Some aspects of energy transfer in molecular and atomic collisions. [Cross sections, statistical model, quenching; S matrix] (original) (raw)
Related papers
Cross sections for the excitation energy transfer induced by collisions with He, Ar and Na atoms
Journal of Physics B: Atomic, Molecular and Optical Physics, 1997
The cross sections for the Na(4D) → Na(4F) excitation energy transfer induced by collisions with He, Ar and Na ground-state atoms have been measured. The sodium atoms in the 4D state have been produced by two-photon excitation using cw laser radiation at 578.89 nm. Within a model which takes into account collisional population and depopulation of the 4F level as well as radiative relaxation (direct or cascade) of the 3D, 4P, 4D and 4F states ending in the 3P level, the rate for the Na(4D) → Na(4F) collisional transfer turns out to be related to the ratio of the steady-state populations created in 3D and 4D states. They have been determined by measuring the fluorescence intensities of the 3D → 3P and 4D → 3P transitions. The measurements yield the following cross section values for those processes investigated: σ Na−He 4D→4F = (60 ± 10) × 10 −16 cm 2 , σ Na−Ar 4D→4F = (28 ± 6) × 10 −16 cm 2 and σ Na−Na 4D→4F = (410 ± 50) × 10 −16 cm 2 , at T ≈ 630 K.
Theoretical investigation of collisional energy transfer in polyatomic intermediates
International Reviews in Physical Chemistry, 2013
Quantum scattering calculations on collisional rotational and vibrational energy transfer in small hydrocarbon reactive intermediates are highlighted. This review focuses on recent theoretical studies of energy transfer in methylene (CH 2 ), in both its ground tripletX 3 B 1 and low-lying singletã 1 A 1 electronic states, and in the methyl (CH 3 ) radical. Propensities in the state-to-state cross sections are shown to depend upon the two types of anisotropies that are present in potential energy surfaces of systems involving nonlinear polyatomic molecules. Computed rate constants for rotational and vibrational relaxation are compared with available experimental data. In addition, collisional transfer between the CH 2X andã states is addressed. Collision-induced intersystem crossing is shown to be mediated by spectroscopically perturbed rotational levels of mixed electronic character.
A molecular dynamics investigation of energy transfer efficiency in collisions of diatomic molecules
Chemical Physics, 1993
Diatomic collisional energy transfer has been studied wtth the aim of aiding the development of accurate representation of the activation-deactivation mechanism in gas phase reaction rate theory. We seek to determine the dependence of the energy transfer efficiency on the parameters describing the colliding molecules, i.e. mass, vibrationaf frequency, inte~olecular potential and initial energy or temperature. While weak van der Waals type interactions give rise to very poor energy transfer efftciency scaling up the mteractions to the strength of weak chemical bonds increases the energy transfer efftciency dmmatic~ly up to nearly statistical bmit values. The dependence on mass and vibrattonal frequency is found to be weaker. The interaction strength is resolved into two factors, hardness of encounter and attractive strength. Varying these factors independently by a modification of the intermolecular atomic pair potential, both are found to contribute strongly to the observed energy transfer efftciency. Two temperatures, 160 and 1500 K, are consIdered and the sensitivity to the lntermo~ecuiar potential 1s found to be distinctly greater at the lower temperature. The average energy transferred per colliston (AE) is obtamed as a function of the internal energy of the target molecule at normal and high interaction strength.
Chemical Physics Letters, 1993
Efficient Monte Carlo sampling methods are used to investigate the effect ofthe conservation of energy and angular momentum on energy transfer in Br,+Ar and Br,+Br, collisions. The effect of the conservation of spatial configuration in the collision is also considered. The results show that angular momentum conservation has a small ( 10%20%) but significant restraining effect on the energy transfer rate while the conservation of the spatial configuration reduced it by nearly 50%. The effects of anharmonicity in the bond potential of the diatomic molecule are also studied and found to be significant. Comparison with trajectory calculations reemphasizes the observation that for small molecules the energy transfer approaches the statistical limits only for strong interaction potentials.
Progress on the modeling of the collisional energy transfer mechanism in unimolecular reactions
Berichte der Bunsengesellschaft für physikalische Chemie
Words: Computer Experiments / Elementary Reactions / Energy Transfer / Molecular Collisions Statistical Theory The RRKM theory of unimolecular reaction rates is a statistical mechanical theory based on an assumption of microcanonical equilibrium in the reactant phase space. The energy transfer in reactant medium collisions was originally described by a canonical strong collision assumption, i.e., an assumption of full thermal equilibration in each collision. In our work we first introduce a microcanonical strong collision assumption which gives the RRKM theory a consistent form. we then introduce parametrizations of the degree of weakness (nonergodicity) of the collisions. A concept of collision efficiency is defined. The weakness of the collision is expressed in terms of reduced subsets of active reactant and medium degrees of freedom. The corresponding partially ergodic collision theory (PECT) yields physical functional forms of the collisional energy transfer kernel P (E ' , E). In order to resolve the energy and temperature dependence and the dependence on interaction strength a multiple encounter theory is introduced (PEMET). Initially each encounter may be described by a semiempirical PECT model. Eventually the encounters may be resolved by quantum dynamical calculations of the semiclassical or CAQE (classical approach/quantum encounter) type. Simple statistical collision models only distinguish between "hits and misses". In reality the energy transfer efficiency exhibits characteristic fall off with increasing impact parameter b. This b-dependence can be explicitly accounted for in the master equation for the reaction rate coefficient.
Reaction probability and energy transfer in collisions of sodium atoms with sodium dimers
Chemical Physics, 1977
The Monte Carlo classical trajectory technique was employed to study reaction probability and energy transfer in collisions of sodium atoms with sodium dimers. The calculations were carried out for a fixed total collisional energy of 14 kcal/mole. The relative amounts of initial relative translational. rotationaLand vibrational energy were varied in order to study the effects of different partitionings of the total energy. In addition, two fixed initial configurations of the atom and dimer were investigated. It was found that the behavior of the reaction probability plotted as a function of impact parameter, the reactive cross section, and the amount of energy transferred far both reactive and nonreactive inelastic collisions were very sensitive to both the amount of initial relative tnnslationll energy and to the partitioning of the initial dimer energy between rotational and vibrational energy. Far the higher values of relative translational energy, nonreactive collisions were much more important than nonreactive ones in effecting relas;ltian of the dimes, while the opposite situation was found at low relative translational energies.
Energy transfer in reactive and nonreactive collisions of Na with Na2
Chemical Physics, 1976
Stimulated by the experimental fading of vibrationally and rotationally cold dimers in supersonic nozzle molecular beams of sodium, we have studied energy transfer in collisions of Na with Naz over a wide range of initial relative translation energies E and impact parameters b by a classical mechanical trajectory method. The vibrational and rotational enef gies were initialized using Boltzmann distributions characterized by temperatures Ttib = 150 K, Trot = 50 K. We fmd that for large values of E the energy transfer in reactive colliiions 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 in&d relative tr&tional 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.
The study of internal energy transfer processes in N2-N2 collisions has found a renewed interest over the last years, in connection with the role such events play in a wide range of temperature regimes, in atmospheric chemistry and physics and in the development of plasma and aerospace technologies. One of the most efficient approaches to calculate vibration to vibration (VV) energy transfer relies on a quantum-classical method, which couples a rigorous quantum mechanical treatment of the vibrations and a quasiclassical description of the other degrees of freedom , allowing for the calculation of energy exchange probabilities for a large body of state selected processes at a reasonable computational cost. The accuracy of the results however depends on the ability of the potential energy surface to correctly describe both long and short range interactions which dominate the outcome of the collisions at different temperatures. In this work we examine the effect of using alternative potential energy surfaces, differing either for the value of the employed parameters and for their formulation, on VV cross sections and rate constants.
1997
The cross sections for the Na.4D/! Na.4F/ excitation energy transfer induced by collisions with He, Ar and Na ground-state atoms have been measured. The sodium atoms in the 4D state have been produced by two-photon excitation using cw laser radiation at 578.89 nm. Within a model which takes into account collisional population and depopulation of the 4F level as well as radiative relaxation (direct or cascade) of the 3D, 4P, 4D and 4F states ending in the 3P level, the rate for the Na.4D/ ! Na.4F/ collisional transfer turns out to be related to the ratio of the steady-state populations created in 3D and 4D states. They have been determined by measuring the fluorescence intensities of the 3D ! 3P and 4D! 3P transitions. The measurements yield the following cross section values for those processes investigated: Na He 4D!4F D.60 10/ 10
Trajectory Calculations of Intermolecular Energy Transfer in H2O + Ar Collisions
Journal of Physical Chemistry A, 1999
The collisional energy transfer between excited SO2 and Ar has been simulated by classical trajectory calculations. The influences of the intramolecular and intermolecular potentials, of temperature, and of excitation of SO2 on vibrational and rotational energy transfer are demonstrated. Average energies transferred and collisional transition probabilities are reported. Permanent address: Australian Defence Force Academy, Canberra. measure this quantity. Instead, in general only first and second moments of k with respect to AE = E '-E are accessible ex-perime~~taIly.'-'~ With the assumption that total rate coefficients (1) Troe, J.