bond causes the surface bond to rupture and the vibrationally relaxed adsorbate is released from the surface. We present a theoretical model which allows an estimation of the residence time of a vibrationally excited adsorbate on a surface. Because of uncertainties in the nature of the surface bond, the lifetimes obtained from the analytical expressions presented have only qualitative significance. The results are interpreted in terms of Franck-Condon overlaps between the wavefunctions which describe the adsorbate-substrate complex and the released adsorbate. Lifetimes are calculated for hydrogen isotopes adsorbed on sapphire surfaces. Guide-lines are given for estimating lifetimes of other systems in terms of a few easily calculated parameters.Let us summarize this guide to spontaneous desorption of physically adsorbed vibrationally excited molecules. The most efficient desorption processes will occur for adsorbates with a small number of bound states (d₀ small) and when released the adsorbate has small translational momentum (small q_m). This momentum gap correlation is most succinctly revealed by fig. 3. Smaller translational momentum will be achieved if the adsorbate can take up energy into its internal motions. Absorption of energy into lattice modes of the substrate will also serve to reduce the translational momentum and provide for more efficient desorption. However, if the vibrational frequency of the adsorbate is in near resonance with surface polarons or plasmons of the substrate, energy transfer to the solid will be so efficient that desorption will be quenched.A test of these possible relaxation channels awaits the first experimental measurements of desorption of vibrationally excited molecules.     

Energy transfer in collisions of highly vibrationally excited tetrahedral molecules

Model trajectory calculations of the energy transfer processes in collisions of Ar with highly vibrationally excited CH₄, CD₄, SiH₄ and CF₄ are performed. Special attention is payed to the calculation of the energy transferred to active (vibrational) degrees of freedom. The results support the diffusion model of excitation-dissociation and give the low pressure collision efficiency β_c which qualitatively agrees with experiment in magnitude and temperature dependence.     

The formation of vibrationally excited HD from atomic recombination on cold graphite surfaces

HD molecules formed in v"=3 and v"=4 have been detected by laser spectroscopy when a cold (15 K) graphite surface is irradiated with H and D atoms. Population of the v"=3, J"=0-6 and v"=4, J"=0-6 levels has been detected and the average rotational temperatures of the nascent HD were determined. These results are compared with previous data collected for the formation of HD in v"=1 and 2 under similar conditions. This comparison indicates that the nascent HD flux increases with increasing vibrational quantum number for v"=1-4.     

Reactivity of vibrationally excited methane molecules in reactions with chlorine atoms

At 148–298 K, the rate constant for the reaction of methane molecules excited into bending vibration with atomic chlorine does not exceed by more than 30 times the corresponding constant for methane in thermal equilibrium. Consequently, at low temperatures and thermal equilibrium the reaction of methane with atomic chlorine proceeds through the vibrational ground state of methane.

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, 148–298°K , , 30 , . , .

     

Decomposition of chemically activated dimethylcyclopropane molecules vibrationally excited to various extent

The photolyses of ketene (at 313 and 280 nm) and diazirine at 313 nm in the presence of cis-butene-2 were studied. Vibrational relaxation of chemically activated dimethylcyclopropane was shown to occur as a multistep process, and 17 ± 4 kJ mol^?1 was obtained for the average energy transferred per collision with butene-2 collider. Activated cis-dimethylcyclopropane is formed in the reaction of singlet methylene and cis-butene-2 with broad energy distribution which originates from the energy partitioning in the photolytic act. About 30% of the energy released in the photolysis of the methylene source is carried by singlet methylene as vibrational energy at the time of reaction, and this fraction was found to be practically independent of the radical source.     

Non-equilibrium vibrational kinetics of CO pumped by vibrationally excited nitrogen molecules: General theoretical considerations

The non-equilibrium vibrational kinetics of CO pumped by vibrationally excited N₂ has been calculated by solving the vibrational master equations for both N₂ and CO molecules, linked by V-V (N₂CO) energy exchange processes. The results have been obtained for different values of the parameters governing the kinetics (in particular gas temperature, initial vibrational content of N₂, different mixing ratios N₂/CO). Emphasis is also given to dissipative channels present in CO, due to bimolecular reactions involving highly vibrationally excited CO molecules. The results shows the essential features of the temporal evolution of CO and N₂ vibrational distribution as well as the strong coupling existing between them.     

六氟化铀振动激发态的紫外吸收变化

本文以脉冲CO2激光激发SF6分子,经碰撞V-V传能产生UF6振动激发态,在波长220-330nm范围内研究其紫外吸收瞬时变化.在静态池的条件下,分别测定了脉冲CO2激发能量以及SF6和UF6的分压对紫外吸收变化的影响,曾对2.0TorrSF6和2.0TorrUF6混合气体在激光脉冲能量为160mJ时,振动受激的UF6分子的紫外吸收变化与波长的关系作了研究,发在在240,248,290和313nm处有明显的吸收变化的峰值,这可能系几种不同的高振动态SF6分子的吸收截面的变化所引起的.     

Production of a beam of highly vibrationally excited CO using perturbations

An intense molecular beam of CO (X(1)Σ(+)) in high vibrational states (v = 17, 18) was produced by a new approach that we call PUMP - PUMP - PERTURB and DUMP. The basic idea is to access high vibrational states of CO e(3)Σ(-) via a two-photon doubly resonant transition that is perturbed by the A(1)Π state. DUMP -ing from this mixed (predominantly triplet) state allows access to high vibrational levels of CO (X(1)Σ(+)). The success of the approach, which avoids the use of vacuum UV radiation in any of the excitation steps, is proven by laser induced fluorescence and resonance enhanced multi-photon ionization spectroscopy.