Collision partner

The inert collision partner M is assumed to be present in large excess ... 

It is clear from figure A3.4.3 that the second-order law is well followed. Flowever, in particular for recombination reactions at low pressures, a transition to a third-order rate law (second order in the recombining species and first order in some collision partner) must be considered. If the non-reactive collision partner M is present in excess and its concentration [M] is time-independent, the rate law still is pseudo-second order with an effective second-order rate coefficient proportional to [Mj. 

In contrast to the bimoleciilar recombination of polyatomic radicals ( equation (A3.4.34)1 there is no long-lived intennediate AB smce there are no extra intramolecular vibrational degrees of freedom to accommodate the excess energy. Therefore, the fonnation of the bond and the deactivation tlirough collision with the inert collision partner M have to occur simultaneously (within 10-100 fs). The rate law for trimoleciilar recombination reactions of the type in equation (A3.4.47) is given by... 

The collision partners may be any molecule present in the reaction mixture, i.e., inert bath gas molecules, but also reactant or product species. The activation k and deactivation krate constants in equation (A3.4.125) therefore represent the effective average rate constants. 

Some of the internal (rovibronic) energy of the atomic and molecular collision partners is transfomred into extra translational energy AE = (or consumed, if A E is negative). If one averages over a thennal... 

After defining the input parameters, the calculation of the trajectories of an incident ion begins with a randomly chosen initial entrance point on the surface. The next step is to find the first collision partner. 

Taking advantage of the synnnetry of the crystal structure, one can list the positions of surface atoms within a certain distance from the projectile. The atoms are sorted in ascending order of the scalar product of the interatomic vector from the atom to the projectile with the unit velocity vector of the projectile. If the collision partner has larger impact parameter than a predefined maximum impact parameter discarded. If a... 

Inelastic scattering produces a pennanent change in the internal energy and angrilar momentum state of one or both structured collision partners A and B, which retain their original identity after tire collision. For inelastic = (a, P) — /= (a, P ) collisional transitions, tlie energy = 1 War 17 of relative motion, before ( ) and after... 

Apart from the natural lifetime due to spontaneous emission, both uni- and bimolecular processes can contribute to the observed value of T. One important contribution comes from coiiisionai broadening, which can be distmguished by its pressure dependence (or dependence upon concentration [M] of tlie collision partner) ... 

Elastic scattering. An ion/neutral interaction wherein the direction of motion of the ion is changed, but the total translational energy or internal energy of the collision partners remains the same. 

Superelastic collision. A collision that increases the translational energy of the fast-moving collision partner. 

The attractive energies 4D(cr/r)6 and ae2/2 r4 have two important effects on the vibrational energy transfer (a) they speed up the approaching collision partners so that the kinetic energy of the relative motion is increased, and (b) they modify the slope of the repulsive part of the interaction potential on which the transition probability depends. By letting m °°, we have completely ignored the second effect while we have over-emphasized the first. Note that Equation 12 is identical to an expression we could obtain when the interaction potential is assumed as U(r) = A [exp (— r/a)] — (ae2/2aA) — D. Similarly, if we assume a modified Morse potential of the form... 

Collisions at low ion energies (where Equation 1 can be applied) lead to a short-lived complex between the ion and the molecule—i.e., both collision partners move with the same linear velocity in the direction of the incident ion. The decay of the complex may be described by the theory of unimolecular rate processes if its excess energy can fluctuate between the various internal degrees of freedom. For example, the isotope effect in the reaction of Ar+ with HD may be explained by the properties of... 

Ne is metastable neon produced by electron impact. Ne transfers its excitation to hydrogen molecules. The hydrogen molecules participating in these energy transfer collisions are produced in highly excited preionized states which ionize after a time lag sufficient to permit the initial neon and hydrogen collision partners to separate. The hydrogen ion is formed in the v = 5 or 6 quantum states and reacts with a second neon... 

Numerous oxidation reactions of sulfur compounds have been described in which S2O or its precursor SO are formed as intermediates but most of these reactions are not suitable to investigate the properties of S2O because of the low yield or the interference from by-products [1]. A relatively clean process is the reaction of oxygen atoms with COS producing SO and CO. The formation of S2O from gaseous SO is a stepwise process according to the following equations (M is a collision partner) [21] ... 

The formation of S2O from gaseous SO proceeds according to the following equations (M collision partner) ... 

Mechanism I is a three-step process in which the first step is rate-determining. When the first step of a mechanism is rate-determining, the predicted rate law is the same as the rate expression for that first step. Here, the rate-determining step is a bimolecular collision. The rate expression for a bimolecular collision is first order in each collision partner Rate = j i[03 ][N0 j Mechanism I is consistent with the experimental rate law. If we add the elementary reactions, we find that it also gives the correct overall stoichiometry, so this mechanism meets all the requirements for a satisfactory one. 

Based on the molecular collision cross-section, a particle might undergo a collision with another particle in the same cell. In a probabilistic process collision partners are determined and velocity vectors are updated according to the collision cross-section. Typically, simple parametrizations of the cross-section such as the hard-sphere model for monoatomic gases are used. 

In reaction C the N02 itself does not react but plays the role of a collision partner that may effect the decomposition of the N03 molecule. The N02 and N03 molecules may react via the two paths indicated by the rate constants k2 and k3. The first of these reactions is believed to have a very small activation energy the second reaction is endothermic and consequently will have an appreciable activation energy. On the basis of this reasoning, Ogg (4) postulated that k3 is much less than k2 and that reaction C is the rate controlling step in the decomposition. Reaction D, which we have included, differs from the final step postulated by Ogg. 

The third feature is the recruitment of a molecule, in this case Sos, to the membrane in order to fulfill its function. Both increased local concentrations at the membrane and two-dimensional diffusion of the collision partners, Sos and Ras, lead to enhanced encounter of the two proteins and thus to accelerated nucleotide exchange on Ras. 

The notion of a collision implies at least two collision partners, but collision-based theories are applicable for theories of unimolecular reactions as well. 

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